HNB 2.1 The Headphone Trick

You can read about the "headphone trick" here. Basically, I need to orient the power and output transformers to minimize a sympathetic 60-cycle hum. What happens is that the wildly swinging wall voltage is creating magnetic havoc in the PT. This can be sensed in the OT, even though it it is not plugged in. It seems kind of scary, but you have to wire up a power cord to the primaries on the PT, and then hook up some headphones to the secondary of the OT. Then screw around with the locations of the two transformers until you don't hear the disturbing hum.



If this is not taken care of at this stage, then the small hum will never be removed from the amp. This is a magnet thing, and unless I wanted to get into mu-shielding, I would be stuck. Oddly enough, the hum was smallest when they were aligned parallel to each other. Every thing I read says to try it perpendicular. Well, that hummed!



Now the chassis is marked for drilling the mounting holes and the holes for the primaries and secondaries of each transformer. I might have gotten excited and drilled these holes. However, I just realized that I did not have the rubber grommets that protect the wires from the chassis. I didn't want to cut 1/2" holes, and then realize I couldn't find grommets that fit this! Luckily, our hardware stores carry these.

--gh