John Curl's Blowtorch preamplifier part II - Page 2431 - diyAudio
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Old 8th July 2012, 10:32 PM   #24301
DF96 is offline DF96  England
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Why does a Mylar cap cause trouble as a coupling cap (high pass filter) but not as a low pass filter in a DC servo? Could it just be that one arrangement has less signal voltage across the cap because the CR time constant is bigger? Clearly any audio distortion created by the cap is fed back into the amplifier input by the servo so in this respect the servo is no different from using a coupling cap.
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Old 8th July 2012, 10:33 PM   #24302
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It IS a single op amp PACKAGE. Just a dual in a mini-dip. Think it through. Moving to a dual in the same space, is cheaper than using 2 Mylar caps. Today, the offsets on fet input op amps are good enough that the added stage will not significantly add to the inherent offset. (sorry about the pin numbers, pushed the wrong key)
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Old 8th July 2012, 10:39 PM   #24303
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Quote:
Originally Posted by DF96 View Post
Why does a Mylar cap cause trouble as a coupling cap (high pass filter) but not as a low pass filter in a DC servo? Could it just be that one arrangement has less signal voltage across the cap because the CR time constant is bigger? Clearly any audio distortion created by the cap is fed back into the amplifier input by the servo so in this respect the servo is no different from using a coupling cap.
Because of the long time constant there could only be harmonics of a sub 1Hz tone from the servo cap. Theoretically, that is.
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Old 8th July 2012, 10:40 PM   #24304
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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.

Last edited by john curl; 8th July 2012 at 10:54 PM.
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Old 8th July 2012, 10:41 PM   #24305
SY is offline SY  United States
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Originally Posted by vacuphile View Post
Sy, how can you have a coupling cap in what is a DC amplifier?
You can't; that was my point. Returning DC to the + input requires a cap between the input jack and the return point of the servo, which defeats the whole purpose of putting the servo in for this sort of design. Otherwise, you inject DC into the source and if the source has a low DC impedance (e.g., a DC output from a preamp or DAC), the servo's voltage will be shorted out, causing the output offset to rise.
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Old 9th July 2012, 04:39 AM   #24306
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Any other questions about servos?
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Old 9th July 2012, 06:25 AM   #24307
a.wayne is offline a.wayne  United States
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John , HCA2200 II or HCA3500 ... ?
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Old 9th July 2012, 08:28 AM   #24308
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Both Parasound power amps have servos similar to what were put up.
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Old 9th July 2012, 08:32 PM   #24309
a.wayne is offline a.wayne  United States
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John , Which is the better model to mod .... ?
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Old 9th July 2012, 09:23 PM   #24310
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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.
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