Hunting wabbits...and 17v PS's

Posted: Wed Feb 28, 2007 9:13 pm

Umm...not actually hunting wabbits. That's Elmer Fudd's job. My job is tracking down a +/- 17v power supply for a 40 ch. Soundtracs MRX. I spoke with Steve Magalnick, who is pretty much THE US Soundtracs guy, he advised me to pick up a few Power One supplies (48v 1A and 24v 2A and +/-17v 3A per leg) and wire them up in a box. Sounds good, except all my searches on Digikey and Mouser couldn't turn up the 17v supply. Questions:

1. Anyone know where I can pick up a +/- 17v (3a/leg) supply?

2. Failing that, could anyone tell me how to regulate a +/- 18v (3A/leg) down to +/- 17v?

3. Failing both of those strategies, could some kind soul point me towards a schem for a +/- 17v supply (you guessed it; 3A/leg)

Thanks in advance, I appreciate any and all suggestions.

Posted: Thu Mar 01, 2007 6:45 am

Try model number SRW-115-2007 from TRC. It's rated at +/-17V at 3.4A.

Data sheet: http://www.trcelectronics.com/IPD/PDF/srw_115.pdf

Otherwise, if you would like to build one yourself, check out the ProdigyPro forum, where they're perpetually discussing power supplies and other exciting things: http://www.prodigy-pro.com/forum/

Posted: Thu Mar 01, 2007 10:02 pm

Thats perfect! Thanks so much! Would you mind if I asked how you came across this power supply? Was it something you previously knew of or did you do a search on some magic website somewhere? Thanks again for your time.

Posted: Fri Mar 02, 2007 4:25 am

I didn't know of that particular one until I found it in response to your post, but I'm a professional electronic design engineer (by day) and frequently have to unearth unusual components to solve tricky design problems.

I tried initially to find a linear power supply, as I generally prefer those for powering mixers and other low-level audio gear, but the only linear types I could find were the adjustable bench supplies, which I judged unsuitable for this application. The SRW range from TRC are switchers, but I think you should be OK with that one. I would use screened cable from the supply to the console power input connector.

If when it's all installed and working you can hear faint birdies or a whine from your console output when wound full up and no input, try clamping a couple of large common-mode ferrites across the power supply cable, one at the supply output and one at the console input. Although the switching frequencies used in this type of power supply are well above the limit of human hearing, you can get rectification effects at P-N junctions within the audio circuitry causing demodulation into the audio range.

Good luck!
Boswell

Posted: Fri Mar 02, 2007 4:30 am

Double post, sorry.

Posted: Sun Mar 04, 2007 8:50 pm

How strange... I posted a reply but it doesn't seem to be on the thread?

Anyway, mostly what I said was something about I am assuming you mean common mode ferrite chokes when you said "common mode ferrites" and is there a particular cutoff freq. for those or is it determined by the max current? Also, when you mention rectification at the P-N junctions, are you refering to the diodes specifically? I apologize that I'm not an EE and still figuring out a lot of things, thanks.

Posted: Mon Mar 05, 2007 10:39 am

Yes, there's something a bit odd about the posting on this board at the moment. I was surprised to find two copies of my previous post when I know I only clicked once on "submit".

Ferrite chokes: this is the type I had in mind, particularly the sort similar to the ZCAT-C models where the cable takes a whole turn inside the ferrite core: http://www.farnell.com/datasheets/89293.pdf

Rectification at P-N junction is a common but little-mentioned problem in transistor and integrated circuit electronics. I won't go into great detail on these forums, but what happens is that the base-emitter junction of transistors within the audio amplification chain can rectify components of signals around or above the ft of the device and pass the demodulated signal to the output. So harmonics of the PSU switching frequencies in the hundreds of MHz can end up being demodulated and appear in the audio. Since their fundamental frequencies are usually R-C oscillator rather than crystal controlled, small frequency variations can be heard as faint warbles or "birdies" in the audio output. Ferrites on the PSU cables help prevent these high harmonics reaching the audio electronics.

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