On 15 Oct 2006 09:24:45 -0700, "
wrote:

0.5 volt across each of the two 0R5 resistors. There is no obvious
difference in heat generation if I don't feed it a signal, or if I turn
down the volume and gain dials (which only adjust the signal levels at
the preamp).
I could only a detect a very low DC voltage across the speaker (without
signal/volume). The battery of my digital multimeter ran down, so at
the moment I only have an analogue one at my disposal, and this sensed
a voltage in the neighbourhood of 0.1V. I'm no expert, but that seems
fine. Not feeling comfortable with disconnecting the speaker
completely, and I don't have a spare resistor on me at the moment (I
assume a 1/4W one would not suffice).
No problem.


For the BD's to heat up like this with literally no input signal,
shouldn't they have to conduct a large DC current instead?
Yes - if you have 0.5 volt across the resistor then there is 1 amp flowing all
the time and with 50 volts available you are dissipating 50 watts of heat.
I would guess (not a design engineer) that there should only be a small
quiescent current - just enough to get the transistors out of their non-linear
region. The schematic does not show any adjustment for the bias which seems to
be fixed by the voltage across the 3 1N4148 diodes. You could check this - maybe
about 2.1 volts?


Geo