Seventh Circle Audio

Hi Tim or anyone that can help. I am getting ready to seal up my chassis when I noticed my outputs volume of my j99's are different from each other. Both are set to -18 +18 but the volume is louder on my newer one that has the bias trim section. Any thoughts?

EDIT. My older j99 without the bias section is louder. Any thoughts.

anyone have any ideas? I want to close my chasis up

The latest rev is set up so gain matches the other modules more closely. I don't think you have a problem.

ok thanks what if I wanted to sum through it? Just have themes rev one louder?


Nigel tuffnel

You can easily trim the gain to match. You could also swap the old rev gain resistors to match the new ones.

sorry tim
I'm a hack. Which resistors are they? What procedure to chane the gain?

Hi Tim

If it's not too much trouble. Can you tell which resistors to change and what values to change them to?

Thanks in advance.

Does anyone know?

You can change R5 - R9 and R11 - R15 on rev 3.3 and 3.4 to match R8 - R11 and R18 - R22 on rev 3.6. If it's not clear how to do it from the schematic, it's probably not a good idea to try it yourself. Realize also that changing the gain resistors still won't make a matched pair, since the sum of R1 - R4 on rev 3.3 needs to equal the sum of R3 - R7 on rev 3.5. That total resistance sets the input impedance, which is 1200 ohms for rev 3.6 but slightly lower for older revs. R1 - R4 would have to be recalculated to match the higher impedance and still retain the 5dB steps.

What voltage/gain do you recommend I set the older rev J99 to? Will this affect the preamps performance/headroom?