Fun with Feedback

What people are working on at the moment
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Cressy Snr
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#1 Fun with Feedback

Post by Cressy Snr »

I completed this amplifier earlier today:
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6SL7 voltage amp, into 6SN7 driver stage, feeding triode strapped EL34.
Global feedback to 6SL7 cathode.

Testing with scope and function generator:
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Niiice 👍
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A bit of fun I thought, but I can’t believe how good it sounds. It isn’t powerful but it makes up for that with sheer presence.
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izzy wizzy
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#2 Re: Fun with Feedback

Post by izzy wizzy »

Looks great. I must have missed the memo about you coming out of DIY retirement and returning to valves ;)

Couldn't stay away from the fun eh?
Cressy Snr
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#3 Re: Fun with Feedback

Post by Cressy Snr »

:D It's not a return to valves exclusively, but it does give me an opportunity to use both valve and solid state amps, choosing whatever I fancy at the time. The amp itself is not really a 'new' device as such. In fact it is 14 years old. It was an SE feedback less amp I built for a mate for his 50th birthday. It has now come back home after it packed in on one channel after 14 years of daily 16 hours use.
Like everyone else in this God-forsaken country, he became conscious of the electricity it was using and bought himself a class D something or other, to connect his TV to his Bose Acoustimass system and to stream AppleTV and Amazon through, so I'm left here with this anachronism, which given today's energy crisis, is becoming increasingly difficult to justify using on a regular basis. Sad really.

Anyway, here we are, and you're right Stephen: I was getting seriously bored - not with the system per se, which sounds the best it has ever sounded in over forty years of messing about. The main reason I was bored was because DIY amps get into your blood and It is difficult to wean yourself off them. They keep the brain working. In terms of the actual system, it's all a bit ironic really, because a pair of 30 year old Mission bookshelf speakers has restored the listening pleasure I had with the big Metronomes (Greg, I accept you were spot on, telling me I had veered off into an unproductive speaker backwater) You have to make the mistakes to make the progress I suppose.

Back to the amp which is the subject of this thread, I had spent weeks reading about negative feedback as applied to both solid and vacuum state situations. This was far more challenging than anything I've ever done in the past. But finding out a bit about it enabled me to improve the sound of my NVA based solid state amp, and quite substantially too, though I say so myself.

One of the main 'eureka' moments was the gain question. Like many dilly-dalliers in DIY electronics I had assumed that negative feedback reduces the gain, of the system except it doesn't, well not in the sense we think it would :wink: Another one was the idea that the feedback signal from the output stage goes back to the input then keeps circulating round and round the amp, slugging everything in its path...er no. Something else that very quickly became apparent, was that if one is going to attempt to design a valve amp that uses global negative feedback, then one is going to have to get to grips with the engineering maths required, otherwise success is not going to happen. Out of band oscillation, shrieky destruction of tweeters and various other bits of electronic mayhem are worryingly easy to produce, and decent sound without the dreaded 7th and higher is not as easy to achieve as the intrepid tyro would believe. As the more experienced among us certainly know, global feedback and valves is an area where tweak, listen and hope is absolutely not the right thing to be doing.
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Nick
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#4 Re: Fun with Feedback

Post by Nick »

As you are getting into this sort of thing, want to have one of these? I don't use it anymore, and it should still work fine. Though will need cleaning up a little.



Good for finding harmonic levels down to -90dB
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Cressy Snr
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#5 Re: Fun with Feedback

Post by Cressy Snr »

Nick wrote: Sat Jun 25, 2022 10:16 am As you are getting into this sort of thing, want to have one of these? I don't use it anymore, and it should still work fine. Though will need cleaning up a little.



Good for finding harmonic levels down to -90dB
Thanks Nick :D
I'll watch the video later. Then PM you this evening as I have to go out now.
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Greg
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#6 Re: Fun with Feedback

Post by Greg »

Cressy Snr wrote: Sat Jun 25, 2022 10:01 am In terms of the actual system, it's all a bit ironic really, because a pair of 30 year old Mission bookshelf speakers has restored the listening pleasure I had with the big Metronomes (Greg, I accept you were spot on, telling me I had veered off into an unproductive speaker backwater) You have to make the mistakes to make the progress I suppose.
That’s gracious of you, thank you. I thought your big Metronomes were fabulous. I understand why Thermionic Idler is so fond of them.

Go on, build another pair. You can’t go wrong doing that with what you now know.
Cressy Snr
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#7 Re: Fun with Feedback

Post by Cressy Snr »

I’ve got a nice speaker project and a pair of finished speakers, coming my way later this year from a couple of forum members, so I’ll be spoiled for choice on that front, which is a good position to be in. A pair of new Mets is not something I was considering, but it is a possibility as I’m hoping to expand the hi fi into another room, now I’ve actually got somewhere else to put a system.
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IslandPink
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#8 Re: Fun with Feedback

Post by IslandPink »

If you need some FF225WK's, let me know.
"Once you find out ... the Circumstances ; then you can go out"
Cressy Snr
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#9 Re: Fun with Feedback

Post by Cressy Snr »

IslandPink wrote: Sat Jun 25, 2022 11:59 pm If you need some FF225WK's, let me know.
Will do.
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Cressy Snr
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#10 Re: Fun with Feedback

Post by Cressy Snr »

Did a few tests whilst bringing the amp into service and all the HF phase compensation (***pF) cap values I have available to put across RFb (not many) end up overcompensating the 10KHz square wave readout, which would lead to :sleepy4: so I’m going to have to get a few in. They’re not expensive at the values envisaged. No ceramics goes without saying.
For the moment then it’s not quite finished, but having carefully executed global feedback around a single ended amplifier is not the sonic disaster some punters make it out to be. Nothing wrong with it at all as far as I can hear. In fact it gives a nice improvement in terms of sonic properties, music retrieval and speaker driving ability. Win win on all fronts, apart from the fact that my brain has undergone a serious mathematical workout, the magnitude of which I was singularly unprepared for: phase lag and lead, poles and zeros, parallel RC circuit analysis, Nyquist diagrams. And as for them fecking imaginary ‘j’ numbers - gawd help me. :shock: :D
Last edited by Cressy Snr on Sun Jun 26, 2022 10:46 am, edited 1 time in total.
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Nick
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#11 Re: Fun with Feedback

Post by Nick »

Also consider playing with HF comp on the first stage anode that was often also used. Also, if ceramic, C0G is not as evil as other dialectics. And check you are not adding LF peaks into the system without noticing.
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Cressy Snr
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#12 Re: Fun with Feedback

Post by Cressy Snr »

Nick wrote: Sun Jun 26, 2022 10:46 am Also consider playing with HF comp on the first stage anode that was often also used. Also, if ceramic, C0G is not as evil as other dialectics. And check you are not adding LF peaks into the system without noticing.
Aye, it’s all good fun. I’m enjoying it, which is quite a turn up for the books. There is a bit of ringing on a 100Hz squarewave which needs looking into. I’ll be looking at the value of the output stage cathode bypass cap as one of the possible ways out of that. Put one thing right and something else goes wrong is an aspect of engineering, I’m coming to terms with at the moment. But that’s the fun of it.
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Nick
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#13 Re: Fun with Feedback

Post by Nick »

I would look at the driver bipass. The output stage bipass will start to interact with the output transformer if too small.
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Cressy Snr
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#14 Re: Fun with Feedback

Post by Cressy Snr »

Nick wrote: Sun Jun 26, 2022 11:21 am I would look at the driver bipass. The output stage bipass will start to interact with the output transformer if too small.
Cheers Nick.
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Cressy Snr
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#15 Re: Fun with Feedback

Post by Cressy Snr »

Half a morning and an afternoon of scoping and hoping (not really, it just rhymes nicely) later and the circuit is now as below. I managed to find a couple of 100pF caps for the Fb resistor compensation network. I figured these would be at least in the right ballpark and the results weren't too bad (I think, but square wave interpretation is not something, I'm confident with quite yet.) The OPT is only a cheap Hammond 125FSE and the -3dB point at the bottom end is 100Hz, so I didn't want to go too big with the output stage cathode bypasses and start saturating the poor little thing with record warps, or Gregory Porter Claptone remixes.

Circuit:
FWFb.png
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Scope traces follow. I noticed I failed to keep the 100Hz square wave levels the same, but I', not getting the test gear out again. The lower input level was the one that showed ringing anyway, so it's not that bad a cock up.
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