High noise floor & low gain on one A12

[Jezza]

Hi Tim,

In my rack of 8 I have one A12 that behaves differently from the rest. The noise floor is much higher than the other 7 and it has less gain than the A12 next to it by a couple of clicks. The problem is not very noticeable on loud sources but it's terrible past gain 8, and I can't record stereo with my A12s.

I have exhausted my limited troubleshooting abilities so I need some pointers. Any help will be greatly appreciated. Thanks so much.

- Jezza


Here is what I can tell you:


Swapping the op-amps has no effect

All the solder joints look good to me

Visual comparison of the good & bad preamps reveals nothing to me, component placements all match, etc.

R23 & 24 are correct (since you asked in another A12 low gain thread)

There are a few dents in the leads where I bent them because I used tiny needle nose pliers to do it. It's not ideal, but I don't think it's a show-stopper.

When driven with a 1k tone:

Gain 6, trim fully clockwise, output on DAW meters:
Good A12 -3dB
Bad A12 -15dB !!!

Output voltage at gain 1 & 6, trim fully clockwise:
Good A12 1.04V & 19.1V
Bad A12 1.06V & 19.2V

Input transformers seem to have no troubles
Good A12
Primary (pins 1-4) .065V
Secondary (pins 5-8) 1.6V
Bad A12
Primary (pins 1-4) .055V
Secondary (pins 5-8) 1.6V

Output transformers are wired correctly and also seem to be fine
Good A12
Primary (brown-red) .633V
Secondary (orange-blue) 1.04V
Bad A12
Primary (brown-red) .638V
Secondary (orange-blue) 1.05V

[Jezza]

I'm sure you'll ask so I also double checked the DC voltages at the op-amp, and all is well. On both preamps I have +/- 18V on pins 3 & 5 and 0V on pin 6.

Thanks again in advance.

[tpryan]

Good A12
Primary (brown-red) .633V
Secondary (orange-blue) 1.04V
Bad A12
Primary (brown-red) .638V
Secondary (orange-blue) 1.05V

By this measurement, the "bad" A12's output is 0.1dB higher than the "good" A12. What do you measure at the output connector? Does the level change if the polarity switch is flipped? Can you measure continuity from T2, ORG/BLU, to CONN2, pin 2/3?

[Jezza]

The following measurements were taken at the output connector, pins 2 & 3, at two different gain levels:

Output voltage at gain 1 & 6, trim fully clockwise:
Good A12 1.04V & 19.1V
Bad A12 1.06V & 19.2V

This seems to indicate no problems, but the audible difference and DAW meter difference is still there.

Nothing changes with the polarity switches, R38 looks good, jumpers are good.

The continuity between T2 and CONN2 looks good, only 1.5ohm in both preamps for ORG/2 and BLU/3. (35ohm if I cross paths to ORG/3 and BLU/2, for what it's worth.)

The noisy/bad A12 was next to the power supply in chassis slot 1 so I swapped it out with the adjacent A12 in slot 2 with no change or result.

Another thorough inspection this morning seems to reveal that all the components are installed correctly. I can not find my error...

[tpryan]

Have you swapped cables between preamp and converters?

[Jezza]

Unbelievable!

Channel 1 in my A/D converter seems have become faulty right around the same time I finished the preamps. I naturally assumed that my SCA build was at fault since it was the newest and most error-prone piece of gear, but it seems like the HD box is to blame.

No wonder I couldn't find the problem! I ripped apart my preamp for nothing.

Thanks again Tim for the support and guidance, and sorry to take your time. Apparently all 8 preamps are just fine after all...

[Jezza]

This is totally off topic for an SCA forum at this point, but further testing this evening seems to indicate that the problem is in the PCI card in my computer, not in the interface itself.

Now THAT is a wild goose chase.