DC offset adjustment Add-on

[diydidi]

Hi.
Recently completed build of discrete transistor amplifier. Would like to add a Dc offset trimpot. Any ideas, The atached circuit is almost similar to mine in the input stage.
Any ideas?

[Steve Conner]

Try swapping R6 for a trimpot.

[diydidi]

Steve.
I tried it. It works, but one of the LTP transistors heat up like crazy. Someone suggested making R4 variable... What do you think?

[Steve Conner]

That probably means you have a mistake somewhere else in your circuit, and you're having to haul the input stage out of balance to correct it. How big a DC offset were you getting? This circuit should do about 100mV without adjustment.

The LTP transistors should only be running at about 1mA current and should never get hot. Unless you're using a Douglas Self style input stage, with emitter degeneration and lots of current.

The last amp I designed had a current mirror for the input stage. The current mirror has two emitter resistors. To get a DC offset adjustment, I replaced these with a 100 ohm trimmer, between the emitters, wiper to the rail. This had a much bigger range of adjustment than I needed.