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Ground loop between phone/computer

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Anonymous
October 16, 2004 3:42:18 AM

Archived from groups: rec.audio.tech (More info?)

I'm trying to record telephone interviews using a Mac iBook. It doesn't
have an audio input so I bought a Griffin iMic, which runs off the USB port.

My problem: I can't seem to get rid of ground loop hum. I bought a Radio
Shack ground loop isolator to put between the phone and the iMic but that
doesn't seem to help much. Any bright ideas would be appreciated.

My set up....
* Radio Shack phone jack from the headset cord, to 1/8" inch mini plug.
* 1/8" female to RCA mail adapter
* RCA female to female adapter
* Radio Shack ground loop isolator (RCA male in, RCA male out)
* RCA female to 1/8" mini male adapter
* Griffin iMic, using either mic or line settings (hum is worse on mic
setting).
* G3 iBook 700
* Audio In recording software for OS X
Anonymous
October 16, 2004 3:42:19 AM

Archived from groups: rec.audio.tech (More info?)

In article <uM0cd.60601$3C6.2309357@news20.bellglobal.com>,
"Sushi" <sushiboy21@sympatio.ca> wrote:

> I'm trying to record telephone interviews using a Mac iBook. It doesn't
> have an audio input so I bought a Griffin iMic, which runs off the USB port.
>
> My problem: I can't seem to get rid of ground loop hum. I bought a Radio
> Shack ground loop isolator to put between the phone and the iMic but that
> doesn't seem to help much. Any bright ideas would be appreciated.
>
> My set up....
> * Radio Shack phone jack from the headset cord, to 1/8" inch mini plug.
> * 1/8" female to RCA mail adapter
> * RCA female to female adapter
> * Radio Shack ground loop isolator (RCA male in, RCA male out)
> * RCA female to 1/8" mini male adapter
> * Griffin iMic, using either mic or line settings (hum is worse on mic
> setting).
> * G3 iBook 700
> * Audio In recording software for OS X

The ground loop isolator doesn't isolate well enough for telephone use.
It is meant to isolate against about 2V, not the 25V or more that's on
telephone wires. You need a real telephone audio transformer. You can
add it to the Rat Shack transformer for maximum isolation.

Put a non-polarized capacitor of a few µF and at least 50V between the
telephone transformer and the phone if you have problems with low
voltage.


<- Phone Laptop ->
<------ -------------- ------> Line in
) || ( ) || (
) || ( ) || (
) || ( ) || (
) || ( ) || (
<------ -------+------- ------> Line in
|-------------------> Laptop Ground

600 Ohm Rat Shack
Telephone Ground Loop
Transformer Transformer
Anonymous
October 16, 2004 3:46:19 PM

Archived from groups: rec.audio.tech (More info?)

Normal telephone conversations work around 500Hz.. your hum will be much
lower than this. I could be wrong here (and would like to know if i am) but.
Perhaps a simple High Pass Filter could cut out this hum.

Rob

"Kevin McMurtrie" <mcmurtri@dslextreme.com> wrote in message
news:mcmurtri-2781F5.21414715102004@corp-radius.supernews.com...
> In article <uM0cd.60601$3C6.2309357@news20.bellglobal.com>,
> "Sushi" <sushiboy21@sympatio.ca> wrote:
>
> > I'm trying to record telephone interviews using a Mac iBook. It doesn't
> > have an audio input so I bought a Griffin iMic, which runs off the USB
port.
> >
> > My problem: I can't seem to get rid of ground loop hum. I bought a Radio
> > Shack ground loop isolator to put between the phone and the iMic but
that
> > doesn't seem to help much. Any bright ideas would be appreciated.
> >
> > My set up....
> > * Radio Shack phone jack from the headset cord, to 1/8" inch mini plug.
> > * 1/8" female to RCA mail adapter
> > * RCA female to female adapter
> > * Radio Shack ground loop isolator (RCA male in, RCA male out)
> > * RCA female to 1/8" mini male adapter
> > * Griffin iMic, using either mic or line settings (hum is worse on mic
> > setting).
> > * G3 iBook 700
> > * Audio In recording software for OS X
>
> The ground loop isolator doesn't isolate well enough for telephone use.
> It is meant to isolate against about 2V, not the 25V or more that's on
> telephone wires. You need a real telephone audio transformer. You can
> add it to the Rat Shack transformer for maximum isolation.
>
> Put a non-polarized capacitor of a few µF and at least 50V between the
> telephone transformer and the phone if you have problems with low
> voltage.
>
>
> <- Phone Laptop ->
> <------ -------------- ------> Line in
> ) || ( ) || (
> ) || ( ) || (
> ) || ( ) || (
> ) || ( ) || (
> <------ -------+------- ------> Line in
> |-------------------> Laptop Ground
>
> 600 Ohm Rat Shack
> Telephone Ground Loop
> Transformer Transformer
Related resources
Anonymous
October 16, 2004 4:46:29 PM

Archived from groups: rec.audio.tech (More info?)

In article <fS7cd.279$_Y.103@newsfe3-win.ntli.net>,
"Rob Beech" <Mail@robbeech.com> wrote:

> Normal telephone conversations work around 500Hz.. your hum will be much
> lower than this. I could be wrong here (and would like to know if i am) but.
> Perhaps a simple High Pass Filter could cut out this hum.
>
> Rob

The signals on phone lines are very badly distorted with respect to
ground. A high pass filter will let through all the high frequency
distortion induced by the base 60Hz signal. You still need to start
with a telephone transformer so that the input is well balanced, causing
the noise present on both wires to cancel out. A HiFi ground loop
filter has too much capacitive coupling between windings to do that well
for telephone line voltages.
Anonymous
October 16, 2004 9:40:28 PM

Archived from groups: rec.audio.tech (More info?)

"Sushi" <sushiboy21@sympatio.ca> wrote in message
news:uM0cd.60601$3C6.2309357@news20.bellglobal.com...
> I'm trying to record telephone interviews using a Mac iBook. It
doesn't
> have an audio input so I bought a Griffin iMic, which runs off the
USB port.
>
> My problem: I can't seem to get rid of ground loop hum. I bought a
Radio
> Shack ground loop isolator to put between the phone and the iMic but
that
> doesn't seem to help much. Any bright ideas would be appreciated.

I have a telephone that will record both sides of a phone
conversation--but only for 15 minutes. Is this long enough? I only
paid $9 for it!

Norm Strong
Anonymous
October 17, 2004 4:20:03 AM

Archived from groups: rec.audio.tech (More info?)

Thanks for the replies.

This would explain why the Rat Shack ground loop isolator didn't do much.
What/where is a telephone transformer? Can i easily make one? I can
solder a connection but I'm a novice at just about anything beyond that.




"Kevin McMurtrie" <mcmurtri@dslextreme.com> wrote in message
news:mcmurtri-E711DD.12462916102004@corp-radius.supernews.com...
> In article <fS7cd.279$_Y.103@newsfe3-win.ntli.net>,
> "Rob Beech" <Mail@robbeech.com> wrote:
>
>> Normal telephone conversations work around 500Hz.. your hum will be much
>> lower than this. I could be wrong here (and would like to know if i am)
>> but.
>> Perhaps a simple High Pass Filter could cut out this hum.
>>
>> Rob
>
> The signals on phone lines are very badly distorted with respect to
> ground. A high pass filter will let through all the high frequency
> distortion induced by the base 60Hz signal. You still need to start
> with a telephone transformer so that the input is well balanced, causing
> the noise present on both wires to cancel out. A HiFi ground loop
> filter has too much capacitive coupling between windings to do that well
> for telephone line voltages.
Anonymous
October 18, 2004 5:09:03 PM

Archived from groups: rec.audio.tech (More info?)

"Rob Beech" <Mail@robbeech.com> writes:

> Normal telephone conversations work around 500Hz..

Ttypical telephone specifications list the analogue telephone system
(PSTN) frequency range to be 300 Hz - 3.4 kHz.

> your hum will be much
> lower than this.
> Perhaps a simple High Pass Filter could cut out this hum.

There can be considerable mount of mains frequency (50 Hz or 60 Hz
dependign on country, plus harmonics of them) noise on the telephone
line signal. In typical telephone line there is both common mode
and differential mode noise.
Normal telephones are built in such way that they do not play
back this noise (high pass filter on the circuitry, acoustic
design of handset speaker etc..)

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
!