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I'm a tinkerer but not an electrical engineer. I'm constructing a stereo->mono summing box for my stereo (to listen to Beatles LPs properly, natch). I'm using the design & schematic here for this. Essentially it's two pairs of panel mount RCA jacks (input and output) for stereo signal, running through a 4PDT switch with a couple resistors to prevent feedback. Here is a copy/paste of the schematic from that blog:

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What I am a little fuzzy on is what the minimum and/or reasonable conventions would be for "shielding" of this component. Ie, against RF interference from outside the box or stupid wiring ideas inside the box.

The web page linked there suggests just using plain 22AWG hookup wire inside the box for the +ve signals and just linking all four of the RCA sleeves (-ve) together. I have a few questions:

  1. I was planning on using an ABS plastic enclosure for this, but then realized that I might prefer a metal enclosure to help with shielding but honestly I don't really know how this works?

  2. Is basic hookup wire reasonable here? Or is there like some kind of single-conductor shielded wire that should be used for audio signals? Or something else? I know that two-conductor shielded wiring is often used for audio internals and carries signal/ground with the shield connected to ground at one end. But the circuit I'm building is really all signal lines and no ground, so even if shielded-single-conductor exists (and it's certainly not common on e.g. amazon) I don't know what I'd connect the shield to-- unless figuring out how to wire it all out to the RCA sleeves.

  3. What do I need to think about, if anything, about physical layout of the wiring inside the box? If I use some two-conductor wires I happen to have handy either leaving one conductor unused or using the conductors for two different things (ie not the +/- of the same signal) is that a problem?

  4. Any other obvious gotchas or obvious best practices that I should be aware of?

I am sure that using the plastic box and the radio shack hookup wire would "work" just fine most of the time, but I'm trying (a) to learn what I should be paying attention to for this sort of project and (b) my stereo is nice enough that I'm trying to not actively degrade the signal through basic incompetence. :)

Thank you so much for any insight.

BZo
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  • I only read enough of the drawing to project in my mind where you are going. So I may have missed something important. Just build something reasonably well and you should be fine. ABS is fine. Metal enclosures are fine and also cheap enough. Don't worry so much about this just yet. The worst that happens is that you feel it's too noisy. If so, ask again with you observations and construction details. But I don't think you'll have any observable troubles. That said, your circuit will attenuate your inputs. That's life with simple circuits like this. – jonk Mar 15 '21 at 03:09
  • @jonk Thanks. Fair enough. Just wanted to avoid doing anything obviously dumb for the first attempt if I could avoid it :) Re: the attenuating inputs-- is that the case even in the straight-through switch condition (the diagram looks inconsistent on this to my eyes-- should be the case when inputs are switched directly to outputs for original stereo)? Or does it only attenuate when doing the mono summing (ie, the resistors are active) – BZo Mar 15 '21 at 03:15
  • If things go straight through (I didn't really look) without going through a divider, then no attenuation. But yes, the mono mode with resistors always means attenuation. You can avoid that by powering the box and using active devices. But then it's a more complex project, too. – jonk Mar 15 '21 at 03:16

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