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Old 22nd January 2013, 10:12 AM   #1
effebi is offline effebi  Italy
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Default Simplest poor man DC protection

I had to put together in a hurry a power amp for a live event.
So, using what I had available, I used some dusty 1Kw rated class B buffer that I had available from industrial application and rigged up WITH A 100 mA VAS + input on a veroboard.
Since speakers were not mine, I had to think to a DC protection. I had no adequate Relays available, BTW. The final stage was supplied 50-0-50 VDC (same 1kW stuff), so the only thing that I could do was to put two 10.000 uF/ 50V electrolytics back to back (the two minus wired together)in series with the buffer otput, plus a F (fast) fuse (only 4 A !) after. Something like you would do with aclass A single ended stuff.
They are the red ones on the picture, taken on the test bench (using 3 Ohm power resistors).
The thing looked to work quite well at the scope and sounded good enogh, so the event went on about 4 hours of play including rehersals and the stuff survived.
Was it pure luck or the concept stands somehow?
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Old 22nd January 2013, 12:27 PM   #2
Bigun is offline Bigun  Canada
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Output caps do a pretty good job of protecting from DC as far as I can see.
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Old 22nd January 2013, 01:58 PM   #3
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Not very poor. 10000 uf caps are about $20 each, here.
I've tried this as an experiment with 5000 uf caps on 8 ohm speakers. It sounded okay on guitar source material but sounded a little funny on top octave solo piano. Some responder in Australia suggested I put a "small film cap" across the pair of electrolytic caps, but I haven't tried that yet.
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Old 22nd January 2013, 03:36 PM   #4
effebi is offline effebi  Italy
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Quote:
Originally Posted by indianajo View Post
Not very poor. 10000 uf caps are about $20 each, here.
I've tried this as an experiment with 5000 uf caps on 8 ohm speakers. It sounded okay on guitar source material but sounded a little funny on top octave solo piano. Some responder in Australia suggested I put a "small film cap" across the pair of electrolytic caps, but I haven't tried that yet.
Well, the capacitors I picked them up around 1991, when we cleaned up the company "old stuff" stock. The remained inactive for at least 20 years im my garage, sometimes -10 celsius in the winter. Surprisingly a time-constant test shows they are more or less charging and discharging normally. They are Italian made or al least branded "Italcond".

BTW the big blue ones that you can see on the Power supply are Mallory USA, 26mF 75V more or less same age, and they still work fine, but these where in operation now and then, for short periods.
Good old world stuff....
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Old 22nd January 2013, 03:48 PM   #5
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Also, there is a small practical advantage. if you use a slightly smaller size (2 x 4700 or 2 x 2200) you get automatically a sort of first order High pass filter in order to protect your bass speakers from dangerous subsonics.

The smaller (1 uf ?) film capacitor proposed by Indianajo probably helps, I believe too.
But I had 3-ways boxes and treble was not bad (to my fifty y/o hears), Yust maybe a little more "opaque" or "boxed" than my hi-fi, but, hey, that was for PA.
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Old 22nd January 2013, 05:31 PM   #6
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I still would add a couble of brigde rectifiers to this.
Only a dollar a piece for those capable of 50A.
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Old 22nd January 2013, 06:05 PM   #7
effebi is offline effebi  Italy
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I'm not following you, sorry .
Where would youu connect those 2 x bridges (8xdiodes)?
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