HUM in unique (AFAIK) config of the boards. Hopefully if I give a decent overview of how I have this wired and grounded, someone could help me fix this?
I've built a pair of the DIs in a single chassis with an input switch to run the unit as dual mono or parallel. For parallel, input 1 feeds both board 1 and board 2, so I have 2 amp outs and 2 XLRs of the same signal. In dual mode each board works fine with a low noise floor. But in parallel, the amp output of channel 2 has a moderately loud hum rendering the signal unusable. This is not present on channel 1's amp out. I haven't bothered testing the XLR outs yet since this is a show-stopper. Components values are all as specified with the exception of changing 1G R to 2.2G R on both boards
The switch is implemented as follows:
Input 1 jack's tip is wired to both board 1 and a SPST, which also receives input 2 jack tip on other lateral solder lug, and the middle lug of course goes to board 2 input. The grounds of each of the 4 TS jacks stay with their respective 'original' boards. The chassis wire for each board is connected to the case at a standoff with the coating of case scratched to ensure good continuity (verified). Pin 1 of XLR goes to both the board and to the XLR chassis tab (no paint scratched off around the XLR screws).
Some specific and broader questions:
- For the TS jacks, I am using Neutrik NMJ4HC-S, which I saw recommended in the thread, but also see Bo recommends Cliff CL1160A. Both appear to be 'switched', is there a difference otherwise between them? (Mouser doesn't sell the Cliff, but I can get them through DigiKey if this may be the suspected culprit.)
- I also stripped the wire long enough so each each wire reaches both solder lugs for tip and sleeve, respectively. Does this defeat the switching or shouldn't be done regardless?
- Any other ideas why I'd get a hum in this admittedly non-traditional use-case? Tests I should run with DMM?
Here's a pic, but regretfully since I used black wire for the audio signal, it's a bit hard to see the routing against the black faceplate, hence the detailed description above. Yes, the wires are a little longer than necessary because I plan to put this into a reorganized faceplate once I have the time to get the layout right (failing at FPD). That said, since the boards work fine in 'dual' mode, I'm doubtful the extra length of the wires has much detrimental effect. Just trying to cover my bases here with details and head off replies like the one above about transistor parameters.
Thanks,