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R/E/P => R/E/P Archives => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: maxdimario on October 10, 2005, 01:58:02 PM

Title: u77 bias
Post by: maxdimario on October 10, 2005, 01:58:02 PM
I just fixed a u77 which had a bad 'driver' transistor.

the mic works fine, so far, but I have seen that the bias resistor had been replaced (or was never there?) by a jumper.

to get approximately 1/2 the voltage on the drain I have to put a 20 ohm resistor.

Isn't that a bit low?

attached is a schematic.

the voltage on the power supply rail is 33 Volts (from the dc to dc converter) with no load and 22 volts when biased for 10 volts on the drain.

does anyone know what the proper voltage/current is supposed to be on this mic for the fet?

Title: Re: u77 bias
Post by: maxdimario on October 11, 2005, 07:38:20 PM
today I biased the fet for minimum distortion for highest signal, and got 380 ohms! as a bias resistor.

Low isn't it?

any of you got one at home to check out the resistor value?