Audio by Van Alstine is now shipping all its Fet Valve amplifiers built since the first of the year with a built in DC blocker circuit.
This protects the amplifier from low level DC voltage riding on your AC power lines. The DC voltage can appear from home appliances that use half of the AC wave to adjust power, such as hair dryers, dimmers, and other similar devices (even ones you may not know are in use in your home).
This enhancement has not increased the price of the amplifiers, just their adaptabiliity to more extreme use situations.
Regards,
Frank Van Alstine