I own a pair of ART SLA-2 amps that I use with two separate Bag End subwoofer systems in a small home studio.  They are great amps with very flat response to 10Hz plus a decent amount of power (200w stereo or 560w mono into 8 Ohms), all in a solid 1U rack package.  Hey, guys need amplifiers.  Actually everybody needs amplifiers…and subwoofers…ones that can go into the infrasonic range like the Bag End subs can…but that’s for another post.  Anyway both of my SLA amplifiers had issues with fairly loud buzzing right out of the box.  With the first one it was an intermittent buzzing/crunching sound that would come and go.   My second amp just had an annoying and steady 60Hz buzz.  With both of these amps the problem would occur with no input connected and even with both level controls at -∞.  It turned out that this was caused by a very simple problem in both amps that only took a few minutes to fix.

Note:  I don’t advocate that anyone without proper experience work on their own amp.  This post is merely intended to bring to light a simple assembly issue with these amps that makes them prone to buzzing, and what I did to fix it.

In a nutshell the problem is usually caused by an internal audio line being either directly on top of, or underneath, the toroidal transformer.  The transformer’s magnetic field then induces 60Hz hum in the cable, or sometimes a sort of intermittent crunching/static sound.  On my first SL-2 the wire was tucked underneath the transformer and moving it out to the side fixed the issue.  On the second SL-2 ART had added what appears to be some mu-metal shielding around the transformer which blocks the space underneath.  On this amp the wire was on top of the transformer.  Not much point in shielding the transformer if you are going to compromise it in this way.  Below is a photo with the top panel removed and the wire highlighted.  The panel is held on with several screws on top, as well as three through the front panel of the amplifier.  As you can see the cable is on top of the toroidal transformer and was sandwiched between the transformer and the top of the case. 

The next photo below shows the wire moved off of the transformer and affixed to the circuit board with a drop of hot melt glue.  I could alternately have used a small cable tie attached to the adjacent wire bundles, some silicon glue, etc. Simply moving this wire off of the transformer fixed the issue immediately:

Here’s another view showing the relocated wire.  You can also see the shielding around the transformer.  My other SLA does not have this shielding.  

This fix took mere minutes with both amplifiers and they are both completely quiet now as they should be.  How these amps could pass inspection at ART with this problem is hard to understand, particularly since they seem well built otherwise.  I mean we are not talking about some subtle hiss.  This was a loud buzz audible across the room.  Anyway I do like these amps despite this and would not hesitate to buy more if needed, particularly now that I know how to fix this common issue.  Now it’s time to sit back with a hot cup of joe and listen to some tunes!  Sitt’n time.