[Sycraft]

I got my DMM back, so now I'm playing with the biasing on the amp. I've read the directions but I don't fully understand. So I'm measuring across R10 and seeing about .42 volts across the resistor on each channel. My confusion comes from the term "voltage drop". Does this imply negative voltage, meaning I should crank it up to around 1v to start with (since I'm using 1k R10s), or does this imply volts under 24v, in which case my amp is biased all to hell and the resistors are turned the wrong way?

Thanks.

 

[thrice]

Voltage is relative between two locations so the term "voltage drop" refers to the voltage between two points (i.e. the two leads of R10 in this case)

As far as the polarity is concerned when measuring across R10, that will depend on where you put your leads. You will get the same reading either way, one way it will be + and the other way - . that doesn't matter in this case.

On tangent's site he states the following:

"If R10 is 1K?, then each volt of drop across R10 equals one millivolt of current through the cascode, so it's easy to trim R9 to get a particular op-amp bias current. 1mA is a good starting value, but you should try other values to see if you can hear an improvment in the sound at different bias levels."

So in this case if R10 is 1K then each volt that you measure across it is equivalent to 1mA of current. So if you're getting .42 V across R10 then that's equivalent to .42mA. You would want to turn the R9 trimpot until you reached 1V. Or you could measure the mA across R10 and adjust it that way. Either way you get the same result.

That's how I understood it at least, but someone please correct me if I'm horribly wrong.

 

[Sycraft]

Yep, that's how it works. I sat down and tweaked it to 1.400 volts and sure enough, the current draw on the amp overall slightly increased.