Seventh Circle Audio

Hi there,

I just completed my two T15 units and they work so beautifully with dynamic microphones (clean, beautiful tone, no noise of any sort). Problem comes when I use a condenser mic - when I turn the phantom power all I get is a hum. When I went back to the dynamic once again I had some great signal.

What did I do wrong?

By the way, I did not mention that I followed the instructions word by word (with the exception of the apparent discrepancies) and am quite confident in my work. I am not saying that it is impossible to have made a mistake, but I think it is worth mentioning that I triple checked everything and all looks correct. Also, both of the modules do exactly the same thing... hmm.. :-/

I have reasons to believe that the instructions could be the culprit - there are more than a few differences and inaccuracies, the kits I received are a different revision.. and again - both units have the exact same symptoms.

I am a bit of a neat freak, my soldering is very clean... hmm. I am about to repeat the testing and measuring, if I discover anything I will post it right here.

Okay, here is what the tests show:

1. When I measure the resistance between pins 1 and 2 of J3 I get:

Unit A: 81 mO
Unit B: .74 mO

2. When I measure the resistance between pins 2 and 3 of J3 I get:

Unit A: 4.5 mO
Unit B: 4.7 mO

3. When I push power through the units and measure DC between PGND and + 18V pad I get:

Unit A: 18.25 V
Unit B: 18.24 V

4. When I push power through the units and measure DC between PGND and -18V pad I get:

Unit A: -18.5 V
Unit B: -18.45 V

5. When I connect PGND and pins 2 or 3 on both units I get:

Switch on: 46.5 V
Switch off: 0.4V


According to the instructions this should be good, right...?

Ideas?

Do you have a jumper installed at J1 as shown in step 10?

Yes sir, I have jumpers both on J1 and J2. Both of them placed at pins 2 and 3 as the instructions indicate. You think that could be the issue?

Can you post a photo of the component side of the board that shows the orientation of all the capacitors?

Absolutely!

I am not sure how to post photos right on this thread, that is why I am going to throw this link at ya - https://picasaweb.google.com/hachaduryan/T15#

Currently uploading, should be all there in a coulpe of minutes. Hope that helps.

By the way, I just want you to know that I deeply appreciate you spending time and helping me out with this. All my interactions with you guys have been wonderful! Thank you!![/img]

Are you absolutely sure that your mic cables are good? Can you measure +47V from pins 2 and 3 to pin 1 at the mic end of the cable?

I will perform a test to check that, but I am 98% sure that my cables are good (almost brand new Monster cables that have been tested and used by me for about a month). Of course, one can never now...

Will post an update in a couple of hours with the result.

Okay, I guess you were right. I did manage to get clean audio signal with different combination of cables. The phantom power needs about 13 seconds to get fully started, but that doesn't really bother me that much.

What bothers me is that one of my modules got fried. I stupidly forgot to power down the unit while connecting J3 and saw a spark. Now it simply won't pass audio... I did all the post assembly measurements and all looks good, but won't work neither with dynamic nor condenser microphones. I do hear a pop when I switch the phase and there is some slight noise when I play with the trim and gain, but that's as far as it'll go. I really wanna kill myself now (well, not really, but pretty darn close)

Any suggestions how to identify what got crapped out?

The phantom power needs about 13 seconds to get fully started

Right, that's by design.

I did all the post assembly measurements and all looks good, but won't work neither with dynamic nor condenser microphones.

What voltages do you measure at

U1, pin 6
U2, pin 7
U2, pin 1
U4, pin 6

Let us know what you find.

U1 pin 6 = 0.6mV
U2 pin 7 = 3.3mV
U2 pin 1 = 18.49V
U4 pin 6 = -0.2mV

Make sure that D1-D4 are not shorted.
Make sure R6, R7, R8, and R9 are not open.
If everything looks good and still no signal, replace U1.

Thank you! I will do that and I will let you know, but that will be next week, gotta go out of town. I appreciate your help!