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Old 11th July 2012, 01:12 PM   #1
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Join Date: Nov 2010
Default New power supply for Quad 405

I have a Quad 405 amplifier which I have been repairing.

So far I have replaced all the board mounted caps, the opamp with an improved modern version, the out of tolerance resistors, the driver transistors, the current dumpers, the inductors with higher current ones, and all of the connections (phono and LS posts). I've modified it to reduce the gain slightly, to reduce the power on/off pop, to have more stable supplies to the opamps, and less distortion at high frequencies.

The result is fantastic - it sounds excellent, and performs very well in all the numeric tests I've done.

The only part I haven't yet touched is the power supply circuit. The amp is 2x100W, and has +/-50v rails.

Currently there is:
- A 300W transformer with two paralleled secondary windings,
- A 7A bridge rectifier,
- Two 10,000uF 63v capacitors.

The obvious upgrade is to split the power supply into two mono supplies. As modern capacitors are smaller than the 1980 ones, I could adapt it to use:
- A 300W transformer with two one secondary windings for each channel,
- A 25A bridge rectifier per channel,
- Two 10,000uF 63v capacitors per channel.

Option 2 would be to keep the windings paralleled but increase the capacitors (and the rectifier to handle that):
- A 300W transformer with two paralleled secondary windings,
- A 25A bridge rectifier,
- Two 22,000uF 63v capacitors.

Option 3 is the cheapest - just replacing the capacitors with modern ones. That would leave:
The obvious upgrade is to split the power supply into two mono supplies. As - - A 300W transformer with two paralleled secondary windings,
- A 25A bridge rectifier,
- Two 10,000uF 63v capacitors.

The replacement capacitor I am considering for options 1 or 3 is this one (low ESR, high ripple, long life, compact size).

Any thoughts or opinions are much appreciated!

Andy
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