A12 hiss and screeching noise

[Delicieuxz]

Hi, so you may recall me from my "Blinking LED" post in the chassis forum - where my issue was that the A12 was shorting "against the chassis divider due to improperly trimmed component leads."

Well the N72 I built works perfectly, and that's great, but the A12 emits a loud hiss when in use, and if the gain is cranked even one notch up from 1 all sorts of screeches and noises flare up, as though demons were escaping though the preamp from the bowels of hell.

Sooo, I'm wondering... any ideas? Does this have anything to do with that shorting? That'd be a real bummer. Well, I guess it's a bummer anyways.

[tpryan]

Go back to step 32 and measure the supply voltages again. Also measure the voltage at pin 6 of the op-amp. Let us know what you find.

[Delicieuxz]

Ok, I was finally able to get back to working on this, and what I've found is that with the API hooked up to the PSU, but the op-amp not yet installed, between pins 1 & 2 of J7, as well as pins 2 & 3, I get a reading of 30.

This is the step that says "Just to make sure you haven't created any blatant shorts, measure the resistance
between pins 1 and 2 of J7. Do the same for pins 3 and 2. You should measure a very high resistance. If you measure a steady resistance under 100 ohms, don't apply power. Carefully check your work until you find that short."


I hope I'm measuring the pins under the right conditions - hooked up to the PSU, without the op-amp.
I don't know if this reading is affected by having already adjusted R25 and R29 for working op-amp levels.

[Delicieuxz]

Help?

[tpryan]

Verify that the DC voltages are correct at the op-amp pins. Also measure the DC voltage at the op-amp output. If the supply voltages are correct and DC offset is close to 0v then it's possible the op-amp has failed. Let us know what you find.

[Delicieuxz]

I measured Pin 4 & 3 of U1, and read 14v.
Then I measured Pins 4 & 5 and read 6v.
Then I put the op-amp in and measured the output, which was about 0v.
Then I took the op-amp out again and remeasured Pin 4 & 3 and got 18v.


These voltages were fine the first time I measured them, before I shorted the card against the metal divider in the case. The pieces that touched the metal divider are U2 and U3.

[tpryan]

As you can see from the schematic, the positive supply is connected to U1, pin 3 and the negative supply is connected to U1, pin 5. Measuring with respect to AGND, which is at U1, pin 4, you should measure +18V at pin 3 and -18V at pin 5. If the voltages are fine with the op-amp removed but change dramatically when the op-amp is installed, the op-amp has probably been damaged. If the voltages are wrong with the op-amp removed one or both of the regulators has probably been damaged.

[redstudio]

what might cause this? From not wiring it up and shorting a piece?

[Delicieuxz]

I didn't cut some of the pieces that stick out underneath the board short enough, and so when the card was put into the case they touched against the metal divider and shorted.

At least I think that's what it is.

tpryan, I sent you a pm over a week ago asking how much it costs to get replacement regulators.

[tpryan]

You should send parts inquiries to sales@seventhcircleaudio.com. You can also order replacement parts directly from the distributors listed on the BOM. You may get them faster and cheaper that way.