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Power Amp output inductance or not?
Power Amp output inductance or not?
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Old 11th March 2006, 11:44 PM   #51
Johan Potgieter is offline Johan Potgieter  South Africa
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We are somewhat off-thread now but I hope it will be allowed to emphasize that what you (and Baxandall earlier) hinted at is vitally important but sometimes neglected. The fed-back signal at the point of encountering the input must be the exact replica of the output signal. Changes here are not corrected by feedback. While this is logical when concentrating on the matter, it is often overlooked in the total picture. When distortion of the order of 0,01% or such is relevant, the scale of this aspect becomes apparent.

It depends of course on the nature of the "non-linearity", and that not being the thread subject I will stop there. I also purposely referred to "encountering the input" - in designs where discreet active stages are used to combine feedback with input, even in a differential mode, increase in distortion can occur when these devices are not equal. For this reason I have on occasion used passive (resistive) mixing of these signals at the input (as per inverting mode), even though that meant suffering some noise degradation due to the finite input resistance. This is all the more true in the case of tube amplifiers (which I still occasionally dabble in) where the input is usually on a grid and feedback via a cathode circuit - but that is yet another thread!

Just to confirm especially for less experienced members, the importance of this aspect.

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Old 12th March 2006, 02:05 AM   #52
Eva is offline Eva  Spain
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This is an interesting subject, because I have seen lots of poor PCB layouts where absolutely no care was taken to get a feedback signal resembling well the output signal. Instead, they take the feedback reference from whatever point that comes more handy, like from some emitter resistor in some corner of the PCB where asymmetric voltage drops across the output track are more than measurable. Note that in class B and all its derivatives only half the output current waveform flows through each set of output devices, so the designer must ensure that feedback is taken exactly from the mid point, preferably from the output connector).
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Old 12th March 2006, 10:37 AM   #53
Bobken is offline Bobken  United Kingdom
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Hi Forr,

For what this is worth, I originally used the term "direct signal path", in this case intending to mean the path which the signal takes 'directly' on its journey from input to output of the amp. i.e. the entire 'wanted' signal passes through this area.

As I think we both recognise, C7 in this case forms part of a high-pass network which determines the lower -3dB roll-off of the amp.
This network running from the feed-back path (connecting the amp's output back to the inverting input) together with the remaining network components R8 (and R9 // RV3) shunts a proportion of the feedback to ground, and also establishes the overal gain of the amp.

I agree with Baxandall, though, about the importance of the feedback components, as already mentioned, and they do need to be considered in much the same way as the 'direct' signal path parts here.

None of this is material to the main issue here, though, and it was only to indicate to Fab, that a 'dyed-in -the-wool' objectivist as JLH was at one time, accepted that component choices can, and do, have some effect on the sound, exactly as you appear to have done with these resistors you mention. And, as I also said, this designer was in a rather unique position with his main 'occupation' to be able to study/measure and consider from all aspects, the performances of individual components used in electronics.

I am very familiar with the 1972 HFN design (I think this must be what you refer to, not '78), which I also built, and I still have one in working order, although I don't use it except if I am feeling particularly 'nostalgic', and for a bit of fun.
Regrettably, it was far too complex with all of its technically- excellent filters/ tone controls etc., and especially the use of tantalum bead caps in various critical areas was not a good choice here, as the designer later accepted. These (then very new) tantalum bead caps I soon found to be poor 'subjectively', resulting in a 'grainy' coarse kind of effect, which I couldn't accept.

Merely substituting these tants with some ordinary almn. electrolytics, considerably improved the subjective results to my ears.
Incidentally, it was not a bad design, overall, in its day, and there were several unauthorised commercial 'rip-offs' by unscrupulous audio makers in the UK! This happened quite a lot over the years with various designs of JLH, as presently appears to be the case with Nelson Pass.

Eva and Johan have made some interesting and important points here relating to feedback and non-linearities etc.

For many months I ran my own 'derivative' of the 1984 circuit, with the input signal being taken directly to the inverting input of the amp. (Q6), as opposed to Q1. This 'sounded' to be an improvement to me, but (as always in life, and with its necessary compromises) there were some snags, which finally encouraged me to abandon this trial. Of course, this had the effect of inverting the signal at the output (which was of no practical consequence) but the consequentially much lower input impedance was less comfortable to deal with.

The most significant problem (IIRC) was that in this configuration, the output characteristics of any pre-amp had some effect on the overall frequency response of the amp, which was not acceptable to me.

After being impressed with the results I had (temporarily)achieved here, JLH took this amp back to his lab for some tests, as at that time my own test gear did not include the ability to measure THD etc., very accurately. When returning it later, he commented ".... it is superior in harmonic distortion and in handling transients.....this is because the signal and NFB are taken to the same point and this avoids errors from one limb to the other of the input long-tailed pair".

This is another example of when I initially 'heard' an improvement during empirical trials, but this was later confirmed by measurements following some suitable tests. Regrettably, so far I don't believe anyone has yet managed to measure many of the sonic effects attendant to different types of components, but maybe one day, this will come.

Apologies to Fab if this is rather straying from his original query, but it seems to be relevant to what has just been touched upon.

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Old 12th March 2006, 02:01 PM   #54
forr is offline forr  France
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---[...] poor PCB layouts [.(..]---
Subjectivists should have much more respect to the Douglas Self's writings than he receives, at least, because of his simple recommandations for the taking of the feedback point and the power supply - grounding schemes which apply to any kind of amps. I still have to see masters of subjectivism, who loved to spread upon the sound of components, to spare some time upon these points of primary importance.

Sorry, I made a mistake but it is not what you think. I am refering to a JLH design published in the Hi-Fi News issue of january 1980 . This is a very simple amp (+ preamp), using bipolar output triples capacitively coupled to the output and a singleton input. In the same magazine, in december 1980, JLH published an updated version using Hitachi Mosfets. I have built two bipolar examplaries of the bipolar version, they are still in use, and one mosfet. What fascinated me in these articles were the square waves on capacitive loads : no overshoot at all. I then thought it was mandatory for good sound ! D. Self explains what happens exactly. This drives us back to the subject of the thread : in most of his designs, JLH uses a 0R22 resistor instead of a small inductor, this partly explains the absence of overshoots. Quite funnily, the problem Baxandall encoutered with a passive component was with a small value sensing resistor in a (real) current feedback, high impedance output, power amplifier.

Belonging to the subjective camp at my beginnings, years of experience made me very cautious about sonic claims, albeit without rejecting them systematically. So I am quite interested by what you say about comparing amps in direct and inverting configurations. Bar the noise (unnoticeable in most conditions), the inverting configuration is always measurably better. I wonder why subjectivists do not explore this connexion which is objectively more significant than so many exotic receipes.

~~~~~~ Forr

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Old 12th March 2006, 04:06 PM   #55
AndrewT is offline AndrewT  Scotland
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Hi Forr,
I don't much care for the way D.Self espouses his doctrine; distortion is the be all etc.

However I agree with you totally that D.Self has given us a whole series of building/assembly advice that if carefully followed can and does reduce amplification errors that are avoidable. Hence his title blameless.

Read and adopt with care, you can probably improve most any amplifier.

I believe, if you ignored LTP current mirror and Miller compensation around VAS, his amps could be even better.
regards Andrew T.
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Old 12th March 2006, 04:09 PM   #56
Bobken is offline Bobken  United Kingdom
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Hi Forr,

No need for any apologies. I just assumed as you mentioned the '70s that it must be the amp I referred to as I was sure JLH had not published any design around the time you mentioned (1978).

I must be honest and say that this amp you refer to was about the only circuit of this designer which I never really had any involvement with. By that time, I had started experimenting with (Hitachi) mosfets, which you also mention, and I was therefore not really interested in bi-polar designs then, although I have nothing against bi-polars, themselves.
I was just more interested in seeing what I could get out of the newer devices, and this did present a few new challenges which were occupying me.

What you say is interesting though, and I take some pleasure in having encouraged JLH to reduce his 'component count' in some of his much later designs (not all of them having been published), for example his 're-hash' in 1996 of the much earlier and very simple Class 'A' design, going back to 1969, in WW.

I had discovered that, although technical improvements/complications seemed preferable on paper and often when being measured, there appeared to be a trade-off, sonically, with almost a law of diminishing returns here.

Frankly, this was an uncomfortable discovery, but it couldn't be ignored, and it may well relate to the fact that no component is quite perfect, and accordingly, the fewer parts used meant fewer imperfections in the chain. I cannot say for certain.

I demonstated this to JLH over a period of some years (with his encouragement, and advice) mainly with a design of his published in WW in 1982, which started out with such 'lily-gilding' as positive feedback within the neg. feedback loop etc. For whatever reason, and in spite of this apparently superior technical topology, it simply sounded better without this particular addition. It progressed along from this in every area I could experiment with, and provided the measured performance was not seriously compromised, it nearly always ended up with better sonics.
Over a few years, I tried out all kinds of other circuit variations, too, along the lines of that I mentioned before, and measured (or had measured, when I could not do this for myself) the effects, and spent month after month listening to what these actually did to the sound. Swapping things back and forth time after time, and discovering as best I could the correlation between various topologies and their resultant sounds.
In all, I have been doing this for over 35 yrs., and I am still learning all the time.

Regrettably, because of conventional wisdom and the lack of time and inclination, most people will never spend the time needed to carry out serious long-term listening tests, but nowadays most high-end manufacturers will make some effort to 'optimise' their products through some listening tests, before their products hit the shelves.

I fully agree with the comments from Douglas Self over care where the neg, feedback is taken from, and, of course, this is quite intuitive. However, when I read his articles in EW & WW some years ago, this was the first time I had seen any published reference to this simple, but very important, issue.

So many apparently insignificant issues contrive together to produce either a very good-sounding result or maybe otherwise, and attention to every last detail is vital in my experience if you are keen to get the very best 'sonic' results with any audio equipment.

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