Switchable bias for the A75?

[jbateman]

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]

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]

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

 

[PassFan]

Jbateman:
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]

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]

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