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 package. My preference for transistors is the MJL3281A and MJL1302A from ON Semiconductor. Of course, different pin compatible transistors can be used as well.

Schematic

Input overvoltage protection

The audio input of the module features overvoltage protection for the bases of the transistors in order to protect them from some events that could damage the output transistors. During normal operation, this protection circuit is inactive and not affecting the signal in any way.

Power distribution network

The output stage is actually part of the power supply as it features two massive capacitors with 18.000uF each. For higher supply voltage, lower capacitance value would need to be installed due to the height limit of 65mm of this module. The audio ground star point therefore is on the output stage PCB and this ensures small loop area of the amplifier circuit wiring. The front end is powered by the output stage module and also connects to the ground star point of this module.

Small 100 mΩ 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 90 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.

Base to collector snubbers

Snubber networks from the transistors bases to the supplies increase the electrical stability of this emitter follower stage and suppress ringing that will occur without. First I added this as a provision in case I need it and chose capacitance too small and resistance too high. Later I optimized the network with the values shown in the schematic. This snubber network has proven to be crucial for electrical stability.

Zobel networks

I chose a distributed Zobel at the NFB node, which means each output transistor has its own Zobel network to ground in order to have smallest loop area and therefore making this circuit element most effective. Due to distributing the power dissipation to eight resistors in total, affordable low power resistors can be used.

There is also an extra Zobel network after the output inductor in order to deal with effects from the speaker cable. The secondary Zobel uses two resistors in parallel for better power handling. I started a bit too optimistic with rather low resistance (2x 20Ω) and found out the resistors power rating is insufficient in case the amplifier oscillates. So either the resistance is increased or a much more powerful resistor needs to be installed.

Ferrites

I first built this output stage with 2R2 base resistors and found electrical stability insufficient. The base to collector snubber networks help to improve stability, but my experiments and simulation have shown that this is not enough. I experimented with ferrites in series with the base series resistors, but this did not yield significant improvement and has its own disadvantages. The best results were obtained by increasing the base series resistors to 12Ω and installing ferrites in parallel to the resistors.

The ferrites do not have an extra footprint on the PCB, but can be installed in parallel to the base series resistors easily on the opposite side of the PCB. Actually, I installed the ferrites in place of the base series resistors and connected the resistors on the secondary side of the PCB instead.

For details, please read my paper about audio amplifier output stages optimization, where I iterate several output stage configurations, isolating the root cause for instability and iterating possible options for improvement.

PCB Design

The PCB is designed using four layers. This is quite unusual for audio electronics, which mostly uses single side plated PCBs for cost saving or dual side plated PCBs for better 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, i.e. P - N - P - N and so on. This avoids half wave rectified currents inducing magnetic fields everywhere. 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 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, yielding low loop area for the whole power circuitry.

Assembly in CAD

The mounting holes of the module fit on the 20mm grid which the whole modular audio amplifier 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 radiators 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 is less likely to couple back into the front end. Using a bobbin for the inductor ebnables to wind any inductance that is required. This is a huge benefit over most designs where the inductor is more or less self-sustaining since the inductor can be tuned as required. I just filled the bobbin completely with a single layer inductor as this arrangement has low inter-winding capacitance and forms an inductor of appropriate value.

Assembly in reality

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 somewhat linear, but have limited power capability. By using four pairs of bipolar transistors in parallel, a pretty high power output stage can be built. For even 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 safe operating area.