Ultimate Amp with High offset voltage
I am working with an Ultimate TA2700 amp.
I just finished repairing a blown channel (the other
channel was ok). Iím getting a fairly high DC Offset
reading on both. Iím getting 75mv on the one I repaired
and about 90 on the other. I did run the amp and it doesnít seem
to get overly warm anywhere. Not sure what is normal for this
unit but this does seem awful high to me. Am I wrong?
Below is a schematic,
Measure the DC voltage between the base of Q101 and Q103.
Do this with no input signal.
If the voltage across the bases is the same as the output offset, the problem is likely mismatched transistors in the differential amplifier. Replacing them with more closely matched transistors should solve the problem.
Other possible problems:
*Badly mismatched emitter resistors in differential amp.
*Leaking bypass capacitor in the feedback voltage divider circuit.
I'm assuming that the amplifier works well otherwise.
Thanks for the reply,
Yes the amp seems to be working fine otherwise.
I ran the unit for several hours under load and it held up
just fine. I did have the RCA input jacks disconnected when I
made the voltage measurements.
I just noticed that when I set the input switch
(High frequency / bass filter) to a different position
that the voltage on one of the channels dropped to 45mv.
Thereís a possibility that the input section could be
feeding something into the main amp. If thatís the case the
main amp section itself could be ok. I will isolate the main
amp itself from the input section and see how it checks out.
I don't think I'd be concerned about 45mv on that amp. It's not going to be a problem.
The voltage across the bases of the differential amp transistors is the offset of the 'power amplifier'.
The voltage between the left side (input) of the differential amp and the signal ground (ground symbols using a hollow triangle) is the offset of the pre-amp section.
When you switch to high pass, some of the DC offset of the op-amps is blocked. Depending on the offset of the preamp section, this could make the offset at the speaker terminals better or worse.
Fair enough Ė Iíll leave it alone.
As I said earlier, I ran the unit pretty long and at a fairly
high level and it did just fine. Nor did I hear any abnormal
noise coming from the output.
Thanks for your help.
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