Phase Linear 700 II
A 200-volt power supply rail, a pair of LED display boards that changed mid-production, and a factory service department that kept issuing bulletins years after the last one shipped.
Overview
The best way to understand the Phase Linear 700 II is to read its service literature. The company documented its own running changes with a candor most manufacturers kept for internal memos. The 700 II is a dual-channel, direct-coupled linear power amplifier - two independent amplifiers sharing one chassis - built around a PL36 driver board and a separate PL37 display board. The service manual lays out the circuit plainly. A wideband, low-noise FET-input op-amp handles the low-level stage. A class-A common-emitter stage swings the full 200-volt supply rail for voltage gain. A unity-gain output buffer delivers the current. A bootstrap arrangement feeds the common-emitter stage a high-impedance load while boosting drive current to the positive predriver, allowing symmetrical saturation. Idle current is set by a transistorized Vbe multiplier mounted on the output heatsink for thermal tracking. That is a deliberate anti-runaway measure.
That 200-volt rail is the number to remember. It is not a marketing figure. It is the voltage swing the common-emitter stage has to survive, and it shapes how the amp behaves, how it fails, and what it demands from its output devices. Owners of Series Two Phase Linear gear tend to describe the sound in terms of grip rather than warmth - the amp does not flatter, it controls.
The 700 II's most visible feature is its LED display, and it is also the most revised part of the amplifier. Phase Linear's service bulletin #5179 marks the transition: starting at 700 II serial number 10000, the company moved to a "universal" LED display board. The changes were specific. R304 became 15K on both the 400 II and 700 II. The 700 II's -20 dB range switch resistor R137 became 120K. The blue trim pot was rewired so its wiper fed R305, and Q301 became a GES 97. Earlier boards came with a blue trim pot, a red trim pot, or no trim pot at all. Phase Linear stated flatly that those older boards are not compatible with the new design in terms of equal tracking. The company kept both types in stock and required model, serial number, and board type before shipping a replacement. For anyone restoring a 700 II today, that bulletin is the difference between a matched display and one channel that reads visibly off.
The display circuit is more elaborate than the front panel suggests. Z302 and Z303 are a pair of voltage-controlled comparator/logic ICs scaled by external resistors and driven by the compression amplifier. Their outputs are current sources, which is why the LEDs hold constant brightness regardless of supply voltage sag. A retriggerable monostable built from Z301B and Q302 fires when the compressor trips a level set by zener D303. R316, R317, and C309 set the flash rate - that is the peak-hold flash on the last four paralleled LEDs. It is clever 1970s analog logic, and it is also the part of the amp most likely to need attention, since the two board generations do not interchange cleanly.
The input stage is where the 700 II's production history gets messy, and where the service bulletins earn their keep. Early Series Two runs used a TO-99 packaged Fairchild or Signetics LF356 BI-FET op-amp on the PL36 board. That package caused an unacceptable turn-off thump in some amplifiers, and Phase Linear's own bulletin admits the discovery came the hard way. The company moved to an 8-pin mini-DIP LF356 from National, which reduced the thump. Then, around May 1979, it switched all PL36 production to a National LF351 - Phase Linear part number 126-0114 - chosen for better slew rate, slightly lower distortion, and substantially reduced delayed turn-off noise. The bulletin notes, without embarrassment, that the LF351 was also cheaper.
The follow-on bulletin from September 1980 ties the op-amp change to the surrounding resistors. The LF356 drew relatively high supply current. The LF351 draws less. So the dropping resistors R1 and R2 were changed to 7.5 kilohms/5 watt, replacing 2.4 kilohms/5 watt on the 700 II and 1.8 kilohms/5 watt on the 400 II. Phase Linear stopped stocking the old values entirely and told service technicians that if either resistor needed replacement, both resistors and the op-amp - if the amp still had LF356s - should be updated together. The retrofit is direct, with no board modification required. For a buyer inspecting a 700 II today, LF351s and 7.5K dropping resistors are a strong signal the amp saw competent service at some point.
The output stage changed even more. Service bulletin 10-81 announces that 700 IIs beginning with serial number 7A10701 dropped the quasi-complementary topology entirely for a fully complementary output stage. The Delco XPL909 and Fairchild FPL909 devices were gone. New production used either Motorola MJ15024 (TP9054, NPN) and MJ15025 (DB15025, PNP), or NEC 2SD555 and 2SB600 pairs. The stated benefit is that the same devices now serve as both output and output driver transistors. The drive boards between the 400 II and 700 II are identical except for R127 and R128 - 56K on the 700 II, 82K on the 400 II. This is the most consequential revision in the model's life, and it means two 700 IIs with different serial number ranges are electrically different amplifiers. The quasi-complementary and fully complementary versions are separate purchases, not interchangeable ones.
The manual is unusually fussy about disassembly. Unplug the line cord, pull the bottom cover's seven Phillips screws, then set the amp front-panel-up and remove the four handle screws to lift off the handles, ferrules, and front panel. The manual explicitly warns not to scratch the appearance parts. The two middle and two lower sub-front mounting screws come out. The two upper screws stay in place, loosened only, so the sub-front can rotate to vertical and be locked there with one screw. Then the main PCB retainer nuts come off, and the board lifts from its mounting bolts and flips forward to expose the foil side. The manual's caution applies here: do not energize the unit with the board out unless you have confirmed nothing on the PCB or its jumpers is shorting to the chassis or heatsink. Reassembly reverses the procedure, but the front panel goes back in a specific order. Insert the six sub-front screws without tightening, secure the front panel with the handles and tighten firmly, then push the sub-front snug against the back of the front panel while checking that the switch buttons align with their holes, and only then tighten the sub-front screws. Skip that sequence and the buttons bind.
Service access to the display boards is separate: four retainer nuts, slide the PCB off its mounting bolts, and on reassembly make sure the foam rubber gaskets are properly aligned before the nuts go back on. Ignore that and it becomes a rattle or a short.
The 700 II does not come with a protection circuit worth the name, in any of this documentation. The service literature is a record of a company iterating on turn-off thump, display tracking, and output device complements, not on fault protection. That is consistent with how these amps are discussed among people who repair them, and it is the reason a 700 II in unknown condition should be brought up on a variac or a dim-bulb tester rather than plugged straight into the wall. The 200-volt rail does not forgive a shorted output device.
Pricing, production totals, and the 700 II's original list price are not documented in the material available here, so no figures are offered. What is documented is a service history detailed enough to date almost any example: TO-99 versus mini-DIP input op-amps, 2.4K versus 7.5K dropping resistors, serial 10000 for the universal display board, serial 7A10701 for the complementary output stage.
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