I've undertaken a rebuild of a completely screwed up, commercial preamp for a friend of mine. By "completely screwed up" I mean mutilated by a solder happy modder or two. It looked like the last guy that played with it couldn't get it to work, so he just routed the line stage input to the output and gave it back to my friend!
Anyway, I've gotten the amp circuit back to where it needs to be, but now there is a hum problem...lots of hum. What we have is a regulated supply, but I'm not all that familiar with these things. I'm guessing that U1 is an error amplifier (the negative input is at a reference DC while the positive input has some B+ ripple injected into it through C17) controlling Q1 which works as a (shunt?) regulator. Again, that's a guess. I'm a complete novice when it comes to regulators.
It appears that U1 and U2 may have been "upgraded" to OPA604 and AD711, respectively. If calibration is important maybe this is why it no longer nulls the hum? B+1 has 5mV of hum which is way too much for the two(!) gain stages it feeds. There ends up being 5mV hum on the final stage's output. I suspect the regulator circuit around U1 is not working and/or adjusted properly. B+2 (supplies the phono section) seems OK for now, but it's hard to tell with all the hum from the line stage.
The voltages on U1 seem OK. 24V on pin 7, 0V on pin 4. I can't measure pin 2 without affecting the measurement because it's set through such a high impedance voltage divider. It is close to the 12V it should be.
It's not a ground loop. Clipping in a 470uF cap across C14 reduces the hum by 2/3. I could just do this, but I'd prefer to have it working properly and as original.
Could someone offer some advice on how to proceed?
Thanks in advance.
Anyway, I've gotten the amp circuit back to where it needs to be, but now there is a hum problem...lots of hum. What we have is a regulated supply, but I'm not all that familiar with these things. I'm guessing that U1 is an error amplifier (the negative input is at a reference DC while the positive input has some B+ ripple injected into it through C17) controlling Q1 which works as a (shunt?) regulator. Again, that's a guess. I'm a complete novice when it comes to regulators.
It appears that U1 and U2 may have been "upgraded" to OPA604 and AD711, respectively. If calibration is important maybe this is why it no longer nulls the hum? B+1 has 5mV of hum which is way too much for the two(!) gain stages it feeds. There ends up being 5mV hum on the final stage's output. I suspect the regulator circuit around U1 is not working and/or adjusted properly. B+2 (supplies the phono section) seems OK for now, but it's hard to tell with all the hum from the line stage.
The voltages on U1 seem OK. 24V on pin 7, 0V on pin 4. I can't measure pin 2 without affecting the measurement because it's set through such a high impedance voltage divider. It is close to the 12V it should be.
It's not a ground loop. Clipping in a 470uF cap across C14 reduces the hum by 2/3. I could just do this, but I'd prefer to have it working properly and as original.
Could someone offer some advice on how to proceed?
Thanks in advance.