Sending audio between 2 Boards / Noise Problems

#1

Hi There,

for quite I while now I'm working on a setup that has 1 external Input that is being sent to two boards and audio that is being sent back and forth between the two boards via the rts pinouts on the back of the board.

I tried to make a drawing of it, I'm sorry I'm really bad at documenting this technical stuff:

Foto am 05.04.17 um 18.05.jpg1080x720 135 KB

Just once more in text here:
External Input, goes into Board 1, Jack In and is being tapped to Board 2 to Pin T (left input)
Connection from R Out to R In between both boards, back and forth (right output / input)
Ground Connection.
Each Board sends out via T (Left) in Mono and the outputs are summed in a little mixer, this one:

The Boards and a little Mixer run of the Same 12V DC power supply.

Furthermore I only send Midi from Board 1 to 2, and got GPio connections between the boards.

The Problem is:
I've got a lot of noise with this configuration. It is not that 50hz grounding noise But more like white noise and a really high pitched drone.

I tried powering all from a battery but I've got same noise still.

When I skip the little mixer and go directly into the main Sudio Mixer the Noise gets even worse. So it seems as if the little china mixer cleans the signal up, but the whole setup is still quite noisy.

Furthermore the leds of my midi controllers (Nanokontrol 2, Launchcontrol XL) add some more high pitched drones.

Any suggestions how I could get rid of the noise?

Is the Groundconnection between the boards important? Right now I went from the Input "S" (sleeve of the jack) to a GND of the other board, but they also share a ground via the powersupply.

I also tried powering the little mixer with seperta PSU, no effect.

This is what Ableton's EQ analyzer shows me (I bosted the noise a lot)

Bildschirmfoto 2017-04-05 um 18.29.24.png1296x383 63.1 KB

With a really loud peak at 3k, low frequency rumbe and two hearable tones at 150 and 200 hz (harmonics of 50hz?)

#2

It could be just coincidence, of course but I think 3Khz is the scanning rate of the GPIO inputs.

#3

One more thought -- if you were getting AC line harmonics I'd still expect to see a large 50 Hz peak, together with those higher frequencies. Unless perhaps internal to your equipment there is some hidden notch filter which would attenuate the main 50Hz and somehow leave the harmonics in the system. But that doesn't seem as likely.

One thing I have learned from working with too many grumpy op-amp chips: Use shielded/coaxial cables if you can, and put everything in a metal box if you can.

#4

I think the 50hz is also there but not as strong as the harmonics. But it could be something als than main cycle hum of course.
But it's not the most annoying. The 3k is really bad. And there is noise all over the spectrum wich is really bothering me.

Just tried removing the gpio interconnection, the 3k is still there.

#5

Sorry I'm not more help! That's a mystery for sure! If they share a ground from the power supply that seems like it should be good. How long is your cabling? Does any change happen if you use much shorter cabling?

#6

No problem thx for looking into this!

I use 20cm jumper/dupont wires for the pin connection, and 20cm long jack cables for connection the small mixer.

I'm planning to use shorter cables but still need to buy or solder them.

#7

20cm should not be too long. If it were a 3 m cable I'd point at that first! Unless your room has a lot more EM noise than normal. Often there is a bad ground connection leading to noise problems.