Audio baluns for sound card input?

[burbeck]

that link again

http://www.maplin.co.uk/Module.aspx?ModuleNo=33172&TabID=1&source=15&WorldID=&doy=1m3

 

[John Woodgate]

I read in sci.electronics.design that Joerg

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 don't know exactly what you are doing (e.g. how many microphones and
how long are the cables), but I think you need to do some tests before
you assume that the power supply is the problem. While these things emit
into the mains supply, they don't normally emit enough radiation to
cause problems with audio unless the audio and power supply cables are
much too close together.

How are you proposing to use 'baluns'?

 

[John Woodgate]

I read in sci.electronics.design that Joerg

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".

It sounds as if it is faulty. Does it do the same thing when used
elsewhere with a different signal source? If so, It IS faulty.

 

[John Woodgate]

I read in sci.electronics.design that Michael A. Terrell

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.

I REALLY would not recommend this. Why transform the audio up to 25 V or
more only to attenuate it again?

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

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.

What Y caps? For a 2-wire, there is no point in including a Y cap.

 

[Jim Gregory]

Assuming your mixer and other apparata are earthed/grounded to a reputable
ground,
try connecting audio screen to mains-operated (grounded) destins *only* at
receiving ends, so dissing them (o/c) at mixer send end, ie, use cores only
which you say are commoned.
But keep the screen for the laptop feed as unbalanced and it can then be
experimentally isolated with a 10k:10k i/p transformer at *receiving* end.
J

 

[Nico Coesel]

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

It depends on the quality of the transformer. Where I used to work we
used a particular type/brand of these telephone transformers for
everything including hifi (roll-off above 20kHz). Also beware not to
saturate a transformer.

 

[Jim Gregory]

Or why not just change the switched-mode(?) power supply to something really
repectable to activate the laptop?
From your description of its filthy waveforms superimposed, it sounds
verboten to deliver decent clean DC outputs.
Jim

 

[Fred Bloggs]

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

Run the laptop off the battery and run a small jumper from laptop case
ground to the power supply safety ground while it is plugged in. If the
noise is present then a small isolation transformer, in conjunction with
screened twisted pair mic cable, should improve the situation and will
be the best you can do. If no noise is present with the jumper, then you
are wasting your time, the power supply is corrupting the sound card
circuitry itself- nothing you can do except go after that power supply
to increase attenuation of high frequency components by cutting cable
and interposing small box with multiple LC lowpass chain and ferrite chokes.

 

[Pooh Bear]

I read in sci.electronics.design that Pooh Bear


What Y caps? For a 2-wire, there is no point in including a Y cap.

See some applications schematics at www.powerint.com. I'll see if I can
dig out the relevant ANs in the office.


Graham

 

[Pooh Bear]

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

If you're going to have active circuitry a simple differential input
op-amp configuration will do the job at better quality.

Graham

 

[Pooh Bear]

I read in sci.electronics.design that Pooh Bear


What Y caps? For a 2-wire, there is no point in including a Y cap.

Correction. Not in the EMI filter.

See the following example....
http://www.powerint.com/images/schematic/di43.gif


Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

If you're going to have active circuitry a simple differential input
op-amp configuration will do the job at better quality.

But those have very poor CMRR for radiated RF! Just what is NOT wanted
in this case (if what the OP thinks is the cause is indeed the cause).

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

See some applications schematics at www.powerint.com. I'll see if I can
dig out the relevant ANs in the office.

I don''t know what you wanted me to look at but I looked at the EPR11
schematic, which is for a 2-wire and ... no Y-cap.

 

[Michael A. Terrell]

I read in sci.electronics.design that Michael A. Terrell


I REALLY would not recommend this. Why transform the audio up to 25 V or
more only to attenuate it again?

1: I did this because it was usually the only decent audio transformer
available when I needed to build another interface in a hurry. I just
use the pair of pots when there is no hum problems. BTW, the highest
wattage tap on the transformer has the lowest voltage. If you use the
16 ohm winding and a 5 watt tap the voltage doesn't go up very much.
The highest voltage is on the lowest tap, or 25 volts. Not only that,
but the voltage only reaches 25 volts at the full rated output of the
amplifier.

2: A good 600 ohm to 600 ohm transformer can be expensive and hard to
find in a hurry. I have a pile of crappy mil spec 600 ohm to 600 ohm
transformers that roll off a little over 2.5 KHz and others that aren't
much higher quality.

 

[Michael A. Terrell]

If you're going to have active circuitry a simple differential input
op-amp configuration will do the job at better quality.

Graham

How does this remove the hum? one input of the op amp will be
grounded because it is an unbalanced source.

 

[John Woodgate]

I read in sci.electronics.design that Michael A. Terrell

I did this because it was usually the only decent audio transformer
available when I needed to build another interface in a hurry.

Yes, the show must go on, of course. I know someone who used a PENCIL
LEAD as an attenuator in those circumstances! But I wouldn't put it
forward as a recommendation.

 

[J M Noeding]

I read in sci.electronics.design that Pooh Bear

But those have very poor CMRR for radiated RF! Just what is NOT wanted
in this case (if what the OP thinks is the cause is indeed the cause).

remember when they started local radio in this area (like BRMB or
Radio City 199), they had no understanding for unbalanced equipment
and balanced telphone lines,we got a lot of complaints. Transformers
cured the problem. I don't understand what is so difficult about
transformers, you find them in allmost any sorts of modems

-JM

 

[J M Noeding]

1: I did this because it was usually the only decent audio transformer
available when I needed to build another interface in a hurry. I just
use the pair of pots when there is no hum problems. BTW, the highest
wattage tap on the transformer has the lowest voltage. If you use the
16 ohm winding and a 5 watt tap the voltage doesn't go up very much.
The highest voltage is on the lowest tap, or 25 volts. Not only that,
but the voltage only reaches 25 volts at the full rated output of the
amplifier.

2: A good 600 ohm to 600 ohm transformer can be expensive and hard to
find in a hurry. I have a pile of crappy mil spec 600 ohm to 600 ohm
transformers that roll off a little over 2.5 KHz and others that aren't
much higher quality.

useful transformers are not difficult to find if you know how an audio
transformer looks like, you probably have a lot of equipment where
they are without considering it. Impedance is not important when the
equipment hasn't been made to a specific impedance

Jan-Martin

 

[Michael A. Terrell]

useful transformers are not difficult to find if you know how an audio
transformer looks like, you probably have a lot of equipment where
they are without considering it. Impedance is not important when the
equipment hasn't been made to a specific impedance

Jan-Martin

I have several hundred transformers and most have poor frequency
response for this application. I also pointed out commercial "DI Boxes"
sold in music stores at high prices.