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Old 28th July 2011, 06:58 PM   #1
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Default Salvaging old amps/PSU's - LM3886 build

hi, I'm somewhat new to chip amp building and have a basic question -

I have a couple of old amps and want to use the PT's for an LM3886 build. The issue is the Transformer voltage. I've seen recommended voltage of +/- 30 VDC - but little about the maximum I can safely use. The datasheet says max is 84 volts, but frankly I'm not comfortable I'm reading it correctly.

My simple question - if I have a PSU that supplies in the range of +/- 42VDC to +/- 46 VDC (no load, no signal), is that too much? My transformers supply 30VAC x2 and 33VAC x2 respectively.

thanks in advance!
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Old 28th July 2011, 07:32 PM   #2
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You read the datasheet correctly. Operating voltage is 20V to 84V, with a no-signal voltage of 94V allowed. It looks like you're near the max voltage, so much depends on the voltage drop between the transformer secondary and the LM3886.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 28th July 2011, 07:47 PM   #3
sesebe is offline sesebe  Romania
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The voltage is too high for LM3886. You can use a linear regulator (high power lose) or you can rewinding the transformer. Another idea is to use TDA7293 (max voltage without signal 120V with signal 100V). TDA7294 have almost the same voltage limits. The problems is that at this voltage the maximum power dissipated by any usual IC is too big.
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Old 28th July 2011, 08:01 PM   #4
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The 30V transformer may be ok. 30X 1.41 ( - ~2v lost in the bridge) = +& - 40.7v
Kind of puts it realy close to max though.
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Old 28th July 2011, 09:32 PM   #5
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thanks guys. That's very helpful.
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