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[bruno putzeys]

Jim Williams wrote on Thu, 30 November 2006 17:36

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.

Similar question: given that slew rate is not the only performance spec in which the 6172 is better than the 4562, why attribute the sonic advantage to slew rate?

Higher slew rates are found in op amps with greater GBW. As an example, I'll refer to PSRR once again. PSRR in a miller opamp equals loop gain. At frequencies above the dominant (real) pole, loop gain scales linearly with GBW and so does PSRR.

You can (easily) convince me that op amps with higher GBW will sound better, but that alone is not enough to draw the conclusion that specifically slew rate is involved.

[zmix]

maxdimario wrote on Thu, 30 November 2006 11:00

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

Max,

If you are going to continue to make the sort of bold assertions that you litter your posts with, you will need to allow your methods to be examined. As a bystander, it appears that you have no method at all, just a 'gut feeling' that what you want to believe is actually true.  

Bruno has asked you to elaborate why you think these artifacts are feedback related, and not due to some other aspect of the design.

In other forums, when you have been asked direct questions you have abandoned the topic altogether.

Bruno is new here, he is a brilliant scientist and for the moment is accommodating all comers. Please don't waste his time.

[Sahib]

I apologise in advance for being out of topic.

maxdimario wrote on Thu, 30 November 2006 13:47

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..

Max,

I would take a great offence from this. I have a very good friend of mine whom is a hi-fi sales man. I don't think you realise that you are making the same subjective comments for which you look down on hi-fi salesmen. If I am remembering wrong then I apologise but you are the guy who said that you could anticipate the sound that a design would produce by simply looking at the schematics. Pleeeassseeee...

Finally, hi-fi is also a part of audio electronics.  

Cemal

[maxdimario]

Feedback related.

I take a circuit built with a variable feedback control..

I have built my own from scratch using both tubes and transistors..

...granted it is almost impossible to make a tolerable amp with zero feedback on say a FET amp, or a BJtransistor. usually tubes tend to do this better.

unvariably I hear the same thing in my circuits or different preamp models with different amounts of global feedback engineered in etc.

sometimes raising the gain on a preamp and applying a pad afterwards or before so that it is not working with high feedback will give better sound in this respect, although specs become worse for noise, thd and freq. response.

taking a sound which has the detail sucked out by passing it through a low fdbk (high thd) circuit will not 'revive' itself through a low feedback amp so this is proof that what I hear is not euphonic distortion. If this were true passing the sound through multiple stages of said amps would create an improvement instead of degredation.

obviously without feedback you are prone to problems with load mismatching, distortion artifacts etc. which are corrected by the feedback.

If the NFB is 'just enough' to get the amp to work properly and if the active components are good to begin with so that very little feedback is needed I find that the amp does not lose those elements of the sound which create the 'real space' illusion.

as far as the term 'corrective circuitry' I use it loosely perhaps, but what I mean is circuitry which is engineered in to improve a system which is not ideal.

I don't know if I am explaining myself in a complex enough manner.

basically I have just heard a lot of circuits: similar, different, opamp, discrete, transformer coupled, dc coupled, etc. etc.

hence my observations..

BTW some opamps sound better with less feedback but most need quite a bit.

[maxdimario]

Sahib wrote on Thu, 30 November 2006 20:15

I apologise in advance for being out of topic. maxdimario wrote on Thu, 30 November 2006 13:47 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.. Max, I would take a great offence from this. I have a very good friend of mine whom is a hi-fi sales man. I don't think you realise that you are making the same subjective comments for which you look down on hi-fi salesmen. If I am remembering wrong then I apologise but you are the guy who said that you could anticipate the sound that a design would produce by simply looking at the schematics. Pleeeassseeee... Finally, hi-fi is also a part of audio electronics.   Cemal

right, well it WAS a bit toungue in cheek but I did spend a lot of time in my teens in hi-fi shops and I grew to hate that 'we know better' pushy attitude that MOST hi-fi salesmen working (with commission) would give me..

add to that the 'miracle knob' syndrome and... that's what I meant.

as far as HI-fi being anything other than consumer sales nowadays, I don't really see that happening.

there are nice passionate people out there I'm sure and I hope your friend is one of them.

[maxdimario]

Jim Williams wrote on Thu, 30 November 2006 17:36

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.

That IS one of the better opamps.

instant gratification..

that was my 'secret mod' to shock my friends who had bought expensive CD players.. made a cheap CD player sound better than a high-end CD!

[maxdimario]

Quote:

you are the guy who said that you could anticipate the sound that a design would produce by simply looking at the schematics. Pleeeassseeee...

can you tell what a harley will sound like by looking at it's engine?

can you tell what an ac30 will sound like by looking at it's schematics?

can you tell what a jcm 800 will sound like..

how about a neve,

an api?

a 50's tube preamp? a V76 a v72 a v77..

an st-70, a macintosh tube amp, an SET

when you've worked with many different circuits and tried mods on many different circuits you can tell what a circuit will sound like..

I can recognize the difference and I know that one type of circuit will be good for one thing, and another for something else.

it's NOT such a feat.

[dcollins]

Jim Williams wrote on Thu, 30 November 2006 08:36

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.

I dunno.  We'll have to agree to disagree here, as my experience with super-slewing parts has not been good.  One mans open is another mans irritating, I guess.  And it wasn't oscillating.

What was Buddy Holly's slew-rate?  That sounded pretty good!

I still use discrete opamps in some places, but unlike Max, I don't think the emotional content of music is damaged by an IC stage....

DC

[bruno putzeys]

maxdimario wrote on Thu, 30 November 2006 20:35

Feedback related. I take a circuit built with a variable feedback control.. I have built my own from scratch using both tubes and transistors.. (...etc)

I recognise this story, I've done similar batteries of tests. The result of this was that I've been "against feedback" for quite some time too. In fact, I've long preached precisely the same gospel as you do (uhh including that I could guess the sound quality by looking at a circuit).

What I've learned since is that "a little feedback is a dangerous thing".

Part 1: Free Harmonics
Imagine you've got a circuit with only a 2nd order non-linearity i.e. a second order transfer like f(x)=x+0.1*x^2. Next, you put an ideal 20dB gain stage before it and close the loop (P control only for the time being). The result will be a transfer which is neither linear nor quadratic:

Before feedback: y=f(x)=x+0.1*x^2
After feedback: y=f(10*(x-y))
(write out f()) y=10*(x-y)+0.1*(10*(x-y))^2
(expand) y=10*x-10*y+100*x^2-100*x*y+100*y^2
(collect for y) 0=100*y^2-(11-100*x)*y+(10*x+100*x^2)

Right. That's a quadratic equation. I don't need to solve it for you to see that there are going to be square roots in this thing. The series expansion of a square root contains an infinite number of second order terms. You start with a circuit with only a second harmonic and you end up with a full set of harmonics after applying feedback. What this means is that a moderate amount of feedback will have created higher harmonics which are more audible than the second you started with. When we apply more and more feedback, the new harmonics will eventually come down again as well. In the end we'll need to apply quite a bit more feedback before the net improvement in second harmonic outweighs the presence of a (now small quantity) of higher harmonics.

Part 2: Making matters worse
Clean P controllers don't exist. Neither the controller, nor the system being controlled has infinite bandwidth. A normal control loop is asymptotically integrating. A circuit that has 60dB of loop gain at 1kHz will have 50dB at 3kHz, 40dB at 10kHz and 34dB at 20kHz. The 3rd harmonic of a 1kHz test tone will be reduced by 50dB. The 10th by 40dB. As a result, integrating loop control will skew the "harmonic balance" towards higher harmonics. This in addition to the fact that the problem outlined previously will also become greater as loop gain declines.
The curious result of this is that an amplifier with moderate loop gain will sound more "natural" if the loop gain is knocked flat below 20kHz (ie. if the dominant pole is real and at 20kHz).  

Conclusion: Don't Be A Whimp
We now have two good reasons why low amounts of feedback may not sound all that good. When using feedback, use tons of it. In op amp terms that means: use wideband op amps. Make higher-order loops if your math skills allow you to (ever seen an inductor in a degenerated input pair). If you can't get really high loop gains at 20kHz, set the dominant pole at 20kHz. In this case, however, do not expect total freedom from colouration. The best you can get is a nonintrusive sound, but which at least leaves the music intact (and which is sometimes euphonic).

[Larrchild]

Here's a paper by James Boyk and Gerald Sussman which says feedback, in smaller quantities can be worse, as Bruno says.

Norman Crowhurst touched on this in the 50's and basically this takes examples of BJT, FET and Tube, and models them with no feedback and small amounts of NFB. Quite an eyeopener.

     http://www.its.caltech.edu/~musiclab/feedback-paper-acrobat. pdf

Indeed, upon building the test circuits mentioned in the paper, I saw the "upper harmonic skew" Bruno just mentioned:

Note: NFB = "no feedback" (sorry) FB= degenerative feedback from 1k emitter resistor.

I think the moral is..that you are supposed to use large amounts of global feedback to drive that stuff into the noise floor or lower.

[Jim Williams]

dcollins wrote on Thu, 30 November 2006 22:52

Jim Williams wrote on Thu, 30 November 2006 08:36 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. I dunno.  We'll have to agree to disagree here, as my experience with super-slewing parts has not been good.  One mans open is another mans irritating, I guess.  And it wasn't oscillating. What was Buddy Holly's slew-rate?  That sounded pretty good! I still use discrete opamps in some places, but unlike Max, I don't think the emotional content of music is damaged by an IC stage.... DC

Many earlier high slew opamps have high THD as well. Many conclude that since "this" fast opamp doesn't sound so great therefore all of them must be the same. The later generations of fast wideband opamps have higher gain-bandwidth (some approaching 90 db in the audio band) which pushes down THD well below common audio grade opamps with their 60 db open loop gain at 10k hz. Some have mondo current output stages like the THS4051/4011 that are heavy into class A. Even the current miserly LM6172 (2.3 ma supply current) will output over 100 ma.

One problem with these parts is they reveal. A harsh top end is not a result of these parts, it's from downstream and now you are hearing it without filtration effects. Fix the source of the irratations and all is well. Just don't expect Pro Stools to do this, it won't.

To those who still dismiss this new technology, I say try them yourself. Report back your findings, just don't dismiss something you haven't heard or tried yet. Everything sounds great until you hear something better. It's our job to do that.

BTW, I believe Buddy Holly's slew rate was 12 v/ us. That's what a 12AX7A can do. Pretty good compared to your average 5532/5534 opamp at 6 v/us. I still believe he sounded better live. Great artists have a special something that no microphone can encode. This I've learned working for Stevie Wonder. If you all could hear him without a mic, it's amazing. All recording gear takes away something. I can only imagine the presence and clarity if Buddy were to record with some of this new technology and I don't mean Pro Stools.

[Larrchild]

Jim Writes:

Quote:

BTW, I believe Buddy Holly's slew rate was 12 v/ us.

Not through the input iron and output iron and magnetic recorders of the day.

Hardly anyone liked that custom Cherokee console with video opamps in it in the 80's if I recall. Hmm.

[Andy Peters]

Bruno Putzeys wrote on Fri, 01 December 2006 02:28

When using feedback, use tons of it. In op amp terms that means: use wideband op amps. Make higher-order loops if your math skills allow you to (ever seen an inductor in a degenerated input pair). If you can't get really high loop gains at 20kHz, set the dominant pole at 20kHz. In this case, however, do not expect total freedom from colouration. The best you can get is a nonintrusive sound, but which at least leaves the music intact (and which is sometimes euphonic).

Just to clarify: "tons of feedback" could mean low closed-loop gain?

-a

[maxdimario]

Quote:

BTW, I believe Buddy Holly's slew rate was 12 v/ us. That's what a 12AX7A can do. Pretty good compared to your average 5532/5534 opamp at 6 v/us. I still believe he sounded better live. Great artists have a special something that no microphone can encode. This I've learned working for Stevie Wonder. If you all could hear him without a mic, it's amazing. All recording gear takes away something. I can only imagine the presence and clarity if Buddy were to record with some of this new technology and I don't mean Pro Stools.

excellent points.. this is exactly what I've been trying to put across.. especially preserving the live quality of a great artist.

stevie wonder will sound best without a mic, and therefore the presence needs to be preserved as much as possible..

there is a very 'fine' and etherial presence which can be felt being in front of an artist which some circuits capture better than others.

with a non-artist.. you can camouflage with 'euphonic distortion' and perhaps improve the result, but with a true artist you must try and get the essence of the performance..

it only takes a bit of smearing on the highs or phase distortion and you lose the ....feel...

good sound reproduction with a clear and open high end and midrange makes you emotionally alert to the sound therefore you become more RECEPTIVE to the emotion of the performances.

dull splatty highs make the music sound unreal, therefore not worth taking seriously from an emotional standpoint.

[maxdimario]

Quote:

When using feedback, use tons of it

I am not a mathematician, so I cannot dechipher what you stated, although I do not doubt it's validity.

From a philosophical point of view it is better to use a system which has either no feedback and therefore exhibits all the strong, good characteristics of a no-feedback design, or if you're going to use feedback, you might as well go all the way and emphasise the good positive aspects of that high-fdbk design.

low feedback amps have typical distortion artifacts and positive qualities...and are cost-inefficient.

high feedback amps have their own subtle distortions.... even though the feedback is a corrective device to effectively cancel distortion, and the distortion can be made very low because of this correction. they are also very cost-effective

obviously you would aim to exploit the strong points of either type: high or low fdbk.

No-feedback designs are almost impossible to realize 100% because active components distort naturally..

the balancing act in getting the least amount of feedback  includes the effort of making feedback as 'local' as possible and avoiding network feedback altogether if it can be done..

for this reason an amplifier with 2 active stages which uses low feedback will represent a more ideal 'low feedback' model than a low-feedback amp with 6 or 12 stages and global (network) feedback..

in opamps, which usually have many active components low feedback therefore is unapplicable.