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The 400 amplifier was, perhaps, Phase Linear's most popular amplifier due to its more than adequate 200 watts per channel and a price tag roughly two-thirds that of the 700. The 400 was introduced in 1972 with a list price just under $500.

Model 400 amp with output device covers removed.

Like the 700, the 400 featured twin output meters. It had six output transistors per channel, plus two drivers per channel, right below the output transistors. The heatsinks were of a different design than the 700, having less surface area. Dimensionally, it differed slightly from the 700 only in height, measuring 19.00w x 10.00d x 7.00h.  The 400 had a factory rated frequency response of 5-250k Hz.

Phase Linear 400 Series Two

In 1978, the 400 Series Two was introduced. This model was new, inside and out. The owners manual gave a rated output of 210 watts per channel. More important were the improved specifications. The old-style VU meters were gone. In their place were two multi-LED output meters, shared with the 700 and D-500 Series Two amps. Output from each channel could now be controlled by a knob beneath each meter.  Rack handles were now standard.

400 Series Two (back)

The 400 Series Two also featured redesigned heatsinks with more surface area, like those use on the 700. When Pioneer acquired Phase Linear in 1979, it sought to improve the specs on the amplifiers even further. In 1980, the 400 had lower THD and IMD of only 0.009%. The price of the 400 had risen throughout the late 1970s, and in 1980 it listed for $700. In 1981 it rose to over $800.