Phase Linear 300 Series 2

Here is another Phase Linear I have in my collection, a 300 Series 2. She is a slightly scaled down version of the 400 Series 2. More than that, her outputs are in Full-Complementary.

Similar to the PL400 Series 2, the PL300 Series 2 uses an op-amp for the front end. Instead of the LF356, it is now a LF351.

Phase Linear 300 Series 2 Power AmplifierFig 1 – Phase Linear 300 Series 2 Amplifier Schematic

The PL300 Series 2 works differently from the PL 400 Series 2. She can be construed to be a mirror image typology, in that the top and bottom halves are the same. I think there’s some confusion when people describe this kind of typology as Fully Balanced. It is not. The word Balance refers to signal transfer using a non-inverting and inverting signal for noise cancellation. It has nothing to do with an amplifier’s topology.

The Phase Linear 300 Series 2 was manufacture in 1979, a few years after the PL400 Series 2. One can clearly see how the designer evolved with new transistors. The PL300 series 2 is an exceptionally elegant circuit. Various manufacturers over the years have used the same topology in one form or another with varying success. One of which is the Peavey PV-1200. When she blows up, she’s a mess.

Summary

This concludes my journey with the Phase Linear power amplifiers. For their time, they were quite remarkable in that high power amps weren’t available then. All the horror stories you come across in the internet about the Phase Linear emitting smoke and fire is no different from other amps when they break down. Some die gracefully while others are quite dramatic. Nonetheless, we are looking at a 50 yr old amplifier. Components like capacitors dry up and when they do, the amplifier becomes unstable. Any power amplifier will suffer the same fate over half a century.

With modern day components, these vintages can have a new lease of life. It’s only a question of whether one has the expertise to restore them to their former glory.