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Old 14th March 2005, 05:41 AM   #1
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Default Sample/hold DC servo circuit

hi,
I've started tube/mosfet differential buffer project, which need for the DC servo circuit, but i heard from high-end people that DC servo affect sound quality anyway, if very slow reaction even. Will be the deal better if i'll do measuring/regulating discrete and only 1 time for each 60s(ie. for 1-3 minutes just a hold last value)? Sapmle/hold with switched cap seems will work nicely and not so much complicated. Thanks for any advices!
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Old 14th March 2005, 06:31 AM   #2
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If you choose a high quality opamp, I think the sample and hold will give you more disturbance. I have used lately AD8620 and this is not a bad opamp
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Old 14th March 2005, 06:41 AM   #3
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I agree with P - A, output of DC servo make in circuit " virtual ground ", connected in parallel to feedback loop. If you use at this position high quality opamp, degradation of sound quality is insignificant.
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Old 14th March 2005, 07:10 AM   #4
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Holding for more then one minute, do you think that's necessary? BTW, probably bigger cap cheaper solution vs ad8620.
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Old 14th March 2005, 07:44 AM   #5
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Yeah, if the DC servo is slow enough and low frequency noise also, then degradation of sound quality is insignificant.
IMHO, it will be better then nonpolar BG in the feedback pass anyway, but I have decided, that sample and hold will be more solid argument vs ordinary continuously regulating DC servo.
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Old 14th March 2005, 08:01 AM   #6
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Each time you sample you will get a step out and depending of how much it is and the rise time you will maybe hear it. How about the S/H circuitry? Far more complex than a single opamp.

The servo speed should be around 0.5-3 Hz I think and this will mean that some signals will slip through up to 100 Hz and a very very very small part up to 1 kHz. AD8620 has extremely good performance up to 1 kHz and up 100 Hz the sonic impact is virtually zero and especially if you compare to the tubes.
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Old 14th March 2005, 08:44 AM   #7
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Schematic, well, i think HC4016(though leakage is too much) +J-FET QUAD(e.g. opa4134) opamp and passive something.
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Old 14th March 2005, 08:08 PM   #8
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Oops, seems the leakage is too much for multiplexer and for opamp also.. 60s hold gave several millivolts ripples, so choice is a continuously regulating or a digital solution.
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