Power Transformer Hum in PM-1000 (not the audio path)

Hi folks,

So I’ve finally got one channel of PM-1000 racked up. I’ve got another ready to drop into the case but I wanted to get this first channel running smoothly first. This has been one of my first DIY projects and it’s been tons of fun – a huge learning experience too. Thanks to everyone who has posted their projects on here in the past…you’ve all been a huge help!

I got everything wired up yesterday and finally turned it on. Tested voltages. Everything checks out. Pushed audio through it. It works! Wahoo! :grin:

[Well, mostly wahoo. The EQ doesn’t seem to be working, but these channels had broken inductors. I replaced them with some other inductors but the EQ is still dead. Something is screwed up somewhere. :? But I digress…]

Here’s my issue:

I’m using the PowerOne HB48-0.5-A power supply that a lot of people recommend for this project. The thing hums. Not quite as loudly as a fluorescent light fixture but enough for me to hear it. Certainly louder than the CRTs in my control room.

Now, luckily, the audio path is clean as a whistle (in fact, it is dead silent – I did the recap and changed out the transistors – sounds great!). But I’m pretty darn sure that my Baby Bottle is going to pick up the sound of this transformer humming away unless I record at a significant distance. And given the size of my tiny "studio" I can only get about 10 feet away at best. :roll:

So I’ve been looking around on here, but I’m not finding any solutions that make me feel particularly hopeful.

http://www.groupdiy.com/index.php?topic=12354
(drop the transformer in a can of varathane – hmmm, I don’t really want to take apart the power supply, but if I have too I suppose I could. Sounds messy.)

http://www.groupdiy.com/index.php?topic=7167
(Replace the transformer. This is a “duh” but I’ve already had this power supply too long to send it back and I’d rather not be out another $40… I suppose this is where I’ll end up if nothing else turns up.)

http://www.groupdiy.com/index.php?topic=3093
(Tighten transformer lams, maybe do some DC filtering? I don’t think I have loose lams, but maybe the DC thing could be the issue? None of my other equipment seems to exhibit this problem though, and it’s all on the same star-grounded circuit.)

And that’s pretty much all I’ve found so far. Does anybody have any other thoughts on this kind of situation? Any other PM-1000 owners come across this problem?

Mechanical hum?

What are the specs?

0.5 or 5 amps?
48 volts?
Supply more info than you think we need.

When a transformer has zero load on it, the flux that the core sees, believe it or not, is at it's highest.
Why?
Because no energy is being transfered, so the magnetizing energy going into the core stays there.
The core can even saturate, especially in the case of a current transformer, where the sec voltage of an unshorted CT will rise until the B max makes the core weak in the knees.
So what?

Well, that means core rattle is at a max.
You might quiet down that xfmr by loading it with some power resistors.

Weird, I have bought 3 or 4 of those guys, and loaded or unloaded, not much hum.

Good luck.

cj

Thanks, CJ! That makes sense, but I never would have thought of that. The power resistors are a brilliant idea.

This is the 0.5 amp 48 volt model power supply. As I recall from another thread on here, someone measured their PM-1000 drawing about 100 milliamps per channel. I've only got one channel plugged in right now, but even when I get the second one in I'll only be pulling on 2/5 of what the supply can provide. Plus or minus what little curent a phantom powered mic will draw.

I'll be able to wire in the second channel in a few days and then I'll see if that quiets it down any. If it helps I can try throwing in some power resistors to see if that tames it even more.

Jed,

Did you ever come to a conclusion for the hum in the 48v power supply transformer?  I have the same supply and have had it installed and running phantom on my console for over a year.  It has been perfectly quiet.  Then one day I turned on the supplies and it started to hum.  I have a HDD-15 and the Power One 48v supplies mounted in a chassis and I tracked the hum down to the 48v supply.  The output is perfect and clean!  I bought 2 of them so I might pop in the backup but I'd like to get this one hum free if possible.