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jbateman 28th February 2002 05:35 PM

Switchable bias for the A75?
I am using Mr. Pass's A75 design for the front end of an amp I'm buiding. The output stage will be much scaled down, with maybe half the output devices and a lower standing bias current. Even still, it would be nice to have the bias switchable for a "standby" or non-critical listening mode.

The simplest way might be to just use a toggle to switch a resistor in parrallel with R80. However that means running wires right past the power Xfrmr. Could that induce hum?

The next simplest might be to have a small signal relay right at the pc board to switch in a parrallel resistor. But in my experience small relays eventually fail.

Does anyone have a superior way to do this...perhaps using one of my leftover mosfets as an fet switch?

Thanks for any recommendations.

Nelson Pass 28th February 2002 07:11 PM

Your approach of paralleling a resistor with R80
would be the correct one. Simply twist the
wires that run past the transformer and it should
be just fine.

jbateman 28th February 2002 07:31 PM

Thanks very much for responding. I really wasn't expecting to get it "straight from the horse's mouth".

PassFan 1st March 2002 12:08 AM

That sounds interesting. I was thinking of using mine in the studio but was concerned because of the heat. If you care to, keep us posted. I'd like to try it.

jbateman 2nd March 2002 04:07 AM

Since I haven't even built the enclosure yet, it will be quite awhile before I can try. If you already have a working amp, it should be fairly simple. Connect a voltmeter across a source resistor to monitor bias current. Connect a 50K pot, set for maximum, across R80. Slowly decrease the pot untill you reach a bias current approx. 2/3 of normal. Remove the pot, measure it's resistance, and select a fixed resistor of the closest value to use with a toggle switch.

The output mosfets should still be in their linear region as long as you give them >100ma idle current each. And the temperature reduction should be significant.

PassFan 2nd March 2002 04:17 AM

Thanks. I'll give it a shot first chance I get.

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