Hello there.
I have this amp with a rather high level of DC offset that I would like to sort out. But I've run into a little problem.
I'm used to adjusting this by replacing the differential pair transistors with matched equivalents, or by simply using a trimmer if available... but this amplifier is something different. It seems to use an opamp in the input stage, and I am guessing the DC offset problem might be caused by it not having a high enough gain or something such. Although that's just my not enough educated theory.
I would totally appreciate some pointers. 🙂
Here's the circuit in question:
I have this amp with a rather high level of DC offset that I would like to sort out. But I've run into a little problem.
I'm used to adjusting this by replacing the differential pair transistors with matched equivalents, or by simply using a trimmer if available... but this amplifier is something different. It seems to use an opamp in the input stage, and I am guessing the DC offset problem might be caused by it not having a high enough gain or something such. Although that's just my not enough educated theory.
I would totally appreciate some pointers. 🙂
Here's the circuit in question:
Hi
how much Dc offset and is it the same for both channels .
Have you checked the voltages against those shown on the schematic .
how much Dc offset and is it the same for both channels .
Have you checked the voltages against those shown on the schematic .
The Op Amp is your differential transistor stage but has an open loop gain of many thousands, so gain won't be the issue here. Check that there is precisely 0V on pin 2 and pin 3. If pin 2 (feedback follows the DC output from the emitter resistors at the output transistors) goes +ve pin 7 of the Op Amp will go -ve and the output stage should follow. Suspect the diodes connected to pin7 and the feedback blocking capacitor 2259 for leakage.
Thanks for your replies, guys.
The voltages are the same as on the schematic (well, about 1-2mV on pin 2 and 3). The offset level is around 100mV -- on both channels exactly the same. Makes it seem like everything is as it should be. I think 100 is a bit much and would like to try to decrease it.
The voltages are the same as on the schematic (well, about 1-2mV on pin 2 and 3). The offset level is around 100mV -- on both channels exactly the same. Makes it seem like everything is as it should be. I think 100 is a bit much and would like to try to decrease it.
Your offset must be present on input ground, I don't see any other explanation. Maybe the connection back to star ground has some issues? Or maybe a bad solder joint at 3253/3259 that increases input offset? Wouldn't be present on both channels though.
A Philips amp with Class G and a bootstrapped collector load? What an odd combination. Which model is that?
A Philips amp with Class G and a bootstrapped collector load? What an odd combination. Which model is that?
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