Shielding a passive audio switchbox

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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?  




Re: Shielding a passive audio switchbox
On Tuesday, 13 February 2018 21:46:57 UTC, snipped-for-privacy@tubes.com  wrote:
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it will if it connects, which is nontrivial with ali.


NT

Re: Shielding a passive audio switchbox
On 13/02/18 23:04, snipped-for-privacy@gmail.com wrote:

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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

Re: Shielding a passive audio switchbox
On Wed, 14 Feb 2018 07:40:53 +0000, Jeff Layman

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


Re: Shielding a passive audio switchbox
snipped-for-privacy@tubes.com wrote:

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  Likely just over a hundred dollars would get you a <scoff> 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.


Re: Shielding a passive audio switchbox
On Tue, 13 Feb 2018 23:10:45 +0000 (UTC), Long Hair

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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.



Re: Shielding a passive audio switchbox
On Tue, 13 Feb 2018 18:13:08 -0600, snipped-for-privacy@tubes.com wrote:

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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
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
We've slightly trimmed the long signature. Click to see the full one.
Re: Shielding a passive audio switchbox
snipped-for-privacy@tubes.com wrote:

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  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.

Re: Shielding a passive audio switchbox
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HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA

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

Re: Shielding a passive audio switchbox
On Wed, 14 Feb 2018 00:47:01 +0000 (UTC), Long Hair

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



Re: Shielding a passive audio switchbox
snipped-for-privacy@tubes.com wrote:

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  Have a nice strife, Bozo.

Re: Shielding a passive audio switchbox
On Wednesday, 14 February 2018 00:14:14 UTC, snipped-for-privacy@tubes.com  wrote:
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nah, he posts it regardless


NT

Re: Shielding a passive audio switchbox
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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.

Re: Shielding a passive audio switchbox
On Tuesday, February 13, 2018 at 4:46:57 PM UTC-5, snipped-for-privacy@tubes.com wrote:
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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.

Re: Shielding a passive audio switchbox
mpm wrote:

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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  

Re: Shielding a passive audio switchbox
On Wed, 14 Feb 2018 04:41:18 -0800 (PST), Phil Allison

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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.

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Re: Shielding a passive audio switchbox
  snipped-for-privacy@downunder.com wrote:

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** Ever looked inside a hi-fi pre-amp ??

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


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**  https://en.wikipedia.org/wiki/Ground_loop_ (electricity)#Sources_of_ground_current

  


.....   Phil  


Re: Shielding a passive audio switchbox
On Wed, 14 Feb 2018 12:23:14 -0800 (PST), Phil Allison

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I build my first hi-fi audio amplifier about a half century ago. In
those days it was (at least in Europe) standard practice to keep
chassis ground and signal grounds separated as long as possible and
use a _single_ signal ground connection to the chassis.  

You may dislike DIN connectors at much as you like, but  the connector
shell could be installed directly to the chassis, the signal ground
(pin 2) was by default separate from the chassis ground.  

Unfortunately brain dead RCA and 6.35 mm jacks had by default a
connection between the signal ground and chassis ground.


You may get rid with this in small systems, but in larger (stage)
systems with multiple devices with more or less leaky mains
transformers, you end up with all kinds of hum problems.

I have solved some guitar amplifier hum problems by simply using
isolated 6.35 mm jacks with all signal grounds connected directly to
the input tube.    

Of course, XLR is even better, with the connector case connected to he
device chassis, the shield possibly connected to pin 1 and the actual
signal lines separated from all this ground mess :-).

This applies to mains/oudio frequencies, for RF (at least UHF/SHF) you
really have to use BNC/TNC/N/SMA connectors with direct chassis
connection for proper transmission line operation.

However, if you want to feed and audio signal to such a device (e.g.
amateur radio transceiver), you should use some audio isolation
transformer.


Re: Shielding a passive audio switchbox
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chassis ground and signal grounds separated as long as possible and
use a _single_ signal ground connection to the chassis. "

Yes but all the signal grounds were common TO EACH OTHER before being connected to the chassis.

Re: Shielding a passive audio switchbox

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a single, common ground plane.
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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. http://crcomp.net/ee/Altoids%20jelly%20bean%20mixer

Thank you,

--
Don Kuenz, KB7RPU

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