Fun with Feedback

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Cressy Snr
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#16 Re: Fun with Feedback

Post by Cressy Snr »

Before and after square wave scope traces - 100Hz
100Hzbeforepeg.jpeg
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100Hzafter.jpeg
100Hzafter.jpeg (64.67 KiB) Viewed 1891 times
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Cressy Snr
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#17 Re: Fun with Feedback

Post by Cressy Snr »

Before and after HF compensation - 10KHz:
10KHzbefore.jpeg
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10KHzafter.jpeg
10KHzafter.jpeg (99.07 KiB) Viewed 1891 times
I'd be interested in the panel's opinion of the '10KHz after' trace. I know cheap OPTs tend to produce crap square waves and this one is no exception. I looks adequate to my untrained eye (maybe a tad slow at HF ?) but I'd like to know what you guys think.
I'll see what it all sounds like later on.
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Nick
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#18 Re: Fun with Feedback

Post by Nick »

Maybe worth trying a zobel. It looks around 60k or so.
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Cressy Snr
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#19 Re: Fun with Feedback

Post by Cressy Snr »

I’ll look into that option.
Thanks Nick.
Last edited by Cressy Snr on Sun Jun 26, 2022 9:06 pm, edited 1 time in total.
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Cressy Snr
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#20 Re: Fun with Feedback

Post by Cressy Snr »

Meanwhile, the sound of the amp has improved again. Detail is improved’ as is the transparency. All very nice I’m sure but more interesting to me is the fact that the measurements, such as they are, correlate well with what goes on with the sound.
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Nick
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#21 Re: Fun with Feedback

Post by Nick »

Is the scope AC or DC coupled in your square wave examples, as the LF response looks a bit lacking.
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Cressy Snr
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#22 Re: Fun with Feedback

Post by Cressy Snr »

Nick wrote: Sun Jun 26, 2022 9:55 pm Is the scope AC or DC coupled in your square wave examples, as the LF response looks a bit lacking.
Is that due to the downward tilt on the 100Hz waveform?
The scope is in AC coupled mode.

But then again the Hammond 125SE transformers are not terribly clever at LF, being down 3dB at 100Hz and only 4H worth of inductance. Hammond don't recommend them for hi-fi applications only as replacements for old domestic equipment trannies. They were the very first OPTs I ever bought when I built the Loftin-White 2A3 amp by Philip at Bluebell Audio, my first ever scratch build in 2005.
They are not bad sounding in practice, but obviously there are far better options available. On the plus side, they do have a lovely midrange balance.
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Nick
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#23 Re: Fun with Feedback

Post by Nick »

Yep. That was it.
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JamesD
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#24 Re: Fun with Feedback

Post by JamesD »

The 10KHz square wave edges are showing signs of slew rate limiting when asked to exceed 1Vper10usec that is 1 V slew at 100KHz - probably OK for audio at our age :-)

Agree with the 100Hz or so roll off visible on the 100Hz waveform.

It could do with some tweaking to optimise the hf compensation but its not too bad already and the phase doesn't look too far rotated - I would guess about 60 degrees at 50 -60KHz ish...

James
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#25 Re: Fun with Feedback

Post by Cressy Snr »

Cheers James, It's not bad for a first attempt then. :)
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#26 Re: Fun with Feedback

Post by Cressy Snr »

Forgot to mention yesterday, but I added an HF compensation network to the anode of the input stage (6SL7) I didn’t have the time to do any waveforms, so just put the amp back in the system and did a bit of subjective analysis.

The job for this afternoon, is to do another set of square waveforms and compare them to the previous; particularly the 10KHz wave. I’ll also put the cursors on and do a frequency sweep, a crude one obviously as I don’t have a proper sweep generator. It’ll just be a case of setting the cursors top and bottom, setting the scale so that the waveform amplitude top to bottom, spans 90% of the Y axis then, sweeping up the signal generator tone and watching the sine wave amplitude for any signs of variation. That will give at least a general idea of the flat zone bandwidth. It will also show up any out of band peaks, troughs or other indications of errant goings on.

The subjective analysis seems to indicate that all is well, but it’s the measurements that will indicate if that is truly the case.
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JamesD
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#27 Re: Fun with Feedback

Post by JamesD »

When doing the frequency sweep it is sometimes instructive to compare the input on one channel with the output on the other - sometimes an amplifier loads the input device such that the input sweep isn't flat. If that is the case then subtracting one trace from the other will show the amplifier response - assuming your scope does the channel subtraction...

ciao

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Nick
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#28 Re: Fun with Feedback

Post by Nick »

assuming your scope does the channel subtraction...
But watch out for phase shifts creating an amplitude difference where there isn't one.
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#29 Re: Fun with Feedback

Post by JamesD »

good point - I've certainly seen that before - usually when chasing stability issues...
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Nick
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#30 Re: Fun with Feedback

Post by Nick »

usually when chasing stability issues
Which almost certainly will involve phase shifts in some form.
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