Friday, May 29, 2015

200 degrees C

The schematic for the Krell 200S indicates MJ15024 and MJ15025 power transistors, although I have read that for best results you need the custom versions Krell actually ordered from Motorola which are only similar to MJ15024/25 but better.

The specs say the 15024 has a maximum Normal Operating Temperature of 200 degrees C.  (That is like 2.4 times higher than any temperature I've actually measured on the heatsinks.)

Device dissipation is 250W per device, and 16 amps.  The derating for power is 1.43 watts per degrees C, so at 80 degrees C the max dissipation would be 171 watts.

I suspect the transistors used in my FPB 300 are at least this good, and I believe there are at least 10 transistors per channel (and maybe 20 or more).  Therefore, even at 80 degrees C, one channel could be dissipating at least 1700W (and that's not counting the power consumed in regulated power supplies, drivers, etc).  and there are 20 outputs devices in each channel.  Therefore, even at 80 degrees C, each channel is capable of dissipating more power than the power supply could possibly deliver, 3400 watts.  And that's not counting the power supply and driver transistors, etc.

Bottom line is that this amplifier is safe to operate at the full maximum temperature of 80 degrees C, which is about 180 degrees F, and still deliver full power.

In my most recent testing, it does not seem like the left channel is rising to the next plateau.  Instead, it looks like it is simply drifting high.  The plateau bias starts it at the right level, but then it slowly drifts up, more like 50-100W than 150W.  Of course if it ultimately drifts up to 80 degrees C, plateau bias computer puts it back down again.  After an hour of idling with inputs connected to the Audio GD Dac, I was seeing about 500W consumption, with one channel being about 145F and the cool one being 115F.  Actually, the hotter channel is probably in a better temperature range for good sound.  After a few hours of playing the amp was happily consuming about 600W, which is perfectly fine, with  155F/135F temperatures, also fine, once again, the drifty hot channel actually being more linear.  Transistors tend to get more linear when they are hotter and/or run with greater current.

If the amp is kept playing music, it might cycle through the 180F max but then settle down to 155/135F with about 600-700W continuous consumption, which is about optimal.

If I could set the bias level to level 2 (600W) that would be nice, but I'm effectively getting about the same thing, just with an occasional higher reach that warms the amp up quicker.

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