This may sound crazy. Putting R (2 or 5watt) between output and ground, value about 220ohm makes the amp stabile. Far more stable than putting Zobel or inductance.
But since I never saw this stabilizing method anywhere else (besides NP's XA), is it not good to replace zobel/inductor with just 220R plain resistor to stabilize an amp?
I'm afraid that putting this 220R in output-gnd will not stabilize an amp with certain load (reactive maybe?) but I don't know how to test it. I've tested it by putting 2.2uF capacitor between output (no load, just 220R), nothing happens, rock stable.
With zobel (or+inductor without this 220R), putting 2.2uF with no load makes the amp oscilates.
Can this 220R replace all zobel and output inductor?
But since I never saw this stabilizing method anywhere else (besides NP's XA), is it not good to replace zobel/inductor with just 220R plain resistor to stabilize an amp?
I'm afraid that putting this 220R in output-gnd will not stabilize an amp with certain load (reactive maybe?) but I don't know how to test it. I've tested it by putting 2.2uF capacitor between output (no load, just 220R), nothing happens, rock stable.
With zobel (or+inductor without this 220R), putting 2.2uF with no load makes the amp oscilates.
Can this 220R replace all zobel and output inductor?
lumanauw said:Hi, Kanwar,
I'm still fine
Is it not working ?(still need zobel+inductor). Maybe that's why I never saw it used (replacing zobel/output inductor) in almost all designs.
Thank GOD,
You are Fine!
The Zobel is Impedance Balancing Electronics, if not used results in some terrorizing curses......
lumanauw said:
Hi David,
Missed you! The stabilising with the 220 will most probably work with all (capacitive) loads, IF you connect that load with a length of speaker cable. The inductance of the speaker cable will in effect isolate the cap load from the output node. The 220 ohms will ensure that the amp is loaded (and stable) all the time. If you connect those cap test loads directly to the output node, some values may again cause instability.
Jan Didden
Two-pole compensation?
I don't like bumping my posts, but just in case people missed my previous post because of other discussions, I'll repeat my question:
"As often in various threads, there are claims by many that Miller comp is bad. Is this assumed to be because of the low-frequency pole, giving non-flat OLG in the audio band, or is it also assumed to be because of some other effect? The point I am aiming at is that if it is the first of these reasons, then what about using two-pole compensation around the VAS? It is not very common, but some amps use it to achieve a flat OLG in the audio band."
I don't like bumping my posts, but just in case people missed my previous post because of other discussions, I'll repeat my question:
"As often in various threads, there are claims by many that Miller comp is bad. Is this assumed to be because of the low-frequency pole, giving non-flat OLG in the audio band, or is it also assumed to be because of some other effect? The point I am aiming at is that if it is the first of these reasons, then what about using two-pole compensation around the VAS? It is not very common, but some amps use it to achieve a flat OLG in the audio band."
Hi,
I am not sure of the reason for the Miller comp method sounding less good.
I think it more likely it has to do with "other reasons" than open loop gain or 2pole correction.
My thoughts, maybe it has something to do with
overloading the input LTP collector?
or slew rate?
or wrapping very few stages (VAS=1) inside the bulk of the amp feedback?
or very little feedback left? to attenuate non linearities from the other stages particularly at higher audio frequencies.
or almost no feedback left to correct high frequency intermodulation descending into audio band?
or almost no feedback left to taper the gradual attenuation of high frequency harmonics that form the supersonic spectrum?
Audiolab went down the Miller comp cap route and although they are reviewed as accurate they are often criticised for sounding sterile or grey. Tag Mclaren copied this part of the design. In my opinion D. Self's designs are accurate but, I believe, his too have a sound quality issue.
I am not sure of the reason for the Miller comp method sounding less good.
I think it more likely it has to do with "other reasons" than open loop gain or 2pole correction.
My thoughts, maybe it has something to do with
overloading the input LTP collector?
or slew rate?
or wrapping very few stages (VAS=1) inside the bulk of the amp feedback?
or very little feedback left? to attenuate non linearities from the other stages particularly at higher audio frequencies.
or almost no feedback left to correct high frequency intermodulation descending into audio band?
or almost no feedback left to taper the gradual attenuation of high frequency harmonics that form the supersonic spectrum?
Audiolab went down the Miller comp cap route and although they are reviewed as accurate they are often criticised for sounding sterile or grey. Tag Mclaren copied this part of the design. In my opinion D. Self's designs are accurate but, I believe, his too have a sound quality issue.
Hi, Janneman,
I'm looking for you in post#2, but you showed up here
Not long ago, I have experimented with the so called "Zobel, ie 100nF+10R" and what it does to an amp.
I found something strange. In the case of no load, first I try to variate the value of the cap first. I try from 1nF to 1uF. It shows that until a certain value, this zobel can help, but below that value it has no effect to the oscilation. OK, I can accept this.
When I have found the minimal capacitor value, then I tried the resistor value. I tried it with 100ohm VR. I use the minimal cap value to know what effect the variation of R.
I found that a zobel works like "U" shape for resistor value. If the optimum value is 47ohm, then putting 100ohm the amp oscilates, putting 10ohm also makes the amp oscilates. It only likes 47ohm, nothing else.
This makes me think. If the curvature of resistor value is "U" shape, or there is optimum R FOR A CERTAIN LOAD, there is no guarantee that a certain zobel will work with other speaker brand?
Until I put that 220R. You are right. The key is this R loads the amp all the time. This makes the amp stable.
But why not many designs use this (only Nelson Pass?) Does it has something awfully bad?
I also found something else. There are some designs that don't like Zobel. No matter what value for R and C's they (all combinations) just makes the amp oscilates, instead of stabilizing it. The most obvious one is with feedback amp with CFP output stage. I suspect this for kind of amp Zobel is "Forbidden" ?
Hi, Christer,
Hi, AndrewT,
Difficult questions.....
If someone said the dominant miller cap is not good, you should try the feedforward cap (VAS-inverting input) effect. The value is only 20pF, but I feel that it is more destructive to the sonics compared to miller cap.
I'm looking for you in post#2, but you showed up here
Not long ago, I have experimented with the so called "Zobel, ie 100nF+10R" and what it does to an amp.
I found something strange. In the case of no load, first I try to variate the value of the cap first. I try from 1nF to 1uF. It shows that until a certain value, this zobel can help, but below that value it has no effect to the oscilation. OK, I can accept this.
When I have found the minimal capacitor value, then I tried the resistor value. I tried it with 100ohm VR. I use the minimal cap value to know what effect the variation of R.
I found that a zobel works like "U" shape for resistor value. If the optimum value is 47ohm, then putting 100ohm the amp oscilates, putting 10ohm also makes the amp oscilates. It only likes 47ohm, nothing else.
This makes me think. If the curvature of resistor value is "U" shape, or there is optimum R FOR A CERTAIN LOAD, there is no guarantee that a certain zobel will work with other speaker brand?
Until I put that 220R. You are right. The key is this R loads the amp all the time. This makes the amp stable.
But why not many designs use this (only Nelson Pass?) Does it has something awfully bad?
I also found something else. There are some designs that don't like Zobel. No matter what value for R and C's they (all combinations) just makes the amp oscilates, instead of stabilizing it. The most obvious one is with feedback amp with CFP output stage. I suspect this for kind of amp Zobel is "Forbidden" ?
Hi, Christer,
It can do that (Flat OLG)? If it can, I'm very-very interested.
Hi, AndrewT,
Difficult questions.....
If someone said the dominant miller cap is not good, you should try the feedforward cap (VAS-inverting input) effect. The value is only 20pF, but I feel that it is more destructive to the sonics compared to miller cap.
Hi, Janneman,
If an amp is 3 stages, ie : differential, VAS, output stage.
If I can choose various transistors with various fT (bandwith).
Which one should have the highest bandwith and which one should have the lowest bandwith to make an amp that is stable (without tricking with compensation caps anywhere)
The benchmark maybe the output stage. 2SC5200 or 2SC2922 pairs have bandwith of 20-30Mhz.
The VAS transistor should have fT higher or lower than 30Mhz? Lower I can use MJE340 type, for higher I can use 2SD669 or BD139.
The same question goes to the differential transistors. If the VAS and Output stage has been matched in fT, what is the criterion of fT for differential transistors? It should be higher or lower than VAS or than output stage transistors?
Maybe it seems funny to ask this, but many designs that uses super high speed transistors (300Mhz, low internal Cop/capacitance), ending up using compensation cap around B-C, B-E, C-C, etc, what's the use of picking up the 300mhz in the first place? Better use slower one without any compensation caps anywhere, I think.
The stability is made by matching/tailoring the fT required for each of that 3 stages, but can end up not using small compensation caps anywhere.
Is it good to make an amp with the same type (like pair of MPSA06-A56) anywhere from differential to VAS to predriver to driver (all the same with these 2). Will this makes the amp more stable due to same fT from differential up to just before the output stage (bigger transistor needed here offcourse)?
I ask the above, because if we use many kinds of transistors with different fT's for each, different gain curve for each, wouldn't the total transfer function becomes very complex (due to different transfer functions for each stage's transistor, with different pole's and zero's for each transistor), more prone to oscilation?
If an amp is 3 stages, ie : differential, VAS, output stage.
If I can choose various transistors with various fT (bandwith).
Which one should have the highest bandwith and which one should have the lowest bandwith to make an amp that is stable (without tricking with compensation caps anywhere)
The benchmark maybe the output stage. 2SC5200 or 2SC2922 pairs have bandwith of 20-30Mhz.
The VAS transistor should have fT higher or lower than 30Mhz? Lower I can use MJE340 type, for higher I can use 2SD669 or BD139.
The same question goes to the differential transistors. If the VAS and Output stage has been matched in fT, what is the criterion of fT for differential transistors? It should be higher or lower than VAS or than output stage transistors?
Maybe it seems funny to ask this, but many designs that uses super high speed transistors (300Mhz, low internal Cop/capacitance), ending up using compensation cap around B-C, B-E, C-C, etc, what's the use of picking up the 300mhz in the first place? Better use slower one without any compensation caps anywhere, I think.
The stability is made by matching/tailoring the fT required for each of that 3 stages, but can end up not using small compensation caps anywhere.
Is it good to make an amp with the same type (like pair of MPSA06-A56) anywhere from differential to VAS to predriver to driver (all the same with these 2). Will this makes the amp more stable due to same fT from differential up to just before the output stage (bigger transistor needed here offcourse)?
I ask the above, because if we use many kinds of transistors with different fT's for each, different gain curve for each, wouldn't the total transfer function becomes very complex (due to different transfer functions for each stage's transistor, with different pole's and zero's for each transistor), more prone to oscilation?
Hi, Pingrs,
Good question I don't know much about theories, but it seems CFP works the best at classA or heavy biased.
I cannot find load invariant in my Doug Self book (mine is 1st edition). But I can find 2 design of his in my book.
The first one is "Blameless amp" where this is "optimally biased classB" or cold amp and the other one is "ClassA" amp.
Interesting that DougSelf uses 2 different output stages for classB and classA. For blameless he uses EF, for classA he uses CFP. Both has zobel 10R+100nF.
He don't use CFP for low (optimally) biased classB.
I forgot to wrote that my experiment is with cold amp (low biased classAB). In this state, CFP seems don't like Zobel.
As far as I know classA (whether EF or CFP) seems far more stable (in any angle) than low biased classAB. You can een omit zobel or inductor with good designed classA.
Good question
I don't know much about theories, but it seems CFP works the best at classA or heavy biased. I cannot find load invariant in my Doug Self book (mine is 1st edition). But I can find 2 design of his in my book.
The first one is "Blameless amp" where this is "optimally biased classB" or cold amp and the other one is "ClassA" amp.
Interesting that DougSelf uses 2 different output stages for classB and classA. For blameless he uses EF, for classA he uses CFP. Both has zobel 10R+100nF.
He don't use CFP for low (optimally) biased classB.
I forgot to wrote that my experiment is with cold amp (low biased classAB). In this state, CFP seems don't like Zobel.
As far as I know classA (whether EF or CFP) seems far more stable (in any angle) than low biased classAB. You can een omit zobel or inductor with good designed classA.
AndrewT said:
Thanks Andrew,
You raise many interesting points.
The slew rate problem should be handled by designing for sufficient slew rate, and then choose the input LP filter such that we never get a slewing problem. However, as you point out, the Miller cap still presents a heavy load for the diff pair at high frequencies. I have thought about that too, and maybe that will cause a rising distorsion at high frequencis that could be a problem?
I find the point about HF IM interesting. Of course, the problem won't go away if we do the compensation otherwise, but it will shift to higher frequencies, at least, which might be a good thing. However, I think two-pole compensation would do an equally good, or better job here? Or maybe not, since we get more OLG up to a certain frequency, but less OLG above it, due to the steeper slope?
Anyway, lots of interesting things to think about.
lumanauw said:
Hi Luamanauw,
the idea of two-pole compensation is to use a CRC network instead of a single Miller cap. This means you can keep the OLG flat to a much higher frequency, since it will fall off twice as fast above that (12 dB/octave instead of 6 dB/octave). An example of two-pole compensation can be found here
http://www.diyaudio.com/forums/attachment.php?s=&postid=80932&stamp=1037049279
although in practice it is usually better to to tie the resistor to the rail than to ground, to improve PSRR. Randy Slone discusses this technique in hit book, and I think Self does too.
Andrew and Christer,
Re Miller compensation: Andrew, just about all of your points apply depending on the circuit, as I found it. To sum up:
The LTP collector(s) could be punished in a case of large Cdom, though this is seldom the major problem.
Slew rate will suffer if VAS can't cope (remember it usually must go the complete output swing). Recall how JLH and others feed Cdom from the output rather than the VAS collector for extra drive capability.
Also, as feedback at h.f. falls off, most amps (even Douglas Self's one) are left with increased distortion above some 8 KHz. This can be especially bothersome at supersonic frequencies where audible intermodulation products can be generated.
I found (in Spice analyses) that high order harmonic generation is sensitive to Miller compensation probably through a combination of the above factors. As said, it depends heavily on circuit design, transistor parameters,etc. That also brings in another member's remark regarding high ft's chosen. With BJT spreads one would rather have compensation under control using a 5% capacitor, instead of relying on varying ft's.
In the quoted Ellis article the case is made for the more instantaneous effect of lead compensation (i.e. a capacitor over the feedback resistor). In conclusion, some well-designed circuits do work well with Cdom compensation; one does not condemn that outright. But in my experience and perhaps that of others, I feel more comfortable in keeping such compensation as far out of the audio spectrum as possible (I already mentioned that I succeeded in reaching a -3 dB point only at 30 KHz). I cannot confirm with personal listening tests - sadly my hearing goes no higher than 7 - 8 KHz at present, but others seem to hear an improvement in such a system. But then hearing is such an individual thing that I would not rely on that solely to characterise an amplifier as good or bad (and no offense intended; it is just simply a human characteristic, plus the psychology involved - but that is another story.)
Regards.
Re Miller compensation: Andrew, just about all of your points apply depending on the circuit, as I found it. To sum up:
The LTP collector(s) could be punished in a case of large Cdom, though this is seldom the major problem.
Slew rate will suffer if VAS can't cope (remember it usually must go the complete output swing). Recall how JLH and others feed Cdom from the output rather than the VAS collector for extra drive capability.
Also, as feedback at h.f. falls off, most amps (even Douglas Self's one) are left with increased distortion above some 8 KHz. This can be especially bothersome at supersonic frequencies where audible intermodulation products can be generated.
I found (in Spice analyses) that high order harmonic generation is sensitive to Miller compensation probably through a combination of the above factors. As said, it depends heavily on circuit design, transistor parameters,etc. That also brings in another member's remark regarding high ft's chosen. With BJT spreads one would rather have compensation under control using a 5% capacitor, instead of relying on varying ft's.
In the quoted Ellis article the case is made for the more instantaneous effect of lead compensation (i.e. a capacitor over the feedback resistor). In conclusion, some well-designed circuits do work well with Cdom compensation; one does not condemn that outright. But in my experience and perhaps that of others, I feel more comfortable in keeping such compensation as far out of the audio spectrum as possible (I already mentioned that I succeeded in reaching a -3 dB point only at 30 KHz). I cannot confirm with personal listening tests - sadly my hearing goes no higher than 7 - 8 KHz at present, but others seem to hear an improvement in such a system. But then hearing is such an individual thing that I would not rely on that solely to characterise an amplifier as good or bad (and no offense intended; it is just simply a human characteristic, plus the psychology involved - but that is another story.)
Regards.
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