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Old 15th July 2013, 03:56 PM   #121
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I'm now inclined to make a classic VAS (enhanced for trial's sake) with a TMC using the same OPS as I've been using, as well as the same voltages etc to make it as close as possible for comparission, to compare THD.
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Old 15th July 2013, 04:40 PM   #122
mcd99uk is offline mcd99uk  United Kingdom
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What about cherry (Nested loops)? Only throwing this out as an idea.
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Old 15th July 2013, 05:06 PM   #123
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Quote:
Originally Posted by MagicBox View Post
Harry, no; C3 was part of a snubber that's now gone again.
Sorry, should have been clearer:

I was talking about C3 in the schematic from post #79.
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Old 15th July 2013, 05:18 PM   #124
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Yeah that's correct, that's the Cmiller.

I now have a version with TMC and I can clearly see the increase in slope on the gain plot, but the phase remains icky. I'm now testing phase margin with a pure capacitive load.
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Old 15th July 2013, 05:36 PM   #125
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Quote:
Originally Posted by MagicBox View Post
Yeah that's correct, that's the Cmiller.
Right. So you could have changed ULGF by changing the capacitor's value as well as by degeneration of the input stage.

Don't forget that you need to do a bode plot of the miller loop as well as the global loop.

Ditto if you're sticking with TMC. I believe that with TMC one plot should look second-order and another should look first order. Both need to have decent stability margins.

Last edited by HarryDymond; 15th July 2013 at 05:39 PM.
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Old 15th July 2013, 08:59 PM   #126
dadod is offline dadod  Croatia
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Quote:
Originally Posted by HarryDymond View Post
Right. So you could have changed ULGF by changing the capacitor's value as well as by degeneration of the input stage.

Don't forget that you need to do a bode plot of the miller loop as well as the global loop.

Ditto if you're sticking with TMC. I believe that with TMC one plot should look second-order and another should look first order. Both need to have decent stability margins.
You can have it with one shot, just do a bode plot of the global loop and connect TMC resistor after the Tian probe.
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Old 16th July 2013, 10:32 PM   #127
Struth is offline Struth  Canada
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Hi Guys

This VAS is actually interesting!

Do you have a souped up Audio Precision system to measure a real product with? Are there measurements or just sims?

Have fun
Kevin O'Connor
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Old 17th July 2013, 04:40 AM   #128
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Nah I don't, but I can wish =) So far the measurements are just sims and the results are used as a guideline, not an absolute.

I've given my VAS a little break as I'm now at a point where I don't know well how to continue:

On one end, with a simple miller compensation from VAS-out to VAS-in of 56p, (and a small tweak to the buffer local feedback (82pf with 22Ohm series) I can get it rock stable. The THD is very bad this way; around 0.01%. When I take out the OPS and put back in an ideal OPS, the THD is back to 0.000055%.

It's clear that the OPS is the main contributor to THD and when excluded from the feedback, it rears its head bad.

If I use OPS inclusive feedback, I get this extra phase shift which leaves me with a phase margin of less than 30 degrees on two locations in the band. This is not acceptable.

So. I think I'm stuck. Using plain miller defeats the purpose of this VAS as I throw away all that loopgain.

I've tried TMC, TPC and variants, but no matter, where there is improvement in gain, there's a degradation in phase.
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Old 17th July 2013, 06:29 AM   #129
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Originally Posted by MagicBox View Post
Using plain miller defeats the purpose of this VAS as I throw away all that loopgain.
That's what compensation does in all circuits! You have to get the loop gain to cross zero before all your high-frequency singularities kick in and bring their evil phase shift.
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Old 17th July 2013, 07:24 AM   #130
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haha yeah, I've experimented with this schematic like a mad-man. I think I made some improvements though. I now have a combination of both Cherry and TMC with the C/R path from VAS out to OPS out is rather low-ohmic.

The question though is what an acceptable phase margin is. When I assume the OPS series resistance (regardless of Cload) is at least 50mOhms, I can get so much more performance from the circuit than if I would stabilize it for an ideal C.

In this situation I now have around 0.000023% into 8Ohm and 0.000046% into 4Ohm. As long as the series resistance doesn't drop below 50mOhm it remains stable at all capacitances (within sensible range as they still load the OPS).

So what's practical?
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