312 troubleshooting -- loose ground?

[oobedoob]

I've been chasing down some noises in one of my 312-ish amps lately...

Jensen JT-110 -> SPA690 -> profile 4804
standard API 312 schematic topology/values

Given known good decoupling caps and power inlet diodes, and assumed good amps, what kinds of things can cause 60Hz noise at about -75dB?

The debug story so far:

Have two of these amps, one had a crackling, intermittent noise, and a much higher noise floor than the other, so I put the bad one on the bench and measured AC on all of the rails (about +/- 5V on powers and gnd). At that point I shotgunned the decoupling and diodes and measured again. Noise was still there. Replaced the 250u cap to gnd from the gain trim (with a crappy one I had in a parts bin), and the AC went away on the scope. (I also changed out the transformer termination to better match the Jensen datasheet).

Unfortunately,

The AC is still there, albeit at -75dB. I recorded both amps with no input (should I have shorted the input?), and amplified up the noise in the DAW until I could see/hear it well, and the 60Hz was there clear as day. The "good" amp had no 60 cycle component in the noise that I could see/hear. The only two ideas I have now are that I put in a really crap 250u cap, or that I've got a bad solder joint on the opamp or ground somewhere.

Any ideas on removing the remaining AC? Anything I've missed in my initial troubleshooting? If there's already a thread on a similar problem, I couldn't find it. Sorry if this is a duplicate....

thanks.

 

[Rob Flinn]

This is most likely to be an earth hum. How have you connected your earths within the unit?

The best way to wire this up is the star earth method:

Essentially you want your mains earth connected to the chassis at a point close to the input socket/s. This is your star earth point. The zero volt rail from the PSU should also connect at this point. Earths to the pcb`s should also connect here, thus connecting to zero volts via this star earth point.

The in & out sockets earth should connect here also as long as they are not earthed through the pcb connections.

 

[oobedoob]

thx Rob

I ruled out system grounding as the problem early on, because the problem was definitely following the card with the faulty cap. It's a power supply/scheme I use all over the place and it never hums. I will swap cards and record again and see if the problem still follows the original card.

The amps are from a commercial outfit, but I find the overall build quality to be a little iffy. The only thing I suspect at the board level is a few traces that could have accidentally had a few mils gouged out of them. Thats my project for this evening--put some solder in a few questionable places....

will also replace the junk 250u cap I put in there with something better.

any other sources for 60Hz at the card level that anyone can think of?

 

[oobedoob]

finally found the source of the 60Hz hum....

the output transformer core was grounded. 2 mounting screws, each with a ground trace running nearby. One was carefully cut at the "factory", one was shorted to ground.

ugh.