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Old 14th December 2007, 10:50 AM   #1
Khron is offline Khron  Romania
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Default Feedback cap voltage?

I've quite often seen that the electrolytic capacitor in the negative feedback circuit (to ground) revolves around 220-470uF, most commonly rated at 16V, and recommended to be low-ESR if possible. I've gathered a nice "stockpile" of Rubycon and Sanyo low-ESR electrolytics off some mainboards, but they're all rated at 6.3V.

What would be the risk of blowing the caps in this situation? If my thinking's correct, the voltage across them sould be vaguely equal to the amplifier's input signal voltage...


Thanks in advance,
Chris
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Old 14th December 2007, 01:36 PM   #2
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Anyone???

Please?...
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Old 14th December 2007, 01:36 PM   #3
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I've never measured it, but your thinking seems reasonable. Doug Self uses doubled diodes in both directions across the cap, just in case the voltage exceeds the rating. That would certainly happen if something in the amp failed and the output went to the supply rail. At least the cap wouldn't explode. IMO, very low voltage electrolytics are to be avoided. I've serviced a lot of test equipment and audio stuff, and it always seems the high value 6.3V caps have failed first. Either they have high DF (lots of ESR), or leakage, or the value is way off. Even new ones might not be that great. Whenever I see something like 500uF@6.3V I check it very carefully and am often rewarded with a quick repair.
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Old 14th December 2007, 01:40 PM   #4
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Hi, No AC should appear across the cap unless you are testing at very low frequencys when the reactance of capacitor becomes significant. The average D.C. voltage is zero EXCEPT under fault conditions.It is permissable to connect reverse biased diode across cap to protect from failure if the output were to put a negative voltage across the cap.The feedback resistors would normally limit any current to safe value but it is good practice to design so that all eventuallities are covered.The voltage seen by the cap also depends on the value (ratio) of the feedback network.All the above also assumes a normal topology on split supplies i.e. not a single rail design.
Karl
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Old 14th December 2007, 01:53 PM   #5
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Given the functioning of a differential pair and negative feedback (in a "conventional" split-supply solid-state amp), i'd expect that the voltages at the inverting and non-inverting input to be quite similar, if not identical.

Thanks for the 2-diode tip, Mr. Hoffman

The caps I have are 1000-1200-1500uF low-ESR Rubycon ZL/YXG series and Sanyo WG series, recovered from a few Asus socket-370 motherboards.
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Old 14th December 2007, 02:58 PM   #6
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Quote:
Originally posted by Khron
Given the functioning of a differential pair and negative feedback (in a "conventional" split-supply solid-state amp), i'd expect that the voltages at the inverting and non-inverting input to be quite similar, if not identical.
Under normal conditions, they will be identical.
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Old 14th December 2007, 03:32 PM   #7
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Depending on the impedances in the feedback network, I would sidestep the question and just use a film cap. I did this to my power amps recently, using a cheap and cheerful polypropylene intended for speaker crossovers, and it was one of the most cost effective upgrades I have ever done - the whole spectrum sounded better, but the bass was totally transformed. On one track that I know really well ("She's Lost Control" from "Unknown Pleasures" by Joy Division) I noticed a sub-bass synth part that I had never been aware of before.
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Old 14th December 2007, 03:47 PM   #8
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The idea isn't bad, but 220-1000uF (of any voltage rating) film caps aren't exactly "on every corner"

Not to mention that they'd most likely be the size of your fist at those values...
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Old 14th December 2007, 04:29 PM   #9
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In many amps, you don't need that a big a value, if it is a film cap.

Large values are used to reduce the distortion contribution from the electrolytic, but films have several orders of magnitude less distortion.

For example, if the feedback resistor in series with the cap is 1k (a very typical value), 22uF gives a 22ms time constant, which would be -3dB at 8Hz. 22uF costs a few dollars each, and is maybe an inch in diameter and two long.

If you are making a subwoofer amp, or care about the last fraction of a dB of low bass response, maybe you need an octave more extension, but that is still practical and cheap. But for most purposes, -3dB at 8Hz is just fine, and getting rid of the mud and grunge in the bass is so musically rewarding that you don't miss any roll-off - indeed, in my case the bass sounded much more powerful, as I could hear what notes were being played!

If you have an amp where the feedback resistor is much lower in value, then I agree that this tweak may be impractical.
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Old 14th December 2007, 05:06 PM   #10
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That brings up another question- Why are the divider values as low as they typically are? It seems like they could be a factor of ten higher without causing a big problem, and the cap could thus be smaller. Yet, almost every amp I've seen uses something like a few kohms and a few hundred ohms.
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