Hi,
I’m servicing an A and R A60 amp. The amp was working well, but had 1.66v DC offset on one channel. I’ve replaced all electrolytics and some key resistors known to drift along with a few that measured out of spec. The amp sounds better and bias is stable but the DC offset is unchanged. This amp (see attached circuit diagram) doesn’t have any adjustment for DC offset. So I’m asking for help in:
1. Finding out exactly what components control the DC offset in this circuit
2. Any insights anyone has into this problem happening before etc
Thanks for any help!
I’m servicing an A and R A60 amp. The amp was working well, but had 1.66v DC offset on one channel. I’ve replaced all electrolytics and some key resistors known to drift along with a few that measured out of spec. The amp sounds better and bias is stable but the DC offset is unchanged. This amp (see attached circuit diagram) doesn’t have any adjustment for DC offset. So I’m asking for help in:
1. Finding out exactly what components control the DC offset in this circuit
2. Any insights anyone has into this problem happening before etc
Thanks for any help!
Offset is normally controlled by the input pair and the components round about it as long as you are sure its not a fault further on.
I would start there first ,that's why many differential input pairs consist of two BJT,s/JFets in the one envelope or if discrete have semi variable resistor connected to the supply via a CC source so that adjustment can be carried out due to the different gain factors of the individual BJT,s.
Check the feed first .
I would start there first ,that's why many differential input pairs consist of two BJT,s/JFets in the one envelope or if discrete have semi variable resistor connected to the supply via a CC source so that adjustment can be carried out due to the different gain factors of the individual BJT,s.
Check the feed first .
Thanks for the pointers. I also can't figure why the protection circuit doesn't kick in with such an offset on the output? The protection circuit is formed from D1, D2,Q16,Q17,Resistors 54-59, and caps 28 and 29. Could the protection circuit be malfunctioning and cause such an offset?
David
David
Nothings impossible but have you already ruled out the component checks both Jon and I mentioned ?
Its engineering logic that you fault find in a well known set order unless its obvious where the fault lies .
Its engineering logic that you fault find in a well known set order unless its obvious where the fault lies .
With respect those components are the over current protection I don't see any DC offset detection circuit unless there is another page to the circuit diagram .
Compare the voltages ( around the transistors ) between the 2 channels to see if there are any anomalies .
Thanks again. I will of course start with the the pointers from Jon and you but am just trying to understand the full context /circuit and any other possibilities before I get back to tackling it. From what you say it's less likely to be in the protection circuit area. I'll report back next week when I get back to it-hopefully positively🙂Thanks again for all help which is much appreciated
Since the amp has unity DC gain an offset of 1.66V means the input pair is not working, you'd expect far less than 0.1V offset otherwise.
Possibly one or both of the devices is fried, or a connection is broken.
Have you ruled out oscillation BTW?, that can give spurious DC offsets all over the place.
Possibly one or both of the devices is fried, or a connection is broken.
Have you ruled out oscillation BTW?, that can give spurious DC offsets all over the place.
Thanks Mark. I havn't ruled out oscillation yet. Having done the basics of cap and some resistor replacements, I thought I'd take stock and get some pointers as to where the problem lies and if nothing comes up from further investigation, put it on the 'Scope and check for oscillation, distortion etc but the amp sounds very sweet despite the offset and does not produce any thumps on switch on /switch off either