Shielding a passive audio switchbox

I picked up a passive audio switchbox. I intended to use it to switch the computer outputs going to a small stereo amplifier. In other words, I have this small bookshelf stereo, and I have three computers that I want to amplify the audio signal.

One is my desktop computer which is intended to only be used for storage and playing music and videos. It is not connected to the internet. The second computer is the desktop that I use on the internet. The third is my laptop computer.

For years I have been swapping plugs from computer to computer. And while that is not the biggest task, the cables always get tangled up and it's a small hassle.

I thought I solved it when I bought this 4 button switchbox. It sas intended for audio and video. (Has the two RCA audio plugs Red/White, and the Yellow video plug). In this case I just do not use the yellow ones.

This switchbox is nothing but a plastic box with four push button switches and corresponding inputs that go to one set of stereo outputs (RCA jacks) and the unused video ins and outs. There are no active electronics in the box. No power supply or semiconductors.

The problem is that it picks up hum. The reason is obvious. The box is plastic and is not shielded. So while all the input and output cables are shielded, this box is not.

Aside from buying another switchbox, or transferring all the components from this one into a metal box, I was wondering if I could simply coat the entire insides of this box with aluminum foil. Glue it to the box, and make sure it's bonded to the grounds (shielded part of the cable connectors).

Will this work?

Reply to
oldschool
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it will if it connects, which is nontrivial with ali.

NT

Reply to
tabbypurr

Likely just over a hundred dollars would get you a FET fired, microcontroller operated HOME AUDIO/VIDEO RECEIVER. That would let you switch shielded audio inputs, then you take the tape monitor output. No need to use those pesky non-tube FET output amps... just use it as a switch / pre-amp for your inputs.

Or you could just go get hypnotized and have him or her tell you that tube amps are not the end all be all of the world items, and you will no longer be obsessed, and you can go on in your life embracing the digital age without inane biases fed to you by other obsessed tube only dingledorfs.

Reply to
Long Hair

Maybe if you could read for comprehension, you would not post your ill informed bullshit. And whether this was a tube or solid state amp, dont matter. The amp works fine, it's just a non-shielded switchbox that is the problem. And for the record, although I work on a lot of tube stuff, this is not a tube setup. it's just a fairly decent solid state, low powered stereo amp/receiver.

I also do not have $100 to blow on this. This is just a slightly better alternative to those cheap crap (so called) "amplified computer speakers". If I wanted to spend over $100, I could buy an older analog mixer-amplifier on ebay, and use that. (Similar to a DJ setup). But I have no intention to spend that kind of money on this project.

I could probably buy another switch box in a metal enclosure for under $20. But I dont plan to spend anything if I can jsut sheild what I already have.

Reply to
oldschool

Caution: On occasion I've found the hum to be due to ground loops source-to-source... had to add a 1:1 transformer to get rid of it.

I'm presently returning to my youth and building my own audio system. Most of my sources now have digital audio outputs... makes life a wee-bit more pleasant ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Maybe if you ever had a grip on reality, you would not be wasting personal time trying to fudge together an audio switch.

I guess you are not all that knowledgeable in audio after all.

Line it with some foil, ditz... yeah... that'll fix it.

The only thing ill informed here is you.

My personal time is worth more than that, and I will spend the hundred, and then spent the hours I did not waste flying my drone or the like.

But go ahead... try to fudge it together and spend inordinate hours just to save a few bucks.

Reply to
Long Hair

Some of those really cheap switch boxes tie all the grounds together (common shields). That's another source for hum.

Does any one of the sources known to hum still hum if it's the only thing connected? If not, then you can probably rule out lack of box shielding.

Reply to
mpm

nah, he posts it regardless

NT

Reply to
tabbypurr

formed bullshit."

Copy that.

This does not make sense rightnow because all those sources are low impedan ce. they can drive headophones and if you understand electronics you know t hat means the low imperdance will short out the picked up hum. You should b e able to run non shielded open wires frkm those sources and not have any h um, unless you are in a hell of an environment or as someone mentioned it i s ground currents.

Gt a bunch of those three prong to two prong adapters they use for older ho uses that had ungrounded outlets and interrupt all the grounds in the syste m. If the hum goes away it is ground currents. Aslo, if there is a cable or digital decoder box anywhere in the system,disconnect the antenna or cbale input to it first. If the hum goes away then the ground current is induced by the cable installer using the wrong ground. If that is the case get an antenna isolator for an older direct line operated TV and wire a second cab le connector to the other side where it would normally connect to the tuner in the TV. I had to do that once, it was a big house and there were two gr ounds. But it can happen for other reasons. For example if some things are grounded to an earth ground rod pounded in outside and others go to a cold water pipe. There ARE ground currents,in fact ground gradients can kill you in a lightning storm even if you are not hit. You get zapped through the b ottom of your feet ! But these ground currents only need generate a voltage in the millivolts to cause a hum, The time I had to do it it was actually causing a hum bar in the guy's video projector. But it is the same thing.

If the hum is NOT ground currents I highly suspect something wrong with tha t little switch box. Try each output into the amp one at a time, if none of them causes the hum that switch box is junk. Your cal;l, see if you can ge t it apart or return for refund. If you return it, realize you don't need a switch at all. Just build a doodad that will combine all the inputs throug h like 3.3K resistors. That is high enough to not overload the outputs of t he devices and high enough to short out the hum. If it doesn't work try a l ower value, you can probably go down to about 100 ohms. mybe just start wit h 220, it will probaly just work at that point. the one problem is to make sure you have enough gain because the inputs might drain off each other. Al so all units should be switched on or audio distortion may result. But it i s nice because you do't even have to switch anything. I don't know about yo u but most of my stuff runs 24/7. It really doesn't pull much power unless it is a class A amp or tubes. Computers only pull power at certain times. A ll you have to do is what I do, either shut the monitor off or set the powe r save on the PC to do it. Do it especially if you use CRT monitors. They p ull more and it can wear out the CRT cathodes. LCD monitors not so much, bu t still, save those inverters.

That is it in a large size nutshell. Sorry if there are typos, can't see we ll and my back requires leaving the PC for a tie now.

Reply to
jurb6006

HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA

Best laugh I had all day, brainiac who never saw a line choke.

Reply to
jurb6006

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** It should not matter - or nobody would ever make such a switching box.

Check that the RCA jacks on the box have their grounds linked internally and that your RCA plugs really do link their cable shields to these jacks.

Because the audio circuits are low impedance and operate around line level, hum injection should be minimal.

If you still feel inclined to shield the box, tin plated steel sheet sold by hobby shops is much nicer to use then aluminium foil.

.... Phil

Reply to
Phil Allison

For a one-off, copper tape works really well.

Reply to
krw

Yep. Might be better to cut up and flatten old soft drink or beer cans if he really wants to do it for free. They are thick enough tot take a nut and bolt for a connection.

--

Jeff
Reply to
Jeff Layman

And you can solder to it. But as I said shielding should not be an issue when all the outputs are low impedance. Should not...

Reply to
jurb6006

The reason is not at all obvious. Most likely the cable shields from the computers and to the amplifier are constantly tied together (not switched).

This is not a problem as long as all devices are battery powered or at least have double insulated (Class II) power supplies.

However, if two or more of these devices are powered from a grounded socket with a grounded mains cable, you easily get such problems. The signal ground is more or less directly connected to the chassis ground which is connected to mains PE (Protective Earth) connection. The mains filter Y-capacitors are also connected to the chassis. Filter ripple current is flowing from one device chassis through the audio cable shield to the other device chassis. If the amplifier is actually a receiver with a connection to the Cable-TV socket, there are additional current paths.

This ripple current (50/60 Hz and harmonics and power supply noise) will cause a small voltage drop in the cable shields, which is directly added to the audio signal as hum on the unbalanced interface.

One thing that may help is to connect all computers and amplifiers to a single extension cord, thus the chassis potentials are closer together. If possible, experiment with each mains cable polarity to minimize the hum even further.

Provided that the amplifier is double isolated use a switch which also connects/disconnects the shields only to the computer cable shield in actual use.

For severe hum problems, use audio isolation transformers on all switch input and output connections.

Reply to
upsidedown

...... Into the Bozo Bin .......

Reply to
oldschool

That does sound like a better material to use. I was wondering how I'd get a solid ground connection to the foil....

Reply to
oldschool

Correct

The desktop computer has an earth ground (3 prong cord). I should meantion that I currrently only have two computers connected, because I need to get another audio cable for the 3rd one.

I did find something interesting though. With the laptop connected to the line cord power supply, there is the most hum. If I run it off it's battery, there is no hum. (however. the hum level drops a lot, if I unplug the desktop computer while running the laptop from the line cord. (I usually use the AC adaptor when I use the laptop at home).

Even if I eliminate the switchbox and run the laptop to the amp directly, there is a slight hum if the laptop is connected to the AC adapter.

Therefore, it appears the problem is really that AC line cord adapter for the laptop. Yea, it is the one made for the computer.

I'm not sure what I can do about that. That adapter only has a two prong plug on the AC cord, and it plugs into the laptop as a DC 12volt source.

All of my devices are connected to one power strip, except the laptop, which I plug into a different outlet because of the location where i put it. I do know the outlets are on the same breaker (all outlets in thsi room are on the same breaker).

That would indicate getting another swithcbox, with more contacts on the switches. I do not know much about the amp's power supply. i have not found a schematic for it. It's a Koss brand.

Are these something that is more or less generic, or are they specific to the impedence? I am not familiar with them.

Reply to
oldschool

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** Funny how hi-fi pre-amps have all the input and output grounds linked - often by a single, common ground plane.

You have never even seen inside one have you ?

Minimises both ground hum and eliminates RFI entering the box.

.... Phil

Reply to
Phil Allison

The aim is to have _exactly_ one connection point for the signal grounds in an unbalanced system.

Things get ugly if there are multiple ground connection points e.g. through mains earth (PE) connections, which are typically at slightly different potential. A small current will then flow in the mains earth and signal grounds trying to eliminate the potential difference. Unfortunately, both the mains earth and signal cable shields have a small resistance (actually impedance), which will cause a voltage drop, which is added directly to the unbalanced audio signal.

Reply to
upsidedown

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