| capslock |
This is a post by ppl, quoting AKSA and adding comments of his own. It is taken from an old thead on the Halcro amp:
[QUOTE]Originally posted by AKSA
[B]I would like to affirm John's observations, and add a few of my
------------------------------------------------------------------------------------
1. Excessive negative feedback does indeed bring back single tone distortion measurements, but creates a
myriad of high-order, subliminal, and often odd-order artefacts which are highly objectionable to the human
ear. For example (I read this somewhere years ago, but cannot remember the source!): A trumpet played
hard and loud has an additional 0.05% of H5, H7 and H9 over a quietly played trumpet, yet if adjusted for
amplitude and heard from some distance, sounds very different despite a H2/H3/H4 spectral composition
essentially the same. This very clearly draws attention to the spectral distribution of the distortion,
something not given much credence in anything but tube circles.
2. Because of the near infinite impedance presented to a voltage amplifying device by a current source,
tubes and SS, the device is thus able to offer its full voltage amplification. In a tube, this is mu, in a
transistor, it is the ratio of the collector to the emitter impedance, with the influence of beta thrown in, and
is typically 60dB. The stage gain of a transistor with a near-infinite collector load is very different to a finite
load. For a given overall gain, this naturally increases feedback factor, which, beyond a certain point as John
points out, is undesirable for sound quality.
3. A current mirror or source is also very fast, and furthermore makes the gain of the stage extremely
sensitive to impedance changes in the following, driven stage. If the output stage is push pull, the variation
in impedance of this load with signal is quite radical, yet this is rarely discussed in light of the uniformly high
impedance presented by the current source supplying current to the amplifying device. Global negative
feedback is expected to 'fix' this problem, and yet the impedance changes, like a tube grid moving into
positive bias, is quite sharp at the crossover transition.
4. We need to give more attention to the voltage amplifying device itself, since we need to pull its open loop
gain back to below unity at the pole frequency by adding lag compensation across its input/output
(base/collector). Rather more lag compensation is required with a current mirror load; this is because we
must pull the OLG back to below unity by the pole frequency to avoid instability. Because the usual
6dB/octave single pole compensation is contending with more OLG to begin with, this compensation is more
savage than it otherwise might be. Lag compensation is bad because it slows the amplifier and traditionally
the voltage amplifier is the slowest stage of a global nfb amp. The amp must be nimble. If it were possible to
pull back OLG by using a finite load which rapidly increased its loading at higher frequencies for other,
unrelated reasons, then such savage compensation might not be necessary.
5. In closing, I would say that all amplifiers sound worse as one increases the lag compensation. Too much is
leaden and flat; too little is fuzzy - and risky for tweeters - as the amp lapses into short term instability on
transients. The trick is to arrange it so that this compensation is both optimal and minimal - a tall order, but
one with sonic rewards.
------------------------------------------------------------------------------------
Hugh has made some importent observattions regarding Audio Amp Design. I for the most part agree with the
Above statments and totaly agree with respect to Lag Compensation. Years ago when i would do Mod's on
Audio Amps,One of the first thing to get removed or Reduced was the Phase lag Network. this greatly
improved the Midrange and High End by reducing the harshness and glare of the Amp. To maintain Stability I
would then reduce the Openloop gain by eather using Emmiter resistors on the Input stage diff Amp and or
Puting a load resistor on the Output of the Second Vas stage. Sometimes using a higher value of emmiter
resistor than was used on this stage was done to futher reduce the Open Loop gain if needed. The Load
resistor would also somewhat offset the Dramatic impedance changes seen by the Second vas stage from
the Output Stages input Impedance Changes. The result was alot Cleaner and more stable Amp.
When I did my own Amp designs i would then not fall into the trap of using all sorts of Compensation
methods to stabilize the Amp Circuit. Phase Lead compensation is also bad as it slowes the Amp down I like
to have stability come naturaly by using the exsisting capacitence of the devices and selecting the open
loop gain so as the have the Unity gain crossing frequency happen prior to the first pole. This will produce
higher THD numbers and in DC coupled designs require care with DC offset, however correction methods are
available to take care of this.
I also like to have the open loop gain be constant across the Audio bandwidth as this produces a Consistent
THD number with frequency and not have the rising THD vs Frequency typical of Circuits using high open
loop gain and limited open loop bandwidth.
Current sources & Mirrors can be quite usefull if properly used and used in the right places. using a current
source to supply current to the Emmiter's of the Input Diff amp is a good thing as it improves both the
comon mode as well as the Power supply rejection ratio of the Circuit. Current Mirror's used on a folded
cascode Voltage gain stage can also give good results, however on the more conventional cascode or comon
Emmitter stage thay as Hugh pointed out can create problems unless the operating current is set so high so
as to be able to drive the Output stage's non linear impedance at it's worst case.
Using Feedback around just the output stages is not IMHO a good thing as it reduces the Stability and speed
of the output stage and this stage must by nature be alot faster than the Vas stages to avoid instability. |
|
|
| capslock |
AKSA and ppl don't seem to like Miller compensation, but I am really curious about the reasoning behind this is.
I can understand their argument to a degree in that it is probably not a nice thing to have the open loop gain starting to roll off within the audio band. Also, the input stage has to be able to deliver the charging current for the Miller cap, otherwise slew rate limiting will occur.
Assuming that DC open loop gain is limited by emitter degeneration of the input long tailed pair and maybe the VAS, so that the open loop rolloff point can be placed at say 25 kHz, and assuming that it can deliver ample current, would one try to do the frequency compensation on the input stage or by adding a Miller capacitance on the VAS transistor?
I suspect it may be better to go for the Miller compensation, because it will:
a) linearize the parasitic inherent Miller capacitance of the VAS transistor
b) be more efficient than rolling off the input stage gain because of the pole slitting action, i.e. greater overall bandwidth can be achieved
c) reduce distortion caused by the nonlinear input impedance of the output transistors because it lowers the output impedance of the VAS stage because of its feedback action
Comments? Flames? |
|
|
| dimitri |
Nothing can be better then Miller compensation
This is a quote from http://www.normankoren.com/audio/
In the original PAS, the feedback loop is stabilized by 33 pF capacitor CLFB in shunt with feedback resistor RLFB. In the present modification, we eliminate CLFB, replacing it with Miller capacitor C3M connected between input stage grid 3G and plate 3P. We also add C3C connected between input stage cathode 3C and ground to shunt RF interference from the output cable. R3GS and C3M provides the dominant pole that controls open-loop rolloff and assures stability. The validity of this technique was confirmed by a recent article in the Journal of the Audio Engineering Society, which used a highly mathematical analysis to determine that .feedback with Miller compensation is a superior approach to error-correcting amplifier design |
|
|
| nemestra |
Hi all,
in this month's Electronics World (Mar. 2003) John Ellis makes a case for the phase lead, input lag (PLIL) technique as an alternative to Miller capacitor. He uses a number of amplifiers as a test beds including Self's 'Blameless' design. I was wondering if any of the posters had read this?
On a related note he observed low level background oscillation with some designs at around 1MHz and attributed this to a combination of marginal stability and inductance in the output stage emitter resistors. I have noticed a background oscillation of around 500 kHz in a recently completed example of Self's load-invariant amp. design and am about to change the emitter resistors to non-inductive Vishay thick film types. Haven't had a chance yet - they arrived about 15 minutes ago! Has anyone previously noticed this problem or tried this cure?
James |
|
|
| Nelson Pass |
You can go round and round on this, but sometimes a
few pF lag is just what you need, and as long as it
isn't excessive, doesn't tend to create problems.
Myself, I find that nice simple amplifiers don't tend to
need lag compensation, but they often like a couple pF
across the feedback resistor to perfect the square wave. |
|
|
| traderbam |
"... am about to change the emitter resistors to non-inductive Vishay thick film types"
My prediction: it won't cure the 500kHz noise. :Popworm:
Just a word about stability in general FWIW. Obviously if a circuit is oscillating out of control it won't sound too good and may well cook your speaker, but what about some minor instability? What should the phase margin be? Why does it matter whether an amps response is totally stable or not? Provided the voltage gain is relatively flat to 20kHz does it matter that it, say, doubles at 200kHz? So what if a square-wave has an HF ringing on it? You won't hear it!
Honestly, we cannot hear over 20kHz.
There are indirect reasons why phase margin and compensators may cause audible effects. Figure out what these are and you can make better choices. |
|
|
| nemestra |
Hi Traderbam,
Your prediction was correct. It didn't change the low level oscillations - just moved them in frequency from approx. 500 kHz to around 320 kHz.
I fully realise that this is not audible but its always there - a small sine wave even with the input shorted to ground. I might not be able to hear it but I'm going to fix before I start to use this amp in earnest.
Thanks,
James |
|
|
| blmn |
Hi Nemestra,
How low is this oscillation? I mean, How many mili or microvolts under nominal load?
I'm interested on it because I'm thinking about trying the load invariant circuit on next month.
Regards, |
|
|
| pinkmouse |
| quote: | Originally posted by Nelson Pass
You can go round and round on this, but sometimes a
few pF lag is just what you need, and as long as it
isn't excessive, doesn't tend to create problems.
Myself, I find that nice simple amplifiers don't tend to
need lag compensation, but they often like a couple pF
across the feedback resistor to perfect the square wave. |
So, for instance, as you can't get much simpler than a gainclone, ( Sorry Nelson!), would a small cap accross the feedback resistor be advantageous in that application as well? |
|
|
| AKSA |
Hi Nemestra,
Try inserting base stoppers on the outputs of 10R and on the drivers of 100R, if they are not already there.
Cheers,
Hugh |
|
|
| dkemppai |
| quote: | Originally posted by capslock
I suspect it may be better to go for the Miller compensation, because it will:
a) linearize the parasitic inherent Miller capacitance of the VAS transistor
b) be more efficient than rolling off the input stage gain because of the pole slitting action, i.e. greater overall bandwidth can be achieved
c) reduce distortion caused by the nonlinear input impedance of the output transistors because it lowers the output impedance of the VAS stage because of its feedback action
Comments? Flames? |
Let me ask a question. Will linearizing the miller capacitance of the VAS stage matter if the feedback through the output stage back into the long tailed pair is fast enough to let the long tailed pair do the compensation? If the slowest portion of the amplifier is the long tailed pair, shouldn't many of the non linearities be corrected for? (Assuming that no 'gross' levels of distortion exist in other stages)
In my experience, it is always better to improve the speed of other stages in the amplifier before you try to compensate via added miller capacitance. Increasing bias on some stages can help, depending on circuit configuration. Only add miller capacitance as a last resort. (Again, as Nelson said, nothing excessive) Even then, to prevent phase shifts at higher frequencies it may be worth trying some sort of pole splitting. I have at times found a resistor in series with the added miller cap will compensate the amplifier and still keep phase shifts relativley low at higher frequencies. However at lower levels of gain, these options may not be avaliable.
| quote: | Originally posted by traderbam
What should the phase margin be? Why does it matter whether an amps response is totally stable or not? Provided the voltage gain is relatively flat to 20kHz does it matter that it, say, doubles at 200kHz? So what if a square-wave has an HF ringing on it? You won't hear it!
There are indirect reasons why phase margin and compensators may cause audible effects. Figure out what these are and you can make better choices. |
That is a good question, what should be pahse margin be? I guess it would depend on your load. If running a purely resistive load, you could probably get away with a few degrees. However if you start adding any complex loads, things may change. I would say phase margin needed would depend on the load impedance. If your speakers don't cause the amp to oscillate, then you're fine. However, if you don't know what the impedance will be, you may be better to err on the side of caution.
Let me ask another question...
What if the gain is flat to 20Khz and the phase shift was around 180 degrees? Would that affect the sound of the amplifier?
I believe that a bigger factor to sound quality is the phase shift verses frequency, not amplitude vs frequency. Think of an instrument, and shift each of the harmonics. Will the sound still be the same? My guess is no. The pressure waves that will hit your ear will not the same as from an amplifier that has no phase shift.
For example, I have inclosed an image, which shows four traces with identical spectral amplitudes. Each trace has the same fundimental frequency with a second 'harmonic' frequency of 20% of the fundimental frequency, differing only by phase shift. Could the human ear detect the difference? Maybe, maybe not, I have not done any audio testing of these waveforms yet. The point is, that phase shift changing with frequency can affect a complex waveforms amplitude vs. time trace.
-Dan |
|
|
| sonnya |
| quote: | Originally posted by AKSA
Hi Nemestra,
Try inserting base stoppers on the outputs of 10R and on the drivers of 100R, if they are not already there.
Cheers,
Hugh |
It is a good advice..... Like written in another thread, the BJT can act as a "inductor" so adding a base resistor will lower the Q of the "inductor" and remove the tendency to oscillation in the transistor.
ad a give capacitive load this tendency to oscillation will rise. This will give you a oscillating current on the base of up to 1 - 3amp in peak.... This will affect all the way back to the VAS stage..
With mosfets (BUZ900DP/BUZ905DP) in the outputs this current on the gate would be 1/10 the size of the BJT (MJL3281A/MJL1302A).
Sonny |
|
|
| capslock |
James/Hugh/Sonny:
I am not sure that this is a local oscillation of the output transistors because in my experience this occurs at much higher frequency, usually at half or even around the f_T of the transistor. Then of course, I don't know what kind of transistors James is using. A local oscillation will usually occur with capacitve loading, and it will start on one polarity. A ferrite bead can cure the problem.
James:
Could you summarize the points of the article, especially what the topology of this PLIL technique is and why it would be superior? Or might I even ask you for scans?
traderbam:
I like to keep the amp as fast as I can but be stable with all kinds of loads. The reasoning behing this is:
- If there is instability, it is usually due to some positive feedback. This means it can grow as the load changes slightly or even the temperature of some components changes, maybe even snowball and fry something.
- It's true we cannot hear beyond 20 kHz, but the input junctions are essentially rectifiers, so they will generate a DC or LF component from this. And assuming the HF varies nonlinearly with the audio signal, we get our audio band signal modulated. There was an Analog Devices app note on HF rectification in metrology amps.
dkemppai:
You may be right that the input stage can linearize the rest of the system. But intuitively, this feels like trying to balance something heavy and unwieldy with a stick made of rubber. Sorry, just a feeling, no solid argument.
I am not sure we have the same definition of pole splitting. In my understanding, a Miller cap without the resistor does the pole splitting. The argument is as follows: Usually the output stage is the slowest stage, so it dictates how the other stages must be rolled off. The Miller compensation makes for a lower impedance drive to the output stage, so it becomes effectively faster. This means that the overall gain rolloff need no longer be as severe as it would have been estimated from looking at the uncompensated circuit. In other words, you can choose a higher open loop rolloff point with VAS Miller compensation compared to input stage rolloff.
Regards,
Eric |
|
|
| sonnya |
| quote: | Originally posted by capslock
James/Hugh/Sonny:
I am not sure that this is a local oscillation of the output transistors because in my experience this occurs at much higher frequency, usually at half or even around the f_T of the transistor. Then of course, I don't know what kind of transistors James is using. A local oscillation will usually occur with capacitve loading, and it will start on one polarity. A ferrite bead can cure the problem.
Regards,
Eric |
Yes you are right, the oscillations also tends to die out. and it is mostly into a capacitive load.
Sonny |
|
|
| traderbam |
Hey, I've just bought this CD by Marie Frank. Never heard of her. It's really good!
dkemppai wrote: "Will linearizing the miller capacitance of the VAS stage matter if the feedback through the output stage back into the long tailed pair is fast enough?"
In theory, provided the circuit is stable the more feedback the less this non-linearity will matter, yes. IME it is very difficult to make the vas stage stable without a compensation cap to slow down the collector voltage (or really killing its gain). I've tried this many times: various difficult-to-track-down parasitic feedback paths tend to terrorize (topical or what?) the performance. So I'd say you need several 10s of pF minimum and a sensible pcb layout to avoid parasitic instability. The latter cannot be fixed at the LTP stage. With a vas BJT with no emitter resistor this creates a pole at some several kHz - and a 90 deg phase shift. Then the rest of the amp is normally designed to add no more than 45deg up to the point where loop gain is unity. |
|
|
| traderbam |
Namestra,
I am not familiar with the Self circuit, can you post it? The oscillation you are seeing could be caused by several things.
For a start I would check the basic things first - like your psu and grounding arrangements. Make sure you are using separate ground wires appropriately and your are star earthing (there is a whole big thread on this in here somewhere) and that psu cables are tightly bound together to minimize loop inductance.
Then we need to check the design.
BAM |
|
|
| capslock |
There are at least three ways to tame the VAS stage:
a) emitter degeneration: brings down gain pretty independent of frequency, so constant loop gain in the audio band can sometimes be achieved, linearizes VAS stage, increases input impedance, does little to change output impedance
b) Miller compensation cap between C and B: frequency dependent feedback, lowers input and output impedance as frequency increases, may sometimes be the only compensation needed.
c) resistor between C and B: never tried it, would also bring down gain independent of frequency same as a), would decrease input and output impedance similar to b), but independent of frequency;
relaxes demands on overall compensation same as a), but compensation must still be done on some other stage
comments? |
|
|
| dimitri |
Dear capslock,
first pole time constant is (VAS input resistance)*(Cbc VAS)*(VAS gain)
second pole time constant is (output stage input resistance)*(C associated with VAS output node)
in your cases a) and c) the second pole time constant will remain unchanged and you should make much higher first pole time constant to move second pole below unity loop gain. Only in case b) you will get the second pole time constant lower (then original one) due to NFB via Miller capacitor. With higher frequency second pole you can use not so low the dominant one.
another nonlinearity is associated with VAS output node - nonlinear input current of the output stage. To makes this nonlinearity lower you should keep VAS output resistance low - again by Miller compensation. Here the advantage over c) will be that the VAS output resistance will be lower with frequency, that helps to keep associated disto at low level (please keep in mind that the overall loop gain is lower with frequency)
so for the discussed topology LTP-VAS-output stage b) is superior |
|
|
| capslock |
| quote: | Originally posted by dimitri
second pole time constant is (output stage input resistance)*(C associated with VAS output node)
|
May be this is too simple. Suppose you put a class A emitter follower buffer (which is made of very fast low or medium power transistors) in front of the output stage. It will increase input impedance by say a factor of 200, namely the AC current gain. Input capacitance will not be brought down significantly (or actually not at all if original predrivers and the additional buffer stage are both using the Sanyo video transistors discussed above) + there is nothing we can do about the output capacitance of the VAS. So will an additional buffer bring down the second pole by a factor of up to 200? My guess is it will not cause a change by more than 10%. |
|
|
| dimitri |
capslock wrote---------
I also like to have the open loop gain be constant across the Audio bandwidth as this produces a Consistent
THD number with frequency and not have the rising THD vs Frequency typical of Circuits using high open
loop gain and limited open loop bandwidth
---------------------------------
Then you can use another topology with several gain stages and individual frequency compensation in the each stage, like in 29 years old Otala amp: http://home.online.no/~tsandstr/OtalaStory.htm
capslock wrote---------
I suspect it may be better to go for the Miller compensation, because it will:
a) linearize the parasitic inherent Miller capacitance of the VAS transistor
b) be more efficient than rolling off the input stage gain because of the pole slitting action, i.e. greater overall bandwidth can be achieved
c) reduce distortion caused by the nonlinear input impedance of the output transistors because it lowers the output impedance of the VAS stage because of its feedback action
----------------------------
a) - false statement. The sensitivity to changes in nonlinear Cbc will remains unchanged with addition of the external capacitor. The solutions are cascode or emitter follower before VAS
b), c) - true |
|
|
| dimitri |
capslock wrote --------
It will increase input impedance by say a factor of 200, namely the AC current gain.
----------------------------
This will be only on DC. Check the equation for the follower gain, having in mind Cbe and real load Rl + j(omega)Cl |
|
|
| nemestra |
Hi all,
Thanks for all the advice. No updates until now due to the usual - "Work is the curse of the drinking classes" - Oscar Wilde.
To answer several questions in no particular order - blmn, the oscillation is around 600 mV and is constant regardless of input. When I quickly measured it late last night I thought it was around 320 kHz - is actually 420 kHz.
I have found that by repeatedly connecting and disconnecting a 16ohm resistor test load that the oscillation will occasionally disappear when the load is disconnected. This is quite rare ( maybe 1 in 30/40 load removals ) and the amp will not oscillate until the load is reconnected.
Hugh, this amp design already contains 100R in the driver bases. The output stage is complementary feedback type (Sziklai-Pair) and I haven't yet tried the 10R resistors.
BAM - I will have to borrow a scanner so I might not get a schematic posted tonight.
Eric - I'll not try to summarise this article in case I miss some of the important detail, but for the above reason scans will take a couple of days. Output transistors are MJL3281A/MJL1302A BTW.
BAM - I have followed good psu and ground practices as you have described.
Thanks everyone,
James |
|
|
| dimitri |
--------------------
Hugh, this amp design already contains 100R in the driver bases. The output stage is complementary feedback type (Sziklai-Pair) and I haven't yet tried the 10R resistors.
--------------------
Please draw how you arrange the power supply rails, output devices and capacitors. Sounds like you have some parasitic inductance in supply rails. Unless you will stop the self generation, you will never get good sound, you will have a lot of in-band intermodulation products! |
|
|
| AKSA |
Nemestra,
You may have oscillation of the CFP.
The negative rail CFP (with PNP driver) is most susceptible.
Try 100pF from collector to base of driver.
If this fixes it, you know where it's coming from. If it does, I'll have more to say later! :irked:
Cheers,
Hugh |
|
|
| nemestra |
Hi Hugh,
added 100 pF capacitors to both drivers - unfortunately no change - low level oscillation still present.
James |
|
|
| dkemppai |
| quote: | Originally posted by nemestra
Hi Hugh,
added 100 pF capacitors to both drivers - unfortunately no change - low level oscillation still present.
James |
Do you have and rail capacitance close to the amplifier itself? If so, do you have any capacitors directly across the rails (from + rail to - rail). If not try add several hundred uF from rail to rail right at the amplifier. (I had a similar problem once, and this seemed to cure it)
-Dan |
|
|
| AKSA |
Hi Nemestra,
The problem clearly is not in the output stage.
Suggest it might be the voltage amplifier, which leads us to compensation considerations, OR damping of the ubiquitous oscillations in the output stage.
The latter is scotch with 10R and 100nF in series from output to ground, and a 1.5uH choke in parallel with 10R at the output, OUTSIDE the feedback loop.
If the oscillation continues even after this treatment, next step would be around 15pF from output of VAS (collector is fine) to the feedback node. This pulls back OLG at very high frequencies and makes the output stage tolerant of capacitive loads. It's a favored technique of John Linsley Hood.
Cheers,
Hugh |
|
|
| capslock |
| quote: | Originally posted by dimitri
capslock wrote---------
I also like to have the open loop gain be constant across the Audio bandwidth as this produces a Consistent
THD number with frequency and not have the rising THD vs Frequency typical of Circuits using high open
loop gain and limited open loop bandwidth
---------------------------------
Then you can use another topology with several gain stages and individual frequency compensation in the each stage, like in 29 years old Otala amp: http://home.online.no/~tsandstr/OtalaStory.htm
capslock wrote---------
I suspect it may be better to go for the Miller compensation, because it will:
a) linearize the parasitic inherent Miller capacitance of the VAS transistor
b) be more efficient than rolling off the input stage gain because of the pole slitting action, i.e. greater overall bandwidth can be achieved
c) reduce distortion caused by the nonlinear input impedance of the output transistors because it lowers the output impedance of the VAS stage because of its feedback action
----------------------------
a) - false statement. The sensitivity to changes in nonlinear Cbc will remains unchanged with addition of the external capacitor. The solutions are cascode or emitter follower before VAS
b), c) - true |
The first quote is not from me, but I can agree with the reasoning. Thanks for the Otala link!
I still think a) is correct. Consider an ihnerent Cbc that will vary with VCE from say 2-6 pF, a 300% variation. When you parallel a 22 pF, you only have 17% variation left, which is much better. |
|
|
| capslock |
Hugh, can I incite you to tell us why you think Miller compensation is a bad thing? Obviously, you have come to the conclusion from lots of listening tests, but I gather you'd also have some theory.
Eric |
|
|
| AKSA |
Eric,
On my commercial amp during development, I tried all sorts of compensation schemes. As a general rule, I found that with each successive reduction in lag comp from 100pF down to 36pF the sonics improved.
Below this, the sound worsened as the amp would lapse into short term oscillation on music.
I believe the theory relates to slew rate, which we could call the 'nimbleness' of the amplifier. As lag comp is reduced, so OLG increases at a given high frequency. As long as OLG gain is just below unity at the Bode frequency, all is well. This frequency is also determined by the load, the Zobel at the output, and the devices used throughout the amplifier, particularly the common emitter device.
On this basis, I searched through the design for ways of reducing OLG at very high frequencies without reverting to too much lag compensation of the VAS. I found one of the most useful was John Linsley Hood's technique of taking feedback directly from the collector of the VAS to the feedback node. This too has a bad effect on the sonics, but it makes the amp much more tolerant of capacitive loads - no bad thing as capacitive loads often precipitate amp instability anyway - since we invert the phase shift conferred by such loads with an interstage cap onto the feedback node. I also found that lag comp and interstage phase lead tend to act synergistically, making the amp easier to stabilize. The quality of these caps is also important.
Cheers,
Hugh |
|
|
| dimitri |
capslock wrote--------
I still think a) is correct. Consider an ihnerent Cbc that will vary with VCE from say 2-6 pF, a 300% variation. When you parallel a 22 pF, you only have 17% variation left, which is much better.
---------------------------
Hi, capslock
Imagine an amp with inherent Cbc that will vary with VCE from say 2-6 pF. Assume it has closed loop gain 1000 at 20 kHz. The output variation will be 300%/1000=0.3%. Then you add 22 pF, the loop gain becomes 10 time lower. The output variation will be 17%/100=0.2%. The SAME value :wave2:
You should read Cherry papers from JAES - very authoritative info |
|
|
| capslock |
| I was not talking about the variation in bandwidth, I was talking about nonlinear distortion due to change in capacitance with voltage swing. Reducing this variation by a factor of 15 will reduce the associated open loop distortion by a factor of 15. Bandwidth will also be reduced, causing lower feedback, but assuming you have to reduce it anyway, it may be better to choose the VAS rather than some other point to do the rolloff. This is all I was trying to say. |
|
|
| nemestra |
Hugh,
the Zobel networks you describe 10R+100n to gnd and a series 1.5uH || 10R are present. I have added a 22pF from the VAS collector to the feedback point ( base of long tailed input pair ) but no change.
I've borrowed a scanner but it's got a SCSI interface - it's going to be few more days before I can scan stuff - need to buy a SCSI card first.
James |
|
|
| dkemppai |
| quote: | Originally posted by capslock
Hugh, can I incite you to tell us why you think Miller compensation is a bad thing? Obviously, you have come to the conclusion from lots of listening tests, but I gather you'd also have some theory.
Eric |
I've come to believe that miller compensation in large quantities is not a good thing. (I won't call it a bad thing... ...because sometimes it is necessary)
Listening tests have proved this... ...to myself and my wife that is :) I'm not sure if it's the phase shift or the limiting of the slew rate of the amplifier that is the real cause. but, beyond a few tens of pF the sound becomes dulled and muddy.
-Dan |
|
|
| dimitri |
capslock wrote------------
I was not talking about the variation in bandwidth, I was talking about nonlinear distortion due to change in capacitance with voltage swing. Reducing this variation by a factor of 15 will reduce the associated open loop distortion by a factor 15.
--------------------------------
I was also talking obout nonlinear disto. Your proposed method of reduction this variation by a factor of 15 will reduce the associated open loop distortion by a factor 15 and will also reduce loop gain by a factor 15, thus the distortion remain the same.
You start the thread, you ask questions and you don't like the answers that aren't coincide with your opinion - so what? |
|
|
| traderbam |
"I'm not sure if it's the phase shift or the limiting of the slew rate of the amplifier that is the real cause"
Can you think of any other causes?
dimitri:
You are assuming that the only impact is at low frequencies. The miller cap does reduce variation of amplitude and phase wrt output voltage. Negative feedback may mitigate this effect at low frequencies but will certainly not mitigate it at high frequencies where the gain margin is small. Is this important? |
|
|
| dimitri |
Hi, traderbam
The Miller cap does provide feedback around VAS, so VAS is in two feedback loops – in local Miller loop and in overall loop. Due to extra loop gain around VAS, the nonlinearity associated with VAS is effectively suppressed. The input stage and the output stage are only in the overall loop with low loop gain.
If the Miller cap is connected between the VAS output and the amp inverting input (Linsley Hood) the input stage will be in both loops the nonlinearity associated with input stage also will be effectively suppressed.
If the Miller cap is connected between the amp output and the VAS input (Edward Cherry) the output stage will be in both loops, and the nonlinearity associated with output stage also will be effectively suppressed.
--sorry for the hand drawing |
|
|
| capslock |
| quote: | Originally posted by dimitri
capslock wrote------------
I was not talking about the variation in bandwidth, I was talking about nonlinear distortion due to change in capacitance with voltage swing. Reducing this variation by a factor of 15 will reduce the associated open loop distortion by a factor 15.
--------------------------------
I was also talking obout nonlinear disto. Your proposed method of reduction this variation by a factor of 15 will reduce the associated open loop distortion by a factor 15 and will also reduce loop gain by a factor 15, thus the distortion remain the same.
You start the thread, you ask questions and you don't like the answers that aren't coincide with your opinion - so what? |
Before you accuse me of not liking your answers, you should read carefully what I wrote. I was well aware of the reduction in open loop gain by a factor of 15. However, my point was that if you had the chance to roll off by the same amount either in the input stage or the VAS, maybe it would be advantageous to do so in the VAS. Doing it in the input stage does nothing about nonlinear distortion but reduces the open loop gain. Doing it at the VAS reduced loop gain by the same amount (or maybe even less, if advantage can be taken of pole splitting) while at the same time swamping out some of the C_bc nonlinearity. |
|
|
| dimitri |
capslock wrote ---------------
my point was that if you had the chance to roll off by the same amount either in the input stage or the VAS, maybe it would be advantageous to do so in the VAS
----------------------------------
You are right. How can we roll off in the input stage - only by the brutal force, to put a certain capacitance to the ground. We will get stability and nothing else. There are three ways to put Miller cap and to organize the extra inner feedback loop - see my previous post. |
|
|
| dkemppai |
| quote: | Originally posted by traderbam
"I'm not sure if it's the phase shift or the limiting of the slew rate of the amplifier that is the real cause"
Can you think of any other causes?
|
Uhhh, My point was I don't know if it was one or the other or both...
-Dan |
|
|
| fdegrove |
Hi,
| quote: | | Uhhh, My point was I don't know if it was one or the other or both... |
Both.
Cheers,;) |
|
|
| capslock |
| quote: | Originally posted by dimitri
capslock wrote ---------------
my point was that if you had the chance to roll off by the same amount either in the input stage or the VAS, maybe it would be advantageous to do so in the VAS
----------------------------------
You are right. How can we roll off in the input stage - only by the brutal force, to put a certain capacitance to the ground. We will get stability and nothing else. There are three ways to put Miller cap and to organize the extra inner feedback loop - see my previous post. |
Well, actually a lot of Elektor designs used roll-off in the input stage. It was a cap in series with a rather small resistor connected between the collectors of the input pair. It worked because the first diff pair only drove another diff pair which then drove the VAS. |
|
|
| phase_accurate |
Anybody ever tried a combination of Linsley-Hood- and Cherry-style compensation ?
Regards
Charles |
|
|
| janneman |
| quote: | Originally posted by capslock
Well, actually a lot of Elektor designs used roll-off in the input stage. It was a cap in series with a rather small resistor connected between the collectors of the input pair. It worked because the first diff pair only drove another diff pair which then drove the VAS. |
There is an interesting article in EW March issue precisely on this. The author makes the case that a combination of input lead and lag compensation in the Vas stage gives the best results. The Vas compensation is modified from the usual cap from C to B, to circumvent the problems of slew limiting and decreasing Vas gain with frequency which would cause increasing distortion with frequency. Check it out.
Jan Didden |
|
|
| capslock |
| I guess I will have to find out how to subscribe EW. What does it stand for, Electronics and Wireless World, maybe? |
|
|
| janneman |
Eric, I'll include it with the other stuff.
Jan Didden |
|
|
| Christer |
I am somewhat surprised that nobody has brought up
two-pole compensation of the VAS. Proponents of this
technique, e.g. Randy Slone, seem to consider it superior
to a single c-to-b capacitor. It seems good to me fromt a
theoretical point of view, but maybe there is some obvious
drawback that I have failed to see. What do you guys think
of two-pole compensation. |
|
|
| capslock |
| Can you explain a little more about two-pole compensation? |
|
|
| traderbam |
dimitri,
Thanks for the diagram. I don't think I understand what you are arguing yet.
What causes the VAS output pole that you show?
Why does the non-miller cap curve coincide with the miller cap curve at the closed loop gain point - is this a coincidence?
bam |
|
|
| Christer |
The idea of two-pole compensation is to have two capacitors
in series between the base and collector of the VAS. Let C1
be closest to the collector and C2 closest to the base. Further
connect a resistor R from the common point of these capacitors
to ground (usually it is connected to the closest rail instead,
but that is equivalent from a theoretical point of view). C2 is
usually choosen much lower in value than C1, say 10 times
lower.
C1 and R act as a voltage divider giving a pole at frequency
f1. However, the reactance of C2 is much higher than the
resistance of R here, so there is very little feedback at f1.
Let f2 be the frequency where the reactance of C2 equals R,
where f2 is much higher than f1.
The combined effect of this is that the pole at f1 formed by C1
and R will have almost no effect for frequencies below f2. As
a net result we get a flat frequency response up to f2, where
it starts to fall by 12 dB/octave, not the usual 6 dB/octave.
Eventually, at a yet higher frequency the response will switch
over to a 6dB/octave behaviour. One might say that we try
to simulate a double pole at f2 by this method.
With a single Miller capacitor we will usually have to put the pole
at a rather low frequency to ensure stability. With two-pole
compensation, we can often have the simulated double-pole
above the audible band and still ensure stability, since we
roll off the response more sharply. |
|
|
| traderbam |
Christer,
I am not familiar with Slone's explanation of this. I understand your explanation: you are asking why not start the O.L. roll-off at a f above 20kHz, then roll it off at 12dB/oct, then introduce a zero to return the slope to 6dB/oct ahead of the C.L. unity gain f. I suppose you can. You would need to look at the affects upon step response of this more complicated roll-off.
My first question is how do you calculate the pole frequency? How do you know whether it will be at 1kHz or 20kHz? What drives this frequency?
My second question is - why does it matter where the pole is? |
|
|
| phase_accurate |
I have once done the maths for this one applied to op-amps. If someone is interested in the formulas I can supply these ones (which would take some time since I don't have it in electronic form).
The two-pole compensation was once described in an article series within EW+WW (I think it was a series by Douglas Self).
Regarding step response of the amp one has to pay attention to the phase marging then it shouldn't be a problem.
Regards
Charles |
|
|
| traderbam |
Charles,
Sinc you've done the cals can you confirm that to determine the pole of the VAS stage you need to be able to determine the open loop gain of the op-amp, something which is highly variable?
Bam |
|
|
| Christer |
| quote: | Originally posted by traderbam
My first question is how do you calculate the pole frequency? How do you know whether it will be at 1kHz or 20kHz? What drives this frequency?
My second question is - why does it matter where the pole is? |
The first pole, at f1, is decided by C1 and R. This pole decides
the basic 6dB/octave roll-off and should be selected to
give stability, just as with a single Miller cap. The second pole
is decided by C2 and R, and "hides" the first pole for frequencies
below f2.
The point of this is that the first pole at f1 will have no effect
for frequencies below f2. That is, even though you place it at
a sufficiently low frequency to guarantee stability, it "does not
affect" the OLG, which will remain flat up to f2, which may often
be above the audible band. Whether it is an advantage to have
a flat response over the audible band is another question. Many
seem to believe this is important, and I think it was one of the
things brought up in the first postings. |
|
|
| traderbam |
Christer,
Can you relay what Slone says about the pole frequencies? It is just that I don't think the poles and zero are related in a simple way to C1, R and C2 due to the feedback around the transistor and due to the influence of the Z on the collector.
If you have some example values for C1, C2 and R and an assumed collector Z I'd like to have them so I can simulate it and confirm it.
Bam |
|
|
| Christer |
| quote: | Originally posted by traderbam
Christer,
Can you relay what Slone says about the pole frequencies? It is just that I don't think the poles and zero are related in a simple way to C1, R and C2 due to the feedback around the transistor and due to the influence of the Z on the collector.
If you have some example values for C1, C2 and R and an assumed collector Z I'd like to have them so I can simulate it and confirm it.
Bam |
I see now that I made an error previously, mixing up C1 and C2,
which affected the whole explanation. I am sorry for that.
Maybe that is the reason for your scepticism. Actually I was a
little bit puzzled myself after rereading Slone and posting my
previous explanation. I sensed something was not quite right.
I am not sure if I had forgotten how it works, or if I had never
quite understood it before. Anyway, I'll make a new attempt at
explaining it.
In order not to confuse for those who happen to have Slones
book, I swap the capacitors to agree with him. That is, the
circuit is as before, but C1 is connected to the base and C2
to the collector. As for the choice of values, Slone says that
C1 is choosen in the same way as if we were doing single-pole compensation, ie. using a single Miller capacitor. He then suggests as a rule of thumb that R = 1kOhm and C2 = 10*C1.
If we were only using C1 to do single pole compensation, we
would get a pole at f1, decided by C1 and the input impedance
of the VAS (and/or the output impedance of the input stage?).
With two pole compensation, C2 and R form a voltage divider,
with a pole at f2, where f2 > f1. Below f2, C2 will have a high
impedance compared to R, so there is almost no feedback
signal reaching C1. Above f2, C1 will start to act as a short
circuit, so assuming the impedance of C1 < R here, the circuit
simplifies to the single capacitor C1.
I have made Spice simulations of some of Sloanes amps, and
it seems to work fine. You can find an example in the Optimos
amplifier at Slones web page:
http://www.sealelectronics.com/kits/index.htm |
|
|
| traderbam |
Hi Christer,
Ok I see now. The Slone circuit does increase the pole f at the expense of phase margin in the interval before the zero kicks in.
I didn't get much benefit from using C1=20pF, C2=100pF, R=1k into a 100k collector load, the VAS fed from a current generator. The Slone increased the -3dB f from 1kHz (using only a 20pF miller cap) to 2.2kHz and 30deg of phase margin was lost at 500kHz. Very little phase gain (if any) was observed above 1MHz.
Bam |
|
|
| Christer |
| quote: | Originally posted by traderbam
Hi Christer,
Ok I see now. The Slone circuit does increase the pole f at the expense of phase margin in the interval before the zero kicks in.
I didn't get much benefit from using C1=20pF, C2=100pF, R=1k into a 100k collector load, the VAS fed from a current generator. The Slone increased the -3dB f from 1kHz (using only a 20pF miller cap) to 2.2kHz and 30deg of phase margin was lost at 500kHz. Very little phase gain (if any) was observed above 1MHz.
Bam |
I guess his rule of thumb for selecting component values will
not work in all cases. I did a lot of Spice simulations on two or
three of Slones amps, playing around with the values in the
compensation circuits and it could make a lot of difference.
It was usually no problem to shift the roll-off corner one or
two decades up or down. I admit it was a bit tricky to find
suitable values sometimes, though, since I did no attempts
at calculating them, but just tried to change the values and
simulate. |
|
|
| AKSA |
Christer,
Nice description of two pole lag compensation.
Typically you'd use 100pF from collector to R, then 1nF from R to base. I've not examined the math; perhaps someone can elucidate the principles?
This technique was used on Russell Breden's amp publish in Electronics World some three or four years back, if memory serves.
A friend built it; some nasty instabilities as I recall, didn't sound that much better, he moved to other, simpler designs.
This approach should pull back gain under unity by the critical pole without as much effect as a single lag comp cap. However, if the collector cap is identical to a normal, the loading is increased by the value of R, but this is trivial, and so it would be reasonable to assume that the slew rate influence would be unchanged over the normal single pole compensation.
So, I have to ask, what is the real benefit here??
Cheers,
Hugh |
|
|
| sonnya |
only thing is that you get some nasty phaseshift with a margin of only 20 - 30 degress within the area where the openloop gain is higher than the closedloop gain..... Thats what the sim showed....
Anyone confirm this!?
Sonny |
|
|
| sonnya |
grrrrrrrrrrrrrrrrrr stupid attachment system!!!!
:mad:
take a look at the attached files |
|
|
| sonnya |
| Aol set to 10k and R1 set to 100meg. |
|
|
| sonnya |
aol = 10k, R1 = 10k.
Look at the phasemargin! |
|
|
| Christer |
Sonnya,
you should swap the values of C1 and C2 (or swap the
components themselves). I know I caused a bit of confusion
on this previously, I am sorry for that. |
|
|
| sonnya |
| aol = 10k, r1 = 1k |
|
|
| sonnya |
aol = 1k, r1 = 100meg..
I know about swapping C1 and C2 but the resulting phase margin is the same. |
|
|
| sonnya |
aol = 1k, r1 = 10k.
Now look at the phase margin. it is 90 degrees. |
|
|
| sonnya |
aol = 1k, r1 = 1k....
The phasemargin is now 50 degrees....
With two pole compensation you will get 5 to 20db more aol at 20k. and still be safe on the phasemargin.
PS : the opamp is your VAS transistor and C1, C2 is your miller cap. |
|
|
| Christer |
It seems this discussion died out, so I take it that two-pole
compensation is not very common and nobody has any
experience with it, or...?
Sonnya, do I understand you correctly that you also came to
the conclusion that the method seems to work well if judging
from simulations? Unfortunately, I never saved any results of
my simulations. |
|
|
| sonnya |
Yes, if you are able to get a phasemargin of 50 degrees i can see a benefit from it. I do not like to get closer to 180 degrees phaseshift than that! ;)
By the way Douglas self writes in his book "The Audio Power Amplifier Design Handbook. Third edition" when i he applies two pole compensation to his Class B or G circuits. Crossover artifacts nearly dissapears!!!!
It is a good book by the way :nod: |
|
|
| dimitri |
traderbam wrote:
What causes the VAS output pole that you show?
Why does the non-miller cap curve coincide with the miller cap curve at the closed loop gain point - is this a coincidence?
-------------------------
a)VAS output resistance, output stage input resistance, output stage (driver) input capacitance
b) the open-loop curve coincide with the miller cap curve at the closed loop gain point because you don’t need to make miller cap higher to get lower closed loop –3dB frequency
capslock wrote:
Well, actually a lot of Elektor designs used roll-off in the input stage. It was a cap in series with a rather small resistor connected between the collectors of the input pair. It worked because the first diff pair only drove another diff pair which then drove the VAS.
--------------------
I use this RC roll-off usually when I use discrete BJT/FET input stage and opamp. The input stage is optimized for the best noise behaviour, for example in RIAA stage I use p-n-p BJT with 100 uA collector current. Then I put 1kOhm trimmer and big elco, trimmed for the nice response with closed loop, then replace elco by a small ceramic one several times to obtain small overshot. |
|
|
| dimitri |
christer wrote:
It seems this discussion died out, so I take it that two-pole
compensation is not very common and nobody has any
experience with it, or...?
-------------
Try to find data sheet for National 20 years old LM101/201/301 op amp. There are frequency response curves for two pole compensation. I still think that 301 and 2-pole is still great for audio (please don’t kick me) you should add only 2.2 k between output and negative supply rail to switch op-amp output stage in class A |
|
|
| dimitri |
| RC roll-off in the input stage |
|
|
| Christer |
| quote: | Originally posted by dimitri
Try to find data sheet for National 20 years old LM101/201/301 op amp. There are frequency response curves for two pole compensation. I still think that 301 and 2-pole is still great for audio (please don’t kick me) you should add only 2.2 k between output and negative supply rail to switch op-amp output stage in class A |
The datasheet was still available and shows examples of
single-pole, two-pole and feed-forward compensations.
Thanks. |
|
|
| traderbam |
| quote: | | a)VAS output resistance, output stage input resistance, output stage (driver) input capacitance |
Ok, that's what I thought. This "pole" will not be static because these parameters will vary with voltage, current and load Z. The Miller cap will act to stabilize the open loop roll-off and counteract the variation of these parameters.
In your earlier post you discussed different arrangements for the cap such as from output to VAS input. The problem I see is that the stability is compromised in these alternative arrangements. Ultimately, we could agree that putting a cap from output to inverting input puts the maximum feedback around the system, but this would typically result in an unstable system.
Have you found a way to either eliminate the miller cap or apply the cap in a more optimum place and achieve better performance with equal stability? |
|
|
| dimitri |
traderbam wrote:
In your earlier post you discussed different arrangements for the cap such as from output to VAS input. The problem I see is that the stability is compromised in these alternative arrangements.
-------------------
This is exactly so. I made a lot Cherry-style compensation, but the second pole remains unchanged and I put RC network to ground at the VAS output to make second pole time constant lower, but at the expence of gain :-(
traderbam wrote:
Have you found a way to either eliminate the miller cap or apply the cap in a more optimum place and achieve better performance with equal stability?
-------------------
In this particular arrangement - LTP, VAS, output buffer it is superior (yes it should be modified fr two-pole compensation CRC network, or by three-pole CLC network ...) |
|
|
| Dave |
| So what became of the oscillating Load Invariant Amp??? |
|
|
| mikek |
| Not! to single-pole miller compensate....:nod: |
|
|
| dimitri |
Dave, please, what would you like to say/ask?
Mikek, Miller compensation is _dominant_ pole compensation, "not to single-pole miller compensate" is stupid idiomatic expression, because two-pole compensation is not the Miller one:) |
|
|
| mikek |
| quote: | Originally posted by dimitri
Dave, please, what would you like to say/ask?
Mikek, Miller compensation is _dominant_ pole compensation, "not to single-pole miller compensate" is stupid idiomatic expression, because two-pole compensation is not the Miller one:) |
No.....i was particularly carefull not to fall into that trap......
Miller feedback, is at the heart of both schemes......whether it initialy results in a single or double pole characteristic, is of sublime irelevance. |
|
|
| Fred Dieckmann |
"Miller feedback, is at the heart of both schemes......whether it initialy results in a single or double pole characteristic, is of sublime irelevance."
I always thought difference in two pole and one pole response made a large difference in phase margin since two poles are changing the phase twice as fast as single pole. Second order systems ring and settling time is affected. Most of the text I've read advocate moving the other poles as far away from the dominant pole as possible to get as close to a single pole response as possible for the best transient response and greatest phase margin. I believe the dominant pole is usually the one from second voltage gain stage were the largest amount of gain is. Discussing this in the in the absence of the contributions of the of the other poles is nonsense. The assumption of current sources charging the compensation cap is also an over simplification since many amplifiers include resistors in addition to current sources define the open loop gain.
The only thing ridiculously sublime is that all this is being discussed for such an ideal and over simplified model. This seems
to be another example of the difference between the pontificator of some particular pet theory on amplifier design and someone who as actually designed or even simulated an actual amplifier design with real transistor models instead of simplified model of a current source, transconductance stage and a capacitor. The real world is a lot more complicated than some academics would like to imagine. Links to contentious threads instead of actually addressing specific questions at hand is something I would expect for a troll rather than someone interested in actually investigating a topic to inform or learn.
http://grove.ee.iastate.edu/docs/papers/20000807.pdf |
|
|
| mikek |
| quote: | Originally posted by Fred Dieckmann
I always thought difference in two pole and one pole response made a large difference in phase margin since two poles are changing the phase twice as fast as single pole. |
The point of two pole compensation is to keep foward path error as small as possible, for as much of the audio band as posible.
A zero then introduced to cancel one of the poles, so that the two pole roll off reverts to a single pole slope long before unity gain frequency.
Thus as far as stability, phase margin, ringing etc...etc... are concerned, we are effectively dealing with a single pole response...since roll-off changes from two pole to single pole before Ft.
| quote: | Originally posted by Fred Dieckmann
Second order systems ring and settling time is affected. Most of the text I've read advocate moving the other poles as far away from the dominant pole as possible to get as close to a single pole response as possible for the best transient response and greatest phase margin. I believe the dominant pole is usually the one from second voltage gain stage were the largest amount of gain is. Discussing this in the in the absence of the contributions of the of the other poles is nonsense. The assumption of current sources charging the compensation cap is also an over simplification since many amplifiers include resistors in addition to current sources define the open loop gain. http://grove.ee.iastate.edu/docs/papers/20000807.pdf |
I am familiar with the reference you've cited....indeed i have carefully read, analysed, and verified, (or not as applicable), all of Schlarmann/Geiger published work.
It obvious from the stuff you've written above, that you have difficulty understanding even the reference you've cited........i don't think you've even made any attempt to to examine the later with any attention to detail, beyond a casual perusal...
| quote: | Originally posted by Fred Dieckmann
The only thing ridiculously sublime is that all this is being discussed for such an ideal and over simplified model. This seems
to be another example of the difference between the pontificator of some particular pet theory on amplifier design and someone who as actually designed or even simulated an actual amplifier design with real transistor models instead of simplified model of a current source, transconductance stage and a capacitor. The real world is a lot more complicated than some academics would like to imagine. Links to contentious threads instead of actually addressing specific questions at hand is something I would expect for a troll rather than someone interested in actually investigating a topic to inform or learn.
|
Rubbish. |
|
|
| nemestra |
| quote: | Originally posted by Dave
So what became of the oscillating Load Invariant Amp??? |
It's still oscillating. Well not at the moment, because I put it on the shelf and will get back to looking at the problem in due course. In the meantime I built a couple of gainclones because I needed something to listen to. Tweaking those should keep me going for some time.
James |
|
|
| Dave |
nemestra,
Are you able to get the schematics posted? I'm still quite interested to know what the problem is.
2-pole compensation seems like a pretty good idea in theory but how come you don't see it used very often if at all? |
|
|
| traderbam |
| quote: | | The point of two pole compensation is to keep foward path error as small as possible, for as much of the audio band as posible. |
Don't you mean that the point of 2-pole compensation is to maintain a high, uniform feedback factor across the audio band. Unless you are suggesting the pole/zero implementation is beneficial for the forward path on its own. I think you meant to say that increasing the feedback at the higher audio frequencies reduces the closed-loop output error. This is quite different from what you actually said.
I think Dave has a good question. Why don't more amps use pole-pole-zero compensators in the forward path? See if you can figure this out and explain it without dropping a reference to yet another from your plethora of articles and journals.
Are you a librarian? |
|
|
| mikek |
| quote: | Originally posted by Dave
2-pole compensation seems like a pretty good idea in theory but how come you don't see it used very often if at all? |
Hi dave,
i often wonder about this extremely valuable point you've made....
Indeed, most amps. using 2-pole-comp. employ incorrect values......infact only one commercial design with correctly choosen values springs to mind.......one of Randy slone's designs...can't remember which.......
(C1=150p;C2=300p;R=1K).
choose the wrong component values, and your design will probably oscillate...
| quote: | Originally posted by traderbam
Don't you mean that the point of 2-pole compensation is to maintain a high, uniform feedback factor across the audio band. Unless you are suggesting the pole/zero implementation is beneficial for the forward path on its own. I think you meant to say that increasing the feedback at the higher audio frequencies reduces the closed-loop output error. This is quite different from what you actually said. |
Different way of saying precisely the same thing.....:)
2-pole comp. maximises feedback across as much of the audio band as possible.....which merely means it allows the maintenance of a high open loop gain across as much of the audio band as possible......which put differently, is the same as saying it helps reduce foward path error, (the difference between input and feedback signal), for as much of the audio band as possible.
| quote: | Originally posted by traderbam
Are you a librarian? |
No...:) |
|
|
| traderbam |
I know what you are saying and I know what I am saying and they are not quite the same.
Let me ask you a teaser. Are you suggesting that using this double pole-zero method will improve the sound quality? If so, why? What assumption are you making that leads to this conclusion? It needs to be a bit more than "more feedback is better"...there are numerous people who have found the opposite to be the case in practice. |
|
|
| MarcelvdG |
For those who haven't noticed: there is a relatively new thread about an op-amp with a ridiculously high open-loop gain. Looking at its gain versus frequency plot, it appears to have five-pole compensation...
By the way, why do you all want to keep loop gain constant over the audio band? I understand you want as much loop gain as possible at 20kHz, but what is wrong with having even more at lower frequencies? Why should the low-frequency distortion be as high as the high-frequency distortion? |
|
|
| mikek |
| quote: | Originally posted by MarcelvdG
For those who haven't noticed: there is a relatively new thread about an op-amp with a ridiculously high open-loop gain. Looking at its gain versus frequency plot, it appears to have five-pole compensation...
By the way, why do you all want to keep loop gain constant over the audio band? I understand you want as much loop gain as possible at 20kHz, but what is wrong with having even more at lower frequencies? Why should the low-frequency distortion be as high as the high-frequency distortion? |
from my point of view the more LF open-loop gain, with as few as possible open-loop poles, the better....:) |
|
|
| mikek |
| quote: | Originally posted by traderbam
I know what you are saying and I know what I am saying and they are not quite the same. |
Perhaps Mr van de Gevel might like to adjudicate?:)
| quote: | Originally posted by traderbam
there are numerous people who have found the opposite to be the case in practice. |
....how so? |
|
|
| Dave |
| quote: | | What assumption are you making that leads to this conclusion? It needs to be a bit more than "more feedback is better"...there are numerous people who have found the opposite to be the case in practice. |
I think the theory is that is provides more feedback over the audioband without raising gainbandwidth product above the final zero and therefore won't compromise stability.
This is different from just increasing total gain or open loop bandwidth which will increase gain bandwidth product and possibly affect stability. |
|
|
| Fred Dieckmann |
"Miller feedback, is at the heart of both schemes......whether it initialy results in a single or double pole characteristic, is of sublime irelevance."
And the clarification:
"A zero then introduced to cancel one of the poles, so that the two pole roll off reverts to a single pole slope long before unity gain frequency."
Thus as far as stability, phase margin, ringing etc...etc... are concerned, we are effectively dealing with a single pole response...since roll-off changes from two pole to single pole before Ft. "
And we are back to the desirability of a single pole response at the unity gain point. Also worth noting is the fact that most amplifiers do not have a real single pole response at the unity gain frequency and are venturing into the beginning of the second pole response, giving rise to consideration of the phase margin. These are basics in any good engineering text, as well as op amp application notes and data sheets. This is not some obscure mechanism hidden in papers by Ph.Ds and unknown to competent amplifier designers who actually build amplifiers.
All of this seems to be a far cry from being "ridiculously sublime" to me.............
It seems to me that dropping broad statements with little or no detail concerning the implications, is trolling so you can drag out the involved explanations and qualifications to support the initial statement after the usual confusion it creates. In this particular case I can write your next reply in advance.... "
"I said whether it INITIALLY results in a single or double pole characteristic"!
If there is a good reference to make your initial point, why not include it. Put the abstract of what it said with the link. Those interested in the topic can read it if inclined to, instead of trying to figure out the concept from a three or four sentence post. The need for lengthy clarifications and explanations after the fact, as a result of the brief and vague initial post, seems to indicate the intention to start an argument. You can then come in with the details and references to prove you were right when no one can understand what your initial point was. Initial post meant to inquire, illuminate, or to promote actual discussion does not require this frustrating game of 20 questions, to see what you meant. I have no doubt you have read every paper on frequency compensation and slew rates ever written. I do question whether you ever analyzed and built an actual amplifier or ever plan to.
Or do you just post to provoke arguments and drag out the appropriate AES or IEEE paper to prove you were right? I have received Email from several people that feel that is your exact intention. |
|
|
| traderbam |
Hey Fred,
I can't work out why you waste so much text on bickering with mikek. It is obvious to me that you are in a higher league both in terms of experience, engineering knowledge and the desire to impart knowledge to others in a form which they can absorb. Mikek on the other hand "appears" to me to be rather a rather young contributor who's self-assurance is rather larger than his competence, who's ego requires continual point scoring and who's experience is borrowed from the writings of others.
That's ok but don't let your normally poiniant posts become banal because of it. |
|
|
| ppl |
Regarding me liking the Frequency response to be flat well beyound the audio bandwidth has to due with Phase shift. the Closed loop response of the Amp must have a -3 dB point beyound 100KHz to avoid phase shift with in the upper octaves of the Audio range. It is true that Low frequency Phase shift is more noticable to the ear than High frequency phase shift is, I still like to remove any phase shift in the 20-20,000 Hz bandwidth to less than 1 deg. IMHO this is very importent to avoid colorations of the midrange and smearing if the related upper harmonics in relation to the fundamental frequency. The use of DC Blocking capacitors is another Evil of allot of Amps. these Coupling capacitors are most often selected way too small and have the -3 dB points set quite High 5-10 Hz. this will introduce low frequency phase shift. Low frequency phase shift is more of a problem that high frequency phase shift as the wavelength is longer and small timing delays seriously muddy the Bass and lower Mids. Add to this the absorption effects of capacitors and the sound can go to mud real quick with large tonal and pitch changes. IMHO the sonic signatures of alot of Audio Circuits can be traced to the Designers Choice of capacitors. DC or Direct coupling along with no or minimal compensation capacitance and these tone colorations are greatly reduced allowing attention to be directed to the remaining circuits.
It is a common misconception that placing a cap around the feedback resistor is a cure all for capacitive loads. in actuality this is not always true. Consider that this cap if too large will couple some of the load capacitance back into the capacitive sensitive inverting input. I admit at times a slight amount of capacitance is needed to trim the Transient response. in these cases i prefer to be critical of the layout and arrange the resistor values around the feedback network so as the Parasitic Board capacitance will do the Trick. I think that a properly designed Amp will only require a few pF(1-5pF) of capacitance to work properly, this is obtainable on the PC board. if more intrusive methods are needed and a additional capacitor is needed then this should be a series R/C network around the Feedback resistor. if the gain stage is not unity gain stable then a Resistor in series with the Cap is Mandatory so as to prevent the Amp from reaching unity gain too soon from External compensation capacitance.
The Above can be used in both IC Op Amp and Discreet component Designs. Discreet has th | | | |
