When you load a linear supply Tom, there is an increase in the 
mechanical sound - and that's okay.  Maybe it's the same for a 
switcher.  You hear too damned well!!
Pat, K9JAuAt 07:48 PM 12/26/2007, you wrote:
>Hi, all.
>
>I have noticed something when using my TS480 with my 35-amp Samlex switching
>power supply.
>
>When I transmit with 100 watts, in CW mode or in RTTY mode using the stock
>mic, I notice a slight but audible hum coming from the switching power
>supply.  The hum stops when I stop transmitting.  I had my XYL take a look
>at the meters on the front of the supply, and they say I'm running at 13.8
>volts, and drawing about 19 amps, which is about what I'd expect.
>
>And, just for the record, I'm doing this with a resonant antenna, and just
>using the auto-tuner to be sure I have a perfect match.
>
>Is this slight but noticeable hum when transmitting normal?
>
>Thanks, and 73 from Tom Behler:  KB8TYJ