Phase Linear 400 Series 2

I have one of this Phase Linear 400 Series 2 in my collection. It is the updated version of the Phase Linear 400 which uses a discrete front end. In the Series 2, the front end is now replaced by a JFET LF356 op-amp. This improves the reliability of the amplifier compared to the earlier PL 400. Outputs are still in Quasi-complementary.

Phase Linear 400 Series 2 Power AmplifierFig 1 – Phase Linear 400 Series 2 Amplifier Schematic

The Red line in Fig 1 traces the signal flow of the Series 2. The input signal is injected into the non-inverting input of the op-amp. The output drives a transistor which in turn drives the VAS transistor. The signal goes through the thermal controller and drives the pre-driver in the top half. In the bottom half, it drives an inverter which in combination with another transistor (not shown) works as a level shifter essential for the Quasi outputs to work.

Fig 2 – Phase Linear 400 Series 2 Power Stage

Fig 2 is the Current Stage of the Phase Linear 400 Series 2. Three pairs of power transistors are used in Quasi arrangement. Note that the level shifting for the bottom NPN transistors were already done in Fig 1.

Summary

Compared to the earlier PL400, this Series 2 offers better stability. When I restore the PL400, I will likely use the Series 2 circuit instead of the discrete version. There will be changes though. I will abandon the Quasi outputs in favor of Full Complementary. That will eliminate the level shifter which can be a source of instability.

If possible, I will upgrade the power supply caps to 40,000uF if there’s space. In the original, two units of 7,500uF is simply not enough, more so when one is pushing the amplifier. Apart from these, all caps will be upgraded and if necessary, the standing current in the VAS will be increased. These are some of the things that come to mind but very likely, more changes will be made as I go along. When I finished with this Series 2, she will sound vastly different. More like High-end amplifiers.