TPC vs TMC vs 'pure Cherry' - Page 26 - diyAudio
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Old 20th October 2013, 10:03 AM   #251
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Quote:
Originally Posted by kgrlee View Post
Can we continue this on my... thread?

Doing more work, my initial thought that your 'Tian probe between OPS and the feedback take-offs' is looking at the VAS+OPS+Cherry 'inner loop' seems to be not quite true though it is a useful alternative view...
Why do you now think this?

Best wishes
David
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Old 25th October 2013, 06:16 AM   #252
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I have taken the liberty to respond to some of your PM comments back in this thread, in case anyone else cares about analysis of feedback.

Quote:
Originally Posted by kgrlee
I was quite excited when you suggested that the Tian probe on the output looked at the 'OPS loop' ... especially when some quick sims on a VAS amp, showed equivalent results to having the Tian probe on the VAS input (which is how I look at the 'inner VAS/OPS Cherry loop)
My analysis of the theory says the two placements should be completely equivalent.
But I also did a few simulations, just to check and was admittedly pleased to see that they matched perfectly.
That confirmed not only my analysis but that the Tian probe is correctly implemented, which I had always assumed but never actually checked.

Quote:
I did some more work on CFAs to try check equivalence of 'Tian on O/P' and 'Tian on 'VAS i/p' looking at the 'inner loop' and didn't quite get that. These were with zillion H inductors isolating the 'inner loop' from IPS.
I think this shows the inaccuracies of the inductor method rather than any problem with my recommendation.
These inaccuracies are precisely why the Tian method is superior.
Unless you have any other problems I think my analysis stands.

Best wishes
David
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Old 25th October 2013, 07:22 AM   #253
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I think we are both nit-picking here. What is still lacking is a good way to look at 'main/outer loops' for CFA amps. hint hint

I'm ashamed to say I haven't had time to look at the true implications of the VAS emitter resistor (ala Cherry) and its decoupling (ala Lee)
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Old 25th October 2013, 05:10 PM   #254
jcx is offline jcx  United States
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for more head scratching: Bob Cordell Interview: Error Correction

the Tian probe clearly isn't measuring what I'd like - pretty sure the loop gain isn't "flat" at lower frequency - my guess is the EC loop changes sign, becomes positive feedback at low frequency and cancels out the negative feedback in the Tian probe calculation

I've not yet learned the Middlebrook "final solution" extra element theorem GFT despite having Vorperian's book, the Middlebrook CD
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Old 25th October 2013, 08:53 PM   #255
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Quote:
Originally Posted by jcx View Post
for more head scratching: Bob Cordell Interview: Error Correction

the Tian probe clearly isn't measuring what I'd like - pretty sure the loop gain isn't "flat" at lower frequency - my guess is the EC loop changes sign, becomes positive feedback at low frequency and cancels out the negative feedback in the Tian probe calculation

I've not yet learned the Middlebrook "final solution" extra element theorem GFT despite having Vorperian's book, the Middlebrook CD
jcx, do you still have the *.ASC models for this sim? And the FET models?

I won't be able to do any serious work on this for a while but they are certainly worth investigating further.

My $0.02 is that Cherry always strives to maximise NFB around the evil OPS (as is right & proper) so you will always see huge ULGF around that for his designs.
______________

Did you find any cases of the mythical 'unstable inner loop and stable outer loop'? Gotta be unstable. Marginal stability isn't enough. The best example of this is the huge Loop Gain mid peak with TPC. It's extremely marginal but always ju..ust stable.
______________

Also any good 'clear explaination of measuring T and calculating desensitivity with (unstable) minor loops inside stable negative feedback amplifiers'
______________

Quote:
I think this shows the inaccuracies of the inductor method rather than any problem with my recommendation.
Mr. Zan, the experiments combined zillion H inductors AND Tian probes. I was trying to isolate the VAS/OPS/Cherry loop from the rest of the amp.

But either way, I think the Tian probe on the output is a valid and useful view of the VAS/OPS/Cherry loop.

But I think its essential, perhaps even more important to have a good loop at the 'outer/main' loop.

Last edited by kgrlee; 25th October 2013 at 08:56 PM.
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Old 25th October 2013, 10:11 PM   #256
wahab is offline wahab  Algeria
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Quote:
Originally Posted by kgrlee View Post
My $0.02 is that Cherry always strives to maximise NFB around the evil OPS (as is right & proper) so you will always see huge ULGF around that for his designs.
Actualy ULGF is relatively low IIRC , i ll have to check
in a design i use as experimentation beast in simulator
of NDFL concept.

Seems that the global feedback loop is frequency limited
such that the input stage is not fed back with the frequency
range at wich phase shift become excessive , that is , gain
margin is increased this way...

I ll have to dig further in Mr Cherry s writings since it s
not a long time ago that i started to give some attention
to what appeared me at first look as a quite complexe
and hardly practical topology but on the simulator i was
surprised that it seemed to work and even very well as far
as linearity is the main concern.
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Old 26th October 2013, 03:04 AM   #257
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Quote:
Originally Posted by kgrlee View Post
I think we ... both nit-pick...
Sometimes apparently minor inconsistencies are a clue to a fundamental issue.
So I want to make sure we have consistency before I post more.
Don't want to spend the time and then have it nit-picked or, worse, incorrect.

Quote:
Originally Posted by jcx View Post
...the Tian probe clearly isn't measuring what I'd like - pretty sure the loop gain isn't "flat" at lower frequency...
Tian probe doesn't know what you like, of course, but it's a bit counter-intuitive.
At least, it may be, but it's fairly hard to see.
If not an ASC at least a clearer picture would help.

Quote:
Originally Posted by kgrlee View Post
Did you find any cases of the mythical 'unstable inner loop and stable outer loop'? Gotta be unstable. Marginal stability isn't enough. The best example of this is the huge Loop Gain mid peak with TPC. It's extremely marginal but always ju..ust stable.
Any linear model will have difficulty with actual instability since output will be unlimited. This has made me wonder how best to analyse the problem.

Quote:
Mr. Zan, the experiments combined zillion H inductors AND Tian probes. I was trying to isolate the VAS/OPS/Cherry loop from the rest of the amp.
I should have written that "I suspect this shows the inaccuracies of the inductor method." Post it up and let's make sure.

Quote:
But I think its essential, perhaps even more important to have a good loop at the 'outer/main' loop.
OK. I'll post it .

Best wishes
David
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Old 26th October 2013, 07:06 AM   #258
MiiB is offline MiiB  Denmark
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I have tried to make e version of the SSA with pure cherry compensation, at high power it shows quite dramatic improvement in distortion figures . Now I prepare a PCB where the compensation scheme can be altered with different stuffing.
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Old 26th October 2013, 12:43 PM   #259
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Quote:
Did you find any cases of the mythical 'unstable inner loop and stable outer loop'? Gotta be unstable. Marginal stability isn't enough. The best example of this is the huge Loop Gain mid peak with TPC. It's extremely marginal but always ju..ust stable.
Quote:
Originally Posted by Dave Zan View Post
Any linear model will have difficulty with actual instability since output will be unlimited. This has made me wonder how best to analyse the problem.
The best 'linear' measure is simply to plot Closed Loop response. The curve I show in #248 has wonky loads AND compensation cut to the bone.

The 3 peaks are at UGLF of the 'inner Cherry' loop, ULGF of the 'main/outer' loop and the resonance of the output inductor with the wonky load. The 'inner' & 'outer' loops are on the verge of instability which you can see on their Loop Gains. This is reflected in the Closed Loop gain.

If the 'marginal stabilty' isn't a problem (like the mid peak with TPC) you'll see zilch on the Closed Loop response. But if the stability of the inner loop is evil, you'll see peaking on the Closed Loop response.

If anyone has examples that disprove this, please speak up and post the evidence.
________________

MiiB, have a really good look at the Loop Gain of your amp with Cherry with the Tian probe on the output. It's important that this has good stability margins.

When Guru Zan shows us how to put a Tian probe on the IPS of a CFA, use this too.

Bear in mind that improving the stability of the 'inner VAS/OPS' loop using bigger Cherry caps, may decrease stability of the 'outer/main' loop.

Have provision on your SSA PCB to put 100p to 1n across the 10R VAS resistors.

SSA (and VSSA too) only have 2 stages within the Cherry loop so is probably OK. But I'm a firm believer in Murphy's Law.
___________________

Mr. Zan, I'm excited to hear that you are doing an article for Linear Audio but sad that this beach bum probably can't afford to see it
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Old 27th October 2013, 03:13 PM   #260
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Quote:
Originally Posted by kgrlee View Post
If anyone has examples that disprove this, please speak up and post the evidence.
OK. Next post.

Quote:
Mr. Zan, I'm excited to hear that you are doing an article for Linear Audio...
Thanks for the enthusiasm.
The first article is not doin' but done and was published in Volume 6.
It's an introduction to "Advanced Amps", essentially that means better than simple Miller compensation.
The second one is under way at the moment for Volume 7.
That looks at TPC, TMC, OIC and the like.
I have plans for another for Volume 8 on how to analyse feedback and simulate with Middlebrook and Tian probes.
Still need to work out some LTSpice details.

Just so there's no confusion, is this the CFA you want to analyse the outer loop?
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Last edited by Dave Zan; 27th October 2013 at 03:17 PM.
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