The caps on either side of the transformer are fairly typical in this appli= cation, they are to reduce the pops and clicks that can happen when a motor= ized appliance such as a washing machine or refirgerator cycles. The idea b= eing that these spikes make it through the transformer and exceed the main = rectifier's breakdown voltage. I don't think it's as big a problem as it was back in the 70's and 80's. I = think the AC line in our homes is much more clamped, most of us have a doze= n surge strips plugged in, all with MOV's, and lots of other electronic dev= ices that tend to make the AC line more capacitive than it used to be. And = rectifiers are better now. Usually. As far as my hum problem goes, I'm tending to believe that these transforme= rs have lots of feedthrough from primary to secondary, as evidenced by the = reversing of the AC line making a difference. And of course being general p= urpose types they have no interwinding shield.=20 But why I would get a ground loop between the two channels leads me to beli= eve that, although identical-looking, the transformers are far from identic= al. Perhaps when I get more time (haha funny!) I will try to investigate th= at aspect of it. Thanks to all. Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of Bob Bl= ick=20 Sent: Sunday, June 30, 2019 10:41 PM To: Microcontroller discussion list - Public. Subject: Re: [EE]: Audio amplifier power supply problem Hint: reversing the AC plug does produce a little hum in one direction. Bob --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .