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Old 18th November 2015, 06:50 AM   #1
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Default How to protect this DC servo capacitor?

Hello everyone,


Click the image to open in full size.


for a power amp we usually go for this two approaches,
for A with a DC servo, B with a DC decoupling capacitor in the NFB network.
for these two a polarized aluminum capacitor is usually being used.

I just wonder if I should protect this capacitor from being loaded in reversed voltage, where the output may actually contain negative DC.

Well for A of coz it is possible to balance that by DC servo.
But just in case for failing or being unstable.
for B it may be needed to be protected?

If I really ask for safety, to protect this capacitor, what can I do?


PS. Do not have enough place and capacitance to make it bipolar by connect two cap in series.
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Last edited by snowcatmaiden; 18th November 2015 at 06:52 AM.
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Old 18th November 2015, 07:05 AM   #2
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In most editions of Douglas Self's Audio Power Amplifier Design Handbook, the use of a reverse parallel pair of signal diodes across the cap. in the "decoupling" application is recommended. If the voltage is likely to be high, then a second pair in series with these be simply added.
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Old 18th November 2015, 08:40 AM   #3
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Click the image to open in full size.

thanks for quick reply.

just found this circuit.
add diode like this?
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Old 20th November 2015, 07:33 AM   #4
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Basically yes but ordinary silicon diodes like 1N4148 (not a zener) could be best, depending on the peak feedback voltage. However, with a balanced power supply, fit another diode in reverse parallel. As said, you may need to fit more diodes in series with these depending on the operating and DC fault voltages.

By reverse parallel, we mean that 2 diodes are in parallel but with reversed polarity so that they conduct in both directions - necessary where a fault could be of either polarity.
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Old 20th November 2015, 08:26 AM   #5
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I could be way out of line here, but with a 47K5Ω series resistor and a 470F shunt capacitor, one ends up with a 140 second time constant. Is this something that makes sense as a DC servo? I would think that a servo time constant would be too long if it were over a second or so.

Given that, couldn't the 470F cap be reduced by a factor of 100, ending up with a compact 4.7F film cap, which avoids all of these polarity issues?

Again - if I'm missing the point, please let me know, but I'm at a loss as to why a huge electrolytic is needed with a servo - the whole point of a servo is to increase the 'R' so that a smaller 'C' could be used for an equivalent time constant.
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Old 20th November 2015, 08:48 AM   #6
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Use back to back LEDs instead. Yes use film caps for servo, do not use Electrolytics. 0.1 to 1uf with 1 meg resistors are the most common component values.
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Old 20th November 2015, 11:43 AM   #7
jcarr is offline jcarr  United States
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Click the image to open in full size.

Three-way comparison of DC servo with opamp (top)

Large-value electrolytic capacitor in NFB loop (lower left)

Multiple feedback scheme to reduce capacitance value, making film capacitor practical (lower right)

hth, jonathan
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