60-cycle hum from Variac - How to get rid of it?

emmjaydubya · Post by **emmjaydubya** » Sun Apr 14, 2013 10:17 am

I posted this in the repair section but no luck. Recently bought a 5-amp autotransformer with no enclosure, brand new in box from ebay and put it in a steel ammo box to run my 12,000 at 90v. Just tested it yesterday and its making a pretty bad hum when rolling the volume up and on full. Can't be sure it's 60-cycle but that's what it sounds like. My Ohmite Variac doesn't do this and I think the new one is wired correctly since the voltages check out fine, but I could be wrong.

I know a bunch of you guys use Variacs on your plexis, have you run into this problem before? Is there a way to filter the power on the secondary side going to the output plug? Some kind of filter or transformer to kill the hum? Any ideas would be great.

BTW, the speaker motors are not wired up yet, so they are not a factor...

rgorke · Post by **rgorke** » Mon Apr 15, 2013 9:21 am

What do you mean by "rolling the volume"?

ON the guitar? On the amo?

Could you post a clip to hear what it sounds like?

rgalpin · Post by **rgalpin** » Mon Apr 15, 2013 12:16 pm

vanhalen5150 · Post by **vanhalen5150** » Mon Apr 15, 2013 12:37 pm

I use my variac on a completely different output. Different fuse in the main fuse panel.

ampSnob · Post by **ampSnob** » Mon Apr 15, 2013 2:29 pm

Strat78 wrote:I don't think variacs like having other electrical stuff too close to them. I had a horrible buzz coming from my amp and after spending a good hour trying to isolate the problem in the amp, turns out my solder gun (turned off mind you) was sitting too close to the variac, now there is total silence. Don't think the motors will make the variac too happy in that metal box, still, what a great contraption you've created!

I mounted a variac about 1cm from an output transformer in a wacky 100watt build and it was dead quiet. Good grounding like a star ground is essential for this kind of thing though. Variacs are a lot like toroidal transformers. The magnetic fields created by them are pretty much focused in the doughnut.

julkke · Post by **julkke** » Mon Apr 15, 2013 2:40 pm

Guys, DON'T LIFT THE GROUNDS IN YOUR AMPS! They are there for a reason.

Strat78 · Post by **Strat78** » Mon Apr 15, 2013 3:36 pm

I don't think variacs like having other electrical stuff too close to them. I had a horrible buzz coming from my amp and after spending a good hour trying to isolate the problem in the amp, turns out my solder gun (turned off mind you) was sitting too close to the variac, now there is total silence.

Don't think the motors will make the variac too happy in that metal box, still, what a great contraption you've created!

rgalpin · Post by **rgalpin** » Mon Apr 15, 2013 3:38 pm

emmjaydubya · Post by **emmjaydubya** » Mon Apr 15, 2013 5:41 pm

Woah! Easy now! Where do I start... First of all, no grounding is lifted, I'm not a total dumbass.

Ha, but maybe I am for trying this! I wanted something that would do several things in one box, since dragging around an Ohmite with all the cables is a pain in the ass. The Variac is STRICTLY for REDUCING VOLTAGE to the plexi, not as an attenuator, if that's what some of you are thinking. And the speaker motors are for eating power from the second speaker jack (ala vanhalen5150's speaker motor box), so these two things are totally UNRELATED. They just reside in the same box, and they both reduce volume. Guess I shoulda made that more clear.

Now, residing in the same box might be the mistake, since I hear you guys about the possible noise they will cause each other. I just had to try it, could be a failure. But before I find out if it is a noise disaster, I have to iron out what's wrong with the variac making a buzz, and I think Ampsnob helped me figure it out. Hopefully...

In the meantime, here's a pic of the outside...

vanhalen5150 · Post by **vanhalen5150** » Tue Apr 16, 2013 1:39 am

Looks like it needs launch codes to work properly.

So what did ampsnob think was the problem?

emmjaydubya · Post by **emmjaydubya** » Wed Apr 17, 2013 12:56 am

Do you have the launch codes? Cause I don't. Still not sure what's going on, it's wired per instructions. Gonna tear it back apart this weekend.

ampSnob · Post by **ampSnob** » Wed Apr 17, 2013 5:58 pm

We've been talking about it in this thread:
http://forum.metroamp.com/viewtopic.php?f=4&t=41620

The hum comes from ground potential being way way off neutral. It should be within a volt. This is not only annoying put potentially dangerous should he use a mic or something that's grounded right, he could get a shock. It's just a wiring problem though, very fixable.

MrBeasty · Post by **MrBeasty** » Fri Apr 26, 2013 10:21 am

julkke wrote:Guys, DON'T LIFT THE GROUNDS IN YOUR AMPS! They are there for a reason.

+1

Safety first!!

rgalpin · Post by **rgalpin** » Wed May 01, 2013 12:22 pm

although i did lift the ground on my coffee maker this morning, and this thread was echoing in my mind as i did it!

emmjaydubya · Post by **emmjaydubya** » Thu May 02, 2013 9:11 pm

Where in this thread, ANYWHERE, did I say that the ground was EVER lifted on this Variac? Maybe you guys need to re-read it again. Sheesh...

Metropoulos Forum

60-cycle hum from Variac - How to get rid of it?

60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?

Re: 60-cycle hum from Variac - How to get rid of it?