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In Reply to: RE: Why Do You Believe Audio Equipment "Sounds Better" After It's Warmed Up? posted by thetubeguy1954 on May 02, 2008 at 08:38:31
One example of performance change with warmup occurs in bipolar class AB power amplifiers.
For most of these, the DC collector current in the output stage is established by having a DC voltage between the bases of the NPN and PNP devices in the complementary output stage. This ends up being about 1.2 to 1.4 Volts. If this were a fixed voltage, thermal runaway would occur. This happens because if you hold the Vbe of a bipolar transistor constant, the collector current will go up as the device gets hotter. If the collector current goes up, the device heats up more, causing the collector current to go up more, and so on. This vicious cycle would lead to device destruction if not compensated for in some way.
The way it's usually compensated for is to mount either a bipolar transistor or some diodes on the power amp's heat sink to sense the heat sink temperature. As the heat sink heats up, the transistor or diodes sensing the temperature will cause the voltage between the bases of the NPN and PNP devices to decrease a bit. If designed correctly, this decrease will be just the right amount to keep the DC current in the devices approximately constant as they warm up, preventing thermal runaway. Another reason this is important is because bipolar class AB output stages have an optimum bias current where their distortion is minimum. Increase or decrease the current from its optimum, and the distortion becomes greater than its optimum value. So it's necessary for the bias current to be controlled both to eliminate thermal runaway, but also to keep the bias current near its optimum value.
But there is a fly in the ointment. These temperature sensing transistors or diodes are sensing the heat sink temperature, and not the junction temperature of the devices themselves. Because of the thermal mass of the heat sink, it can take some time before the change in heat sink temperature "catches up" with the change in output stage transistor junction temperature. During this period, the bias is likely to drift some, before settling in on its final value.
There have been some recent developments that improve on this situation. OnSemiconductor have fairly new devices called "ThermalTrak" transistors. These devices have temperature sensing diodes built into them. This arrangement has much tighter thermal coupling to the device junction than mounting a temperature sensing device to the heat sink to accomplish this. Apparently they are used in the Ayre MX-R power amp, and probably some other amps that I'm not aware of.
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