Shielding a passive audio switchbox

a single, common ground plane.

My Altoids jelly bean mixer [1] works with line level signals. It ties its input and output grounds together. Everything's enclosed in a cheap metal box. There's no audible hum at normal listening levels. C1 and C2 function as hum busters. They successfully attenuate any wall wart hum that tries to leak in. It also helps to choose your wall wart wisely. You want to use a SMPS instead of a transformer.

Note.

1. formatting link

Thank you,

That won't do much for the hum discussed in this thread.

I think that since the sources are all low impedance, R9 & 12 should be 30 K, not 10 K. This brings back the signal level.

Have a nice strife, Bozo.

Nope. 10k is the correct value for an effective gain of 1 for each channel.

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Mass of RCA sockets on the back, ins and outs, all grounded as they enter the metal case. STANDARD practice.

..... Phil

While Wall Wart hum is one of my problems. I did learn by accident just how badly this switchbox leaks (from lack of a metal box). I am short on enough bench space so I shoved the box over in the corner of my bench, where there is a power strip with lots of line cords. Holy crap, the hum almost blew my ears out. As soon as I moved it away from those cords it got nearly quiet again.

I definitely need to enclose it in some metal....

That's most likely magnetic interference. A metal box isn't likely to help much.

Bullshit. IF you have an old wall wart that has a transformer in it, swap it out for a switcher.

Otherwise I do not believe you in the first place as modern wall wart have no transformer in them and therefore (decidedly) do not hum.

Depends on the metal, eh, wrongboy?

each channel. "

Not if the inactive inputs act as a short because they're low impedance. Th en the other 10 K resistors are effectively in parallel to ground. So you g ot 5 K to ground drawing the level down to one third of what it was. If the unused inputs are left open, no. It would need a 10 K, but the unused inpu ts will not be open circuit.

Easy mistake to make since they're not shown past the box. The mind sees th em as open circuit, but they're not.

Think about it a minute and you'll see.

Really. Underwriters labs would love to hear about that. No isolation eh ? OK genius.

No. The opamp input in that configuration is already a *very* low impedance. Zero ohm for an ideal opamp and a few ohms in practice. Paralleling it with several kohms doesn't make any difference.

Wake up, twerp. Wall warts are typically switchers, and as such do not have a transformer.

All are UL listed and approved, no problem.

Maybe you are thinking of the inline LVPS for laptops and the like. Sorry, but they are typically switchers nowadays as well.

And if you are so stupid that you do not know how they achieve AC line isolation, then you need to go back to school. And listen this time.

No the reason is a ground loop. 10kOhm audio circuits won't pick up hum unless you're in Nikola Teslas lab.

Get yerself some RCA-lead audio isolating transformers.

aparrently it blocks mind-control rays :)

Ground loop. Get an isolation transformer, something like these:

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The 120VAC conductors inside the OPs mess of cords and power strips are all sources of capacitively coupled hum.

So would plugging the various inputs IN and turning the amplifier's damn volume down.

.... Phil

Wrap the outside temporary with foil and test. May have trouble what to ground it to.

Busing grounds in the box together may be bad.

Greg

lol