A and T Labs
K6 Audio Power Amplifier Assembly Manual
2001 A and T Labs Inc.
AMK6a
2
operates at unity gain, splitting the approximately 8 volts of bias generated by Q24 and
applying it to the output transistor gates referenced to the power supply rails. Q24 is
mounted on the heat sink to allow the bias to track the output stage temperature. Q14
and Q15 are emitter followers that drive the output transistors Q16-Q23. Finally, the
drains of the output transistors are the amplifier output, via relay K1. The output signal
also takes two feedback paths, the first via R36 to R31, 32, and 35, setting the output
stage voltage gain at about 30x. The second feedback path, R37 to R14 and the input
differential amplifier, set the overall amplifier gain at about 29x, with a closed loop
bandwidth of about 350Khz. This completes the primary signal path from input to
speakers.
In addition to handling the audio signal, several other functions are performed on the
amplifier board. The first is automatic servo-nulling of DC input offset voltages,
ensuring no DC at the output of the amplifier. This is achieved by IC4, which senses
any DC at the output, and injects a compensating current, through R38, into the
feedback side of the input differential amplifier.
Next, Q7-Q9 detect any clipping of the amplifier by sensing excessive internal drive
signals, such as will occur when the feedback loop becomes non-linear under overload
conditions. IC3, C19, and R57 capture any clipping occurrences and light LED1 for a
minimum of 1 second.
IC4a and b detect the presence of any DC offset (due to a fault) at the output of the
amplifier, and will release relay K1, protecting the speaker. TC1 is a thermal protector,
which can also drop out K1, in case of overheating of the amplifier.
Finally, REG1 and REG2 are voltage regulators that keep the supply voltage for the
input stages at +/- 30 Volts, while the output stage supply voltage may vary from +/- 41
volts to +/-82 volts, as determined by the switching power supply setting. D11 and D12
regulate the +/- 15 volts for the operational amplifiers IC3 and IC4.
