J99 - mismatched levels

[P_N]

Hey guys,

We've built several SCA pres and all are terrific and worked immediately. I am though having problems matching the outputs of the 2 J99s I just built.

The extreme biasing on SW1 works like a charm, but when I cycle through the switch positions expecting the voltage to drop to 0, it doesn't. I've redone the biasing twice (on both units, always with matched extremes) and while the 1st readjustment led to a closer output match, alas SW1 positions are one "click" apart and the trim is also (very slightly) adjusted to match the output.

What should be by next move?

Other than that the pres sound awesome.

PN

[tpryan]

What levels are you talking about, the audio or the DC offset?

[P_N]

I'm most concerned about the audio level. I've done a few test-sessions with the J99's paired with the overhead mics and though it sounds great, I'm worried that in a more controlled performance such as strings/solo instrument not having them matched will yield a poor result.

Like I mentioned above, I've built quite a few things - but like the shop teacher with 9 fingers - it could merely be an oversight which led me to this!

[P_N]

Also I should note that while the voltage during the SW1 "click exam" doesn't drop to ZERO, it does drop to 28 millivolts.

[tpryan]

The bias current compensation has no effect on gain. If the modules really are mismatched by what seems like one switch step, the first thing to check are feedback resistors R12 and R28 and trim control R24. But first, you should make some measurements with a test signal and meter to determine the exact difference in the output levels.