Audio baluns for sound card input?

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

Hello Graham,

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

Regards, Joerg

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

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From experience or are you conjecturing ?

Graham

This or similar might help

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.

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!

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".

Regards, 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

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?

Ratshack anyone.

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

Hello Jim,

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.

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

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?

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.

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'Audio baluns for sound card input?', on Sat, 26 Mar 2005:

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

Hello Rich,

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

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:

See some applications schematics at

Graham

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

Graham