It can only be either C3 with a short across it (or leaky) or a physical short on the board.
So that is great news really... because there has to be a simple cause.
So that is great news really... because there has to be a simple cause.
Mooly, you continue to give me hope! I was just checking this thread before shutting down for the night, so I'll take a closer look for any shorts tomorrow and grab some pictures near C3 if that might help.
Meter to DC volts, black probe to ground (use the buss wire on the back panel)
Red probe on…
The side of R10 nearest to Q3 to get the voltage circled near C3
The other side of R10 (near the pot) to get the R10 voltage
The leg of R9 near Q4 to get the R9 voltage
Any of the legs in the middle where the 4 large resistors come together will give the C1 input voltage (on the right of Mooly’s diagram)
Red probe on…
The side of R10 nearest to Q3 to get the voltage circled near C3
The other side of R10 (near the pot) to get the R10 voltage
The leg of R9 near Q4 to get the R9 voltage
Any of the legs in the middle where the 4 large resistors come together will give the C1 input voltage (on the right of Mooly’s diagram)
Well, I went back and checked for any possible indications of a poor solder or short and… maybe progress?
4 of 5 values in the diagram now check out. The only one that doesn’t is on C3 which is reading only ~9V (it fluctuates a bit, but it’s also tough to get my probe in the gap I left, unfortunately). There’s also still voltage across R10, but it’s now dropped to ~0.6V also fluctuating as I measure it.
So is C3 the culprit?
It might all be OK now...
The two critical measurements that prove its working are:
1/ The midpoint voltage at the junction of the 0.68 and 0.47 ohm resistors (this should adjust to half the supply with the preset). Drift with temperature is fine but it should not be going up and down randomly here. So make sure this is OK,
2/ The voltage across those 0.68 and 0.47 ohm resistors. These should be around 0.37 volts across each 0.47 ohm and around 0.54 volts across each 0.68 ohm. The values aren't an absolute but should be around that mark. Again a bit of drift with temperature is normal.
If those are OK then all is well and we can blame the C3 reading on the meter probe, the meter loading and variations in probe contact resistance. C3 is a 'high impedance' point and so any change in loading by the meter will pull and push the result around.
The two critical measurements that prove its working are:
1/ The midpoint voltage at the junction of the 0.68 and 0.47 ohm resistors (this should adjust to half the supply with the preset). Drift with temperature is fine but it should not be going up and down randomly here. So make sure this is OK,
2/ The voltage across those 0.68 and 0.47 ohm resistors. These should be around 0.37 volts across each 0.47 ohm and around 0.54 volts across each 0.68 ohm. The values aren't an absolute but should be around that mark. Again a bit of drift with temperature is normal.
If those are OK then all is well and we can blame the C3 reading on the meter probe, the meter loading and variations in probe contact resistance. C3 is a 'high impedance' point and so any change in loading by the meter will pull and push the result around.
GREAT GOOGLY-MOOLY IT'S WORKING!!! 
Mooly & 6L6, I can't emphasize enough how much I appreciate your patience, assistance, and detailed troubleshooting. There is no way I would have gotten this running without your help. Thank you!!
Over time, I'm hoping to build a system around a second ACA and a pair of Zu DW6, so I'll have to keep you posted on how that goes as the space and components come together. Right now this single ACA is powering a pair of ELAC Debut 2.0 B6.2, which sound pretty great already, so I'm looking forward to what comes next. I'm already noticing more richness and low-end detail than the 10+ year-old budget Onkyo home theater receiver the ACA is replacing.
Mooly & 6L6, I can't emphasize enough how much I appreciate your patience, assistance, and detailed troubleshooting. There is no way I would have gotten this running without your help. Thank you!!
Over time, I'm hoping to build a system around a second ACA and a pair of Zu DW6, so I'll have to keep you posted on how that goes as the space and components come together. Right now this single ACA is powering a pair of ELAC Debut 2.0 B6.2, which sound pretty great already, so I'm looking forward to what comes next. I'm already noticing more richness and low-end detail than the 10+ year-old budget Onkyo home theater receiver the ACA is replacing.
Well, I may have spoken a little too soon. The music is flowing, but the Rice Krispies mascots seem to be living somewhere inside the left channel.
Even with my preamp, a Schiit Vali 2++, powered off, I'm getting crackle in the left channel, but not the right. Half win?! It does get worse when the preamp is on.
I've really thoroughly checked the board/soldering in the left channel and don't see anything, though I've clearly been wrong about that before. Is there anything else in particular I should be looking for when I review the wiring and search for a cause this weekend? I wasn't getting this with my old amp, so I don't suspect the speaker itself.
Even with my preamp, a Schiit Vali 2++, powered off, I'm getting crackle in the left channel, but not the right. Half win?! It does get worse when the preamp is on.
I've really thoroughly checked the board/soldering in the left channel and don't see anything, though I've clearly been wrong about that before. Is there anything else in particular I should be looking for when I review the wiring and search for a cause this weekend? I wasn't getting this with my old amp, so I don't suspect the speaker itself.
First step is to try it with no input leads connected (or swap left and right input leads around) just to be sure. I can't see how the preamp being on would affect things assuming you are testing with no signal applied.
Try that first, lets get a base line on what's going on.
Although you don't suspect the speaker you should at least just swap left and right speaker leads around just to 100% eliminate things.
Try that first, lets get a base line on what's going on.
Although you don't suspect the speaker you should at least just swap left and right speaker leads around just to 100% eliminate things.
Well I tried a few things over the weekend, cleaned up just about every solder point on both boards, and I've still got issues.
First of all, the static is still present in the left channel. If I switch the speaker wire from the right channel posts over to the left channel binding posts, it's also heard from the right speaker with or without RCA input connected. It doesn't seem to have changed after the soldering cleanup pass.
When listening, I mostly notice two things: there's a volume imbalance (R is louder) and missing treble detail. If I put my ear to the speaker (even with no input connected) I hear the static coming distinctly from the tweeter. It seems like I'm losing the treble detail to the static. Is that possible?
I watched this test video and it made it very easy to notice the level imbalance and following the sweep up the frequencies was helpful in confirming that the issue seems mainly to be in the Left channel treble. Any ideas given all that?
First of all, the static is still present in the left channel. If I switch the speaker wire from the right channel posts over to the left channel binding posts, it's also heard from the right speaker with or without RCA input connected. It doesn't seem to have changed after the soldering cleanup pass.
When listening, I mostly notice two things: there's a volume imbalance (R is louder) and missing treble detail. If I put my ear to the speaker (even with no input connected) I hear the static coming distinctly from the tweeter. It seems like I'm losing the treble detail to the static. Is that possible?
I watched this test video and it made it very easy to notice the level imbalance and following the sweep up the frequencies was helpful in confirming that the issue seems mainly to be in the Left channel treble. Any ideas given all that?
Concentrate on the static.
As far as how loud one channel vs the other is, well normally the only parts that determine that are the two resistors in the feedback network which are R11 and R12. The ratio of one to the other sets the gain.
The treble is a mystery because nothing in the circuitry could really reduce treble content. The caps come into play for audio signals but they can only reduce bass (if something is wrong) and not reduce treble.
Without seeing this I'm going to suggest you either replace (or swap over) the two input FET's, more to eliminate them as much as anything else.
Are the DC conditions still correct? with the midpoint around 12 volts and the FET's hot and with around 0.37 volts across the 0.47 ohm and 0.54v across the 0.68 ohm resistors.
As far as how loud one channel vs the other is, well normally the only parts that determine that are the two resistors in the feedback network which are R11 and R12. The ratio of one to the other sets the gain.
The treble is a mystery because nothing in the circuitry could really reduce treble content. The caps come into play for audio signals but they can only reduce bass (if something is wrong) and not reduce treble.
Without seeing this I'm going to suggest you either replace (or swap over) the two input FET's, more to eliminate them as much as anything else.
Are the DC conditions still correct? with the midpoint around 12 volts and the FET's hot and with around 0.37 volts across the 0.47 ohm and 0.54v across the 0.68 ohm resistors.
| Left (V) | Right (V) | |
| R1 | 0.312 | 0.320 |
| R2 | 0.313 | 0.320 |
| R3 | 0.452 | 0.464 |
| R4 | 0.452 | 0.464 |
| Q1 Middle | 11.85 | 12.03 |
So this is weird, right? Neither side checks to those values. This is after things have been on and in use for the better part of a day. Heatsinks were around 46*C and Q1 & Q2 ~56*C on both sides, no temp difference across channels anymore.