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Old 4th April 2012, 08:58 AM   #1111
AndrewT is online now AndrewT  Scotland
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Cdom or Miller cap, is there to maintain stability.
Set it so that the amplifier is stable.

Increasing the value of Cdom reduces the open loop gain of the amplifier by increasing the local feedback around around the VAS. That reduction of Open Loop Gain reduces the feedback in the global feedback loop and that is what increases the stability margins.
Cordell is correct. Self is correct.

As a consequence of setting the Cdom value as low as possible and still maintaining the required stability, the open loop gain is set to maximum. With the closed loop gain fixed, that results in more feedback. That increased feedback reduces all the various characteristics that rely on feedback. Distortion is just one of them.

In summary.
reducing Cdom reduces the stability margins.
reducing Cdom reduces distortion.
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Last edited by AndrewT; 4th April 2012 at 09:01 AM.
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Old 4th April 2012, 12:33 PM   #1112
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Great explanations, this makes things clearer around that oh-so important cap.

So for people like me that likes to try things and doesn't necessarily have all the required instrumentations, what could be the test path?

I think I can refer to other DX designs, their gain and Cdom value to try and determine a new value for Cdom. I know that DX has tested with 121x gain and 56pF Cdom, but it WAS unstable.

I guess that from 121x gain and 180pF, going down to 32x gain permits going to 100pF without any problem.

But without instrumentation, what is the indicator of oscillations? Will wi hear anything wrong? will the amp self-destroy?

From my knowledge, a way to monitor is to check the temperature of the 15R as shown below, which is the escape route for the oscillations... (see pic)... Any thoughts?

Regards,
Mart.
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Old 4th April 2012, 01:42 PM   #1113
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by canonnica View Post
......... I know that DX has tested with 121x gain and 56pF Cdom, but it WAS unstable.

I guess that from 121x gain and 180pF, going down to 32x gain permits going to 100pF without any problem.
Wrong guess.
As one reduces the closed loop gain, one is also increasing the feedback, making the amplifier less stable. To correct for this one would expect the Cdom to get higher in value as the closed loop gain is reduced.
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Old 4th April 2012, 01:57 PM   #1114
a.wayne is offline a.wayne  United States
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@cannonica ,

Temperature will escalate fast when oscillating , monitor tranny temps ....
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Old 4th April 2012, 02:40 PM   #1115
bonfis is offline bonfis  United States
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Originally Posted by AndrewT View Post
Wrong guess.
As one reduces the closed loop gain, one is also increasing the feedback, making the amplifier less stable. To correct for this one would expect the Cdom to get higher in value as the closed loop gain is reduced.
What Andrew says agrees with the formula in Cordell's book. Cdom is inversely related to closed loop gain. So if the amp is stable even with the increased feedback and original 180pf then there would be no need to increase Cdom. The question would be do you want to risk decreasing it.

For a scope free stability test Cordell also suggests looking at the closed loop frequency response for evidence of any peaking (anymore than 1 db) just before the final roll-off. Any peaking is a "danger sign." He notes however that absence of peaks doesn't guarantee stability. You need to bypass any input low pass filters while running this test.

He also describes tests that can be run in simulation but they are too complicated to include here.
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Old 4th April 2012, 02:50 PM   #1116
evette is offline evette  Canada
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Yes I,ve heard from him,, He,s still a dirty old man. Evette
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Old 4th April 2012, 03:16 PM   #1117
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I see. I've taken temp readings on the VAS, the CCS and the output trannys when I reduced the global closed-loop gain, and everything stayed the same. I hope this indicates it's stable.

With my little knowledge, it would be too risky to blindly tamper with Cdom. I will wait until I understand better its role and method of calculation before changing it.

In the meantime maybe someone will make some real-world measurements and post his results. We miss DX's knowledge right here...
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Old 8th April 2012, 05:32 PM   #1118
emmi is offline emmi  India
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Amplifier can be used for PA ???
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Old 8th April 2012, 05:35 PM   #1119
AndrewT is online now AndrewT  Scotland
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no.
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Old 10th April 2012, 12:29 PM   #1120
emmi is offline emmi  India
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Quote:
Originally Posted by AndrewT View Post
no.
Why so ?
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