Ribbon cables and audio

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Ok, the project is a guitar FX pedal control board. Instead of daisy chaining guitar pedals as is the norm, one will hook the in and out of each pedal to a "slot"
on the controller. It will use a microprocessor and a crosspoint switch chip (an AD75019 specifically) to do the work. The user will pre program a bank of foot switches such that they may have any combo of pedals, in any arbitrary order in one foot stomp.

I have a scaled down version running on a breadboard (4 pedal "slots" instead of the planned 12).

My question is how to get the audio from the PCB with the switching chip to the jacks. It is not my desire to have the jacks mounted on the main PCB, but even if they were, I think my question is still relevant.

What I really want to do is have ribbon cables going from the main board to the board with the jacks.

I'm concerned enough about having all those unshielded signals on the PCB, so I;m really concerned about the noise immunity of using a ribbon cable. Some of these signal will be to and from pedals with a fair amount of gain.

Can a ribbon cable do this noise free? Another consideration is that the crosspoint chip's switch impedance is kinda slippery, so I am buffering the in's and the out's. My inkling is to put the opamp buffers on the main PCB, so my high impedence point is before the cable run. Valid thought?

My only other thought would be individual shielded cables (or twisted pairs), but for twelve slots, thats 24 cables plus the main in and out. That would certainly be the safest, but seems kinda inelegant to me.

Joined 2009
Paid Member
If you put some thought into how you arrange signals and grounds on the ribbon cable, you can probably get the audio across without degrading the signal in the process. I'd run something like ground-signal-ground-signal-ground. If you'd rather use differential pairs, you can get ribbon cable that contains twisted pairs. It's more expensive, but that could work for you. You can also get shielded ribbon cable, but now we're talking serious bucks.
Your biggest enemy is likely to be coupling between inputs and outputs of the various pedals. You may want them a bit apart on the cable.

Keep the ribbon cable as short as you can and keep the impedances down. You'll probably be fine.

You could also use relays and put the relays on the board with the connectors for the pedals. SMD relays don't take up much space. Kemet make some nice ones for small signal switching that I used in my Differential Preamp 8x2. I measure THD of 0.000070 % from input to output of the preamp, so you don't have to worry about any colouration of the sound from the relays.

I've found the punchblock design of ribbon cable connectors to be highly unreliable. I've replaced many of them in Woodman packaging machines. The force is not very high, and oxide builds up between connector and copper wire. Crimped on connectors have higher force and exclude oxygen better between connector & wire.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.