Fun with Feedback
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- Amstrad Tower of Power
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#31 Re: Fun with Feedback
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:
10KHz square wave widened out:
1KHz square wave:
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:
10KHz square wave widened out:
1KHz square wave:
Last edited by Cressy Snr on Tue Jun 28, 2022 4:23 pm, edited 1 time in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#32 Re: Fun with Feedback
100Hz square wave:
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#33 Re: Fun with Feedback
Frequency response plot (OK it's hand drawn, but it gives the general idea):
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.
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.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#34 Re: Fun with Feedback
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.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#35 Re: Fun with Feedback
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.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#36 Re: Fun with Feedback
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
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
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
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#37 Re: Fun with Feedback
Thanks James, I'll read and digest later on.
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#39 Re: Fun with Feedback
As for the sound, I am not going to go into proud papa syndrome, but let’s just say it’s good 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.
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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#40 Re: Fun with Feedback
Hmmm, that article is very interesting.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
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
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#41 Re: Fun with Feedback
As it's a single-ended amplifier with global feedback, I thought I may as well give it an appropriate moniker:
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#42 Re: Fun with Feedback
Splendid!
#43 Re: Fun with Feedback
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#44 Re: Fun with Feedback
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.Greg wrote: ↑Sat Jun 25, 2022 9:18 pmThat’s gracious of you, thank you. I thought your big Metronomes were fabulous. I understand why Thermionic Idler is so fond of them.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.
Go on, build another pair. You can’t go wrong doing that with what you now know.
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
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
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#45 Re: Fun with Feedback
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.Thermionic Idler wrote: ↑Fri Jul 01, 2022 12:31 am …(they really love the Lynn Olson configuration of push-pull DHT's it seems)…
Sgt. Baker started talkin’ with a Bullhorn in his hand.