AMB Laboratories DIY Audio

[ntropic]

Ok, so if everything's supposedly working, then we're back to the part where I read no voltage across R23/R47, no matter what position the trimpots are at and that there's no measurable DC offset voltage. Am I supposed to load the amp, or provide a source?

[amb]

No load should be necessary. Are you setting your DMM to DC mV range? If not, you're not going to be able read the small voltages accurately.
You could also check the voltage across C7. As you turn the trimpot adjustment screw clockwise, the voltage should increase.
Lastly, make sure your trimpots are the correct value. In the photo you posted it appears to say "201" which implies 200 ohms, when it should be a 1K ohm trimpot.

[ntropic]

Good thing it does too. After all, the part number is 3296W-102LF-ND. The resistance only goes up to about 860 in any case. I'm sure the trimpot works, since when I measure the voltage on R17, I can watch it change in real time. My DMM is autoranging, but I'll see if manually setting it has any effect.

[ntropic]

Update: So I decided to hook up a source and see what'd happen when I turned it on. Good news? It passes sound, and the potentiometer works. Both channels output sound as well. Bad news? I can't get past more than a quarter turn of the pot before distortion goes straight up. It'd seem anything over a certain loudness would cause the cheap earbuds included with PMPs to distort quite heavily. I wasn't interested in trying this out on a different pair of headphones. I fiddled with the trimpots (no more than a turn or two), but it didn't seem to have any effect. Any ideas?

[amb]

If the output stage is under-biased then you'll have distortion, especially in an amp with no global NFB like this one. Sounds like you simply haven't biased it up enough.

The Bourns 3296 trimpots you have are 25-turn units. If you only turned it two turns then it's not much of a change. Don't be afraid to crank it up as long as you're watching the voltage across the output resistors, and the output transistors' temperature. Follow the "initial setup" procedure found at the CK²III
website.

[ntropic]

Yeah, I'd love to watch the voltage across those resistors, but like I've said, I simply read nothing when I measure voltage across them. I've turned the pots all the way up before (until they start clicking at the max clockwise position), and I still read nothing. The output transistors don't even begin to warm up their heatsinks no matter how long I wait. I know the trimpot works though because C7/C16 and the resistors next to them have measurable DCV changes when I start messing with the trimpot.

Thanks for the help so far. Hopefully fiddling with the trimpots will help. At this point, all I have to watch out for is making sure the temperature doesn't destroy those resistors and transistors, right?

[amb]

Assuming the circuit is working, turning up the trimpot will increase the output transistors' quiescent current, and that should warm them up. When you measure the voltage across the output resistors, use the DC mV range. Also, try a different DMM In case there is something wrong with the current one.

[ntropic]

Well, I tried turning the trimpots up. The sound got a little louder in the process, but even at the max setting on the trimpot I could get distortion quite easily.

[amb]

What's the voltage across the output resistors at max trimpot setting?

[ntropic]

0V, apparently. Maybe millivolts don't tickle my multimeter. I don't suppose it'd be worth the effort to remove the heatsinks, would it?