[lug] audio interference question

Fri Jan 30 14:24:26 MST 2009

I agree with most of the analysis so far, but some key questions would be
this... 

Does your laptop's power brick big and heavy or light?  

(This generally would help determine if there's a transformer inside of it,
or something like a buck/boost converter and diodes to drop AC to DC.  The
two different types are quite different when it comes to noise problems.
More and more have no transformer -- thus no isolation from the AC system --
and common mode currents and ground-loops are a much bigger problem then.
But people want LIGHT, and they got it.  Many small/light power supplies are
HIDEOUS RF noise generators, and this noise can be rectified by audio
amplifiers when hooked to unshielded audio cabling.)

Next, does it have a three-prong AC plug, or a two-prong?  (This might help
indicate why it has such a ground-loop issue.  But maybe not.  Really need
to open it up to see what's going on, but it helps to make an educated
guess.)

I agree that the fact that it tends to go away when you're on battery power
is either an indication of: 

1. A ground loop.

2. RF/EMI being generated in the CHARGING/POWER circuits of the laptop or
the power brick, being rectified in an unshielded cable into a DC voltage,
and showing up as "noise" to an audio op amp stage that's not properly
isolated/capacitvely coupled at the amplifer.

A fix for the ground loop exists at Radio Shack, if you're willing to chop
into your cable (or other option below) and do a tiny bit of soldering.
It's called a 1:1 audio transformer, and costs $4.  

http://www.radioshack.com/pwr/product-reviews/Cables-Parts-Connectors/Compon
ent-parts/Relays-transformers/RadioShack/p/2103994-1-1-Isolation-Transformer
.html

Okay, electronics 101 here... 

Since transformers only let AC through, and the number of windings on each
side dictates whether or not the voltage changes as you cross from one side
to the other, a small transformer the size of a chicklet candy is enough to
isolate the grounds from each other, and clear up the audio -- if the ground
loop is the problem.  The AC signal in one side, the same on the other side.

(EE's, hush... I know you can sweep one and find some minor losses in
signal.  And some transformers leak DC through... let's not get into the
details of designing a darn circuit here right now.  These little biscuits
work "just fine" for this application, since it's what they're sold for.)

Really high dollar audio gear, includes such isolation as a built-in
feature, either via transformers, or more likely for perfect audio
reproduction -- optical isolators between the input audio circuit and the
first amplifier stage. 

Make yourself up a cable going from the PC to the amp with one of those
little green biscuits in the middle (solder it in-line, there's instructions
for which wires are which side, and it doesn't matter which side of the
biscuit transformer you hook to which device... think of it as a filter...
that works either way...), wrap where you hacked up the cable up in a little
electrical tape, and your first electronics "hack" will be complete.

You'll have two wires coming from your laptop (in a typical audio cable with
1/8" mini-plugs) and two going the other direction.  The biscuit will have
four wires, and the diagram on the back of the packaging shows the "pairs".
Just keep one pair of colors hooked to the piece of cable coming from the
PC, and the other to the cable going to the amp.

(If you need help here, ask.  I probably can even find a biscuit in the junk
box somewhere, to reference during a phone call, if you're REALLY stumped,
but this is easy stuff.)

If you want to get REAL fancy, mount the biscuit in a little box, and wire
up *plugs*, so it just sits in the middle of a couple of cables, going to
each device.   This is nice and tidy and "professional", but not really
necessary.

I've usually just hacked up the cable.  Cables are cheaper than project
boxes, and I don't have to buy female jacks, drill holes... yadda.  If you
make the box, whip out the lable-maker and mark the little project box, "1:1
Audio Isolation Transformer", and you can then use it again someday in
another project or to test for a ground loop at a friend's house (you'll
amaze them with your little magic box you made yourself... you electronics
whiz, you!)...

Disclaimer: I'm not an EE and don't play one on TV, but the FCC seems to
think I know enough about radios not to kill myself.  I've held an Amateur
Extra Class Radio license for many years.  I have at least three of these
biscuits "in production" right now at my house... keeping radios and PC's
from having ground-loops on audio lines between each other. 

Nate (Amateur Radio Callsign: WY0X)

-----Original Message-----
From: lug-bounces at lug.boulder.co.us [mailto:lug-bounces at lug.boulder.co.us]
On Behalf Of karl horlen
Sent: Thursday, January 29, 2009 8:35 PM
To: lug at lug.boulder.co.us
Subject: [lug] audio interference question

i plugged the stereo out of my laptop into a really nice power amp / speaker
combo i normally use for recording.

when the laptop is NOT plugged in (running purely on battery), the sound is
pristine... exactly what i'm looking for.  fwiw, i made sure i've muted the
microphone to eliminate any extraneous sound.  if i don't do that, you can
hear whitenoise when no music is playing.. probably picking up internal
drive and fan noise..

here's the catch.. the minute i PLUG the laptop in to POWER IT (non
battery), i get a an annoying but constant static / hum / interference
sound.  it does NOT go away, even if i totally mute the sound on the laptop.
so it's electrical in nature.

i've tried plugging the laptop in all of the outlets that are within reach
of the power amp.  buzz / hum / static is still there.. it sounds like a
motor or something and or possibly even acting as an antenna...

the odd thing is i've got another pc and 3 other audio gadgets that are
hooked up to this same power amp and none of them have this this same
issue.. pristine sound.  so i can't think it's a grounding problem in the
house or circuit.  only the laptop plugged into the wall outlet generates
the hum.

i tried unplugging all other devices on the power strip, nothing helps.

- is there anything i can do to make this work with power?
- any test to eliminate the problem (though my tests are already telling me
fairly clearly that the laptop (psu or brick?) itself is the source of the
problem)?
- are laptops or their psu / bricks known to not be grounded or shielded
very well?

thanks to anyone that can unlock the mystery.  i'd like to make the laptop i
have rarely ever used become my full time mp3 / media player.

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