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Old 1st May 2013, 06:56 PM   #11
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Yes, let's not throw mud...

but I have yet to encounter this problem, happen across it when repairing and testing assorted and various amplifiers across many years.

otoh I have seen instability due to parasitic oscillations... but not that often.

that's why I am curious as to exactly what sort of instability you are seeing? what would I expect to see on the scope?

perhaps you'd care to grab a random amp and shoot a scope pic of this? seriously, I think it would be useful and educational.

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Old 1st May 2013, 09:06 PM   #12
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Hey Bear,

Has your nick anything to do with, or is it inspired by Owsley Stanley?
See: Owsley Stanley - Wikipedia, the free encyclopedia
Just wondering.

Cheers,
E.
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Old 2nd May 2013, 12:02 AM   #13
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Quote:
Originally Posted by bear View Post
.. but I have yet to encounter this problem, happen across it when repairing and testing assorted and various amplifiers across many years.

otoh I have seen instability due to parasitic oscillations... but not that often.

that's why I am curious as to exactly what sort of instability you are seeing? what would I expect to see on the scope?
bear, I've been a beach bum for well over a decade so have little chance to measure Golden Pinnae stuff today.

But the effect is usually not subtle. The guitar speaker was one of my standard tests of stability.

This pic is of something completely different but if you remove the crossover spikes from the red curve, you get an idea of what to expect. Usually only on part of a cycle. It may or may not go away as the amp heats up, cools down, signal level & history etc.

Many (most?) Golden Pinnae amps, especially those without an output inductor, exhibit this often very obvious 'feature'. No wonder they sound different.

A more subtle example may not be seen on a cheap scope. You may find THD becomes very bad on very specific loads when it is excellent on 8R resistors. Curing the instability restores good THD on these loads.

I'm sure Bob Cordell has mentioned this, either in his book or on some thread here.
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Old 2nd May 2013, 02:15 AM   #14
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Edmond,

Ha ha. No, but I certainly knew who he was, although I never met him. I did correspond with him about a decade ago... I was a pretty big fan of early Grateful Dead too. Someone else who is active here actually knew him back in the day...

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Old 2nd May 2013, 02:23 AM   #15
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kgrlee, very interesting graph. generated by a simulation or by actual measurement?? Can't say I have ever seen this first hand. Makes me want to try your "test protocol". So you are suggesting that this happens into inductive loads rather than capacitive??

And are you wrestling here with an artifact found with class AB switching only?

But of course all this is OT, no not output transformer, to the original question you raised.

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Old 2nd May 2013, 02:35 AM   #16
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I think with this type of artifact, you need to identify whether it is loop gain related oscillation, or whether it is parasitic. If loop gain related, the solution is to sort the compensation out. And I absolutely believe in output coils (with 2r2 or 3r3 damping resistors) and Zobel networks ( on the amplifier side of the coil!).

Parasitics are usually a bit more difficult to put your finger on. I have seen the problem in EF3, but the fixes are easy to implement and they work very well. Cascode instability is also another one, as is parameter modulation ( Cob specifically).
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Old 2nd May 2013, 02:44 AM   #17
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Old 2nd May 2013, 04:51 AM   #18
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Excellent to see such work.
It would be nice to see the Bode plots done so that 180 phase is the same line as 0 dB.
Quicker to interpret, and a sensible standard to simplify comparisons.
Quote:
Originally Posted by kgrlee View Post
Neither of these compensation circuits have been optimised so I invite the various champions to offer improvements...
Has anyone pointed out the obvious, that OIC (AKA Cherry) is just a subset of TMC?
Optimised TMC can do at least as well as OIC. At worst the optimisation may end up with a zero ohm resistor to the output and a zero farad capacitor to the OPS input, to produce OIC.

Best wishes
David

Last edited by Dave Zan; 2nd May 2013 at 05:09 AM. Reason: Rephrase
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Old 2nd May 2013, 05:14 AM   #19
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Quote:
Originally Posted by Bonsai View Post
And I absolutely believe in output coils (with 2r2 or 3r3 damping resistors) and Zobel networks ( on the amplifier side of the coil!).
Hi Andrew
Why do you prefer Zobel on the amp side?

Best wishes
David
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Old 2nd May 2013, 05:38 AM   #20
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Quote:
Originally Posted by bear View Post
... generated by a simulation or by actual measurement?? Can't say I have ever seen this first hand. Makes me want to try your "test protocol". So you are suggesting that this happens into inductive loads rather than capacitive??
As I said earlier, I've been a beach bum for a long time so can't really do sensible measurements this millenium.

The pic is a simulation of something else entirely .. but you asked for something to look for on a scope.

I've seen it on both inductive & capacitive loads.

A capacitance box (like in school lab experiments) is a quick and easy way to check capacitive loads formally.

The guitar speaker is more cumbersome and I don't have a proper formal procedure. But unlike with capacitors, IME the amp is unlikely to go up in smoke so you can take your time.

Quote:
.. are you wrestling here with an artifact found with class AB switching only?
One of the culprits was a large famous Class A Golden Pinnae Amp (actually 2 of their models) in Stereophile/TAS good books.

Your question is entirely relevant as I was hoping some gurus would pontificate on the 'real life' stabilty of the 3 versions I presented.
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Quote:
Originally Posted by bonsai
Parasitics are usually a bit more difficult to put your finger on. I have seen the problem in EF3, but the fixes are easy to implement and they work very well
Bonsai, I obviously have my own ideas but what are your favourite easy fixes to try with 'parasitics'?
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Quote:
Originally Posted by Dave Zan
It would be even better to see the Bode plots done so that 180 phase is the same line as 0 dB.
I'm a newbie with LTspice and am often frustrated by the auto scaling.

Main grouse at present is I must show phase from 0 to -360 so can't have it cycle and show +/-180. I tried to turn the Tian probe round but I failed to get that to work. Suggestions gratefully received.

Quote:
Optimised TMC can do at least as well as OIC. At worst the optimisation may end up with a zero ohm resistor to the output and a zero farad capacitor to the OPS input, to produce OIC.
My obvious contention is that TMC has no advantage over 'pure Cherry' and is usually worse.

I'm hoping someone will show differently using my examples. I've made all my models available and the circuit is simple enough not to obfuscate the issue. I believe they are 'realistic' in as much as a sim can be realistic

I've my own opinions re: TPC vs TMC but in practice, there is little difference.

[deleted: 21 pages of pedantic rant about why it should be called 'pure Cherry']

Last edited by kgrlee; 2nd May 2013 at 05:41 AM.
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