I read somewhere in this forum that to bias a Class A amp, Nelson Pass recommended that the heatsinks needs to operate at 50 degrees C. Wouldn't the heatsink temp be dependent on the size of the heat sink? meaning a larger heatsink would require more idle current to heat the heatsinks to 50 degrees C, so if two identical 50 watt class A amplifiers circuits are made with one having a larger heat sink, the one with the smaller heatsink would have less Class A watts vs the one with a larger heatsinks, is this correct?
A little knowledge is a dangerous thing.
The 50 C number comes from an amplifier reaching thermal equilibrium at a temperature that will not cause excess thermal stress to the output transistors. The bias current is an adjustment for two things – the electrical operating point (along with voltage) of the transistors, and the heat that must be dissipated by the heatsinks.
Some amps run on the cool side, the ACA for example. Others such as the Stasis line can run quite hot, even with very large heatsinks. Setting the bias current is a balancing act between hitting the electrical “sweet spot” of the devices used (Mosfet or Bipolar), and keeping them away from internal temperatures that will shorten their life. Larger heatsinks do allow for higher bias current (and voltage).
The bias current needs to be set while the amp is at thermal equilibrium. This takes about an hour with typical 25W class A amps, but can take longer. It is best to check the final settings a couple times from a cold start.
The 50 C number comes from an amplifier reaching thermal equilibrium at a temperature that will not cause excess thermal stress to the output transistors. The bias current is an adjustment for two things – the electrical operating point (along with voltage) of the transistors, and the heat that must be dissipated by the heatsinks.
Some amps run on the cool side, the ACA for example. Others such as the Stasis line can run quite hot, even with very large heatsinks. Setting the bias current is a balancing act between hitting the electrical “sweet spot” of the devices used (Mosfet or Bipolar), and keeping them away from internal temperatures that will shorten their life. Larger heatsinks do allow for higher bias current (and voltage).
The bias current needs to be set while the amp is at thermal equilibrium. This takes about an hour with typical 25W class A amps, but can take longer. It is best to check the final settings a couple times from a cold start.
Consider this an ideal max. temperature for the heatsink (settling time can be up to an hour or so). Exceeding this temperature by far will in turn lead to an excessive junction temperature of the chip and accelerate aging and lead to failures earlier.
Of course your heatsink can be cooler than that!
They will both have the same output characteristics. Only the one with the small heatsink will run hotter of course.
Mind you there are types of amps that more easily suffer from "thermal runaway" and have to be kept below a certain temperature range but that´s another story.