A12 Assembled, but no sound...

[tpryan]

That looks pretty good. Did you install the insulator under T1? What resistance do you measure between pins 1 and 4 of T1? Between pins 5 and 8?

[Todzilla]

That looks pretty good.

Thanks. Don't tell me you weren't terrified to see what mess I had made, given the idiocy of my previous questions.

Did you install the insulator under T1?

Yep.

What resistance do you measure between pins 1 and 4 of T1?

Didn't see that in the instructions. I'll measure (sans OpAmp, right?) and report.

Between pins 5 and 8?

Didn't see that in the instructions. I'll measure (sans OpAmp, right?) and report.

Thanks for sticking with me on this!

[Todzilla]

Okay, resistance is as follows:

T1 Pins 1 to 4 - .9 MOhms @ 200M setting of DMM - I guess that's 900 Ohms?

T1 Pins 5 to 8 - same measurement

I do have a complete unassembled kit, so I have a reference, if needed.

[tpryan]

Try making the same measurements on your uninstalled transformer. If your meter is not auto-ranging, start on the low resistance ranges and work up until you get a reading.

[Todzilla]

Both spare transformer and installed transformer measure as follows:

T1, Pin 1 to Pin 4 = .03 K Ohms @20K measurement
T1, Pin 5 to Pin 8 = 2.8 K Ohms @ 200 K measurement

My DMM does not do auto-ranging

[tpryan]

OK, so far, so good. Next, and always with power off:

Measure the resistance between pins 2 and 3 of the input XLR, CONN1. The measurement should be just about the same as between pins 1 and 4 of T1.

Measure the resistance from U1, pin 6 to J7, pin 2, or any other convenient PGND point. Then measure the resistance between the BRN and RED leads of T2. The measurement should be the same.

Remove the jumper at J5. Measure the resistance between pins 2 and 3 of the output XLR, CONN2. Then measure the resistance between the ORG and BLU leads of T2. The measurement should be the same, and about twice what you got in the previous step.

Replace the jumper at J5 and measure at pins 2 and 3 again. What do you get now?

Let us know what you find.

[Todzilla]

Measure the resistance between pins 2 and 3 of the input XLR, CONN1.

38.9 Ohms @ 200

The measurement should be just about the same as between pins 1 and 4 of T1.

38.9 Ohms @ 200

Measure the resistance from U1, pin 6 to J7, pin 2, or any other convenient PGND point.

0.7 Ohms @ 200

Then measure the resistance between the BRN and RED leads of T2. The measurement should be the same.

19 Ohms @ 200

Remove the jumper at J5. Measure the resistance between pins 2 and 3 of the output XLR, CONN2.

36.8 Ohms @ 200

Then measure the resistance between the ORG and BLU leads of T2. The measurement should be the same, and about twice what you got in the previous step.

36.8 Ohms @ 200

Replace the jumper at J5 and measure at pins 2 and 3 again. What do you get now?

36.8 Ohms @ 200

Sounds like we're getting somewhere with discrepancies between U1, P6 to J7, P2 and T1 BRN to T1 RED, as well as the jumpered settings.

[tpryan]

OK, pull the op-amp out and measure resistance on the board between U1, pin 6 and U1, pin 4. You should get the same number as when measuring between RED and BRN at T2. If you don't, re-flow the solder joints at T2, L1, L2, and the pin sockets at U1.

Measure the op-amp out of the circuit. What's the resistance between pins 6 and 3, 6 and 4, and 6 and 5? Take each measurement with the red lead on pin 6, then again with the black lead on pin 6. Let us know what you find.

[Todzilla]

OK, pull the op-amp out and measure resistance on the board between U1, pin 6 and U1, pin 4.

19 Ohms @ 200 Ohms DMM range

You should get the same number as when measuring between RED and BRN at T2.

Yep, 19 Ohms @ 200 Ohms DMM range

Measure the op-amp out of the circuit. What's the resistance between pins 6 and 3, 6 and 4, and 6 and 5? Take each measurement with the red lead on pin 6, then again with the black lead on pin 6. Let us know what you find

OP AMP:
Red on 6, Black on 3 - 1.2 MOhms @ 200M DMM range
Red on 6, Black on 4 - couldn't get a steady reading, DMM jumped all around
Red on 6, Black on 5 - 2.6 MOhms @ 200M DMM range
Black on 6, Red on 3 - 2.4 MOhms @ 200M DMM range
Black on 6, Red on 4 - couldn't get a steady reading, DMM jumped all around
Black on 6, Red on 5 - 1.1 MOhms @ 200M DMM range

Just for good measure, I confirmed these Op Amp readings with the second one I have for the kit yet to build.

Thanks for hanging in there with me. Sorry it took a while to reply. I was out of town for a bit.

[tpryan]

Sounds like it should work. Have you tried the simplest possible signal path, say, SM57->A12->amp->headphones, without anything else connected? Are you certain your cabling is good?

[Todzilla]

Sounds like it should work. Have you tried the simplest possible signal path, say, SM57->A12->amp->headphones, without anything else connected? Are you certain your cabling is good?

Yes. I mimiced the setup from my Demeter HM1 preamp, same mic, same cabling same everything. But with the A12 in the preamp chain, it doesn't work. I did it with a dynamic mic, no phantom and with my Neumann U-89 with phantom. Same thing- Demeter works like a champ, A12 not so much.

I also carefully turned all the A12 knobs fully clockwise and I could hear hiss coming through.

What are our next steps? I know your 2006 support policy frowns on it, but would consider taking a look at it for your hourly rate?

Alternatively, my local audio electronics technician could review it, with the standard instructions and schematic.

Your thoughts?

[tpryan]

Send it in, I'm curious to see it. Your tech can probably find the problem in 5 mins with a scope, but if it's a bad transformer or op-amp he won't be able to do much about it. Send an e-mail to rma@seventhcircleaudio.com for return info.

[Todzilla]

Just wanted to report that my A12's quick trip to Oakland solved my minor, yet evasive issue. Tim found a distressed spot on the board, due no doubt to my "overzealous" attempt to desolder a capacitor I had hastily soldered in the wrong spot. I tried it out in my studio and the A12 sounds GLORIOUS. I know I'm biased, but I think it sounds waaaay better than my next-door neighbor's API 512 which he's let me borrow from time to time. Funny that.

Lessons learned:

1) Desoldering takes waaay more time than soldering. Meticulousness most of the time doesn't cut it. Triple/Quadruple check each step.

2) Desoldering is a more precise skill than soldering. I should have practiced my technique.

3) Work on your preamp project before drinking beers, not after.

[Todzilla]

Just built my second A12. Worked perfectly first try, this time.

Just ordered an N72, emboldened by my success.

BTW, the pair of A12s sounds incredibly punchy on drums. Can't wait to start putting everything through my SCA gear. My poor ol' Demeter preamp is very jealous.