trouble adjusting J99's

[tazman]

Tim - we are now measuring at U1/U2. As I have been saying for days, the voltages move in parallell, thus they never get closer to each other. This is happening on both J99's. So either I built them both somehow incorrectly or there is something wrong with either the kits or the pcb's. And both J99's hum like crazy when connected and only at certain gain settings and when the other pres are turned up as well.

[tpryan]

Send them in, we'll get them working.

[dandeurloo]

Tim I'm back at trying to bias my J99's. I was using the stock opamps and they sounded ok but between my pair of J99's they where a whole gain stage off drom each other so it was hard to use with my folcrom or for overheads. So I have replaced them with jenson op amps. I was successful this time around biasing the first one but the second has a few issues.

While biasing I noticed a crack down the top of sw2. And not all of the gain stages really click into place they are a little smoother then normal. So I tried biasing and I got about 6 mv away and the headroom started to click or top out on R50. So I am wondering if the crack on the switch is causeing a problem with biasing or if I need to replace r50 with a stronger value in order to get it closer like I have been able to do on my other set.


dan deurloo
612.799.0396

[tpryan]

but between my pair of J99's they where a whole gain stage off drom each other

These are modules that we worked on? The gain is set by resistor values and has nothing to do with the op-amps themselves. You need to determine if the gain error is consistent at every step or is only present at certain steps.

As for adjusting the bias current compensation, if 6mV of difference is as close as you can get, you're done.

[dandeurloo]

What about the switch being broken. It still works it just doesnt click into each step tightly? Can I order one of those from you or can I buy it somewhere else.

As fars as the 6mv my meter reads 6.30 mv with the that should be close enough. So why then would the two be a full click of from each other in gain? Broken switch is my guess?

Also how would I go about checking each step. are they in 10 db steps?

[tpryan]

Send an e-mail to sales@seventhcircleaudio.com and we'll fix you up with a replacement switch.