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[maxdimario]

How about a discrete transistor circuit with relatively low slew rate but low(er) global feedback?

Would the slew rate influence distortion in opamps considering their complexity and corrective circuitry?

I say this because corrective circuitry needs to be fast and stable in order to work ideally..generally speaking.

I see you talk mostly using OpAmps as reference.

[dcollins]

maxdimario wrote on Mon, 27 November 2006 16:50

How about a discrete transistor circuit with relatively low slew rate but low(er) global feedback? Would the slew rate influence distortion in opamps considering their complexity and corrective circuitry? I say this because corrective circuitry needs to be fast and stable in order to work ideally..generally speaking. I see you talk mostly using OpAmps as reference.

Max, do you like the sound of  IC opamps?

DC

[bruno putzeys]

I explained earlier in this thread that feedback does not affect slew distortion, neither positively nor negatively.

When I speak of op amps I mean what's inside them. Whether this circuit is executed using discretes or on a chip has no import on how the circuit is analysed. Most discrete amplifier circuits are op amps too.

Depending on the amount of abstraction, an op amp has several possible definitions.
1) (schoolbook) a circuit with two inputs and one output. The output voltage is plus infinity times the voltage difference between the inputs. Actual output in a given circuit is arrived at through limit calculus.
2) (practice) a circuit with two inputs, one output and a output reference (usually one of the supply rails, sometimes ground). The output voltage is the time integral of the voltage difference across the input times the gain-bandwidth (expressed in radians/sec), referenced to the output reference.

According to the first definition, an op amp is a P controller with P tending towards infinity. According to the second definition, an op amp is an I controller with I equal to the gain bandwidth product (in rad/sec).

There is also a third "definition" which is more like a recipe:
3) An op amp is a transconductance amplifier followed by a transimpedance amplifier, where the transimpedance of the latter is substantially capacitive.

In most op amps (be they integrated circuits or discrete circuits) there is no "corrective circuitry" other than the gain of the amplifier itself. In circuits employing local feedback loops, these loops are faster than the main loop as a matter of course, or the whole shebang simply wouldn't work at all. No op amp using "corrective circuitry" that is *not* stable would even make it off the lab table.

If the need arises (and when I have more time) I'll put up some op amp theory here.

All too often, I read blurbs (sales literature) suggesting that "stability" and "correction" and what have you are "difficult" and so on. I should warn against the hijacking of technical terms to be used by sales people who charge them with emotions.

It seems that what "the terrorist" is to politics, negative feedback is to audio. Something to be vilified, stereotyped and used to make anything acceptable by.

[zmix]

Bruno Putzeys wrote on Tue, 28 November 2006 06:47

I should warn against the hijacking of technical terms used by sales people who charge them with emotions. It seems that what "the terrorist" is to politics, negative feedback is to audio. Something to be vilified, stereotyped and used to make anything acceptable by.

Bruno,

I do hope that this forum can maintain a utility beyond invoking 'gear hypochondria' in the infirm, but as they say "a little knowledge is a dangerous thing".

Be prepared...

-CZ

[bruno putzeys]

zmix wrote on Tue, 28 November 2006 19:27

I do hope that this forum can maintain a utility beyond invoking 'gear hypochondria' in the infirm

An occupational hazard for anyone trying to make anything clear, I fear.

zmix wrote on Tue, 28 November 2006 19:27

Be prepared...

Uhh don't frighten me more than I already am

[dcollins]

Bruno Putzeys wrote on Tue, 28 November 2006 03:47

It seems that what "the terrorist" is to politics, negative feedback is to audio. Something to be vilified, stereotyped and used to make anything acceptable by.

In Audiophile land, I think this is largely true.  No one advertises that they use high levels of NFB!  

At the Hi-Fi show it's always fun to ask the designer that shuns any type of feedback if he uses emitter resistors....

If they only knew that NFB is not magic, but it's close!

DC

[Andy Peters]

dcollins wrote on Tue, 28 November 2006 23:08

At the Hi-Fi show it's always fun to ask the designer that shuns any type of feedback if he uses emitter resistors....

Bingo!

Oh, those degenerates ...

-a

[arconaut]

You are all so informative - and amusing!

By the way, the Ciletti quote is out of context. His article was about modifying gear, and that sometimes in trying to make something "better" you take away the character that made the gear cool in the first place.

In swapping op amps, one might say "this slew rate is terrible" and  try to find a better amp. That's when he made that statement about slew rate and the output transformer, his point being that the slew rate didn't really matter that much in the grand scheme of things. I just wanted to understand that little detail - but I got so much more.

Thanks all.

Noah

[dcollins]

arconaut wrote on Wed, 29 November 2006 05:56

In swapping op amps, one might say "this slew rate is terrible" and  try to find a better amp.

3000V/us has got to sound better than the primitive gear that made all the records we love today............

DC

[maxdimario]

interesting.

As DC has mentioned I have never found an op-amp which could perform as well as a discrete circuit for critical amplification applications such as mic preamps.

When I have been forced to use IC's, I have noticed that on GOOD (expensive) high-slew rate opamps the feedback-induced artifacts which I hear quite clearly, seem to diminish.

of course 'fast' circuits are prone to oscillation and ringing and other HF anomalies, and there is more to an IC than specs..

wouldn't a faster overall slew rate improve phase-issues in the feedback path? even subtly?

on the other hand, you SHOULDN'T need super-fast slew to make an amp work well with audio... but I have found that in commercial IC op-amps the high slew rate tends to minimize typical opamp artifacts (ear-wise).


have you ever worked with discrete transistor designs ?(incl. discrete opamps).

By the way I believe that hi-fi salesmen are one of the lowest forms of life.

Audio electronics and hi-fi are two different things..

[bruno putzeys]

I've been building my own op amps for a long time. During the course of developing a circuit that really had an edge over IC based circuits I found that most discrete op amps *cannot* compete with good IC op amps, neither in measured terms, nor in sonic terms. Epecially minimalist designs are quite useless if transparency (=input and output indistinguishable) is desired, even though some can be decidedly euphonic.

The circuit I've settled on for the last 5 years was clearly worth the trouble. At around 20 transistors, it is neither simple nor minimalist. The reason why I designed this circuit was to get rid of the implicitly supply-rail-referred compensation in miller type circuits that are universal in IC op amps and almost so in other discrete op amps. PSRR of the standard miller circuit is zero if not for loop gain. I would go as far as to say that it's not worthwhile to build a discrete op amp if not to improve PSRR by moving the compensation reference to GND or to have more drive capability. All other performance specs on good IC op amps are already good.

Another way of improving PSRR on a miller type op amp is of course to increase GBW (PSRR of the "bad" supply rail equals loop gain). Usually though, fast op amps have other flaws. A fast op amp that is good on all counts is the new LM4562 . GBW is over 50MHz so PSRR (at unity gain) on the "bad" rail is at least 68dB at 20kHz (the +psrr plot at 15V appears to be in error).

Could you elaborate on how you deduce that certain sonic artefacts are feedback-induced and not caused by any other aspect of the operation of the circuit?

[maxdimario]

Yeah!
i'd love to but it's impossible to PROVE via internet.

I just basically spent 20 years listening.

a quick way of describing it is that regardless of tonal balance, freq response, percieved detail etc. there is a 'flattening' of the high-freq dynamics and percieved depth.

by percieved depth I mean the instruments which are up close are heard as being foreward or close and distant sounds and room reflections are heard as being distant.

sounds with a strong dry and clean percussive attack stick out a little more.

listening on speakers the effect is that the music is shifted foreward and 'in front' of the speakers.. into the room.

The non-tecnical response to low feedback amplifiers by people who have heard the results is that they can 'see' the musicians and instruments. This OBVIOUSLY depends on the quality of the source material and the recording techniques used.

so it has something to do with localization and percieved space.

when I think of localization etc.. I believe that the ears get their clues from something which is not tonally related but time-related..

in other words the correct reproduction of a transient attack and 'sharp' wave crests.

I've had arguments about this before...

[maxdimario]

I agree about PSRR.

in fact I believe that one of the main design or PRODUCTION advantages of using IC's is that they are built to be operated with cheap power supplies, but the power rail correction circutry creates problems with instability etc.

I can see why they would invent such devices..and why I would never want to use them personally..

[Jim Williams]

dcollins wrote on Wed, 29 November 2006 23:10

arconaut wrote on Wed, 29 November 2006 05:56 In swapping op amps, one might say "this slew rate is terrible" and  try to find a better amp. 3000V/us has got to sound better than the primitive gear that made all the records we love today............ DC

Just might be. Pop in a LM6172 into a circuit using the tired 1975 vintage NE5532, albit with compensation and psu treatment and you will find that 3000 v/us slew rate part really does make a difference. It definately sounds much more open than either the LM4562 or the AD8599, although the noise is higher.

[bruno putzeys]

maxdimario wrote on Thu, 30 November 2006 17:00

a quick way of describing it is that regardless of tonal balance, freq response, percieved detail etc. there is a 'flattening' of the high-freq dynamics and percieved depth.

I am not asking what you like or dislike about the sound of certain circuits. I am asking why you believe this to be related to feedback.

maxdimario wrote on Thu, 30 November 2006 17:05

in fact I believe that one of the main design or PRODUCTION advantages of using IC's is that they are built to be operated with cheap power supplies, but the power rail correction circutry creates problems with instability etc.

What power rail correction circuitry are you referring to?