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Old 12th February 2003, 02:43 PM   #1
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Question to Miller compensate or not to Miller compensate

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.
 
Old 12th February 2003, 02:56 PM   #2
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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?
 
Old 12th February 2003, 07:04 PM   #3
dimitri is offline dimitri  United States
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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
 
Old 12th February 2003, 07:31 PM   #4
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Post Electronics World Article

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
 
Old 12th February 2003, 08:59 PM   #5
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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.
 
Old 12th February 2003, 10:37 PM   #6
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"... am about to change the emitter resistors to non-inductive Vishay thick film types"

My prediction: it won't cure the 500kHz noise.

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.
 
Old 12th February 2003, 11:05 PM   #7
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Default Correct

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
 
Old 12th February 2003, 11:19 PM   #8
blmn is offline blmn  Brazil
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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,
 
Old 12th February 2003, 11:37 PM   #9
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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?
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Old 13th February 2003, 12:03 AM   #10
AKSA is offline AKSA  Australia
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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
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