Questions on transformer hum.

Emission

Supreme [H]ardness
Joined
Dec 6, 2005
Messages
4,420
Okay, so I built this kit not too long ago, and it sounds wonderful:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=320-212

But the issue is, its powered by a dual 12v AC output from this transformer:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=129-035

While the transformer is a perfect match rating wise, I'm getting the feeling that the ripple from the AC supply is causing a hum to be audible in the speakers. Not only that, but I'm wondering if having the transformer so close to the amp is also part of the problem, due to the electromagnetic field its creating, am I correct?

Really what I'd like to know is how I can cancel out as much hum as possible so that listening at low volumes is much more clear. I have ideas as to the direction I should go but I'm a bit confused as well. Something to smooth out the ripple as much as possible so that there is no audible hum from the switching in the AC lines.

Thoughts?
 
If possible, it would be nice to verify that instrumentally rather than just guessing. Also, do you have a schematic of the circuit? If it's passing too much of a ripple, you need to improve the rectifier add/improve subsequent voltage regulation.
 
It's a very simple circuit, I'll pull up a schematic in a few.
 
Amps usually are...it's the power section in this case that might need tweaking. I'm more suspicious that there might be a soldering issue in the power section, which you will need a DMM and loads of patience for.

Could also be something's not grounded that should be...non-insulated jacks connecting to the case, etc.
 
I did a simple test. I unmounted the transformer and placed it a few inches farther away from the amp circuit, and there is no more hum. I can also hear the actual transformer buzzing, so I'm going to chalk it up to EMI :).
 
Wrap it in tin foil?

Shielding is a PITA, especially with amps which won't know noise from signal and will amplify it all equally.
 
I actually did put in some shielding but it doesn't make sense when both the transformer and amp are within it. I just need to place the transformer somewhere outside the box and make a separate box for it, no problem.

It sounds quite beautiful now :).
 
Yeah. Reasonably high-end amps actually do that regularly - separate boxes for the power supply and amplification circuitry.
 
Back
Top