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Old 24th May 2012, 02:49 AM   #1
JZatopa is offline JZatopa  United States
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Default Silly capacitor question

So I have a silly question. If you could get a couple of these 5F caps for really cheap Power Acoustik PCX-5F 5.0 Farad Digital Capacitor (PCX5F) could you use them to make a very large power supply for a class A amplifier? I know that the V limitation is only 24V but you could use 2 in series to bring that up and you would have a whole lot of filtering. I know that it's over kill but I am curious if there is any reason not to do something like this.
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Old 24th May 2012, 03:25 AM   #2
CBS240 is offline CBS240  United States
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5 farads! I think that would be a bit of overkill. The transformer would see a dead short, at least for the time it takes to charge that thing. I think it would be better to design the circuit to have a better PSRR, if that is the problem, and spend the extra expense on other aspects of the amplifier.

Although, it would not be a bad thing to have some of those if the price is reasonable. The aesthetic value is pleasing. Someone with a large car stereo might be interested in those as power filters...
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Old 24th May 2012, 03:40 AM   #3
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Voltage regulator circuits are much smaller and less expensive.
Two in series ups the voltage, but also doubles the ESR.
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Old 24th May 2012, 03:59 AM   #4
JZatopa is offline JZatopa  United States
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I know it is totally impracticable but once or twice I have been offered large caps like this for cheap. I don't believe in using them in car systems because a small battery will always work better. I was just wondering if the next time I get some offered if I should grab them and make some sort of silly huge PS for a JLH or F5 amplifier. Shame they don't handle a much larger voltage as you could make one hell of a F5T PS.
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Old 24th May 2012, 05:44 AM   #5
CBS240 is offline CBS240  United States
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With 24V, I just had a second thought... If the inrush current was limited to a reasonable level for the transformer/fuses to handle by a current limiting circuit, the cost would be a significant dV/dt issue. If you made a control circuit to switch the amplifier circuit on when the initial charge is complete, power a relay, or a combination of such, along with more large semi-local decoupling caps and smaller film cap local decoupling, I suspect you could create a very 'battery like' low impedance power supply that would extend to higher order frequencies with better ESR. If you were only to be able to get 1 cap, you could build a balanced class A amp and float the load between the two outputs directly coupled.

I'd shoot for about 22VDC after rectification. To have two of these +/-22V might not be so bad, if the inrush current is controlled along with the rails powering the amp circuit.
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Last edited by CBS240; 24th May 2012 at 06:01 AM.
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Old 24th May 2012, 06:17 AM   #6
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I agree with CBS249. There are a number of circuits around that allow the slow build up of voltage to the PSU transformer. Jung published one 30odd yrs ago with a timer for delaying full mains voltage. The early JLH circuits had a thermistor in the transformer primary; from memory they were parts numbered CZ1/CZ6 and other "CZ" numbers....They reduced the current flow while charging up. And in addition to sofaspud's comments the series connection will also half the capacitance. Although he did use an unregulated supply on one of his early "economy" versions of the class A 10 watt model JLH himself was more in favour of regulated supplies. Lower cost, lower impedence, lower size and better bass response than unregulated PS.....
Still if you're prepared to experiment with 5F you will be able to speak definitively on these topics later on....! Cheers, Jonathan
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Last edited by Jonathan Bright; 24th May 2012 at 06:20 AM.
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Old 24th May 2012, 06:49 AM   #7
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Quote:
And in addition to sofaspud's comments the series connection will also half the capacitance.
So obvious, I'm embarrassed I didn't think of that.
If I could get those caps cheap, I'd find other uses for them outside of audio power supplies.
Or they might power my O2 for an hour or two.
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