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Single ended preamplifier bias
Single ended preamplifier bias
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Old 27th March 2021, 09:48 PM   #1
wersuss is offline wersuss
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Default Single ended preamplifier bias

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.
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Last edited by AllenB; 28th March 2021 at 12:03 AM. Reason: Fix image
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Old 2nd April 2021, 10:52 PM   #2
ejp is offline ejp  Australia
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You don't adjust their offset at all.
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Old 3rd April 2021, 01:10 PM   #3
wersuss is offline wersuss
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Thank you for absw. What those trim pots do here then? What about current adjust?
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Old 3rd April 2021, 08:54 PM   #4
ejp is offline ejp  Australia
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Impossible to say from just a photo. Do you have a service manual? Schematic?
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Old 3rd April 2021, 08:58 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by wersuss View Post
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.

Last edited by rayma; 3rd April 2021 at 09:01 PM.
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Old 4th April 2021, 02:00 PM   #6
wersuss is offline wersuss
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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
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Old 5th April 2021, 08:57 PM   #7
wersuss is offline wersuss
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Can it be DC offset for op amps? i measured voltages at trim pots are around 200mV.
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Old 5th April 2021, 10:43 PM   #8
ejp is offline ejp  Australia
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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.
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Old 5th April 2021, 11:07 PM   #9
wersuss is offline wersuss
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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.
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Old 8th April 2021, 03:36 PM   #10
andy2 is offline andy2  United States
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It seems like your pre is an opamp based amplifier. Which opamp did you used and how does it sound?
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