John Curl's Blowtorch preamplifier part II

[scott wurcer]

Originally Posted by DF96

It is true that in order to get a flat total response at the output there may be peaks at intermediate nodes, and some designers forget that. To get delays you need storage, and that means resonances (e.g. HF in OPT).

A problem in op-amp based FDNR filters.

[ThorstenL]

Hi Ed,

Originally Posted by simon7000

I have found when I drive anticipating the curves in the road works better than driving by feel.

Well, predicting music is not as easy as predicting the road (lack of signage to start with...).

Originally Posted by simon7000

Feedback should occur in units of time measured by phase. Of course there always is feed forward available.

Well, phase is another way of saying time. So if feedback is not zero or leading for all frequencies, we may conclude that feedback is "post hoc".

Feed forward can only be Ad Hoc if it is in fact Pre Hoc, as in the distortion reduction system used with LP (aka Tracing simulation).

Ciao T

[jcx]

digital source in principle allows as much "look ahead" as you want, up to whatever is acceptable delay after pressing start to load the buffer - could calculate "pre-distortion" to compensate for the known distortions of your amp

of course engineering knows how to treat "delay" and phase shift in feedback systems - the "objection" is the Sophomoric use by popular Audiophile press/marketing to bash "negative feedback" as inherently flawed, unsuitable for audio amplification


I just got a look at Quan's paper that John so highly recommends - it would have been OK if published around 1980 - even then the objection would have been that no "contemporary" recommended "audio" op amp was tested - not TL071 nor SE5534 - for a paper presented in 2010 one has to wonder if the SCA http://www.sca.org/ has a audio electronics subgroup

I strongly suspect that his paper's FM distortion measurements would read - "not visible above instrument noise floor" for most of the past ~ 5 years op amps that are recommended for audio

[simon7000]

Originally Posted by ThorstenL

Hi Ed,

Well, phase is another way of saying time. So if feedback is not zero or leading for all frequencies, we may conclude that feedback is "post hoc".

Feed forward can only be Ad Hoc if it is in fact Pre Hoc, as in the distortion reduction system used with LP (aka Tracing simulation).

Ciao T

DC does not exist as I am not immortal.

Therefore all signals only have phase shifts, just with very long periods.

But from a practical perspective when the delay is a small fraction of a cycle the correction spectra is significantly reduced and as the shift approaches zero, the spectra also does.

So we can play with semantics, but the issue should be clear.

[scott wurcer]

Originally Posted by jcx

I just got a look at Quan's paper that John so highly recommends - it would have been OK if published around 1980

There you go Jan, ask Barrie to "clear" the air. The sparks would fly.

[scott wurcer]

Originally Posted by jcx

digital source in principle allows as much "look ahead" as you want, up to whatever is acceptable delay after pressing start to load the buffer - could calculate "pre-distortion" to compensate for the known distortions of your amp

of course engineering knows how to treat "delay" and phase shift in feedback systems - the "objection" is the Sophomoric use by popular Audiophile press/marketing to bash "negative feedback" as inherently flawed, unsuitable for audio amplification


I just got a look at Quan's paper that John so highly recommends - it would have been OK if published around 1980 - even then the objection would have been that no "contemporary" recommended "audio" op amp was tested - not TL071 nor SE5534 - for a paper presented in 2010 one has to wonder if the SCA Society for Creative Anachronism, Inc. has a audio electronics subgroup

I strongly suspect that his paper's FM distortion measurements would read - "not visible above instrument noise floor" for most of the past ~ 5 years op amps that are recommended for audio

The usual suspects have launched the poo-poo hoping it would not hit the fan but they missed.

[rsdio]

Originally Posted by jcx

the "objection" is the Sophomoric use by popular Audiophile press/marketing to bash "negative feedback" as inherently flawed, unsuitable for audio amplification

To borrow and extend Simon's analogy:
An audio amplifier without feedback is driving on a curvy road with its 'eyes' closed.

[john curl]

Yes, I, too, was disappointed that Ron Quan neglected to put some measured results with some more COMMON, up-to-date IC's. He has tried a few, at my suggestion, perhaps a year ago. He does not design audio products, and he did his paper as a self-funded project to get down to what is important in audio design, which has been an ongoing hobby. He has a professor at Stanford University to vet his work. What I would be really interested in is the 4458-62 series of IC op amps that seem so universal in mid fi design, as they are cheap.
I would like to remind you Scott, that Dick Sequerra would like to talk to you sometime about PIM. He certainly is of another opinion than you appear to have here.

[john curl]

Why don't you find out what changing damping factor does, subjectively, by just adding a series resistor of 4 ohms or so, in series with your loudspeakers? Give it a shot. I have!

[Bob Cordell]

Originally Posted by jcx

I just got a look at Quan's paper that John so highly recommends - it would have been OK if published around 1980 - even then the objection would have been that no "contemporary" recommended "audio" op amp was tested - not TL071 nor SE5534 - for a paper presented in 2010 one has to wonder if the SCA Society for Creative Anachronism, Inc. has a audio electronics subgroup

I strongly suspect that his paper's FM distortion measurements would read - "not visible above instrument noise floor" for most of the past ~ 5 years op amps that are recommended for audio

Hi jcx,

You are absolutely correct. Quan is a nice guy and quite intelligent. I spoke at length with him at the AES convention. But his paper was a diappointment, and adds nothing to the PIM debate, nor does it change the results that have been previously articulated.

The mechanism of feedback-generated PIM is well-understood. Feedback-generated PIM is ALWAYS accompanied by easily-measured intermodulation distortion. It is merely an amplitude-to-phase conversion that occurs via NFB. PIM was just a new name for differential phase and gain, long understood by video engineers. Even amplifiers without feedback have intrinsic PIM, and that PIM is usually actually reduced by NFB. Feedback-generated PIM depends on closed loop bandwidth, NOT open-loop bandwidth.

Cheers,
Bob



Cheers,
Bob