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'[EE:] Gorund loop?'
2004\07\24@164543 by Anthony Toft

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As those following might have guessed I am building an in car mp3
player. A computer powered by an inverter, into a graphic eq/booster. I
finally got everything connected yesterday, however, there is a
horrendous humming, regardless of engine running. I have tamed that a
little by connecting the PC chassis to the inverter 0v with 4 strands of
18 guage (I didn't have anything heavier) but it is still fairly
obnoxious. It is nearly eliminated when I unplug the audio from the PC.

I am about out of ideas, could anyone please offer some hints and tips
how to eliminate (or nearly) this hum?

The gritty details...
   I have the PC output plugged into my EQ using some shielded
        stereo cable (20 or 18 guage I think)
   The 12v and 0v line for the inverter are 10 guage.
   The 12v comes from the battery post through a relay switched by
        the accessory line
   The inverter is in the trunk and grounded under a bolt in the
        trunk.

   The EQ is hooked up in front, power comes from the stock radio
        connectors. Using the supplied cabling (looks like 18-20
        guage)

Thanks,

Anthony


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2004\07\24@172146 by Matthew Brush

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Is the PC in the trunk or close to the EQ?  It may be just a matter of
the cable being too long and picking EMI from someting along it's path
in the car.  I'm no expert, but that's what I would check first.

I'm currently working on a switchmode PC power supply that runs from the
cars battery directly and outputs around 5-10A on the +5v and +12v
lines, which should be enough for a basic PC with a hard drive and
cd-rom.  I plan to build it into an AT power supply housing and use
screw terminals on the back to connect the battery supply.  If you're
interested, take a look on google for car mp3 player power supplies,
there's tons of schematics and good stuff to make one of these power
supplies.  The MAX787 looks like a condidate.  I'm sure the efficiency
of a battery powered switching power supply would blow away the
traditional Inverter setup.

Anyways, good luck...cheers

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2004\07\24@182340 by Carey Fisher - NCS

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  >
  > As those following might have guessed I am building an in car mp3
  > player. A computer powered by an inverter, into a graphic eq/booster. I
  > finally got everything connected yesterday, however, there is a
  > horrendous humming, regardless of engine running. I have tamed that a
  > little by connecting the PC chassis to the inverter 0v with 4
  > strands of
  > 18 guage (I didn't have anything heavier) but it is still fairly
  > obnoxious. It is nearly eliminated when I unplug the audio from the PC.
  >
  > I am about out of ideas, could anyone please offer some hints and tips
  > how to eliminate (or nearly) this hum?

  Isolation transfomers in the audio lines to/from the PC will break up the
grounds and should solve the problem.

Carey Fisher, K8VZ

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2004\07\24@184705 by Spehro Pefhany

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At 04:45 PM 7/24/2004 -0400, you wrote:
{Quote hidden}

Is the PC case intentionally or accidentally connected to chassis ground on
the car?

If so, get rid of that.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\07\24@185530 by Anthony Toft

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> Is the PC case intentionally or accidentally connected to chassis ground on
> the car?

Intentionally, it reduced the noise a bit (from unable to hear music to
able to hear with annoying hum)
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2004\07\24@190154 by Spehro Pefhany

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At 06:55 PM 7/24/2004 -0400, you wrote:
> > Is the PC case intentionally or accidentally connected to chassis ground on
> > the car?
>
>Intentionally, it reduced the noise a bit (from unable to hear music to
>able to hear with annoying hum)

Then you probably need an audio isolation transformer as someone else said.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\07\24@191025 by Anthony Toft

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> Then you probably need an audio isolation transformer as someone else said.

Radio Shack have them, will it be OK to recombine the grounds after the
device?
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2004\07\24@192516 by Harold Hallikainen

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It'd be interesting to listen to the audio output of the PC on a floating
device (headphones, battery powered amplifier, etc.) where the only ground
connection is that plugged into the PC sound card. If the hum goes away,
the audio isolation transformer suggested by others should work. If the
hum is still there, the transformer will probably not do much. In that
case, it's probably a problem with the square wave out of the inverter
getting into everything.

Good luck!

Harold


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2004\07\24@193553 by Spehro Pefhany

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At 07:10 PM 7/24/2004 -0400, you wrote:
> > Then you probably need an audio isolation transformer as someone else said.
>
>Radio Shack have them, will it be OK to recombine the grounds after the
>device?

I assume there is a ground in the amplifier already (or you wouldn't be
having the problem), so you should not put an extra ground in there, unless
you want to bring the problem back again. ;-)

IOW, the primary of the isolation transformer is grounded by the PC.
The secondary of the isolation transformer is grounded to a different point
by the amplifier.

The two grounds have an AC noise signal between them.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\07\24@195253 by Anthony Toft

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> IOW, the primary of the isolation transformer is grounded by the PC.
> The secondary of the isolation transformer is grounded to a different point
> by the amplifier.

This I understand! I guess I will have to get some cables and keep the
signals and returns separate after the transformers. Thanks for the
assistance and the explanation, I am enlightened (think 10W bulbs
here:))

BTW a test with the headphones rendered NO noise at all, I think this
device will fix it!
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2004\07\24@200331 by Anthony Toft

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> the cable being too long and picking EMI from someting along it's path
> in the car.  I'm no expert, but that's what I would check first.

Well, I have tried the cable unplugged, and there was minimal hum, even
less when I moved the audio cable from under the inverter :)

> cars battery directly and outputs around 5-10A on the +5v and +12v
> lines, which should be enough for a basic PC with a hard drive and
> cd-rom.  I plan to build it into an AT power supply housing and use

The problem with this is that this PC is ATX, and needs 3.3 volts and
the negatives. A future design is based around the AMI Indium board
which takes +5 and +12v only. For which I am slowly putting the PSU
design together (it's not ready for a sanity check yet)

> interested, take a look on google for car mp3 player power supplies,
> there's tons of schematics and good stuff to make one of these power
> supplies.  The MAX787 looks like a condidate.  I'm sure the efficiency

Yes, I have 2 of these, 5A, 5v. the big problem I have found is the 12v
rail, National have a nice buck boost solution that will deliver 1.2A at
12v, with 8-16v input. The current stack (indium and drive) draws 0.8A
running and about 3A starting, so I have to figure out the LC values for
it.

> of a battery powered switching power supply would blow away the
> traditional Inverter setup.

Yes, it would, except that in the car, being efficient isn't such a big
deal, there is LOADS of 12v floating around, you might trade efficiency
for simplicity which is what I did last year.

> Anyways, good luck...cheers

Thanks, I'm going to get the rat shack isolator probably tomorrow, I'll
let you know how it works out.


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2004\07\25@083458 by Mike Hawkshaw

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----- Original Message -----
From: "Anthony Toft" <EraseMEtoftatspamEraseMECOWSHED.8M.COM>
To: <@spam@PICLIST@spam@spamspam_OUTMITVMA.MIT.EDU>
Sent: Sunday, July 25, 2004 1:03 AM
Subject: Re: [EE:] Gorund loop?


> Thanks, I'm going to get the rat shack isolator probably tomorrow, I'll
> let you know how it works out.
>
Another way to do this, apart from using isolation transformers, is by using
ballanced inputs. You can do a fairly easy electronically balanced input
using a decent quality op-amp ie NE5534. If you connect the earth side or
ground to one input and the hot side to the other that should work OK.

The action in both cases (transformer or ballanced input) is to get rid of
common mode signals, which is what your hum is.

Cheers......Mike.


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2004\07\25@175135 by Peter L. Peres

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Anthony, I would suggest purchasing two 1:1 audio transformers and using
them in the cable to the equalizer. Almost everything else will cause
trouble. Car "grounds" (notice the quotes), aren't really. From the audio
or rf point of view, a couple of meters of thin sheet steel spot welded
here and there is not ground. Since you are playing mp3 $10 (each) audio
transformers should have good enough quality <g>.

Peter

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