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I have several use cases for MIDI audio switching. Specifically, MIDI controlled AB pedals and mute pedals. Since these don't really exist, I will need to build my own. (Can you find a MIDI controlled stereo mute pedal for example?)

I will be using this device for MIDI control: https://six4pix.net/product/relayswitcher/

I have a high level architecture decision to make, which is the main focus of this question, with an emphasis on low noise design. Do I make this monolithic or modular?

Monolithic approach:

All 1/4" inputs would be in the same housing. All 1/4" inputs would be line level, with the exception of two of the inputs, which would be passive guitar pickup level. (passive vs active pickups debate is outside the scope of my question)

Pros:

  • Ease of construction. Really just a matter of wiring the jacks to the relays and that's it.

Cons:

  • Potentially noisy?

Modular approach:

I build individual pedals that follow a standard 1/4" "tip to ring" shorting convention. Ie, for an AB pedal, I have two 1/4" jacks for input, one for output, and one as the "switching jack". If the tip and ring are connected together, that triggers a relay inside the AB pedal and switches the output. (Similar to some guitar amp channel switching conventions)

Pros:

  • Allows for incremental construction. The whole project doesn't have to be finished at once.
  • Potential re-usability.
  • Expandable in the future, let's say I want to trigger another AB for something else later.
  • Easier repair, for example, if one of the pedals fails I can just swap that out.

Cons:

  • Higher part count / cost, including quite a few additional 1/4" jacks and patch cables.
  • Each pedal needs a pcb, power connector, etc.
  • Having a relay trigger another relay just feels wasteful.

My questions:

  1. If I go with the monolithic approach, will I run into issues with noise? (be it GSM RF, other RF, ground loop hum, 60hz, or crosstalk from the other signals being switched in the same chassis) Keep in mind all inputs are line level, with the exception of two inputs which are passive guitar pickup level.

  2. Do I need to shield the chassis? I was considering 3d printing the chassis to make it easy to design and assemble, but of course plastic won't work for a faraday cage. Can I line with foil?

  3. Assume I go with the monolithic design, let's say I 3d print the chassis and then line it with foil, or I just use a metal chassis. Do the grounds of the jacks have to be disconnected from the chassis? I know when building guitar amps, you sometimes want the jacks isolated from the chassis. If so, which jacks should be disconnected from the chassis and why?

  4. Do any of the audio signal wires I run need to be shielded? If so, how do I connect the shields? To chassis ground or audio ground?

  5. Is there a way to securely attach wires to screw terminals with no chance of them coming loose? This might sound like an unlikely issue, but it's actually a serious concern. I built a guitar amp that worked great until all the nuts vibrated loose from vibration and caused failure. (I built it before I knew what nyloc nuts were) I was considering desoldering the screw terminals from the relay board and soldering to the board directly, but if there is another technique I would love to know about it.

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  • \$\begingroup\$ So basically you are asking how to switch and mute instrument and line level audio with relays? The part about relays being MIDI controlled seems not to affect anything, it does not matter how the relays are controlled. \$\endgroup\$
    – Justme
    Commented Apr 4 at 7:47
  • \$\begingroup\$ I wanted to be sure to include that detail so people wouldn't say "just use an off the shelf AB pedal" etc \$\endgroup\$
    – cat pants
    Commented Apr 4 at 9:35
  • \$\begingroup\$ I'm a musician, but having a hard time picturing your two scenarios. Would you attempt a schematic, please? If you provide a hand drawn schematic, I will even put it into LTspice for you to make it look good. \$\endgroup\$ Commented Apr 8 at 13:30

1 Answer 1

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If I go with the monolithic approach, will I run into issues with noise? (be it GSM RF, other RF, ground loop hum, 60hz, or crosstalk from the other signals being switched in the same chassis) Keep in mind all inputs are line level, with the exception of two inputs which are passive guitar pickup level.

You will always run into noise because every electrical device with resistance or semiconductor creates noise. The question is: is the noise of the part acceptable for my application.

The second thing is shielding, you need to be able to shield the device and return any electric fields (RF or 60Hz from AC mains) back to the source, a good ground is usually the best way to do this.

Do I need to shield the chassis? I was considering 3d printing the chassis to make it easy to design and assemble, but of course plastic won't work for a faraday cage. Can I line with foil?

Probably yes. Shielding is costly, but if the application demands it then design it in. It won't matter much if the cables you are using are not shielded, because the RF will be able to get in through the cables. If you are concerned with low level signals (like your guitar signals) it would be wise to shield them. You can only shield electric fields, magnetic fields can only be attenuated but usually you don't need to worry about them unless you have a close source because they fade fast (with the distance cubed, electric fields are with the distance squared). If you have amplifiers in your box, it would be wise to use shielding and filtering on the front end.

MIDI is a digital signal and is less susceptible to noise (you have to have a large signal to change the voltage from high to low).

You will want to distance the digital MIDI signals and analog audio signals to avoid cross-talk.

Assume I go with the monolithic design, let's say I 3d print the chassis and then line it with foil, or I just use a metal chassis. Do the grounds of the jacks have to be disconnected from the chassis?

With a good shielding design usually you have a chassis shield (the box) and a signal/PCB shield. Usually these are connected at one point, but not always. EMC design is an art and there are no hard and fast rules. Experimentation is usually the best way to determine what is best.

I know when building guitar amps, you sometimes want the jacks isolated from the chassis. If so, which jacks should be disconnected from the chassis and why?

Do any of the audio signal wires I run need to be shielded? If so, how do I connect the shields? To chassis ground or audio ground?

If the jacks are carrying the ground signal, and tied to the case, if there is any current on the chassis that wants to go down the cable it will do so and cause noise. With analog signals it's usually best to not tie the signal or reference ground of the signal to the chassis ground. If you have a shield, tie that to the chassis ground.

Is there a way to securely attach wires to screw terminals with no chance of them coming loose? This might sound like an unlikely issue, but it's actually a serious concern. I built a guitar amp that worked great until all the nuts vibrated loose from vibration and caused failure. (I built it before I knew what nyloc nuts were) I was considering desoldering the screw terminals from the relay board and soldering to the board directly, but if there is another technique I would love to know about it.

Yes, use a washer and a lock washer beneath the nut and tighten it until the lock washer engages and has some compression on it.

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