Sound Quality Vs. Measurements

[bcarso]

Originally Posted by john curl

There is better, today, and not much difference in price, like the lme4562

Even Self, one of the 5534 stalwarts if ever there was one, has acknowledged the 4562 as an improvement.

A tube of 25 LME49710 in DIP arrived yesterday. $1.29 from the distributor in that quantity.

The reason to get them: I lack a reliable model for the part, and want to see how well a modified linuxguru-like buffer, or a buffer with a little gain to increase the output swing, will work if inside the loop. I used the dual parts in SM in the front end of a switchmode power amp and got very good results, so I don't doubt the part itself.

About the only thing one could wish from the 49710 is a voltage swing a bit closer to the rails. Of course it is a bipolar input part, and has as well a little more current noise than the 5534.

[dvv]

Originally Posted by janneman

I agree with your post here - well said. I'm just wondering why you feel that nfb should be limited to 26dB (or whatever number).
If you can maintain stability, what's against the highest nfb factor you can get away with? Is there some point where nfb turns 'bad'?

jan

Jan, 26 dB is a heueristic value for me. The best amps I have ever heard were almost all around that value, give or take 3 dB, with the only exception of H/K models, which range from 12 dB (HK 680) to 25 dB (usually limited to their cheapest model).

However, there were a few exceptions, for example, one being the LAS power amp I highlighted here some time ago, which is a total turnabout, it has VERY little local degeneration, it's almost 100% turned towards global NFB, yet manages to sound wonderful.

As for the top limit, to be frank, I never investigated that point, I never felt the urge to. I suppose there should be no problem, but I only assume this. The reason is that I find that if an amp has been done well, NGB over 26 dB is not likely to bring any notable improvements, except on paper. And I don't listen to paper.

So, in conclusion, I feel the amp simply should not need more than 26 dB to achieve what it can do. Other than that personal feeling, no other reason.

[a.wayne]

Is this possible ..? That is to build a power amplifier with zero feedback and still have low distortion and good performance ..

[dvv]

Originally Posted by nigel pearson

The typical LTP of old was 1 mA tail current with no extra emitter degeneration . I have played with various currents and found 1 mA not such a bad choice . The choice then to run more current or make the VAS a Darlington or whatever needs to be considered . I suspect Douglas Self struggled with this and came to no absolute conclusion . Anyone brave enough to suggest an optimum LTP current ? I will start the ball rolling and say 2 mA . Is 10 mA excessive ?

... .

Personally, I find that a standard bipolar transistor pair in a differential circuit will usually do what it can by the time your PER TRANSISTOR bias value gets to 1 ... 1.4 mA, or 2 ... 2.8 mA for both.

However, I must add that I generally use up to 2k collector load resistors, typically 1.5 or 1.8 k. For a reason I cannot explain, I have a slew of second thoughts about more than that, my conscience (if you believe I still have one at this age ) bothers me when I go above 2k.

FETs are different matter, In my view, the only FET I ever used to any measure is the venerable old 2SK170, and in my experience, it does really well when the total bias current (for both in a pair) is around 3.65 mA, at a voltage of 8.2, 9.1 or 10 V. Always in a cascode configuration, typically BC546B, 2N5551 or BF422 above them.

You can imagine my surprise when I saw the schematics of the H/K 870 power amp from 1985, which also used exactly the same value. Well, even a blind chicken gets the seed every now and then.

[dvv]

Originally Posted by jcx

actually the 10-20 dB is the range where feedback is "worst" - where "harmonic multiplication" gives the highest peak in the feedback generated new, higher harmonics when looping a low order open loop distortion

much higher feedback is needed to have the new harmonics all fall with increasing order as we are told is necessary

the "harmonic multiplication" is fundamental feedback math - applies to degeneration as well as "local loop" or "global" feedback for those who think there is a distinction - it is not in this property

JCX, I am not going to argue the point, I'll take your words at face value.

However, my ears say otherwise. Not always, or by default, of course, but I find it's possible to have just 17 dB of global NFB and literally GREAT sound, as demonstrated by my H/K 6550 (and that's a SEPP design, too).

Just goes to show that despite the fact that we have learnt a hell of a lot since the wild 70ies, we still don't know it all. Far from it.

[dvv]

Originally Posted by john curl

Nigel, this has been thought out thoroughly for the last 50 years. IF you increase the current in the differential pair too much, you get increased current noise contribution, and it is not quiet unless you drive with 50 ohms or less. That is one good reason to not go above 2ma with bipolar transistors. Also, bias current goes up, almost proportionally with increased current. However slew rate will remain the SAME if the gain-bandwidth of the amp remains the same, because the increase in differential stage current increases the Gm proportionally, and then your compensation cap increases proportionally. Only degeneration using emitter resistors or increasing the gain bandwidth of the amp will get you increased slew rate. That is the 'KEY' in Solomon's paper. Unfortunately, emitter resistors WILL add input noise. So, very low noise is out.
The rational alternative is the low noise jfet pair, with inherently lower Gm, so you don't need any source degeneration resistors, and immunity from current noise. NOW you can run any source current you want, mostly, because the Gm does NOT track the current, so you are not 'chasing your tail' so to speak. I hope this helps.
In any case, please look at successful working examples of amps and preamps and note that they are often pretty much the same, as far as operating current is concerned.

I can't speak for Nige, but it sure has made me rethink some things I thought were done and over with.

I have always agreed with the Italian designer Bartolomeo Aloia, a man with impeccable credentials and numerous projects ranging from simple ans cheap to complex but damn ood, who said one that if you want lots of current at the output, you have to have decent current at the input.

It seems I have been going a bit overboard with that thought, meaning that I have probably overbiased some differential stages to a suboptimal level. Kind of shot myself in the foot.

I just realized that from your comment above. To me, it makes operfect sense, and why I didn't see it myself I really don't know. Now that you said it, it seems so perfectly obvious I feel like banging my head against a wall for not seeing it myself.

On the subject of noise - while I agree with what you said, I would still point out that a bit of that added noise could be done away with if you use electronically regulated power supplies, which are assumed to be quite a lot cleaner than unregulated ones (at least, they should be).

One more trade-off to take into account.

[dvv]

Originally Posted by jcx

bjt diff pair noise with higher bias isn't a major issue in power amp front ends where we are expecting line level signal V - today we expect low Z drive at up to 2 Vrms from digital sources/SS preamps - can use lower Z volume pot - or can skip the volume pot for digital

where input current noise isn't limiting then we can use higher bias, degeneration R to make bjt front ends more linear than jfets: "What's your reasoning?" and not "What's your belief?".

I second Jan's comment - a truly GREAT post.

It made me think back to a text I read sometime in the past, somewhere, and was shortsighted enough not to save it, from somebody in Harman/Kardon, who more or less said the same thing. He claimed that a classic bipolar differential pair should have an overall gain of no more than 11:1, and no less than 5:1, and the in-between values would show up the bipoars at their very best, assuming the design has no other flaws, such as a poorly judged bias current.

H/K themselves did 5:1 a few times - for example, the input bipolar differential stage of my 6550 integrated amp has 2x220 for degeneration, and 2x1.2k for load. In the 870 power amp, it's 2x220 for degeneration, and 2x1.5k for load, and that for a FET (2SK170) cascode (2SC2240) input stage. And in their top model from 1998, HK680, it's 2x100 for degeneration and 2x1k for load in a straight 2SC2240 differential stage (bias 2x1.2 mA).

[dvv]

Originally Posted by a.wayne

Is this possible ..? That is to build a power amplifier with zero feedback and still have low distortion and good performance ..

I think not.

The marketing gimmick of "zero feedback" simply meant no overt GLOBAL NFB, but it also probably means that the feedback has all been relegated to individual stages. In other words, it has been made all local.

I have heard a power amp which claimed that, from a Danish manufacturer, Dennsen. While it no doubt worked, it left me with an impression of a loose sound, not quite all together now. The bass seemed a bit bloated, not too much so, but definitely lacking some control and definition.

All very subjective, of course. I can't help it if I banged on a drum kit in my wild and wanton youth, so I instinctively listen to the bass lines first. The friend I did the banging with, while he strummed a classic guitar was much luckier than I was, he went to to eventually become one of the greatest local rock legends of out time, with a career from 1976 until this day, and still going strong. The price he paid for it is that he's now an alcoholic, while I drink a glass of something on special occasions only (birthdays, new year, etc).

[tvrgeek]

"much higher feedback is needed to have the new harmonics all fall with increasing order as we are told is necessary"

One of the things I noticed ( about 6000 posts ago) was the Rotel designs did this where the DH-120 when stock did not. After the mods, it did behave as suggested.

I have not seen this mentioned before, and would appreciate a source so I can read some more. Does it matter what the initial level of distortion is, or just the general profile of harmonics? What is the characteristic sound of one or the other?

[nigel pearson]

Originally Posted by a.wayne

Is this possible ..? That is to build a power amplifier with zero feedback and still have low distortion and good performance ..

It might be . I recently built a zero feedback amplifier which has 0.03 % THD at 0.63W . It was better than that down to 50 uW . It used the opposite distortion of a pentode and triode . Pre distrotion I suppose . If all triode it had considerably more distortion ( 5 % full power ) . The amplifier made the Old DIN4550 just . If I remember correctly 0.1% THD at 1.6 W . It doesn't seem much to shout about , however for a zero feedback SE design it is very good . 2 valves and 0.56V in for 8V out .

I am not comfortable that it is a low distortion design . It looks like low distortion . The sound is rather good . I notice multi-tracked voices like never before , especailly if the same voice . I have been offered some 211 valves to see if a few more watts can be had . Bass is not the greatest . I hope to try it with Quad ESL 63's soon as they do not require high damping factor . I certainly feel the all triode version was inferior . Along the way I tried Schade feedback , I have doubts about it . I would liken the effect of my design to that of Ultra Linear feedback although this is not feedback . Have I become a valve convert ? No , if anything the other way . I certainly have enjoyed doing it .