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In Reply to: RE: we agree on the bit about the OPT at least. posted by morricab on October 12, 2021 at 11:27:21
But if I had to say, those amps that you try to turn up to get them to play ball have too little feedback (20dB isn't near enough), with no way to add the amount needed to get them around that problem.
Sounding loud and sounding dynamic are not the same perceptually.
Yes and no... both come from higher ordered harmonics. Music has a lot of transients- that's where the power usually is needed. In an SET, when these transients occur, if they happen at a power level where the amp's linearity is reduced (usually about 20-25% and above) then the higher orders show up on the transients and nowhere else. That makes them sound dynamic. At higher levels though it becomes 'loud'. This is why so many SET owners seem to think that 85dB is all the 'louder' they need.
Not sure what you're referring to in your first statement. I googled Peter's name but don't know what I'm looking for. Do you have a link?
When you get rid of that issue, you can't even tell that they system is playing that loud. Now the dynamic structure is coming from the recording rather than the electronics. When sitting right next to someone, you may find you have to yell to be overheard. IOW, its lot harder to tell how loud the stereo is playing.
Follow Ups:
"Sounding loud and sounding dynamic are not the same perceptually.""Yes and no... both come from higher ordered harmonics. Music has a lot of transients- that's where the power usually is needed. In an SET, when these transients occur, if they happen at a power level where the amp's linearity is reduced (usually about 20-25% and above) then the higher orders show up on the transients and nowhere else. That makes them sound dynamic. At higher levels though it becomes 'loud'. This is why so many SET owners seem to think that 85dB is all the 'louder' they need."
The answer is just no. You may have heard the term "micro"dynamics? This is where little, seemingly subtle shifts, in intensity can make or break realism. Things like action of a pedal on a piano or a clack on the rim of a snare. The initial draw of a bow on a string etc. These are not loud in the context of SPL but there is a relatively large shift in level.Dynamics is the shift in level, loudness is the level.
For a car analogy: Dynamics is like acceleration and loudness is like velocity.
Speakers and especially amps that are dynamic show this prowess at ALL listening levels. Because the realism is retained throughout the SPL range. Quiet sounds shifting in level, loud sounds shifting in level...dynamics is about the shift, the speed of the shift and how accurately the shifts final level and speed is captured... not the actual SPL.
High order harmonics from the electronics has nothing to do with this perception. It has to do with accurate tracking of the recording of all these subtleties and tracking the shifts accurately as they would have occurred on the recording...or in real life.
One of the most disturbing effects of negative feedback that I have heard is this loss of natural dyanmics. The sound is so tied down that it no longer breathes like it should. I think that when you are feeding back the output to the input you are also somehow blunting these dynamic cues from the music...you are not only removing distortion (or creating new harmonics).
All of the best amps I have heard from a dynamic expressiveness perspective were zero feedback designs.
I would have thought, given your bigger models at least seem to be feedback free (the S30 at least is not I believe) that you would understand what I mean here and that it has nothing to do with loudness per se but the perception of a shift in level that reminds one of what one hears live. This could mean a very high instantaneous SPL but one that would be gone almost before it is perceived. However, it is prevalent (or absent) at all SPL.
No SS with feedback and no Class D I have owned or heard has the same "acceleration" as a good no feedback amp.
I realize that not everyone hears this properly so they are not bothered by it's absence, but once heard and understood it distinguishes true high end from mere hifi.
Edits: 10/18/21
'I think that when you are feeding back the output to the input you are also somehow ... '
I have to agree ... you DO think that!
it has little to do with reality but you're certainly entitled to your opinion
be well,
OUTSTANDING!!
Thank you, Thank you, and Thank you!
This is the most useful post EVER on this forum...
-Dennis-
One of the most disturbing effects of negative feedback that I have heard is this loss of natural dyanmics. The sound is so tied down that it no longer breathes like it should. I think that when you are feeding back the output to the input you are also somehow blunting these dynamic cues from the music...you are not only removing distortion (or creating new harmonics).
Yes- I've heard that too. That is why the feedback in our smaller amps (S-30 and M-60) is really minimal (2dB) and zero in our larger amps.
But that isn't a function of feedback as it turns out. Its a function of not enough feedback. And its not enough to say its not enough, because you can point to a Futterman OTL which claims to have 60dB (which it does at bass frequencies). At higher frequencies (like so many other amps out there) it has considerably less, owing to a lack of enough Gain Bandwidth Product.
Dynamics should come from the recording. The amp should not mess with that in any way. And they don't; the problem is distortion affects how we perceive the dynamic contrasts. If you don't have enough feedback you can have proper presentation in the bass, but as frequency goes up, things get messed up. There's more higher orders so while the bass might right, the highs are not.
If you have enough gain bandwidth product you can get around this problem. What you're looking for at any rate is a distortion figure that is the same at all frequencies. You can do that with zero feedback if you have enough bandwidth. Or, if you can get enough GBP you can do it that way, but you have to have enough GBP to support 35dB or more at 20KHz.
At that point the 'dynamic compression' you seem to hear with lessor amounts of feedback goes away. The sonic signature of the amp is another matter; it still needs to have the proper distortion signature (lower ordered harmonics as the dominant distortion product) even though that distortion might be quite low.
I've seen many 'objectivists'(?) (people who simply look at the specs) denigrate high end audio amps as 'tone controls'. In a way they are right, the problem they are having is they are not being pragmatic to understand that distortion is never going away. So if its not going away, you have to make sure that its as innocuous as possible. That will allow the amp to sound like music.
I have heard ultra high feedback amps (mola mola for example) that still sound relatively dead dynamically. Sure they play loud and clean but not dynamic.
If you feed a large % of the signal back you are in fact combining fedback signal with "fresh" incoming signal...this must create some kind of phase shift and "noise".
In fact, Norman Crowhurst noted that one of the most insidious aspects of feedback was a signal correlated "noise" floor. Which is another way of saying a myriad of distortion products, that covers the whole spectrum none of which is large enough to stick up as a clear harmonic peak. What it does do though is modulate with the signal level because, as you know, true noise is not correlated with the signal.
This is why you can easily hear "below the noise floor" on a recording with tape hiss, which is uncorrelated with the music signal. Your brain can pick out the correlated signal and reject the uncorrelated signal (a bit like an organic lock-in amplifier).
What happens though with a correlated "noise floor" though is that you can no longer hear below that noise floor because it is correlated with the signal, which has the effect of truncating low level information and I would argue probably has a deleterious effect on the dynamics as well.
An amp with huge amounts of feedback would also have a very strong and correlated response to the noise floor and this has negative impacts on perception...I can't see how adding even more feedback would magically take you out the other side for this particular problem.
I think Ralph has a way to balance two
feedbacks against each other, cleaning up
the mess that is created by using only one NFBK .
(By doing NFB ALSO in the opposite direction, we balance-out
most of the unwanted aspects of a single NFB loop).
I like it better, but I still don't like it much because
it still truncates dynamics-- simply by lessening
amp sensitivity to "microsignal" information.
The 2-stage amp is made possible-- partially-- by
not using NFBK.
If a balanced NFBK system is employed, many desirable
items in music are restored. But if we do this, the NFBK
loops still reduce gain. When we do that, the gain must be
restored-- add another stage of amplification if you please.
Now, what do we have? GAIN restored. Microsignal musical
information further reduced by what?----- the extra stage AND the
NFBK loops.
Not too bad when driving a medium-eff speaker using several
watts from the amp.. Sterile (by comparison)
when driving a large surface area, high-eff model that is wired
with silver, and has a great crossover system..
That speaker will tell you that your amp is OK-- better than
most-- but in comparison it lacks quite a bit of life-- it's
got power, it's got bandwidth, it's got pretty good phase alignment.
But there's one thing-- in spite of all of it's "A" scores, that
earns it a "D" score when playing music. It doesn't sound ALIVE--
like music dancing in an actual real life listening area.
-Dennis-
When we do that, the gain must be
restored-- add another stage of amplification if you please.
The gain of a class D amp is the relationship between the triangle wave and the incoming signal, if the amp uses pulse width modulation. You can get quite a lot of gain this way. In practice, we get enough so that we can drive the module directly with our preamps (which have no worries driving low impedances, even though they are all-tube). Most preamps don't have that kind of output voltage or current, so we boost the input signal by about 6dB using a set of low noise opamps. When opamps run with a gain of only 2 (6dB) the feedback is so high that the distortion is really stupid low so they tend to be very neutral, even at 20KHz. We're using some pretty nice opamps for this service, so the class D amp will be low noise on horns.
We do not run any significant feedback in our tube amps, because you have too many problems with phase margins and insufficient gain bandwidth product to support the feedback needed. So the distortion product tends to be a simpler kind with more lower ordered harmonics and less higher ordered, much like an SET in this regard although usually a couple of orders of magnitude less.
Thanks, Ralph! An excellent discussion,
as usual.
I break rules whenever I think that it
might work-- maybe it won't work.
I have been lazy in that regard because
I always wanted to play with a high-gain
solid-state device, and run it Class A, with zero FBK..
Everyone I ever talked to on this said mostly
the same thing-- it won't work with solid-state
devices-- they're not tubes-- they require all that
FBK.
Someday I think I'll try it anyhow-- I just want
to know for myself--- hah!
Who knows what that will sound like!
-Dennis-
-
I have some idea on this because I had in the past an amp from the company NAT out of Serbia.
The amp was the Symbiosis SE, which was a single ended hybrid that had a tube input, tube driver and an output stage with a single large MOSFET that was strapped to the bottom of a huge heat sink...per channel. There were two other heat sink towers that also had a single MOSFET that were regulating the output of the other MOSFET. Being single ended it was pure Class A where it consumed 800 watts even at idle.
This amp in some ways was the most amazing sounding amp I ever heard...if you could wait 2 hours for it to fully warm up (it was 155lbs. so there was a lot to warm up). It got spookily transparent...like an OTL and yet had much of the tonality of a good SET. It didn't quite have the same inner resolution and holography of a top SET but it wasn't absent either.
Of course there was not Feedback on this amp at all and it sounded that way. The most interesting part was that it actually put out 100 watts into either 8 or 4 ohms and was 2 ohm capable.
So, while a MOSFET doesn't sound quite like a triode it could in many ways sound quite convincing when used like a triode.
The only reason I sold it was the very long warm up to where it sounded almost psychedelically good...before that it was kind of like you knew it could do much better so it was a bit disappointing until it really came on song.
Most interesting, and confirms some of
my suspicions about it.
The question for me is-- is that long warmup
a function of zero FBK, or is it just power supplies,
etc., finally getting warm enough to clean-up.
I suspect that, but don't know it, so I'll just
have to build one. Mine would be much simpler--
maybe I'll get away with something-- it could work.
Thanks! -Dennis-
you know, I see where you were going with your earlier post that I ['tongue in cheek'] 'snarked' on and have to say it does resonate with me morricab
thanks for expanding on it here!
with regards,
-
nt
I have heard ultra high feedback amps (mola mola for example) that still sound relatively dead dynamically. Sure they play loud and clean but not dynamic.
Yes- something is wrong with the distortion signature. Its not enough to simply get the distortion low.
I agree (and quite a lot) with most of what you wrote.
An amp with huge amounts of feedback would also have a very strong and correlated response to the noise floor and this has negative impacts on perception...I can't see how adding even more feedback would magically take you out the other side for this particular problem.
Its not magic... its just when you get to that much feedback (and it has to be that much even at 20KHz) then it gives the amp the ability to clean up the mess that feedback otherwise leaves. It corrects phase and even corrects the harmonic and inharmonic noise floor (which sounds like hiss, but cannot be penetrated by the ear the way regular hiss can) as well as the bifurcated harmonics it otherwise creates. Back when Crowhurst was writing, this kind of feedback wasn't possible- the gain bandwidth product needed simply was decades out of reach.
But even though that's different today, you still have to pay attention to the distortion signature even though the distortion is much lower. Its got to be right or the amp will still get shot down.
Thanks again, Ralph. From you we learn more
about feedback. I try everything I can
think of to avoid it, but you point out more--
something else-- that people should pay
attention to.Negative global FBK is a TIMING issue, above
all else-- as far as music sounding real is
concerned.This is what makes your post important.
Read between the lines of your post and
one can deduce:The amplifier and the speaker should be viewed as a single
part-- should follow the dynamic events occurring with the
music sources and their components,, their interconnects, etc.Some setups will require a bit of NFBK in order to have
the timing cues come out -- shall I say-- unmolested.IF, however, one can invent ways to get these timing
cues ("microdynamics", etc.) to come out of the speaker
intact, WITHOUT any global NFBK being necessary, then
you've just won the sonics lottery.-Dennis-
Edits: 10/18/21 10/18/21
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