Audio baluns for sound card input?

[Joerg]

Hello Folks,

When someone at our church tried to record audio into a laptop this
caused a serious racket if line powered. Probably this is due to the
laptop switch mode power supply. Only running the laptop off its battery
produces a decent recording. Is there a simple balun transformer that
you could recommend?

I know the Muxlab (Montreal) versions which have great frequency
response. However, these are rather bulky devices that are ok for a
fixed installation. But this one needs to be small and portable even if
it's not 100% "hifi".

Ok, of course I could make one. But if there is a smaller of-the-shelf
version that would be better.

Regards, Joerg

 

[j.b. miller]

How about using the common 'telephone 600r-600r' type transformers ? Easy to
find,just hack any old PC modem card.or if you have access to an old Amtelco
concentrator, they have great audio transformers in there!
jay

 

[Jim Gregory]

The pulses within the PSU may be playing havoc. Try it again running off
mains, but make sure the PSU is as far away as the cable allows - so its
electromagnetic radiation won't influence the sound card at all. Orientation
of laptop may also minimise the extraneous pulses.

What type of mic (assuming it's a mic) cable and what length is being run
into laptop?
Balancing transformer (it's not that big) not encouraged yet until we have a
scenario.

 

[Jim Gregory]

Usually, if tiny, the modem xformers have the mid-band freq response of a
telco line (200Hz to 4kHz); that's all they ever need to isolat!

 

[Joerg]

Hello Jim,

The pulses within the PSU may be playing havoc. Try it again running off
mains, but make sure the PSU is as far away as the cable allows - so its
electromagnetic radiation won't influence the sound card at all. Orientation
of laptop may also minimise the extraneous pulses.

We did try that. The noise didn't change.

What type of mic (assuming it's a mic) cable and what length is being run
into laptop?
Balancing transformer (it's not that big) not encouraged yet until we have a
scenario.

It is out of the main mixel panel at line level. This source feeds other
things such as a speaker amp, hearing wands etc. Those didn't get
disturbed so it looks like the laptop is "self polluting".

Regards, Joerg

 

[Joerg]

Hello Jay,

As Jim hinted these are for telephone line bandwidths. I tried them
before and while some modem xfmrs do exceed 4kHz substantially they did
roll off below 300Hz.

Regards, Joerg

 

[Jim Gregory]

Joerg
You say the pre-mixed feed also goes to several other destins. Literally in
parallel with the line level to laptop i/p, or are these from different
facility ports/interfaces? Are these balanced o/ps (floating, ground-free),
and do your other bits and pieces have bal or unbal i/ps?
Is your i/p level into laptop too high? 200mV should suffice. Has it got agc
or has it got adjustable (via software) sensitivity?

 

[Mook Johnson]

Ratshack anyone.

they have an audio isolator that is good quality for non audiohpile
applications.

 

[Joerg]

Hello Jim,

You say the pre-mixed feed also goes to several other destins. Literally in
parallel with the line level to laptop i/p, or are these from different
facility ports/interfaces? Are these balanced o/ps (floating, ground-free),
and do your other bits and pieces have bal or unbal i/ps?

Spliced in (parallel), same port. It is non-balanced. This is all on the
output side, all the mikes go into the mixer inputs and some of those
are balanced.

Is your i/p level into laptop too high? 200mV should suffice. Has it got agc
or has it got adjustable (via software) sensitivity?

This one I'd have to check since it was somebody else doing the actual
recording. I believe you had to set the audio level in software (no AGC
function). AGC actually would not be desired here since the Sunday
services have moments of silence.

Regards, Joerg

 

[Joerg]

Hello Mook,

Radioshack is what I vaguely remembered. I didn't know they called it
isolator. Their web site doesn't offer a lot of tech info, could it be
this one?

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-054

Regards, Joerg

 

[Rich Grise]

Hello Jim,


We did try that. The noise didn't change.


It is out of the main mixel panel at line level. This source feeds other
things such as a speaker amp, hearing wands etc. Those didn't get
disturbed so it looks like the laptop is "self polluting".

If it's power supply noise, how is a transformer supposed to help?

Thanks,
Rich

 

[Pooh Bear]

If it's power supply noise, how is a transformer supposed to help?

Thanks,
Rich

Common problem with laptops. The psu is usually a 2 wire job , so there's no nice
clean earth for the Y caps in its EMI filter.

Connect it to an earthed device ( such as much audio gear ) and the current from
the Y caps likes to go there instead.


Graham

 

[Joerg]

Hello Rich,

If it's power supply noise, how is a transformer supposed to help?

An audio transformer can cut down on common mode noise currents quite a
bit. Just hook an EMC current clamp onto the DC line from the power
supply into the laptop. Then look at the analyzer when you connect
anything else to the laptop. You can even see it on the scope and the
picture ain't pretty. This stuff propagates onto almost any cable that
connects to the laptop.

Regards, Joerg

 

[Joerg]

Hello Graham,

Common problem with laptops. The psu is usually a 2 wire job , so there's no nice
clean earth for the Y caps in its EMI filter.

Connect it to an earthed device ( such as much audio gear ) and the current from
the Y caps likes to go there instead.

My impression is that it got worse with the three-wire versions.

Regards, Joerg

 

[Pooh Bear]

Hello Graham,


My impression is that it got worse with the three-wire versions.

Regards, Joerg

From experience or are you conjecturing ?


Graham

 

[Michael A. Terrell]

Hello Mook,

Radioshack is what I vaguely remembered. I didn't know they called it
isolator. Their web site doesn't offer a lot of tech info, could it be
this one?

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-054

Regards, Joerg

No. You need to take the brute force method. An isolation
transformer connected to the PA amp's speaker outputs and a two stage
attenuator to reduce the level to what you want. get a 25, 70 or 100
volt audio line transformer and connect the low impedance side to the
speaker line. Use a 10K ohm pot followed by a second 10 K Ohm pot. The
first is to set the maximum level, and the second is used to adjust for
changes in the system levels, if needed. The first pot should be inside
the metal project box so it can't be tampered with. Add a capacitor to
the output lead in case you have to use the microphone input on the
computer. they are made for electret microphones and have a DC voltage
on the jack. Use a three circuit plug to keep from shorting out the DC
supply and connect the audio to the tip, and ground to the sleeve. I've
used variations of this circuit for over 30 years to make quick
connections to existing sound systems on a moment's notice. Something
similar is sold as a "DI BOX" for musicians but they cost a lot more
than a couple pots, transformer and a metal project box.

Here is the first Google link for a passive DI BOX:
http://www.leonaudio.com.au/passive.htm

 

[[email protected]]

This or similar might help
http://www.rohm.com/products/databook/audio/pdf/ba3121.pdf

 

[Tony]

Hello Folks,

When someone at our church tried to record audio into a laptop this
caused a serious racket if line powered. Probably this is due to the
laptop switch mode power supply. Only running the laptop off its battery
produces a decent recording. Is there a simple balun transformer that
you could recommend?

I know the Muxlab (Montreal) versions which have great frequency
response. However, these are rather bulky devices that are ok for a
fixed installation. But this one needs to be small and portable even if
it's not 100% "hifi".

Ok, of course I could make one. But if there is a smaller of-the-shelf
version that would be better.

Regards, Joerg

I've used common mode chokes successfully in this situation, and also
in the serial port connection, since this too seemed to be necessary.
Compared with an isolating transformer, at very low frequencies, the
transformer has great isolation but poor response, whereas the CM
choke has no isolation but great response, and this seems to be more
appropriate. They are similar at HF, but as the load impedance can be
quite high, there should be no real problem at HF. Try anything that's
available, eg I've recently come across some very cheap CM chokes for
ethernet applications that are 4.7mH with 4 windings. Others are up to
47mH (better still).

Tony (remove the "_" to reply by email)

 

[Ban]

My impression is that it got worse with the three-wire versions.

Regards, Joerg

One thing to be considered is to buy an external USB2 soundcard for the
laptop, because the usual input is only meant for an electret mike and mono.
Even if there are line inputs, they are usually not well shielded and the
interference with the digital noise is at elevated levels. Another
possibility is the use of a wireless link with line inputs. These would
eliminate the need for galvanic connection and solve the A/D conversion as
well.

 

[burbeck]

hi joerb,
this unit is ideal for this use stereo 2 isolating transformers in one
box, used for car
audio.http://www.maplin.co.uk/Module.aspx?ModuleNo=33172&TabID=1&source=15&WorldID=&doy=1m3
availible from radio shack i think
regards
bob