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Old 15th February 2004, 04:09 PM   #1
Rafal is offline Rafal  Canada
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Default Feedback capacitor to minimize DC offset

Hi All,

I'm building a gainclone based on Peter Daniel's design (who, by the way, is extremely helpful and patient with all my questions). I would like to add a feedback capacitor per channel to minimize DC offset to help protect the speakers (just in case). The spec for LM 3875 form National's website calls for a 22uF capacitor. I would like to use a very good quality cap which is carried by partsconexion.com. I am also not sure as to the voltage rating and capacitor type. The cost is also an issue.

Can you help me by recommending a good capacitor from that supplier?

Thanks,
And now to my favorite hobby...

Rafal
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Old 15th February 2004, 04:15 PM   #2
SY is offline SY  United States
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Since the cap is in series with a good-sized resistor, ESR is not really an issue. It also won't see too much voltage, so anything rated at 2x the rail voltage or higher will work. You can bypass it with a film cap of 1 uF or so, but there are lots of good designs out there which don't bother. Fancy designer caps in that position are, IMO, a waste of money; just use a good quality nonpolar cap (or two back-to-back elytics) from a reputable source.
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Old 15th February 2004, 04:22 PM   #3
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I already suggested BG N type cap, as it's as good as electrolytics go, but was not sure about proper voltage rating. How much voltage the cap actually see in there? As that rating affects price substantially, what is the minimum voltage spec. one could use here?
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Old 15th February 2004, 04:38 PM   #4
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In reality, spec'ing at 1x rail voltage should be more-than-adequate, since the cap is connected to ground and fed from the output through a divider. I like to double things for reliability- after all, it's possible that a reactive load could kick back some higher voltage...
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Old 15th February 2004, 04:51 PM   #5
Rafal is offline Rafal  Canada
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Thanks for your helps guys

Just so I'm clear, is the rail voltage, the voltage coming out of secondaries of my transformer?

Cheers,

Rafal
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Old 15th February 2004, 04:57 PM   #6
SY is offline SY  United States
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The rail voltage is the DC voltage that you feed to the chip.
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Old 15th February 2004, 05:53 PM   #7
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Default ummmmmmmm.........

I don't know were people come up with this stuff. The cap to ground from the inverting input feedback resistor sees nowhere near the supply voltage ( for plus and minus supplies) and I have often seen 16V caps used in this part of the circuit. ESR and capacitor quality is very important to sonics in this cap and it would be one the first caps I would (and have, in modifying many amps). Bypassing with film caps is not always a good idea. A film cap across a very low ESR cap like a Black Gate is " The kiss of death) as one of my favorite designers puts it. I am all for conservative design but voltage rating overkill and penny pinching is not even close to a good idea in this part of the circuit. Use very good resistors for the feedback loop as well. This is one of the most critical parts of the circuit for parts quality and I am quite puzzled why anyone would think outerwise.
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Old 15th February 2004, 06:20 PM   #8
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Thanks for clarifying that Fred. I also suspected that this cap shouldn't "see" any high voltage, as normally in my amps this end of 680 resistor is connected directly to ground. So I hoped that 16V or even less should be sufficient. Your observation regarding bypassing BG caps is right on the mark as well, as I tried it before and it actually degrades the sound, IMO.
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Old 15th February 2004, 06:27 PM   #9
SY is offline SY  United States
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Fred, if you've got a few hundred ohms in series with the cap, why on earth should a few milliohms of ESR make any difference? The resistance of the actual series resistor will change more than that if the room temperature changes a couple of degrees.

Speaking of penny-pinching, why not use a decent voltage rating just in case? I've already pointed out the the o/p voltage is divided down, but 63V caps are not significantly more expensive than 16V, so why not go for it? Saves worry in case of fault conditions and temperature derating.

Peter, what happened to your sig? I had a joke all ready to go and you short-circuited it!

Quote:
Use very good resistors for the feedback loop as well.
D'accord. Low voltage coefficient of resistance and inductance are the key parameters.

EDIT: A change of merely one degree C will cause the resistance of a good quality 680 ohm resistor (100ppm/degree) to shift by 68 milliohms. Puts the ESR thing in perspective, eh?
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Old 15th February 2004, 06:35 PM   #10
Rafal is offline Rafal  Canada
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I guess I an try it without the cap first. However, should I decide to use one, would Blackgate 58646 (Standard )22uF 16v or Blackgate 60093 33uF 16V be better? (I can't find a N series 22uF rated at 16v.

Cheers

Rafal
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