chess, I had a few people tell me that most of the HALO's since mid 2004 have a newly designed power supply. I don't think there was a problem with the others but from what I understand John Curl himself wanted a higher current draw. Perhaps this would address any possible hum issues that may have come up with the A23.

As you know, I have never heard the hum but if it was out there, the first place I would look is my power supply.