Ultimate Amp with High offset voltage

glen65 · 15th October 2007, 06:23 PM

I am working with an Ultimate TA2700 amp.
I just finished repairing a blown channel (the other
channel was ok). I’m getting a fairly high DC Offset
reading on both. I’m getting 75mv on the one I repaired
and about 90 on the other. I did run the amp and it doesn’t 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,
Thanks

Perry Babin · 15th October 2007, 08:31 PM

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.

glen65 · 15th October 2007, 10:27 PM

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.
There’s a possibility that the input section could be
feeding something into the main amp. If that’s 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.

Perry Babin · 15th October 2007, 10:49 PM

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.

glen65 · 15th October 2007, 11:19 PM

Fair enough – I’ll 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.

15th October 2007, 08:31 PM	#2
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	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. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

15th October 2007, 10:49 PM	#4
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	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. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

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15th October 2007, 10:27 PM	#3
glen65 diyAudio Member Join Date: Jun 2004 Location: mid Ohio USA	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. There’s a possibility that the input section could be feeding something into the main amp. If that’s 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.

15th October 2007, 11:19 PM	#5
glen65 diyAudio Member Join Date: Jun 2004 Location: mid Ohio USA	Fair enough – I’ll 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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