N72, A12 noise, dead pilot light

[vinylvalet]

My pilot light assembly went dead after about 20 hours of operation. Could you provide a mouser or digikey part number for that assembly?

My chassis has two N72, two A12 and one C84. With my board at maximum gain, I get the same type and amplitude of hum from each N72 and A12. The C84 is dead quite. I searched, read and tried all the noise debug suggestions from all the forums. With only one A12 or N72 in the chassis there is no noise. Adding the C84 has no impact. As I add more A12 or N72 modules, the noise increases. All inputs properly terminated, tried modules in different positions, all holes sanded, bolts tight, chassis moved away from everything else, cables check out, etc. Gain and trim do not affect the amplitude. I have the PS03 supply.

Any other ideas? Thanks.

[BerkeSound]

Unfortunately, I can't help with the noise problem as mine hasn't been doing that (2 n72, 1 a12). However, my pilot lamp also burned out almost immediately. Basically, Tim told me to pull the assembly apart (tough but not impossible), remove all the glue, and replace the LED and resistor within. This has worked for me (for all of an hour now).

Here's the topic if you're interested:
[url]
http://seventhcirclestudios.com/SCA/php ... php?t=1673[/url]

[vinylvalet]

Thanks BerkeSound. I just found the CH01 BOM. I can only find a L1501-24R at Newark for about $13 each. Found the Imlec website. The 1502 is still in production but none of the usual suspects (mouser, digikey, newark, jameco) stock these. Since these are underrated for the application and seem to burn up regularly, it would be nice if Tim kept a stock to sell to his customers. Thanks again.

[tpryan]

Since these are underrated for the application and seem to burn up regularly, it would be nice if Tim kept a stock to sell to his customers.

We've sold over a thousand of these lamps and your two are the only ones I can recall failing, though there have been a few that had incorrectly marked polarity. They're just LEDs with an internal dropping resistor, and R19 on the PS03 provides the additional current limiting for use with a 30V supply. If you can't find a suitable replacement or don't care to repair it yourself, we can sell you a complete lamp and connector assembly for $16.

[vinylvalet]

I still have not solved the noise issue. I saw another poster was having the same problem. The solution suggested was to upgrade from a PS02 to PS03. There was no indication if this worked or not. I have a PS03 (ordered 6/14/09) which I believe is the latest revision.

I took the preamps to another location and hooked it up to a different system. Same problem but louder. C84 is still completely quiet, both N72s and A12s have the same hum. Lifted some or all safety grounds, no change.

[vinylvalet]

Thanks Tim. I'll place an order for the pilot lamp shortly.

[vinylvalet]

I tested all the connections on the wiring harness, everything okay. All three ps voltages measure okay. Noise (ref chassis ground) on all three supplies about 40mV, high frequency. With inputs shorted, all five modules connected to the ps, the output noise from the A12 and N72 modules is 160Hz, kind of looks like a noisy sine wave with the negative swing clipped.

With only the C84 and one of the A12s or N72s connected to the ps, there is no noise. When I connect a second A12 or N72, the noise appears, same 160Hz, same shape, lower amplitude than all four A12 & N72 connected. When I connect the third module, the amplitude doubles, same frequency but the polarity flips. When I connect the fourth module, same frequency, polarity flips but the amplitude only increase a little.

The C84 is completely quiet. It doesn't seem to affect the other modules whether it's connected to the power supply or not.

The position of the A12 and N72 modules in the chassis doesn't have any impact on this issue.

[vinylvalet]

I measured the power supply voltages, loaded (all five modules connected) and unloaded.
Loaded: 47.1, -30.5, 29.97
Unloaded: 47.1, -31.43, 30.16

The wiring harness is neat and tied. Moving these wires around makes no difference in the noise.

I'm 100% certain the modules are assembled correctly. Other than the noise, they function properly.

I'm out of ideas. With trim and gain turned all the way down and whatever system you are driving turned up, does anyone else see this noise? Thanks.

[vinylvalet]

Could this be an oscillation? Maybe there is a similarity between the A12 and N72 modules where a similar inadvertent tank circuit is formed on each module? Once one goes off it excites the others. It's a long shot since, if true, others would be experiencing this problem as well. It's just the weird frequency makes no sense otherwise. If the hum was 60 or 120hz it would be a different story, much easier to troubleshoot. But then, I have very little knowledge of switching supplies.

[tpryan]

I don't think anyone's seen this symptom because not many people use their preamps as you're describing, running the output into a console and adding another 60dB or so of gain. We measure an EIN of -129dB for the A12 and -126dB for the N72. If you'd like us to have a look at the PS03, send an e-mail to rma@seventhcircleaudio.com and we'll provide return instructions.

[vinylvalet]

I'm running into line, not mic, inputs of a control surface. The hum is apparent well before max gain. I'm sure you spec your modules individually. I don't have a problem until I add two or more. With two or more A12 or N72 modules in a CH01/PSO3, inputs shorted, gain and trim all the way down, output gained way up, do you see this 160Hz signal at your facility? If that's the case, I'll move on and figure out a work around. I'm also curious if you see this with the J99 as I was planning to purchase two of these and another C84 to complete my rack.

Thanks for your offer to look at the PS03 but I'm fairly certain it's not the PS. It's time to get out the DVM, scope and dig in unless you confirm that what I'm seeing is normal.

Thanks for your reply.

[vinylvalet]

I had a bit of a breakthrough this morning. I decided to try powering the five modules in every possible combination and measuring the 160Hz hum.

Summary: When I power either just the A12 pair or just the N72 pair, no hum. If I add the C84 to either pair, the extremely low hiss actually deceases a little. If I power any type of N72, A12 combo, the hum appears.

[tpryan]

Check your PS03 and make sure that R11 is 330 and R35 is 18K. If they're not, changing them to the values above may show some improvement.

[vinylvalet]

Thanks Tim. My PS03 is rev 1.4, can't find R11 or R35.

[tpryan]

Try connecting a jumper between the PGND and AGND terminals at the rightmost module or the last MOLEX connector on the wiring harness. Adding a jumper at R16 on the PS03 may make an improvement as well.