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Old 7th November 2005, 06:08 AM   #1
rkc7 is offline rkc7  United States
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Default PSU for QSC amp questions

I looking at the service manual for this amp and wanted to confirm what I thought:

In the power supply, the 8 filter capacitors (bottom right) make up two supplies with 4 caps each in 2 pairs of paralleled in series?

The rail voltages are stated as 54vdc, but the caps are all 3300uf 80vdc. So wouldn't this effectively make 3300uf@160VDC on each rail? That seems rather inadequate... I'm guessing the voltage rating is because all of the USA series amps use the same board with the 900 having 74vdc and the 1310 having 93vdc rail voltages, so they don't have to use a variety of parts in each, but still, 3300uf per rail is really low!

Considering the 400 only runs at 54v, would replacing all the caps with 12000uf/35vdc caps be OK? They're basically the same size, just much higher capacity and the series voltage should be adequate, right?

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Old 7th November 2005, 07:16 AM   #2
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I've learned that it is a very dangerous idea to put two capacitors in series in order to improve maximum voltage handling. ESPECIALLY filter capacitors that are very large and sound like a shotgun when they die!

==> It sounds like you're trying to put two filter capacitors in series (with each having a max voltage of 35V) to get a total maximum voltage of 70V, which would be adequate for a 54V power supply rail. However, due to transients and tolerance matching, if you applied 54V to these two capacitors in series, you can not be guaranteed that the voltage would divide evenly. That is, you can't be guaranteed that each capacitor would only have to withstand 27V. It is possible that during transients (such as initial power-up or power-down), all the voltage may go across one cap or the other, effectively relieving the cap of its magic smoke, and creating a boom in the process (or at least damaging the cap).

==>does this answer your question? I think I understand it, but just making sure.
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Old 7th November 2005, 02:18 PM   #3
rkc7 is offline rkc7  United States
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Nevermind... I found the filter caps in the schematic, they were kinda hidden in there.

It doesn't appear they're in series according to the schematic, I'll have to figure out what's going on. I may just stick to the original design and replace with at least 80VDC rated caps, if they need to be.

Sorry to bother, I'm new to this stuff
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Old 7th November 2005, 02:42 PM   #4
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Quote:
Originally posted by rtarbell
I've learned that it is a very dangerous idea to put two capacitors in series in order to improve maximum voltage handling.
put a pair of resistors parallel to each of the series capactiors -- 470k/1watt for tube supplies for instance.
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Old 10th November 2005, 10:19 AM   #5
mobyd is offline mobyd  New Zealand
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Be aware that the USA400 (like many other QSC amps) paint the wall by holding the paintbrush still and shaking the house up and down (it enables them to get away with neat tricks like having all output devices' collectors bolted to a grounded heatsink with no insulators). Hence each channel has a totally independant power supply and the centre node of the upper and lower filter caps drives the speaker. Study the schematic CAREFULLY before attempting any modifications.
M
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Old 10th November 2005, 06:18 PM   #6
rkc7 is offline rkc7  United States
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Yeah, I was thrown off by looking at it at first... so I'm just replacing everything per spec.
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