Fun with Feedback

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Cressy Snr
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#61 Re: Fun with Feedback

Post by Cressy Snr »

Paul Barker wrote: Mon Jul 04, 2022 11:20 am Steve
...I’m saying the enemy you perceive may not be the biggest issue and you’re feedback might make it worse......
Hi Paul, thanks for the input.
The thing is, the feedback hasn't made anything worse, far from it. The proper application of the feedback had a massively positive impact on the quality of the sound. I know that I might have had a tendency in the past to either build and hope, or copy other people (Alex Kitic comes to mind on that one) but on this occasion I have been through the whole initial process on my own, with old books, a fine tooth comb and a terrible trial by mathematics to put it mildly. Frankly with the maths, I was on the point of giving up, but I gave myself a bollocking and determined that this time I wasn't going to bail out and was going to put in the effort, and sweat required to learn something useful.

Once I had the basic process down and had built something, I tested it with a few records and was frankly gobsmacked at how good the sound was. This was before any scoping of waveforms, poles, zeroes, or HF phase shift compensation calculations had even been thought about. That was another load of maths and I needed a tiny steps strategy to avoid getting overwhelmed.

I was certainly gratified that the maths, theory, whatever you want to call it I had learned thus far, had worked the way Crowhurst et al had said it would and because the sound was so good, I was satisfied that I was on the right lines. You've no idea how that felt after so many years of frustration. Let's just say there was a great deal of fist pumping, hooting and hollering when the first tune struck up from the speakers. It's a good job Melanie was out and the neighbours were on holiday. If she'd seen and heard me, I'm sure she would have been on the phone to the men in white coats toot sweet. :shock:

As you know, the HF phase shifts and where they occur are unique to every make of output transformer and although with a bit of estimation from the published specs and some rough working, it is possible to get in the right ballpark, the fine tuning becomes an iterative process. The mess of harmonics on top of the first 10KHz square wave trace without any phase compensation looked pretty damned ugly, and confirmed the need to do something about them:
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I managed to improve the traces with a worked out then tweaked up cap across the feedback resistor:
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After that, it was a matter of dealing with a bit of slew rate limiting as best I could, then fitting another worked out and tweaked up compensation network to the input stage anode to finish up with this:
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As I edged towards the best I could get out of the OPT, the sound just got better and better, to the point where now I have something that sounds great, but more importantly, I know why it sounds so good because I've used the triple combination of measuring, interpreting (with help) the test results and final subjective auditioning to get what I want.

It's the first time I've followed the process and now appreciate the reasoning behind it. The maths, the measurements and the advice helped me to define a clear set of goals, then make them happen.
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Cressy Snr
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#62 Re: Fun with Feedback

Post by Cressy Snr »

I know also that RD would have had a fit over the number of caps sprinkled around the circuit and would have given the two wrongs don't make a right' argument. Others will no doubt say 'if you had used linear tubes (expensive DHTs) in the first place then there wouldn't have been any need for nasty horrible feedback.'This is a valid point, but I don't have the resources any more for expensive DHTs and even more expensive interstage/parafeed/output transformers. This is a project of necessity as the mother of invention when it comes down to it.

This project's aim is to try to obtain a bit of single-ended magic on the cheap and if by necessity, I have to use inexpensive audio pentodes strapped as triodes, cheap ubiquitous voltage amps and drivers, and cheapo output transformers, then using negative feedback, properly applied is a damned good way of getting the kind of sound quality that punches way above its weight.

TBH, this is probably the only forum where I would have gotten away with doing what I've just done. I wouldn't have dared to put this project on DIYAudio for instance. I'd probably have been chewed up, spat out and run out of town whilst picking shotgun pellets out of my arse.
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Nick
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#63 Re: Fun with Feedback

Post by Nick »

This is a project of necessity as the mother of invention when it comes down to it.
And why not. You also seem to have learnt some things in the process, so its a win win in my book.
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Paul Barker
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#64 Re: Fun with Feedback

Post by Paul Barker »

a good read at least. He who dares to show his 10khz square wave at the output of his transformer is brave. Its quite normal in the set world to swiftly change the conversation when people ask about square waves.
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JamesD
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#65 Re: Fun with Feedback

Post by JamesD »

In terms of linear valves.... the 6SN7 is as linear as any valve and the 6SL7 is not far behind and more linear than any other IHT with its gain (mu) so brilliant choices... particularly in a GFB circuit. :D
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Nick
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#66 Re: Fun with Feedback

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JamesD wrote: Mon Jul 04, 2022 7:27 pm In terms of linear valves.... the 6SN7 is as linear as any valve and the 6SL7 is not far behind and more linear than any other IHT with its gain (mu) so brilliant choices... particularly in a GFB circuit. :D
Yep, may be the case (though I don't know for certain), but looking at the RCA data sheets from 1954 the driver for the 6sn7 being so linear was for its use in the deflection circuits for TV's rather than audio.

Not that it matters.
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IslandPink
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#67 Re: Fun with Feedback

Post by IslandPink »

Cressy Snr wrote: Mon Jul 04, 2022 3:08 pm .... as the mother of invention
Mustn't post picture of ...
:confused3:

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...failed ...
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JamesD
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#68 Re: Fun with Feedback

Post by JamesD »

6SN7 released by RCA in 1939 as an audio tube with datasheet included in the 1940 RCA receiving valve book for the first time. Developed as a double triode version of the 6J5 single triode that was released in 1937 as an audio triode. Later use as suitable for multiple roles including TV sets due to its linearity...

If you look at Pete Millets high Gm pentode data though you can see that they can make lower distortion audio drivers than the 6SN7... http://www.pmillett.com/pentodes.htm so, as always, THD isn't the whole story...
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Nick
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#69 Re: Fun with Feedback

Post by Nick »

JamesD wrote: Tue Jul 05, 2022 6:00 am 6SN7 released by RCA in 1939 as an audio tube with datasheet included in the 1940 RCA receiving valve book for the first time. Developed as a double triode version of the 6J5 single triode that was released in 1937 as an audio triode. Later use as suitable for multiple roles including TV sets due to its linearity...

If you look at Pete Millets high Gm pentode data though you can see that they can make lower distortion audio drivers than the 6SN7... http://www.pmillett.com/pentodes.htm so, as always, THD isn't the whole story...
Yes, I assume frequency multiplexed phone trunks was one of the driving forces for the later pentode designs?
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Cressy Snr
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#70 Re: Fun with Feedback

Post by Cressy Snr »

I’ve got a few new parts coming later today from HFC, so that the audio circuit can be wired up with decent quality components. Apart from the HF phase compensation caps which were new, the rest of the circuit is like Rafferty’s motor car.

You really don’t want to know what it looks like under the hood - not at the moment anyway. :)
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Nick
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#71 Re: Fun with Feedback

Post by Nick »

I would not worry too much about it, you may find that as you spend more time with the measure, analyse, listen process you become less worried about boutique parts.
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#72 Re: Fun with Feedback

Post by JamesD »

Yeah telecoms drove a lot of the better triode and pentode designs from USA & European manufacturers...
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izzy wizzy
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#73 Re: Fun with Feedback

Post by izzy wizzy »

Nick wrote: Tue Jul 05, 2022 10:42 am I would not worry too much about it, you may find that as you spend more time with the measure, analyse, listen process you become less worried about boutique parts.
+1 on that.
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IslandPink
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#74 Re: Fun with Feedback

Post by IslandPink »

-1 on that ( or is it zero ? ) , the more I go on, the more I agree with Jono's adage that 'Everything matters enormously'
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Nick
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#75 Re: Fun with Feedback

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IslandPink wrote: Tue Jul 05, 2022 9:44 pm -1 on that ( or is it zero ? ) , the more I go on, the more I agree with Jono's adage that 'Everything matters enormously'
Yes, agreed, up till the point where it doesn't matter any more. i.e. paying £1 on a cap is certainly worth doing, paying £10 may be worth doing. But paying £100 and up is almost certainly not.

I should say for clarity, when I say "where it doesn't matter any more", the important word is more, I am not suggesting it stops mattering, its just that the amount it matters doesn't increase over a certain point. As it happens, I found a entirely unrelated graph with the correct curve shape yesterday.
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