Fun with Feedback

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andrew Ivimey
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#46 Re: Fun with Feedback

Post by andrew Ivimey »

Has any of you got examples of Metronomes. I've forgotten what they look like!?
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#47 Re: Fun with Feedback

Post by Cressy Snr »

My originals from 2005. Unseen pair of subs provided the bass.
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andrew Ivimey
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#48 Re: Fun with Feedback

Post by andrew Ivimey »

Okay I remember, muchly ta. I did something like this long before I knew Scott and his wonderful inventions. I used fostex 167s and thought - brilliant! But that was just the beginning.

In that vein I finished off with Subaros or was it Saburos. Once I'd built them I couldn't get them in the car but lots of youse will have heard DtB's speakers; fascinating!
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
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#49 Re: Fun with Feedback

Post by Cressy Snr »

I've had an interesting few days reading around the subject of audiophile opinions concerning the use of global negative feedback, looking for positives, but there aren't any. For instance, Martin Colloms reckoned in Stereophile, that corrective feedback was fundamentally unmusical and that the slightest whiff of it in a single ended triode amplifier kills the sound stone dead.

When you've worked your arse off to try and do a proper implementation, and heard the excellent results that are possible if you are prepared to put in the effort, you have call out this so called 'opinion' for what it actually is: cobblers.
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#50 Re: Fun with Feedback

Post by JamesD »

Its all about balance and nuance. And one big difference is between an amplifier for the general public that has to work and be stable in all possible systems and a diy amplifier that can be optimised for one system... Global feedback is hard to get right for all possible systems deployments and much easier to tweak for a given system... really glad your enjoying the 'journey' and the sound of the final system....

HiFi reviewers have a difficult job as they have to satisfy their advertiser swho pay their production costs as well as their readership - so expect them to be biased and opinionated - just don't expect them to be accurate- whatever that is for HiFi....
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Greg
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#51 Re: Fun with Feedback

Post by Greg »

Cressy Snr wrote: Sun Jul 03, 2022 5:38 pm When you've worked your arse off to try and do a proper implementation, and heard the excellent results that are possible if you are prepared to put in the effort, you have call out this so called 'opinion' for what it actually is: cobblers.
I like it. Colloms’ Cobblers😀
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#52 Re: Fun with Feedback

Post by Cressy Snr »

Hi James.
I did all my tests into a dummy 8R load as that’s what my speakers are. I had a look at a DIY SE design with NFB in an old wireless world book from the early 1950s. According to that article, you have not only to change the feedback ratio, because of the differing step down ratios, but also the compensation cap across the feedback resistor, depending on which impedance tap on the OPT you wished to use. So you had better have been sure what speaker you were going to use with it.

This may be one possible reason that many modern amps with NFB just take the feedback signal from the 16R tap and be done with it. However I can’t see this being optimum for the HF compensation arrangements at the same time as the ratio. Yes it will keep the designer from having to provide switching for the differing Fb ratios, but the HF compensation problem would remain unoptimised. Wonder if this is the reason some manufacturers encourage the end user to experiment to see which external ‘red’ speaker terminal gives the best subjective experience. I know David Shaw encouraged me to try the different taps for best sound, regardless of the impedance of my speakers when I bought my Stereo 40 push-pull amp way back in 2004.

Loads of edits. I couldn’t get the bloody grammar right for some odd reason. Still not sure I’m making sense.
Last edited by Cressy Snr on Sun Jul 03, 2022 7:42 pm, edited 6 times in total.
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#53 Re: Fun with Feedback

Post by Cressy Snr »

Greg wrote: Sun Jul 03, 2022 7:04 pm I like it. Colloms’ Cobblers😀
:mrgreen:
Colloms’ assertion was made in in Stereophile, Jan 1998 when SETs without feedback were the new messiahs and all else was gaslight.

This from Norman Koren:
“In his January 1998 Stereophile article "A Future Without Feedback ," Martin Colloms asserts that "measurements do not fully describe sound quality," and goes on to suggest that "corrective feedback is fundamentally unmusical." He poses the question, "has anyone explored the implications of negative feedback for reproduced sound quality in the absolute sense?"

http://www.normankoren.com/Audio/FeedbackFidelity.html

Wonder what his opinion is these days.
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#54 Re: Fun with Feedback

Post by Cressy Snr »

Found the Stereophile article:
https://www.stereophile.com/reference/70/index.html
The letters following the article are also interesting.
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#55 Re: Fun with Feedback

Post by JamesD »

Its interesting that DEL Shorter & Norman Crowhurst back in the late 50s and early 60s described how the harmonic progression of distortion affected our perception of sound quality and how feedback and especially global negative feedback in commercial amplifiers tended to make this worse than it needed to be - along with the effect of typical PP commercial amplifier designs of the time... Their findings are still debated and/or ignored today but were well established at the time... It would be really interesting to see what the harmonic distortion profile of Steve's amp is like... I suspect its rather in line with Shorter and Crowhurst's findings...

Shame Colloms didn't build on what had gone before - maybe he wasn't aware of it at the time???

But nice work Steve, and an example to us all of what persistence and research can achieve but I still miss your 6B4G amps magic!

BTW - Wasn't there an article in Linear Audio showing that more GFB was better as long as at least 60dB of feedback was applied with no further gains being made after 150 dBs of feedback? Not sure I remembered that correctly... Good luck with maintaining open loop stability with that amount of gain in the system!
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#56 Re: Fun with Feedback

Post by Nick »

Yep, but I feel I should point out that:
corrective feedback is fundamentally unmusical.
Is one of those lines that is many things to many people, as I kept asking when Richard D used it, just what does "unmusical" mean? I am certainly sure there is more than one definition,
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#57 Re: Fun with Feedback

Post by JamesD »

Indeed - I sure one can make the argument both ways round as well so its meaningless without context setting out what unmusical means in this statement... or its a debating trick to always enable one to always be right when making the statement by shifting its meaning...

I'm tempted to ask what non corrective feedback sounds like? And what is fundamentally musical? Is it tonal or atonal music - is it occidental or oriental or accidental?

I guess one could say "its complicated" in that feedback extends the harmonic content of music but even that may not be audible as the Linear Audio article I referred to earlier applied so much feedback that the generated harmonics were not only below the noise floor but below the ear/brain systems ability to detect them within the noise floor...

Although I would maintain that competently designed DHT amplifiers always sound nice and sometimes sound magical... but they are not the only way to achieve such end goals and have well understood limitations that other solutions avoid... which brings us back to Steve's exploration of SE feedback designs and the competency of tweaking ones way to nirvana once one understands how to apply feedback and what result one is trying to achieve with it... The lesson, surely, is that the talented amateur can exceed the results of the professional by optimising his system better than the professional can optimise for all systems...
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#58 Re: Fun with Feedback

Post by Nick »

The lesson, surely, is that the talented amateur can exceed the results of the professional by optimising his system better than the professional can optimise for all systems...
I think there is a big and important truth there James. I often disagree with the idea that DIY can do better than commercial because of some set of choices they can make, and conversely commercial is better than DIY because of some magic dust that is only available with a trade account. But the fact that DIY can make all the optimum choices for their ear/music/room that the commercial designer can't is undeniably true.
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#59 Re: Fun with Feedback

Post by JamesD »

Its interesting to see the sort of systems that the talented diyer ends up with too - many and various paths to Nirvana!

ciao

James
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Paul Barker
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#60 Re: Fun with Feedback

Post by Paul Barker »

Steve, the output transformer winding process is a significant part of the sound. As it’s incorporated in global feedback there is more than one influence of a badly implemented opt on sound. I would say you want all sections of all windings should be used, and feedback should likewise apply to all sections.

The other matter which springs to mind is the Quad II opt feedback winding. I employed partial feedback in the phase splitter transformers on the 6b4g pp I brought to an Owston the feedback fed the cathode of the 6em7 power section driver tube.

When you’re applying feedback involving transformers intuitively I think you want to work with all the length of the windings. If you ever test the sound from a series wound tapped 4/8/16 ohm secondary transformer, you would find that the complete secondary has an entirely different character to the partial lengths. Sometimes this is the dominant influence and not the turns ratio. Add feedback and you’ve now got 3 balls in the air. Not many cheap transformers use every contact surface for every impedance secondary.

Ideally simplest and best is specify the one output matching you want, for cheapness and quality. The cheap eBay guitar amp opt’s might not be built to this standard. You’re trying to fight an enemy to what you perceive to be good sound. I’m saying the enemy you perceive may not be the biggest issue and you’re feedback might make it worse. If you were only applying feedback around resistors that would be different.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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