Single ended preamplifier bias

[wersuss]

Hello, i have preamp and would like to make sure everything is in working order. Anyone knows where to measure transistors and how to adjust their offset with trimmers? Because one runs 38c and rest 33-34c

These are Toshiba 2SC2238Y/ 2SA968Y.

 

[ejp]

You don't adjust their offset at all.

 

[wersuss]

Thank you for absw. What those trim pots do here then? What about current adjust?

 

[ejp]

Impossible to say from just a photo. Do you have a service manual? Schematic?

 

[rayma]

What those trim pots do here then?

It's likely they are for adjusting the regulator voltages to an exact value.
Perhaps 15VDC, but check the mfr info first. If the board was supplied assembled,
the adjustments should already have been made.

 

[wersuss]

I have tried to turn them and watch transistors temperature so voltage regulators will be off. Where can i measure voltage? The original board looked like this

 

[wersuss]

Can it be DC offset for op amps? i measured voltages at trim pots are around 200mV.

 

[ejp]

That's the total available control voltage for whatever is being controlled, not the DC offset. No opamp is going to have that much DC offset in any sane preamp circuit. You need to abandon this theory or else back it up with a schematic. If you can't produce a schematic it's difficult to see the point of this thread.

 

[wersuss]

True hehe. Well it sounds good so i will just leave it, no schematic. But before left channel had small noise while controlling volume and now when i adjusted all 4 resistors to same 0.25v that noise is gone.

 

[andy2]

It seems like your pre is an opamp based amplifier. Which opamp did you used and how does it sound?

 

[wersuss]

Burson V6 Vivid, sounds fine,but haven't noticed much difference from original NE5534. Maybe benefits comes at higher volumes i listen only at very low volumes, but sound is pretty clear.

 

[as_audio]

See also here, a second thread to threat health of a good preamp:
Trim pots on preamplifier circuit