Parts for AD797 vs. 990C

[markmazoo]

With the new option of the AD797 op amps, I'm having a
difficult time deciphering which parts to use and which to
exclude when comparing the "instructions" to the "BOM" and
to the "schematics". Unfortunately, I'm not experienced at
reading schematics and the sections that appear separate
from the main J99; like the R45 and R50 sections and the
U8 and U9 sections, confuse me. The photos of the PCB and
the actual PCB I received have several differences.

I will be using the 990C op amps. Do I need to install
C23, C28, C29, C30 and C31 (these are not mentioned in the
instructions but there are places for them on the PCB) and
the associated resistors R41, R42, R43, R44, R46, R47, R48,
and R49 (the instructions say to insert the 1/4 watt
resistors), but I read on the forum not to install R27 and
R39 as well as C21 and C22, and I'm wondering if R41
through R49 should be excluded too. I just can't tell if
some of these parts are needed only for the AD797s and some
only for the 990Cs.

Also, do J6 and J8 and R45 and R50 need to be installed?

The instructions explain how to adjust R28 and R32, but
not how to adust R45 and R50, and don't seem to mention J6
and J8 at all.

Thanks for any clarification you can give.

[markmazoo]

Hi Please take a minute and help me get back on track.
Thanks.

[tpryan]

will be using the 990C op amps. Do I need to install
C23, C28, C29, C30 and C31 (these are not mentioned in the
instructions but there are places for them on the PCB) and
the associated resistors R41, R42, R43, R44, R46, R47, R48,
and R49

Yes.

Also, do J6 and J8 and R45 and R50 need to be installed?

Yes.

The instructions explain how to adjust R28 and R32, but
not how to adust R45 and R50, and don't seem to mention J6
and J8 at all.

Please read the sticky at the top of the J99 forum carefully. If you still have questions, let us know.

[markmazoo]

Thanks Tim

I read the Sticky again.

One question; where it says:

To adjust the input bias current

Do those instructions apply to both the AD797 and the 990C?

[tpryan]

Do those instructions apply to both the AD797 and the 990C?

Yes, they apply for any op-amps with bipolar input transistors.

[markmazoo]

Followed all the above instructions and when I plugged my mic in, the damn thing wouldn't work. The instruction clarifications by Tim were invaluable to me. I was just uncertain enough that I didn't want to make a dumb mistake, if I could avoid it.

The reason the J99 wouldn't work was because I received a "bad" 990C op-amp. Tim diagnosed the problem in seconds. I sent the op-amp back to John Hardy. Got the replacement back a couple of days ago, plugged it in today and the J99 sounds great. I want to build another one.