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anonymous 14 January 2008

In our project studio, we have several pieces of gear that use 9 volt AC wall-wart type power supplies. (When you have 16+ wall-warts, I think the term "infestation" is applicable).

I thought it logical to assume that by purchasing a large current capacity transformer, I could replace the power supplies. I tried an 18VAC center-tapped transformer, and balanced the loads between the legs of the secondary. The hum rendered the project useless.

I thought at first that the hum was induced by the transformer field, so I move it to 20 feet away, with no improvement.

Any ideas on how I can make this work?

Comments

pr0gr4m Mon, 01/14/2008 - 21:50

Sorry man. The original puts out A/C...I didn't look at the specs for their new one...kinda figured it would be the same. My bad.

Anyway, the original has A/C but it only has 4 outputs. That's probably not a cost effective solution.

I have the same problem except for rack synths. It's a pain in the arse. I'm waiting for this mPathx company to release their single rack wall-wart killer.

Boswell Tue, 01/15/2008 - 02:50

By attempting to replace several wall-wart PSUs with a single unit you might get one or two bits of gear to work by chance but you will just as likely blow the rest up. Don't go there!

Each piece of equipment is entitled to assume its power supply is floating with respect to the ground. Quite apart from the different voltages needed and whether they are a.c. or d.c., some equipment design may position the power input supply half-way up in order to get + and - rails, some may have the +ve grounded to get a single -ve rail, and others will ground the -ve to get a +ve rail. As soon as you connect an audio cable from one piece of gear to another you could short circuit the power supplies.

It's a pain to have socket boards full of those wall-warts, but you have to be really, really careful to use anything else. A custom-designed multi-output floating supply is about the only thing that would work safely in this application and not have intrinsic ground loops that can cause the sort of hum you mentioned.

moonbaby Tue, 01/15/2008 - 06:00

I'm no electrical engineer, but as I have understood things over the years, audio gear requires a FULLY RECTIFIED power supply design (as opposed to a "half-wave rectifier"). Without the properly designed filtering, you will get very obnoxious 60Hz. hum intrusion. What type of filtering does your "master supply" provide?

Link555 Tue, 01/15/2008 - 11:36

Boswell makes a very good point. Be careful to check which side each piece of gear uses as a common (circuit return path). If they are all using the negative rail as the common point, or they all using the postive rail as the common, then you might be ok. However if they are not then you will need to have isolated DC supplies on each 9VDC output.

There are plenty of options for this , but most use a switching power supply with a small transformer. for Instance:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1113,P1549,D4364

Contact me via email if you want more info on how to design an isolate power supply.

Best of Luck,
Link

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