Phase Linear 700-AS (1973)
The Phase Linear 700-AS is a professional power amplifier from the company's 700 Series. It was aimed mainly at the PA and recording market, not home hi-fi. It listed for $749.95 in 1973, at the top of the Phase Linear lineup alongside the standard 700 and 700B.
The 700-AS was built on brute force. Audio (May 1972, p.20), discussing amplifier regulation, noted the engineering trade-offs in delivering this kind of power. Very low regulation figures required either an "infinitely large power transformer" or an electronic regulator of comparable cost and complexity. Phase Linear's own advertising from March 1973 boasted that "never before has such power and purity been available in a home stereo amplifier," though the 700-AS was clearly aimed beyond the living room.
The amplifier is rated at 500 watts per channel into 4 ohms (a separate source lists 470W into 4 ohms). Harmonic distortion is 0.05%, signal-to-noise ratio 100 dB, and damping factor 1,000. Input sensitivity is 1.75V RMS into 4 ohms. The output stage is direct-coupled, complementary symmetry, with twenty 2SC2565 output transistors per channel mounted on large finned aluminum heat sinks. The power supply uses a massive toroidal transformer. Because of the heat, the amplifier is liquid-cooled, which was a rare feature in consumer-adjacent gear of the era.
One owner describes the sound as clean, aggressive, and "in your face." It lacks finesse but is incredibly forgiving of poorly recorded records. The sense of authority and presence is unmistakable, if not subtle.
A known issue is the DC protection circuitry, which could fail and send DC to the speakers. Look for a unit that has been carefully serviced, particularly one with updated protection modules.
- Power output: 500W per channel into 4 ohms (470W per variant)
- Harmonic distortion: 0.05%
- S/N ratio: 100 dB
- Damping factor: 1,000
- Input sensitivity: 1.75V RMS (4 ohms), 2.0V RMS (unspecified)
- Output devices: 20 × 2SC2565 per channel
- Cooling: Liquid
- Power supply: Toroidal transformer
- Original price: $749.95 (1973)
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