What size power supply should I get for repair work? - diyAudio
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Old 6th December 2007, 10:29 PM   #1
spooney is offline spooney  United States
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Join Date: Dec 2006
Default What size power supply should I get for repair work?

I was considering a 20 amp power supply but I am kind of curious whether or not this may be over kill.If a 15 amp supply will work out then I would just as soon save some money and get the smaller one.
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Old 6th December 2007, 11:01 PM   #2
glen65 is offline glen65  United States
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Join Date: Jun 2004
Location: Richland County Ohio
Default Re: What size power supply should I get for repair work?

[What size power supply should I get for repair work?
It depends on what you plan to work on.
If for example you are going to be working on large car amplifiers you're going to need plenty of current capacity so
buy as much as your budget will allow.
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Old 6th December 2007, 11:36 PM   #3
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I was fortunate enough to be given a 50A* switch-mode supply recently. It unfortunately it doesn't have a front panel adjustable active current limit. I've been using this supply after I've got an amp functioning again. When I test for instance a Punch 200 for max output before visible clipping, I trip the 36A circuit breaker I've got wired between the supply and amp.

I had been using a home made adjustable linear supply that would source 20-25A. It doesn't have enough heatsinking (for more than a couple minutes at this load) on it's series-pass transistors and I burned it up a couple weeks ago.....

I think it would be nice to have a supply with plenty of current capability that would have an adjustable output voltage and an adjustable current limit (to keep from doing serious damage while troubleshooting a broken amp). I shudder to think what price tag such a piece of equipment might have.

The supply I was given is an Astec 2000W "brick" type that had a few outputs: 5v @ 10A, 5v @ 50A, 5v @ 180A, and a 12v @ 10A. I really wanted more current at 12v (14.4v, etc). I connected the 5v 50A and the 5v 180A in series then turned the trimmer pots setting the output voltages as high as they would output. This gave me a little over 12v. I then took the thing apart and found the resistor in series with the trimmer pot. By paralleling these series resistors with another resistor I was able to adjust each "5v" output to 7v. I've been using it this way (outputting 14v) and it seems to work fine. I imagine that the 50A it was rated for at 5v would equate to only 5/7ths of that current at the now 7v. The output filter caps were 10v so I figured that part should be ok. I wear safety glassed when doing this stuff though!

I've only been playing with fixing audio amps for a month now (a few nights a week) so I'm certainly in no position to give advice. I don't know how necessary this is, but I've been loading the amps to 50-75% of their rated output (one channel at a time) and measuring the voltage drop across the emitter (or source) resistors to see how well they are current sharing. As other's have pointed out, using same-date code transistors certainly helps with the current sharing issue.... I do have an old Techtronics curve tracer that in theory I should be able to use to characterize transistors, trying to find "matches" to a given part, but it's certainly more time efficient to use the same date codes.

My wife is beginning to question if I really need any more broken amps (I've got 8 of them in a little over a months time..). How do you guys handle that sort of thing?
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Old 6th December 2007, 11:50 PM   #4
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Join Date: Nov 2003
Location: Louisiana
I'd suggest 35 amps as an absolute minimum. You can test most sub 2kw amps with ~70-80 amps. You won't be able to test all of them at full power into the lowest rated load but you will be able to tell if there are any problems. If you're going to repair 2kw+ amps to full power for extended periods of time, you'll need ~100 amps per 1kw of power. For a 3kw amp, you'll need ~300 amps of continuous current capability.

I use a 35 amp for initial testing and troubleshooting. When burning in small to medium amps, I parallel another 45 amp supply. I have 2 large supplies that I can strap together for 250 amps of continuous current but I rarely use them.

Testing with music requires less robust power supplies. Sine wave testing requires much more current and is much tougher on the amplifier (especially when using a dummy load instead of a speaker).
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