Phase Linear Model 400
A 1970s American power amp that kept beating its own spec sheet.
Overview
The Phase Linear Model 400 is a dual-channel, direct-coupled solid-state power amplifier built by Phase Linear Corporation of Lynnwood, Washington. The circuit description puts it bluntly: two independent direct-coupled linear amplifiers in a single chassis, "capable of extremely high power output." HiFi/Stereo Review (September 1972, p.38) put the Model 400 on the bench and found it comfortably exceeded its own ratings - 270 watts continuous per channel into 8 ohms with both channels driven, and 150 watts per channel into 16 ohms.
The input stage uses a wideband, low-noise FET-input op amp running high open-loop gain to keep distortion low. A voltage level shifter then feeds a class-A common-emitter stage that can swing the full 160-volt power supply rail. That's where the voltage gain comes from. The output stage is a unity-gain buffer, there only to deliver current. A bootstrap arrangement raises the load impedance seen by the common-emitter stage and increases the drive current available to the positive predriver, so it saturates symmetrically. Idle current is set by a transistorized Vbe-multiplier bias circuit, with the regulator transistor mounted on the output heat sink for thermal tracking. That's the standard defense against thermal runaway, and it's implemented cleanly here.
The payoff of that topology is headroom. The 270-watt figure is what a reviewer measured, not what Phase Linear printed on the box, and the amp's willingness to drive low-impedance loads is part of why it ended up in so many high-end systems of the period. Phase Linear's own advertising in High Fidelity (July 1976, p.68) claimed the company built products "with more dynamic range and bigger, more lifelike sound than any comparably priced components available on the market today" - marketing copy, not an independent verdict. The same advertising says the Research Department was headed by Robert Carver.
The Model 400 is not a fragile amp, but it has specific failure modes, and the service literature is unusually candid about them. Early production used a TO-99 packaged Fairchild or Signetics LF356 front-end op amp on the PL36 driver board, and that package caused an unacceptable turn-off "thump" in some units. Phase Linear's fix, rolled out around May 1979, was the National LF351. It's a direct retrofit with no board modification required, and it has better slew rate, slightly lower distortion, and a substantially reduced delayed turn-off thump.
There's a second service bulletin, dated September 24, 1980. Early Series Two 400 II and 700 II amps used the LF356, which draws relatively high supply current. Later runs switched to the LF351, which draws less. Because of that change, the dropping resistors R1 and R2 were revised from 1.8 kilohms/5 watt (on the Model 400 II) to 7.5 kilohms/5 watt. The old 1.8K resistors are no longer stocked, so if either needs replacement, both resistors and the op amp get changed together. The bulletin notes an added benefit: the LF351 has a much lower turn-off transient.
Output devices are where the Model 400 gets messy. Phase Linear used a shifting roster of transistors across production runs. The XPL909 (Delco, their own number stamped on selected DTS411 devices) was used in original 400s from the start, but XPL909s are no longer used or stocked as replacement parts. Early XPL909s were sometimes marked PL909, and those must not be confused with the Fairchild PL909. Fairchild parts are marked with black ink and should be ordered as FPL909, while early Delco PL909s are marked in blue and treated as XPL909. The XPL910 is a power Darlington no longer in production, and its design markings are identical to the XPL909, so mixing them up is easy and bad. On very early Model 400s you may find Sensitron SE230 devices. There's also the PL909A (Motorola), found only on very early 400s and possibly an early 700. It was found to have inconsistent reliability traits and should all be replaced whether good or defective.
Suitable replacements do exist. The 2SD555 (NEC) was never used in production runs of the 400 or 700 series but is a suitable replacement and slightly more expensive than other outputs. The TP9054 (Motorola, their number on MJ15024 devices) is a higher-voltage, more expensive part used in some Series Two 400 II and 700 II runs. If MJ15024s are available locally they may be mixed with TP9054s in the same channel, which is one of only two exceptions to Phase Linear's mixed-output rule. The other exception is the XPL909/DTS411 pairing. Output drivers are a separate matter: the 66546 (RCA 410 devices), SJ2741 (Motorola, found on early 400s and 700s, also listed as MJ2741), and MJ1800 (Motorola) are the driver types, and the manual states flatly that drivers must not be used as output substitutes or vice versa. Drivers may, however, be mixed within a channel without hurting performance or reliability.
Phase Linear also issued a bulletin about the Infinity Quantum Line Source speakers. The 3000 mfd coupling capacitors in those speakers created a potentially hazardous condition when the power was switched off and back on before the caps fully discharged. Field reports showed an abnormally high failure rate on power amps - explicitly including the Phase Linear Model 400 and 700(B) - coupled to QLS speakers. Infinity authorized a modification: an approximately 500-ohm, 5-watt resistor installed across the terminals of each speaker. It isn't a fault of the amp, but it's the kind of interface problem that killed otherwise healthy 400s.
The other piece of tribal knowledge is about testing. Phase Linear's engineering department determined that because of the low-gain/high-current characteristics of the output devices, standard transistor testers are "totally invalid" for checking whether an output is bad in this application. Field techs kept reporting outputs as leaky when they weren't. If someone condemns a Model 400's outputs based on a bench transistor tester, get a second opinion.
The Model 400 is a period American high-power amp with a measured 270 watts per channel into 8 ohms and 150 into 16, and its service history is well documented in Phase Linear's own bulletins. The PL36 driver board is the heart of it, and the PL37 display board handles the front panel. Schematics, layouts, and X-ray views for both boards are in the service literature. The catch is the output transistors: replacements are a mix of obsolete, exchange-only, and locally sourced substitutes. Phase Linear's own exchange policy allowed them to swap good non-defective outputs for whatever they had in stock, with the caveat that TP9054 and 2SD555 devices couldn't be sent as direct replacements due to cost. Anyone restoring one today should expect to replace a full channel's worth of outputs if duplicates aren't available, and should not trust a standard transistor tester's verdict on the originals.
Phase Linear's advertising from the mid-1970s said the company started "where state-of-the-art leaves off." The Model 400 was a straightforward, high-headroom design that measured better than it claimed. Production-run variation is large enough that two cosmetically identical 400s can have meaningfully different internals.
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