4 Pairs of BJT output stage for the modular high end audio amplifier

Project presentation

Abstract

The amplifier output stage presented here is part of my modular audio amplifier system. It is designed to be equipped with four pairs of transistors in TO-264 housing. My preference for transistors is the MJL3281A and MJL1302A from ON Semiconductor. Of course, different pin compatible transistors can be used as well.

Design presentation

Design considerations

The output stage is actually part of the power supply as it features two massive capacitors with 10.000uF each. The audio ground star point therefore is on the output stage PCB and this ensures small loop area of the amplifier circuit wiring.

Schematic

The schematic features way more components than are usually found in typical audio amplifier output stages. Snubber networks from the bases to the supplies suppress any ringing that may occur without. Each output transistor has its own Zobel network in order to have smallest loop area and therefore making this circuit element most effective. There is also an extra Zobel network after the output inductor in order to deal with effects from the speaker cable. The audio input of the module features overvoltage protection for the bases of the transistors in order to protect them from many events that could damage the output transistors. During normal operation, this protection circuit is never active and not affecting the signal in any way. The front end is powered by the output stage so that everything connects to the star ground and power supplies correctly.

Small 0.1 Ohm resistors ahead of the large storage capacitors bring threefold improvement: First, a low pass filter is formed with the storage capacitors aboard the output stage module with a corner frequency of only 160 Hz. Second, the resistors avoid parallel resonance between the storage capacitors of the output stage and the capacitors of the power supply, ensuring a well behaved supply impedance. Third, inrush current surge is mitigated a bit. Small local capacitors at each output device are used for local decoupling.

All power resistors are non-inductive, low distortion metal band resistors with good surge capability.

PCB Design

The PCB is designed using four layers. This is quite unusual for audio electronics, which mostly uses single side plated PCBs or two side plates PCBs for serious performance. Since the whole amplifier concept is targeted towards high end audio, the extra cost for multi-layer PCBs is accepted in exchange for the dramatic performance increase thereof. The benefit is that the transistors can be placed interleaved so the two groups of transistors consist of each two P and two N types. Also, the supply routing forms the closest loop by having the ground on the outer layers and the positive and negative supply voltage each on a dedicated inner layer. This four layer supply stack is then routed together to each point where supplies are required. This arrangement also shields the supplies using the ground layers, resulting in very low emissions. The output signal is routed on four layers in parallel, which results in super low resistance and inductance of this node. Routing of the output signal is also close to the supplies stack, resulting in low loop area for the power circuits.

Assembly

The mounting holes of course fit on the 20mm grid, the whole modular system is based on. Only the output transistors are located on a 10mm grid, which is only present for the outermost mounting hole rows on the heat sinks in order to optimize transistor placement.

The output inductor is physically located furthest away from the audio input and all small signal stages so that the magnetic field generated by the inductor cannot couple back into the front end. Using a bobbin for the inductor makes it possible to wind any inductance that is required. This is a huge benefit over most designs since the inductor can be tuned as required.

Summary

This output stage can be used with a variety of driver stages and front ends in order to yield many amplifier configurations. Bipolar transistors are linear, but have limited power capability. By using four pairs of bipolar transistors in parallel, a high power output stage can be built, that is still good to drive. For higher power applications, the four pair vertical MOSFET and six pair vertical MOSFET output stages, which are also part of the modular amplifier system, offer higher SOA.