Phase Linear 700-B (1974–1976)
An amplifier rated at 350 watts per channel into 8 ohms when most rivals topped out around 50 to 100 - and a service manual that admits it can thump your speakers when you switch it off.
Overview
The 700-B was Phase Linear's big stereo power amplifier, rack-shaped and rated at 350 watts per channel into 8 ohms. The company's own advertising pushed that figure hard, and it made the amp a genuine outlier when it went on sale in late 1974. It stayed in the catalog and in the advertising pages through the fall of 1976. The headline number is what people remember, but the manual tells a more honest story. This is a high-strung, high-current design that rewards the right owner and punishes the careless one.
Phase Linear's advertising in HiFi-Stereo (December 1974, p.123) pitched the 700-B at speakers that "require tremendous amplifier power," and the headline touted a "700 watt club" - the combined output of both channels. The printed specs were imposing. They claimed 350 watts per channel, minimum RMS, into 8 ohms from 1 Hz to 20 kHz, with no more than 0.25% total harmonic distortion. Hum and noise was quoted as better than 100 dB below 350 watts. The advertised price was $799, and Phase Linear backed it with a three-year parts-and-labor warranty. All of those numbers come from a single source, the company's own ad, so treat them as manufacturer claims rather than lab-verified results.
The 700-B earns its reputation in the manual. The service documentation is unusually candid about the amp's quirks, and collectors should read it before buying. The rated output is real and the protection circuitry is elaborate, but this is not a set-and-forget amp.
Key Features
350 Watts Per Channel Into 8 Ohms
The power rating is the whole point of the 700-B. Phase Linear's ad copy claimed 350 watts per channel minimum RMS into 8 ohms across the full 1 Hz to 20 kHz band, with total harmonic distortion held to no more than 0.25%. The Hi-Fi Hall of Fame notes that top-end amplifiers from other manufacturers, at the time the original 700 was introduced, were generally capable of 50 to 100 watts per channel. That puts the 700-B in a different weight class, and it's why the amp found its way into systems driving difficult, power-hungry speakers. An Audiogon reviewer reportedly sought out a 700-B specifically to drive speakers whose ribbon drivers "present a very difficult load."
Patented Electronic Energy Limiters
Phase Linear's advertising described the protection circuit as "patented electronic energy limiters," and later documentation and product listings refer to a protection relay in the 700-B. Protection matters here because the amp can deliver so much current that a fault can be catastrophic for whatever is connected downstream. The service manual's troubleshooting section notes that if an output transistor develops leakage or shorts in a way that doesn't blow the supply fuse, the full supply voltage can reach the speakers. It explicitly reinforces the need for external speaker fusing. Owners who run a 700-B without fusing their speakers are taking a real risk.
Quasi-Complementary Output Stage
The original 700-B design used a quasi-complementary output stage, according to a diyAudio refurbishment thread. That topology is a period hallmark, and it's also the part of the amp most often modified today. The same thread documents converting a 700-B from the original quasi-complementary arrangement to a full complementary output. The parts cost to upgrade a 400 or a 700 is reportedly the same. It's the single most consequential modification an owner can make, and it changes the amp's character as much as its reliability profile.
VU Meters and the Glowing Logo
The 700-B has front-panel VU meters, with a logo piece between them that glows blue when backlit. That badge has become disproportionately important to owners. An Audio Asylum buyer in 2004 was hunting specifically for the piece between the VU meters with the Phase Linear logo that glows blue. Spare parts are scarce enough that a missing badge becomes a search project.
Normal and Direct-Coupled Input Modes
The service manual describes two input modes. In the NORMAL mode, the inputs are AC-coupled with a subsonic rolloff of 6 dB per octave and a corner frequency of about 15 Hz. Phase Linear recommends this setting for high-fidelity home use, since it protects woofers from DC or subsonic content passed along from upstream gear. In the DIRECT coupled mode, the amp amplifies down to DC, where gain is 1. That mode is aimed at sound-reinforcement work rather than home listening. The front-panel level controls are sensitivity controls only. They affect the input signal and are not connected to the output stage, so they don't change the amplifier's gain.
Historical Context
The 700-B arrived in a market where high power was scarce. The Hi-Fi Hall of Fame notes that when the original Phase Linear 700 was first introduced, there were few high-powered amplifiers available, which explains why the 700-B's 350-watt rating was such a talking point. The 700-B is a later version within the 700 series. A phxaudiotape forum thread traces the line from "Phase Linear 700 - the first 700" through "Phase Linear Pro700 - the last 700 series amp."
By 1976, Phase Linear's corporate advertising had shifted its emphasis. Research headed by Robert Carver was described in the company's own copy as starting where state-of-the-art specifications leave off, chasing "obviously audible improvements" in music reproduction rather than numbers alone (Audio, October 1976, p.122). The 700-B sits at the transition point, a big amplifier from an era when raw wattage was the headline, sold by a company that was beginning to talk about sound rather than specs.
Collectibility & Value
The 700-B has a real following, and prices vary widely with condition and modification. A restorer quoted on a Reverb listing reports routinely getting $1,800 to $2,200 for upgraded 700-B/Series II amps and says they hold their value. That figure is for refurbished, upgraded units, not tired originals, and it's one seller's experience rather than a market index.
Reliability is the thing to understand before buying. The diyAudio thread titled "Phase Linear 700B burns speakers" documents an owner whose amp, after a few months of use, sent feedback down one channel and burned out a speaker. That's the failure mode the service manual's warnings point at, and it's why so many surviving 700-Bs have been through a rebuild. The common refurbishment work documented by owners includes a recap, a new power switch, and a triac board. The more ambitious owners convert the quasi-complementary output stage to full complementary. A service manual covering both the 700 and 700B exists for troubleshooting, repair, and alignment.
Parts are gettable but not always easy. Original Phase Linear XPL909 NPN transistors pulled from a 700 have turned up for sale with the note that they fit other models, and the hunt for the blue-lit logo badge shows how thin the spares supply can get. One Audiogon buyer found a 700-B that had been rebuilt by an electrical engineering professor. A diyAudio owner whose amp had been checked over reported it had "a clean bill of health and sounded brilliant."
The service manual also documents a known quirk: a delayed turn-off thump is characteristic of the 700 and 700B, caused by the power supply discharging through the front-end differential transistors. It's normally low-level. If it's loud enough to threaten the speakers, the manual points to specific components to check on the 700B. It also notes that fitting an output relay eliminates turn-off noise while adding turn-on delay and subsonic/DC protection.
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