We take this solution one step further by using, in the resistor's stead, a thermistor, which is a negative temperature related resistance (a negative temperature coefficient). As the heat increases, its resistance drops. Often used as current inrush limiters, these devices offer a high resistance when cold that will reduce to a low resistance when hot. Unlike the resistors of the previous solution, finding the right value thermistor requires more than calculator, it requires some experimentation, which would prove expensive for the hobbyist, but inexpensive for the professional amplifier maker, as free samples could easily be obtained.



Free DC Power Supply

Here is an old technique that deserves to be revived (I am waiting for my first e-mail scolding me for not realizing that some amplifier designed and built 3 years ago had actually invented this technique. Then when I e-mail a scan of a 45 year old schematic, I will not get a reply). DC on the heater makes for less hum, as high AC current flowing through the hookup wire and heater element generates a strong electromagnetic field that can induce hum in other parallel conductors, hence the need to tightly twist the heater wires and physically place them close to the chassis. Beyond less hum, there is a marked improvement in bass reproduction with DC on the heaters (well actually, my guess is that this improvement is solely due to hum reduction, as the heater's thermal constant is so long, but I could be convinced otherwise).

