John Curl's Blowtorch preamplifier part II - Page 2432 - diyAudio
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Old 9th July 2012, 09:31 PM   #24311
DF96 is offline DF96  England
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But you can detect the DC level if you have a barometer in your listening room. (provided your room is an IB!)
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Old 9th July 2012, 09:32 PM   #24312
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Quote:
Originally Posted by Juergen Knoop View Post
those of us who had been tortured with analog video, know that the DC-level after after the coupling cap follows the average level instead of being zero, thus 'clamping' was invented...a DC-servo may have its merits in audio, however the DC-level, whatever it may be, is unlikely be heard anyway.
It's just the thought of DC through speakers .......
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Old 9th July 2012, 09:37 PM   #24313
wahab is offline wahab  Algeria
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A DC servo is somewhat an easy trick whose worst purpose
is actually to keep the input differential(s) unbalanced.

I could understand that such a circuit is useful to get rid of the last few mV
DC offset wich can yield high current in case a very high OLG amp output
is shorted , even with no input signal.

As such it is useful in PA gears but surely not in home amplifiers.
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Old 9th July 2012, 09:42 PM   #24314
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We use servos for the same reason that many people use coupling caps. We want to keep DC off the outputs and any volume pots. We also want to have lots of AC gain, perhaps up to 100 dB, and still direct couple. Without a servo, we would HAVE to use coupling caps. Good coupling caps are large and expensive, so we use servos, instead.
I use servo designs that allow balanced from input to output, when necessary.
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Old 9th July 2012, 09:43 PM   #24315
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DC through loudspeakers is not good, DF96, and neither is noise when adjusting the volume control.
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Old 9th July 2012, 09:44 PM   #24316
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so how much DC is allowed and much does it change with the signal level? And what are the benefits of a coupling cap?
At a first glance, servos are a addendum to to coupling caps - not a replacement!
Any thoughts?
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Old 9th July 2012, 09:49 PM   #24317
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Quote:
Originally Posted by john curl View Post
OK, now it is time to address Wavebourn's statement.
The ADVANTAGE of a servo is that it ONLY operates to any significant extent BELOW the audio bandwidth, and the attenuation of the final resistor (adj) with the feedback to ground resistor (not shown), will reduce the sensitivity another 20-40dB further from ANY aberration due the IC or the cap. Therefore, the cap and IC are buffered from the audio path.
John; it sounds silly.


I can say the same about coupling caps, that AC voltage on them operates to any significant extent BELOW the signal bandwidth, and open loop gain of the amp is much less than open loop gain multiplied by gain of servo.
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Old 9th July 2012, 09:58 PM   #24318
a.wayne is offline a.wayne  United States
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Kinda like sticking a transformer on the outputs ...
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Old 9th July 2012, 10:58 PM   #24319
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Quote:
Originally Posted by a.wayne View Post
Kinda like sticking a transformer on the outputs ...
Right, and McIntosh did that with great success.
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Old 9th July 2012, 11:33 PM   #24320
Bonsai is offline Bonsai  Taiwan
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Yes, servos do look sexy.

I think in JFET based fully symmetrical front end amps you cannot get away without a servo. This is the real reason you need them (initial offset even after matching and temp drift). For bipolars you do not need a servo, just initial offset adjustment which is typically in the region of 10 to 15mV but can be as high as 25mV. WRT drift, I measured levels from ambient to circa 60 degrees on two prototype power amps recently that was under 1mV.


If you do use a coupling cap in the feedback network, then it has to be oversized to avoid LF distortion, and I would of course say it needs to be decent quality, branded type.

The AP shows 0 (zero) distortion through an oversized electrolytic, and Edmond and Ovidui proved with the PGP amp.
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