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Old 1st October 2001, 03:35 AM   #1
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Join Date: Jul 2001
I have a 30-0-30 transformer running center-taped powering a stereo amp. I want to add a preamp in the same chassis that calls for a 30V (15-0-15) transformer to be used. Can i just use one of the larger transformer's windings, or would this cause undesireable effects?
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Old 1st October 2001, 04:36 AM   #2
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Join Date: Feb 2001
Location: Columbia, SC
Yoda,
There are two issues to be dealt with. One is that you will need to create a ground if you use just one winding. There are various ways to do this, some better than others; none perfect.
The other is that, yes, you will be robbing some amount of current from the one polarity of the power supply, thus slightly unbalancing thigs.
I'd think that the better tactic would be to go ahead and use the full 30-0-30V, but regulate it down to the required end voltage, probably ending somewhere in the range of +-21V, yes? Preamp circuits generally don't draw enough current to cause you any difficulties in heat dissipation in a regulator. As an alternative, you could simply drop the excess voltage in a resistor leading into a cap.

Grey
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Old 1st October 2001, 04:47 AM   #3
paulb is offline paulb  Canada
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Join Date: Jun 2001
Location: Calgary
I agree with Grey. Build your preamp with regulators right on the board. You can use the LM317/337 adjustables, or 7815/7915 (or other voltage) fixed versions. This makes your preamp more flexible for use with other power supplies in the future (we tend to change things around a lot, don't we?). Then pull the DC from the power amp's DC power, or (better) use a separate rectifier/cap arrangement for the preamp and pull AC directly from the transformer.
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