The power supply of the tested amplifier is +/-24 volts and should be rated at 6 amps
continuous duty, and more than 10 amps peak per channel. Ordinary unregulated supplies
will work fine, and if you need suggestions the Zen Variations #5 (AudioXpress, October
2003) and Zen Variations #3 (AudioXpress, August 2002) both have good examples of
regulators and the raw supplies that feed them. I recommend rail voltages from 23 to 25
volts.
Heat Sinking
At 1.3 amps per channel, you will see idle heat dissipation of 62 watts. To keep the
temperature rise of the heat sink to 20 deg C. above the ambient temperature, you will want
a heat sink rated at about .6 deg C./watt for each transistor. An example of this would be a
chunk of finned aluminum, with a series of 2” fins attached to an 8” by 6” base. You will
need two per channel.
The output devices need to be intimately attached to the heat sink. The mounting surface
on the heat sink should be at least ¼ inch thick and smooth and shiny. You can use silicone
pads for insulation, but Mica and silicone grease are the best.
Initial Adjustment
Before applying power to the amplifier, you will want to set the values of P1 and P2 to their
minimum. Verify this with an ohmmeter. When it comes times to “fire” up the amp first time,
if you have a Variac, use it, fusing the AC line to the amplifier with a 1A fast blow fuse. Turn
the Variac up slowly, and if you haven’t popped the fuse, then go ahead and confirm the rail
voltages to the channels.
Each channel does not need to be attached to a load in order to adjust it. If the only load
you have is the loudspeaker, I would advise against using it during adjustment. For each
channel you will be adjusting P1 and P2 alternately in order to achieve 0 volts DC at the
output and .59 volts across R11 and R12. Each time you adjust P1 you will probably have
to go back and adjust P2 again, and so I recommend adjusting the pots in half-measures,
alternately setting the pots half-way to their voltage goals and measuring the DC values.
Unless there is something very wrong, when the output is at 0 V DC, the values across R11
and R12 will be equal.
In spite of the thermal compensation in the circuit, you should assume that there will be drift
as the heat sink temperature rises, and you will need to readjust the values over the course
of an hour or two. Usually it is best to start out bias adjustment low, at maybe 0.4 mV
across R11 and R12 until the amp is warmed up a bit.
You should be able to get the output DC offset down to 10 mV or so, and I would consider
50 mV the highest acceptable figure for this amplifier when warmed up. After the amplifier
has been operated for a few weeks, it is a good idea to check and adjust the offset again
after the parts have been burned in.
Measured Performance
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