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Reference Voltage Generator
R1, R2, R5, D11, and U2 establish a 38-volt reference at node VMIDLOWZ. D11
assures that 20 volts appears across U2, keeping it well within its 36-volt rating. R9
isolates VMIDLOWZ. D12 helps charge C1 more quickly during turn-on, establishing
the output voltage relatively quickly. VMIDLOWZ also sets the output voltage of the
amplifier at 30 volts, half the supply rail. This is the optimum point to allow maximum
linear output swing.
Gain Setting Circuits
R7 and R8, 0.1% tolerance resistors, set the amplifier’s gain so accurately that the
amplifiers can be easily paralleled by paralleling the input pins and paralleling the output
binding post. C6 and C11 help keep the amplifier stable under all conditions. C4 sets the
low frequency -3 dB point at 1.59 Hz. This keeps the signal across C4 in the audio band
miniscule, assuring high linearity. The input -3 dB point is dominated by C2 and R6, at
about 1.59 Hz. R26 provides output inclusive Miller compensation.
C13, the output coupling capacitor, couples the amplifier’s output to your speakers. L1,
wound around C13, and R35 isolate capacitance in the speaker, also assuring stability for
all load types. C13, at 10,000 µF, makes a -3 dB point of 2 Hz when driving an 8 Ohm
speaker (4 Hz when driving a 4 Ohm speaker).
Relay Control Circuit
K1 mutes the output of the amplifier during power up and power down. That avoids any
pops or clicks during those times. R38 charges up C11 during the turn-on period. R40
keeps C11 discharged in the absence of a speaker load.
The relay control circuit keeps the relay open and the input muted until the power supply
reaches about 53.5 volts. R27, R29, and D3 set this voltage. R24 and Q6 keep C1
discharged until the power supply exceeds 53.5 volts. After that, relay opening is further
delayed by the charging time of C1 through R25. D2 sets a 10-volt bias voltage at the top
of R25, making for a repeated turn-on delay.
D6 limits the gate voltage on Q4, which protects Q4, and provides consistent operation of
the delay function. Q10 resets C16, the delay capacitor, when the power supply voltage
drops below 42 volts. R52 limits C1’s discharge current.
R59, R61, R62, R63, and Q6 control Q1, a J176, a P-channel JFET that mutes the input
whenever the power supply is less than 42 volts. C22 delays the muting function and
allows it to smoothly be removed, making for a smooth transition from silence to sound.
R57 is used to set the idling current in the output stage. Q19-Q21, R64, R65 and R27
sense the temperature of the bias stage and adjust the bias voltage to minimize the
variation in idling current with temperature.
