Phase Linear Model 700
345 watts per channel, a protection circuit that actually works, and a turn-off thump that owners simply learned to live with.
Overview
Phase Linear built the Model 700 around one number: 345 watts per channel minimum RMS into 8 ohms, from 20 Hz to 20 kHz, with no more than 0.25% total harmonic distortion. That is not a music-power figure or an "IHF dynamic" figure. It is continuous, both channels driven, full bandwidth. In the early 1970s most flagship receivers were still fighting over 100 watts per side, so this was a different category of amplifier. The 700 was not designed to be reasonable. It was designed to make big speakers do things they had never done in a living room.
The typical distortion figure shows the design best. Phase Linear rated THD at 0.01% at rated power into 8 ohms. That is far below the 0.25% guarantee, and it says the output stage had plenty of headroom. Intermodulation distortion, measured at the standard 60 Hz:7 kHz 4:1 ratio, came in under 0.25% at rated power. Signal-to-noise was rated at greater than 100 dB below rated output. That is a serious achievement for a circuit this powerful, since noise floors scale badly with output stage size, and Phase Linear clearly spent real engineering effort keeping the front end quiet.
The input stage is more flexible than the front panel suggests. A switch lets the 700 run in either "normal" or direct-coupled mode. Normal mode puts an AC coupling capacitor at the input with a 6 dB/octave subsonic rolloff cornered at about 15 Hz. Phase Linear explicitly recommended this mode for high-fidelity home use, and the reason is practical: it stops DC or subsonic garbage from an upstream component from reaching your woofers. Direct-coupled mode amplifies all the way down to DC, with a gain of 1 at DC. That mode exists for sound reinforcement work, not for the living room, and owners who flipped the switch casually sometimes found out why. Input impedance was 100k ohms nominal, dropping in direct-coupled operation.
The front-panel level controls are not gain controls, and this trips people up. They are sensitivity controls that attenuate the input signal only. They sit ahead of the amplifier's voltage gain stage and have no connection to the output stage. Turning them down does not reduce the amplifier's available power. It just means you have to feed it a hotter signal to reach the same output. Owners who treated them as a master volume and then wondered why the amp still produced full output from a hot source were not seeing a fault. They were seeing the design.
The protection circuit was patented, and it is one of the more interesting pieces of engineering in the amplifier. It watches the current flowing through the output emitter resistors. When that current gets dangerously high, capacitors in the sensing network charge up and turn on a pair of transistors that shunt drive current away from the predriver stage. The amplifier cannot be driven into an uncontrolled current increase. Diodes in the network keep the protection circuit from tripping when the amp is driving a reactive load. That is a real problem for lesser designs, which either false-trip on capacitive or inductive speakers or fail to protect at all. Separate flyback diodes catch reverse overvoltage across the output transistors if an inductive load gets switched.
Phase Linear was blunt about speaker protection, and the manual says so in plain language. The supply fuses inside the 700 protect the amplifier first. Speaker protection is secondary. The company strongly recommended additional in-line fuses on the speaker lines, and even supplied the formula: fuse current equals the square root of peak power handling divided by four times nominal impedance. An 8-ohm speaker rated for 200 watts of peak power wants a 2.5-amp fast-blow fuse in series with one speaker lead. Slow-blow fuses were explicitly forbidden for this purpose.
The Model 700 will drive anything from 4 ohms to an open circuit, with nothing connected at all, and the manual permits multiple speaker pairs as long as the combined nominal load stays at or above 4 ohms. Below that, you are on your own, and the manual says not to. Damping factor was rated at 1000:1 at 1 kHz. That is a high figure, and it explains part of why these amplifiers had a reputation for holding large woofers in place when other amps could not. Slew rate was greater than 11 volts per microsecond and small-signal rise time under 1.6 microseconds. Power bandwidth spanned 12 Hz to 40 kHz within +0/-1 dB. Phase shift was 0 degrees at 20 Hz, lagging 10 degrees at 20 kHz. Input sensitivity was 1.75 volts for rated output into 8 ohms.
The 700's reputation in the press came from exactly the kind of measured behavior the specs imply. Reviewers running the smaller Phase Linear 400 through the lab found it comfortably exceeded its ratings, delivering 270 watts continuous per channel into 8 ohms with both channels driven, and 150 watts per channel into 16 ohms. HiFi-Stereo (September 1972, p.38) published those figures alongside the 400's measured performance. The same publication, testing the 700 directly, recorded distortion at 10 milliwatts of just 0.04% and noted that instantaneous peak power measurements were running in the 450- to 500-watt range before the test speakers' own protective fuses gave out. HiFi-Stereo (April 1972, p.62) made the point that the limiting factor in the test was no longer the amplifier.
The turn-off thump is the flaw every owner knows about. A delayed thump on power-down is characteristic of both the 700 and 700B. It is caused by the power supply voltage discharging back through the front-end differential transistors. Phase Linear's own service documentation describes it as normally a fairly low-level noise, but acknowledges that if it becomes loud enough to threaten connected speakers, the 700B has specific remedies (checking or replacing D3, C3, Q1 through Q4). The original 700 has, in the manual's own words, very little that can be done aside from experimentally matching the front-end differential transistors. The recommended fix for both models is installing an output relay, which also provides a delayed turn-on and adds subsonic and DC protection. Many 700s in the field today have had that relay added, and it is worth asking about before buying one.
Servicing a 700 is a job for someone who knows what they are doing, and Phase Linear said so on the cover of the manual: hazardous voltages are present inside, and the document is intended for qualified service personnel only. Bias adjustment requires measuring the DC voltage drop across specific 10-ohm half-watt resistors in each output bank - R47 and R62 on a 700 - and setting it to a 300-400 mV range by adjusting R36 on the PL0171 printed circuit board. The second bank's reading has to fall within 20% of the first. For amplifiers used in sound reinforcement or commercial service, Phase Linear recommended biasing toward the low end of that range at 300 mV, which raises distortion only slightly but keeps operating temperatures far more reasonable. Even then, the manual still recommended a cooling fan. DC offset is adjusted at two separate points on the board - R17 with the front panel control fully closed, R7 with it fully open - and the two interact, so the procedure has to be repeated until offset settles below 50 mV at any level position. If it cannot be zeroed, the differential pairs are the likely culprit.
R36 sets the idling current in the driver transistors, just enough to prevent crossover distortion. It is a small adjustment with large consequences. Set too low and the amplifier gets harsh at low levels. Set too high and it runs hot and stressed. The thermal switches on the heat sinks open if things get too hot and close again automatically once temperatures return to a safe region. That is a backstop, not a license to run the bias hot.
On the PL0171 drive board, the positive half of the signal is handled by an emitter follower driving six paralleled output transistors (Q11 through Q16). The negative half runs through a PNP predriver acting as a phase inverter into a negative output driver, which in turn drives five more paralleled devices (Q19 through Q23). That is eleven output transistors per channel sharing the load. It is why the amplifier can sustain 345 watts continuously, and it is also why a failed channel can be a real repair project. The service documentation flags output transistor insulators as a recurring trouble spot - thermal cycling, overtightening, or a previous repair can pinch metal particles between the insulator and heat sink. Where the rubber self-lubricating insulators are found, Phase Linear recommended replacing them with mica types and thermal compound.
Oscillation is the other classic 700 ailment. The service notes walk through a specific diagnostic sequence: checking the RC output pole network, replacing predrivers, replacing output drivers, checking insulators, and confirming line voltage stays at 117-120 VAC, since low line voltage can provoke oscillation bursts just below rated output at low test frequencies. On 700s that oscillate, ferrite beads may need to be placed experimentally on the output buss wiring. On the 700B, some units left the factory with beads already installed at specific locations chosen per-unit for maximum high-frequency stability, and the manual warns against removing or relocating them. If a 700B has beads on the output buss, those are not a modification. They are factory.
One more point from the service literature: the amplifier will not deliver its full 53 V RMS into an 8-ohm load if the AC line sags. The manual requires at least 117 VAC at the plug when delivering full output. A 700 that seems down on power in a workshop running on a long extension cord is not necessarily sick. It may just be starving.
Anyone chasing a 700 today should know what they are getting. The turn-off thump is inherent to the design and largely unfixable on the original model without an added relay. The bias and offset adjustments are not set-and-forget. They drift, and a unit that has not been serviced in decades is almost certainly out of spec. Output transistor insulators and oscillation are known failure modes with documented fixes. The fundamentals are a genuine 345 watts per channel, a damping factor of 1000:1, a patented protection circuit that does not false-trip on reactive loads, and more than 100 dB of signal-to-noise.
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