Audio baluns for sound card input?

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

Hello Graham,

From sad experience. The first laptops were all two wire and resonably quiet. Ok, some noise on the AM radio but that was it. The "new and improved" laptops all came with three wire and sometimes when I measure subtle stuff in the lab I have to unplug the PS. Never had to do that with the old Wangs and Compaqs.

Their PS has to crank more power though because these are "new and improved systems", meaning you need 256MB RAM, 2GHz clock and stuff to write 'hello world' these days. But the noise still increased disproportionately.

Regards, Joerg

Hello Ban,

That's what I always wanted to have. But there doesn't seem to be enough of a market since you can't buy anything decent around here for wireless audio xmit. Except the devices X10 offers but they are bulky, need a power supply each etc. Not very practical, too much cabling to tangle up.

Regards, Joerg

So, it's not so much that the power supply itself is providing the noise, just that the ground is so long that it makes a nice high impedance path for the common mode noise to excite?

Makes a lot of sense, when you put it that way. Transformers it is! :-)

Thanks! Rich

Hello Rich,

Well, the PS is the source. I believe these things have gotten a lot worse over the years. After hooking up a new printer a couple months ago I couldn't even hear our local AM station anymore. Blew me away. All you heard was hash. Until I cut down on it with a good dose of #43 ferrites. Now we have more #43 toroids in the supply bins than other people have aspirins. Luckily that stuff is cheap.

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.

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.

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Michael A. Terrell
Central Florida

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

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.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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.

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Michael A. Terrell
Central Florida

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

--
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/91n.htm

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)

LF will do any saturating that needs to be done. ;-)

Graham

No they don't ! Depends what you mean by high I suppose.

Graham

To cut a long stort short.... Can you afford $30 ? It's small, portable and does what you want.

Pro audio guys call it a DI box.

Graham

Fred Bloggs wrote

Joerg said his mixer o/p is unbal'd and multi-distributes from the one o/p, so no buffering DA is in use. Nor do we know source Z. Imagine it's about

50r. So could protect mixer o/p distrib star node from accidental shorts with, say, a 100r - 150r resistor in series with *each* send core, and add one spare protecting way for the next probable add-on, or for testing, while he's at it. There isn't a strong TV Tx mast close by, is there? Worth asking, too.

Yes, I now agree the PSU must be the offender and should be replaced by a behaving, smooth one - they can't *all* emit large pulses on their DC lines. (Or use a fully charged, high-capacity accumulator - and thus we would never have read this which appears to have landed on a helpful but unlikely newsgroup anyway!). If he nevertheless tries to use a low inductance AF 10k:10k (or a 10k:2 x

5k secs for series/parallel config) xformer for i/p isolation, it must be canned + connected to source ground and preferably have an Electrostatic screen wire/tag which also *should* be tied to the screen of unbal cable from the mixer - as it is a good ground. Even smallish ones can handle +12dBU at 30Hz. But I don't know how rare or expensive these are across the Pond.

We still don't know what levels come out of the mixer port. If average operating level is still too high, add a 10k to 22k log preset across the secondary, with its wiper and low side to "send" to laptop line i/p jack.

Hello Michael,

Essentially the passive DI boxes are like that Radio Shack solution, probably much better in frequency response though. Money is, of course, an issue at church.

If you ever need a decent quality audio xfmr check these out:

Regards, Joerg

Hello Jim,

Yes, some day I think we have to tear it all down and rig it up again, mainly because the system grew over time. But for now we have to leave it as is and some cable connections are behind walls. So at this point we are going to try what works reasonably well without tearing the whole thing apart. Some noise is tolerable because it is mainly voice (the sermon) that is going to be recorded and not music.

Regards, Joerg

Hello Jim,

That's an option and actually one I did contemplate here in the lab. It's all brand name stuff but when I placed an inkjet/scanner combo there it blew my mind. The noise that its little power brick made was so bad that I had to give it its own outlet with some big #43 toroids in there.

Regards, Joerg

Hello John,

Maybe, but it's not my laptop so I'll have to ask. Some "modern" power supplies seem weird to me. One in a new printer works but produces lots of EMI and makes inconsistent hissing noises. Like a whining bearing. Dell said that's "normal".

Regards, Joerg