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Old 7th June 2004, 12:04 PM   #1
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Default Capacitor Size Opinions

I am looking for opinions on PS cap sizing.
The amp in question is a ML 27.5 Class A amplifier (all solid state). The cap is from the low level PS circuit. It is rated at 1900ufd @150VDC with only 85VDC across it. I've temporarily put 1100ufd in cct and it works fine.

Given that the following are what's available to fit the physical size, and money is not an issue what would you choose? Just curious.
The available values are:
2700@200VDC CDE
2400@100VDC Mallory
5000@160VDC CDE
4700@100VDC United Chemi-con (cheapest)


Dan
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Old 7th June 2004, 12:26 PM   #2
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There's the option of parallelling smaller value caps too, for lower ESR I think
Steve
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Old 7th June 2004, 12:46 PM   #3
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If the cheapest is such an increase, I'd go for that first.
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Old 7th June 2004, 12:48 PM   #4
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The problem is physical size. They are held in with their screw terminals and one very unyielding clamp. Paralleling is not an option because of that.
What I'm basically concerned with is 100V units not standing up with 85V on them (long term) and/or putting too much capacitance on the rectifiers. There's a fair bit of heat in this amp.
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Old 7th June 2004, 12:54 PM   #5
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I think that 100V would be a bit short for the 85V. If you got 10% more mains (not unusual) you're up into the low 90's, and although the cap will not explode it will shorten it's life. Is the 85V what you measured or what is in the manual? This may differ.

I also would be hesitant to increase the capacitance too much, you may actually get more higher-order harmonics on the supply lines. Going from 1900 to say 2700 uF seems OK, but I wouldn't go to 4700.

Jan Didden
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Old 7th June 2004, 01:27 PM   #6
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Your thinking along the same lines I am. The voltage was measured at 84.9 VDC. Actually I had to measure the other channel to get a value. The original cap is reading "0" capacitance, and giving me only 74V of noisy DC. So I will be fairly close to it's limit.
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Old 7th June 2004, 01:29 PM   #7
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Jan, or anyone else who knows, this is a little off topic I guess, but still related. Is it possible to use high value caps in combination with small caps to take out high frequency noise. I've seen this quite often in PS's, where there's a big cap and a small, maybe 0.1uF cap paralleled accross it, and wondered if that's what it was for.
Cheers,
Steve
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Old 7th June 2004, 04:00 PM   #8
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As to my knowledge, it's normal procedure to put smaller caps (size 1:100 on less) in parallel with large caps in the PSU.
I've even tried the mix of 47.000 // 4.700 // 470 // 4.7 // 0.47 uF, and I think the result was quite good, however I have NO documented evidence for this
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Old 7th June 2004, 04:38 PM   #9
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For the stuff I do at work, we derate capacitors by 60%, to give longer life. It's mil stuff, so reliability is critical. For an 85 Volt rail, I would specify a cap with at least 150V rating. The 2700uF/200V part sounds like it's closest to the right capacitance value, and the 200V rating will help insure it's longevity.

The data sheets for capacitors typically give a lifetime of 2000 hours running a cap at it's rated voltage. The more you derate them, the longer they last. Temperature also is a factor. Some of the newer caps, like the conductive polymer types, are claimed to not need derating, but we derate them anyway. (Those are low voltage caps anyway, so they don't apply to your case).


Cheers,

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Old 7th June 2004, 04:47 PM   #10
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Quote:
Originally posted by baggystevo82
There's the option of parallelling smaller value caps too, for lower ESR I think
Steve
Whats feeding the cap? A trafo/ recifier or a regulator? If the latter. You could blow the regulator which may not be designed for a high current charge-up of the larger (4700uF?) cap.

As others, I'd also say 2400uF of at least 160v may make it "live" longer.

K-

PS: In some Krell pre-amp, I saw burnt 63v caps with 56v supplies. The 100v parts were ok... i.e. did not heat up as bad. So in a power on and forget circuit, I'd go with lots of voltage headroom.
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