Fun with Feedback

What people are working on at the moment
Cressy Snr
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#31 Re: Fun with Feedback

Post by Cressy Snr »

OK,
Here are the new plots after having done the HF compensation on the 6SL7 anodes. Unfortunately, although my scope is dual trace and can subtract one waveform from another, I've lost the second probe, so I'm ferked on doing anything other than the crudest of measurements:

10KHz square wave:
10KHzSQwithadditional compsn.jpeg
10KHzSQwithadditional compsn.jpeg (52.64 KiB) Viewed 2017 times

10KHz square wave widened out:
10KHzSQwider.jpeg
10KHzSQwider.jpeg (86.81 KiB) Viewed 2017 times

1KHz square wave:
1KHzSQ.jpeg
1KHzSQ.jpeg (84.86 KiB) Viewed 2017 times
Last edited by Cressy Snr on Tue Jun 28, 2022 4:23 pm, edited 1 time in total.
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Cressy Snr
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#32 Re: Fun with Feedback

Post by Cressy Snr »

100Hz square wave:
100HzSQ.jpeg
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Cressy Snr
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#33 Re: Fun with Feedback

Post by Cressy Snr »

Frequency response plot (OK it's hand drawn, but it gives the general idea):
FWFbFreqresp.jpeg
FWFbFreqresp.jpeg (79.68 KiB) Viewed 2017 times
The top plot has its vertical scale exaggerated to show a trough between 50 and 60KHz, which could either be a remaining phase issue with my compensation or just a characteristic of the output transformer. Either way I don't think I could get it a whole lot better. Actually I'm really pleased that the hard work I put in with the maths appears to have borne fruit.
Bottom plot is the more usual 10dB scale which shows the true nature of the trough, so all things considered, it's really not that much of a problem. There is certainly nothing in the performance in terms of sound quality, that gives me cause for any concern.

Response is flat within 1dB from around 100Hz to 30KHz, with no out of band oscillation or any nasty peaks at the bottom end.
Last edited by Cressy Snr on Tue Jun 28, 2022 4:12 pm, edited 1 time in total.
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Nick
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#34 Re: Fun with Feedback

Post by Nick »

If you have a two channel scope, you may be interested in looking at the input and output signals as it goes past that dip at 50k. You might get extra insight looking how the phase moves either side of that dip.
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Cressy Snr
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#35 Re: Fun with Feedback

Post by Cressy Snr »

Nick wrote: Tue Jun 28, 2022 4:12 pm If you have a two channel scope, you may be interested in looking at the input and output signals as it goes past that dip at 50k. You might get extra insight looking how the phase moves either side of that dip.
I need to buy another probe to do that Nick, but I'll certainly be getting one. I'm enjoying learning about how to look a bit deeper into the engineering process. Not to develop further insight/skills, would be a tad silly now, I think. :)
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JamesD
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#36 Re: Fun with Feedback

Post by JamesD »

The frequency plot shape is pretty typical of an OPT. Here is an indepth measurements of a Willianson OPT with explainations of what the plots are showing - its long and semi-technical not a lot of equations for fully technical :D

https://dalmura.com.au/static/Williamso ... ements.pdf

The frequency plots show the same shape you have an explain what causes it...

But the results are looking really good and even though you have a roll off from 100Hz the -3dB point seems to be about 30Hz ish??? That's well below your little Missions response so your fine there too..

Neat!

James
Cressy Snr
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#37 Re: Fun with Feedback

Post by Cressy Snr »

Thanks James, I'll read and digest later on.
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#38 Re: Fun with Feedback

Post by Cressy Snr »

Schematic:
2B002DFA-7422-43B2-A0A2-C14294C13B8E.jpeg
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Cressy Snr
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#39 Re: Fun with Feedback

Post by Cressy Snr »

As for the sound, I am not going to go into proud papa syndrome, but let’s just say it’s good :wink: It certainly wasn’t an easy task for a non mathematical person like me to take on, but what can I say, except the results are well worth the head scratching and brain pain.

Of course the feedback and compensation has to be worked out to suit the individual output transformer. I’m afraid it’s definitely a case of “you can’t just….” where feedback of this type is involved. Test gear is also required. As far as I am concerned there are many upsides and no downsides that I can hear. And thanks Nick and James for the advice and help with interpreting the scope traces.
Last edited by Cressy Snr on Wed Jun 29, 2022 11:11 am, edited 2 times in total.
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Cressy Snr
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#40 Re: Fun with Feedback

Post by Cressy Snr »

JamesD wrote: Tue Jun 28, 2022 4:59 pm The frequency plot shape is pretty typical of an OPT. Here is an indepth measurements of a Willianson OPT with explainations of what the plots are showing - its long and semi-technical not a lot of equations for fully technical :D

https://dalmura.com.au/static/Williamso ... ements.pdf

The frequency plots show the same shape you have an explain what causes it...

But the results are looking really good and even though you have a roll off from 100Hz the -3dB point seems to be about 30Hz ish??? That's well below your little Missions response so your fine there too..

Neat!

James
Hmmm, that article is very interesting.
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Cressy Snr
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#41 Re: Fun with Feedback

Post by Cressy Snr »

As it's a single-ended amplifier with global feedback, I thought I may as well give it an appropriate moniker:
Heretik.jpeg
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JamesD
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#42 Re: Fun with Feedback

Post by JamesD »

Splendid!
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Nick
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#43 Re: Fun with Feedback

Post by Nick »

:-)
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Thermionic Idler
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#44 Re: Fun with Feedback

Post by Thermionic Idler »

Greg wrote: Sat Jun 25, 2022 9:18 pm
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.
Just saw this. Can't add much other than what I've already said before, but, yep, they have easily kept pace with my ongoing projects - two power amplifier upgrades (they really love the Lynn Olson configuration of push-pull DHT's it seems) and a major phono stage upgrade. I have every confidence they'll be good enough for Dom Harper's cartridge when (if) I ever get the money together to buy one. Two things in my system have been assumed constants for years now - one is the turntable, the other are the speakers.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Cressy Snr
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#45 Re: Fun with Feedback

Post by Cressy Snr »

Thermionic Idler wrote: Fri Jul 01, 2022 12:31 am …(they really love the Lynn Olson configuration of push-pull DHT's it seems)…
Yep, the big Mets love a push-pull amp driving them. I remember Nick bringing a transformer coupled EL34 push-pull breadboard to Owston, some years ago and playing Roxy Music. I’d never heard them sound as good.
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