A and T Labs
K6 Audio Power Amplifier Assembly Manual
2001 A and T Labs Inc.
AMK6a
1
1. INTRODUCTION
Congratulations! You are about to experience the satisfaction of building and owning
one of the finest pieces of audio equipment around. Please read all of the instruction
first, carefully, and then return to the beginning and start construction. We strive to
provide complete information, but it is advisable to check the A and T Labs WEB site
for any additional notes, changes, or hints that may be helpful. If you find errors or
omissions or have suggestions for improving these instructions, please give us your
feedback. We begin with an overview of how the design works.
2. HOW IT WORKS - CIRCUIT OVERVIEW
Figure 1 shows a block/connectivity diagram of the amplifier. The unit consists of a pair
of linear discrete amplifiers and a full-bridge off-line switching power supply to power
them. This design approach has several advantages over the conventional heavy line
transformer and filter capacitor alternative.
The typical transformer and rectifier/filter power supply needs to employ exceptionally
large filter capacitors, since they are charged at the relatively low rate of the power line
frequency. While some manufacturers of amplifiers tout this large energy storage as a
virtue, it is not in fact a necessity for driving audio loads, and constitutes a destructive
threat to the output transistors in the case of abusive loads or short circuits.
The K6 switching power supply runs at 75 KHz, allowing the filter capacitors to be much
smaller, providing much less stored energy to dissipate in case of a short circuit. At the
same time, the supply is capable of providing over 1 KW of continuous power and 2
KW peak to drive the most demanding audio loads. The inverter may be switched
between three different rail voltages, to permit operation at different power levels or for
low impedance loads.
2.1 AMPLIFIERS
The power amplifiers are full discrete designs as shown in Figure 2, (except for the
bridging inverter and differential input adapter stage). IC2 provides differential input
capability, while IC5 acts as an inverter to drive a second channel for bridging.
The normal single ended input is via input blocking capacitor C1 and the R1-R3/C2
input low pass filter network. (This limits the bandwidth to about 160Khz)
Complementary differential input stage transistors Q1-Q4 form the first gain stage. Q5
and Q6 along with R15 and R16 provide the next stage of gain, driving the
complementary cascode inversion stage, Q10-Q13. Q24 develops the gate bias for the
output FETs, and is mounted on the heat sink for thermal tracking. The inverter stage