How eliminate this earth loop?

[Jon D]

I am in the UK.

I have a hi-fi cassette player and I feed the outputs to the line-in
on the motherboard of my PC.

I get a horrible hum on the recording I make on the PC. The
recordings are of phone conversations so the overall quality of what
I am recording is not great but the hum is still very distracting.

(1) I am told that an audio isolating transformer should work.
However I have learnt that basic transformers with only a small metal
core are prone to picking up hum. So I figure any such isolation
will remove noises I don't have but will be liable to getting the hum
I want to get rid of!

(2) I believe another way of reducing hum is to take off the earth
from either the source or destination equipment. However the
cassette player does not have an earth. Would it help to take the
earth off the PC?

(3) Or should I actually add an earth to the cassette recoder? -->
Maybe take the mains earth from the mains socket and connect it to
the outer of the signal lead? Or to the brushed aluminium case of
the cassette recorder?

I get this hum if I take the signal from the line-out or the earphone
sockets of the tape player. Changing the input socket on the PC to
the mic socket does not help. The lead is not damaged because I can
use that lead to replay from a battery-powered dictation machine into
the PC without any hum.

 

[Ethan Winer]

Jon,

I am told that an audio isolating transformer should work. <
Yes.

I figure any such isolation will remove noises I don't have but will be

liable to getting the hum I want to get rid of! <

Unlikely. If you're on a budget Ebtech makes a line of "hum eliminator"
transformers. I wouldn't call these units "audibly transparent," but they're
okay for what they cost. There are a few other companies that make similar
low-cost devices. If you want better quality and are willing to pay for it,
look into Jensen Transformers:

http://www.jensen-transformers.com/

--Ethan

 

[GregS]

I am in the UK.

I have a hi-fi cassette player and I feed the outputs to the line-in
on the motherboard of my PC.

I get a horrible hum on the recording I make on the PC. The
recordings are of phone conversations so the overall quality of what
I am recording is not great but the hum is still very distracting.

(1) I am told that an audio isolating transformer should work.
However I have learnt that basic transformers with only a small metal
core are prone to picking up hum. So I figure any such isolation
will remove noises I don't have but will be liable to getting the hum
I want to get rid of!

You don't want a core to introduce diferential hum signal, but its unlikely to introduce
common mode hum, which is what an isolator is trying to acomplish.

(2) I believe another way of reducing hum is to take off the earth
from either the source or destination equipment. However the
cassette player does not have an earth. Would it help to take the
earth off the PC?

Reversing the plug on the player might work or help.

(3) Or should I actually add an earth to the cassette recoder? -->
Maybe take the mains earth from the mains socket and connect it to
the outer of the signal lead? Or to the brushed aluminium case of
the cassette recorder?

You could try running a wire from the computer case to the player case.

 

[Arny Krueger]

I am in the UK.

I have a hi-fi cassette player and I feed the outputs to
the line-in on the motherboard of my PC.

I get a horrible hum on the recording I make on the PC.
The recordings are of phone conversations so the overall
quality of what I am recording is not great but the hum
is still very distracting.

(1) I am told that an audio isolating transformer should
work. However I have learnt that basic transformers with
only a small metal core are prone to picking up hum. So
I figure any such isolation will remove noises I don't
have but will be liable to getting the hum I want to get
rid of!

I just did some bench tests of Radio Shack's "Ground Isolator" 270-054 using
test signals that maxed out around 2.5 v RMS. ZSource = 150 ohms, ZLoad =
10K ohms.

The measured performance was truely amazing for a pair of transformers case
and cables selling for only $16.65.

All IM, THD, and noise artifacts were at least 80 dB down with most in
the -100 dB range or better. Frequency response showed a 2 dB peak at 20 Hz
and then 10 dB down at 10 Hz. There was a 3 dB peak at about 51 KHz falling
to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz.

No problems with hum pickup were noted.

 

[GregS]

I just did some bench tests of Radio Shack's "Ground Isolator" 270-054 using
test signals that maxed out around 2.5 v RMS. ZSource = 150 ohms, ZLoad =
10K ohms.

The measured performance was truely amazing for a pair of transformers case
and cables selling for only $16.65.

All IM, THD, and noise artifacts were at least 80 dB down with most in
the -100 dB range or better. Frequency response showed a 2 dB peak at 20 Hz
and then 10 dB down at 10 Hz. There was a 3 dB peak at about 51 KHz falling
to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz.

No problems with hum pickup were noted.

Amazing since its dual channel.

I never measured the 600 ohm transformer, but some day I will.

greg

 

[Arny Krueger]

Amazing since its dual channel.

Since this part may vary, I should point out that the UUT differed at least
cosmetically from the picture at:

http://www.radioshack.com/sm-see-all-needs-and-wants--pi-2062214.html

The cylindrical case on the sample tested is more truely cylindrical, with
sharper corners far less nomeclature. The markings are on a black sticker
and lack the pictorals of signal connections and text related to them. A
RCA (female) to 3.5 mm TRS adaptor was provided.

 

[GregS]

Since this part may vary, I should point out that the UUT differed at least
cosmetically from the picture at:

http://www.radioshack.com/sm-see-all-needs-and-wants--pi-2062214.html

The cylindrical case on the sample tested is more truely cylindrical, with
sharper corners far less nomeclature. The markings are on a black sticker
and lack the pictorals of signal connections and text related to them. A
RCA (female) to 3.5 mm TRS adaptor was provided.

The caveat here is the driving requirment. A typical CD player or tape player would
have several hundred or more ohms output, and this unit would not
work. Just how well would it work??

greg

 

[Arny Krueger]

The caveat here is the driving requirement. A typical CD
player or tape player would
have several hundred or more ohms output, and this unit
would not work.

I don't think that is a given.

Just how well would it work??

Presuming that mutual inductance is high across the audio band, the load on
the transformer is equal to the sum of the source and load impedances. IOW
in the case of my test, the total load was 10,150 ohms, which is the
stronger effect. How that is broken up between the input and output would be
the weaker effect.

I did a test where I added an additional 10K in parallel with the output of
the transformer. This reduced the bass peak to about 2 dB, and reduced the
treble peak to 0.6 dB. Both referenced the midband response which was flat
within 0.1 dB from 50 Hz to 10 Khz.

 

[Terry]

I have a hi-fi cassette player and I feed the outputs to the line-in
on the motherboard of my PC.

I get a horrible hum on the recording I make on the PC. The
recordings are of phone conversations so the overall quality of what
I am recording is not great but the hum is still very distracting.

(1) I am told that an audio isolating transformer should work.
However I have learnt that basic transformers with only a small metal
core are prone to picking up hum. So I figure any such isolation
will remove noises I don't have but will be liable to getting the hum
I want to get rid of!

(2) I believe another way of reducing hum is to take off the earth
from either the source or destination equipment. However the
cassette player does not have an earth. Would it help to take the
earth off the PC?

The cassette player has an earth unless it is battery powered. Are you
saying it is battery powered, or are you saying it has just a 2-prong
plug, not a 3-prong plug. A 2-prong plug has a connection to earth on
one of its two wires.

(3) Or should I actually add an earth to the cassette recoder? -->
Maybe take the mains earth from the mains socket and connect it to
the outer of the signal lead? Or to the brushed aluminium case of
the cassette recorder?

I get this hum if I take the signal from the line-out or the earphone
sockets of the tape player. Changing the input socket on the PC to
the mic socket does not help. The lead is not damaged because I can
use that lead to replay from a battery-powered dictation machine into
the PC without any hum.

1) If you are running the cassette player from AC, try reversing the
AC plug.

2) Try plugging the cassette player into the same power strip the
computer is plugged into.

3) Ccan you run the cassette player from batteries? If so, that should
eliminate the problem.

4) Have the cassette player actually play the recording (over it's
speaker), and plug a microphone into the computer to re-record it.
Since it is just telepone conversations, the loss of qualtiy by
re-recording is not really a problem.

5) Isolation transformers should work for this problem, but I would
try the above first.

Terry

 

[David Nebenzahl]

Terry spake thus:

The cassette player has an earth unless it is battery powered. Are you
saying it is battery powered, or are you saying it has just a 2-prong
plug, not a 3-prong plug. A 2-prong plug has a connection to earth on
one of its two wires.

Yikes! No, it should not. That would be a recipe for disaster, man. No,
2-prong devices have a chassis ground that is in no way connected to
either of the incoming line leads.

Even with polarized plugs (which can sometimes be inserted backwards),
this would be asking for trouble.


--
Pierre, mon ami. Jetez encore un Scientologiste
dans le baquet d'acide.

- from a posting in alt.religion.scientology titled
"France recommends dissolving Scientologists"

 

[kony]

Terry spake thus:


Yikes! No, it should not. That would be a recipe for disaster, man. No,
2-prong devices have a chassis ground that is in no way connected to
either of the incoming line leads.

Even with polarized plugs (which can sometimes be inserted backwards),
this would be asking for trouble.

Agreed, it's most likely a two prong plug then a step-down
transformer isolating it.

 

[Terry]

Terry spake thus:


Yikes! No, it should not. That would be a recipe for disaster, man. No,
2-prong devices have a chassis ground that is in no way connected to
either of the incoming line leads.

Even with polarized plugs (which can sometimes be inserted backwards),
this would be asking for trouble.

Well, maybe we're just used to different language on different sides
of the pond. But "earth" does not mean "chasis ground" to me.

 

[Jon D]

The cassette player has an earth unless it is battery powered. Are you
saying it is battery powered, or are you saying it has just a 2-prong
plug, not a 3-prong plug. A 2-prong plug has a connection to earth on
one of its two wires.

My cassette player/recorder (made by Hitachi for the UK market) does not
have an earth.

The mains lead to it has only two wires: live and neutral.

 

[Franc Zabkar]

Well, maybe we're just used to different language on different sides
of the pond. But "earth" does not mean "chasis ground" to me.

You may be thinking of the "mains earthed neutral" (MEN) system. In
such cases, even when there is no earth pin, the appliance can still
be affected by earth loops via capacitive coupling.

See http://www.jaycar.com.au/images_uploaded/humloop.pdf

- Franc Zabkar

 

[Palindr☻me]

My cassette player/recorder (made by Hitachi for the UK market) does not
have an earth.

The mains lead to it has only two wires: live and neutral.

Perhaps even more significantly, the lead probably ends in a
non-polarised plug. The unit must be fitted with a double-insulated
internal power unit. There is no electrical connection from the
mains-side of the unit to the low voltage side.

 

[sQuick]

I am in the UK.

Read above! (as lots haven't)

 

[Jon Elson]

I am in the UK.

I have a hi-fi cassette player and I feed the outputs to the line-in
on the motherboard of my PC.

I get a horrible hum on the recording I make on the PC. The
recordings are of phone conversations so the overall quality of what
I am recording is not great but the hum is still very distracting.

Are you sure the hum is created when the tape is played to the computer?
Have you listened to the tape directly from the player (into stereo, into
headphones, etc.)?

(3) Or should I actually add an earth to the cassette recoder? -->
Maybe take the mains earth from the mains socket and connect it to
the outer of the signal lead? Or to the brushed aluminium case of
the cassette recorder?

That may help. Worth a try, for sure!

Jon

 

[Don Kelly]

----------------------------

 

[Tex]

Are you sure the hum is created when the tape is played to the computer?
Have you listened to the tape directly from the player (into stereo, into
headphones, etc.)?

That may help. Worth a try, for sure!

Jon

I've seen that a couple of times on my system if the input jack is not
fully and/or correctly plugged in. What I get is pure 60 Hz hum. I
find, when it occurs, that I have moved something and partially unseated
one of the jacks. You might want to check to make sure this is not
happening and/or your cords/jacks match and aren't giving you a phase to
ground connection of some kind.

 

[Tex]

I've seen that a couple of times on my system if the input jack is not
fully and/or correctly plugged in. What I get is pure 60 Hz hum. I
find, when it occurs, that I have moved something and partially unseated
one of the jacks. You might want to check to make sure this is not
happening and/or your cords/jacks match and aren't giving you a phase to
ground connection of some kind.

Re: my post immediately above this -- I guess I should not have used the
term "phase to ground" as that sounds like I'm implying 120 VAC across
the connection. What I really meant was some kind of current flow from
power to ground and/or an inductance that causes/allows the ac sine wave
pattern to be present at the audio input.