1. Home
  2. amps-preamps
  3. 196160-1630

Another mechanical 60 Hz noise post

Checked the previous posts on this here, but can’t find anything that seems to apply to my situation.

To start, please check out this pic of my office system:

At the bottom of the rack, you’ll see my vintage Belles Model One, purchased new and well-loved.  It’s been routinely used and never abused since I purchased it in 84 or 85.  It has always been located 10’ or more from my listening position until now.

During a late-night listening session with my Stax SR-404 Signature cans through an SRD-7SBmk2, I heard a hum when I took them off to return the beer.  Did the isolation drill and discovered the amp is making a physical hum.  It does not put any hum out through either the speakers or the ‘phones.

I haven’t ripped it out of the rack yet; as long as I have the volume up, I can’t hear it.  With nothing playing, the hum is about the same level as the radon extraction system that runs outside the right-hand wall.  And time to take my workspace apart is very rare.  Before I put it in the rack, I took the cover off, gently dusted the inside with air and looked things over.  Discovered the plastic on the old speaker terminals had cracked so replaced them.  Didn’t see anything else amiss and the caps aren’t bulged or leaking.  When fired up, it sounds just as good as the day I brought it home.  Listening to it now, in fact.

 

So here’re my questions:  Not being able to find a review on it, is what I’m hearing an artifact of this very first Belles effort that I just never heard before?  Since I will take the amp out and pull the cover off for an inspection in any case, what would be the most likely thing to look for?

effischer
jeffbij
272 posts
 

+1 with @ghdprentice suggestion.

I believe that amp uses a transformer that has steel plates and the copper windings are wound around the plates.  (as opposed to a torridal type).  It is not uncommon that as they age, the plates start to separate slightly and vibrate.  (The "hum") Sometimes the plates are bolted together and just need to be tightened up, or if they are glued together with some kind of varnish, they can delaminate.

Usually it is not a catastrophic problem, but is annoying and can cause heat problems and/or add noise to the circuitry.  If is gets real bad, it can cause voltage issues that can do damage.

There are several ways to repair plate separations.  Just Google "delaminated audio transformer plates repair" for suggestions.

Hope this helps.
 

Jeff

 

erik_squires
17,549 posts
 

So, yes tranformer hum, but also, it can be due to a variety of electrical issues in the house.  If you move it to the previous outlet, does the hum go away? 

The most common reasons for hum are a lifted neutral or DC on the line.  Lots of LED light supplies and PC supplies produce DC on the line.  It happens when they only draw power from the (+) or (-) swing of the AC line.  On a scope this causes the sinusoidal waveform to shift up or down, and are no longer centered on zero. 

erik_squires
17,549 posts
 

Besides moving the amp, another thing you can try is turning everything else off on the same AC circuit, and if that still doesn't work, go to your breaker box and turn off every single breaker.  You may have DC on one leg, caused by another device. 

effischer OP
577 posts
 

Thank you for the input everyone!  I ran the isolation drill again and then powered the amp from a totally different circuit without any DC devices in it. That confirmed @ghdprentice and @jeffbij identified the most likely culprit.  The transformer is definitely not toroidal, FWIW.

When I get a chance this next weekend, I'll pull the amp out of the rack and give the transformer a hard look.  I'd thought it the sound was too pronounced to be anything other than a vibration of some sort, so I'm going to check for loose bolts and such first.

Hopefully, I won't have to rely on Professor Google for more involved repair.  I really don't want to have to go that far if I can avoid it.

Thanks again!

jeffbij
272 posts
 

@effischer  -  No problem.  Glad to help.

Just be careful.  It only takes 100 milliamps to stop your heart.  And at 100W into 4 ohms is 5000 milliamps....

If the plates are too loose or have delaminated enough, it is possible that some of the windings maybe broken and are shorted to the plates.

Make sure the amp is unplugged and let it sit for a while.  You may want to discharge any of big caps hooked to the power supply side.

More to discover