August 2011, Manassas, VA
I had wanted a Phase Linear 700B amplifier since I was in high school in 1972 after reading the first ads and the initial observations by Julian Hirsch in Stereo Review and Popular Electronics magazines. However, I could not afford one back then ($799.00) and I don’t think my parents would have approved of the music at the Sound Pressure Levels (SPLs) I was contemplating. The concept of the amplifier was made possible by the new (at the time) high-power DELCO transistors designed for the high voltages of electronic automotive ignitions. The Phase Linear 400 and 700 designs were made to have extra power to loudly run the relatively inefficient sealed-box speakers like the Acoustic Research AR3/AR3a. Today’s speakers such as JBL and Klipsch are much more efficient, producing as much as 104 db per watt and really exploit the massive reserve power of the Phase Linear amplifiers and their brute force power supplies. These amplifiers earned the nickname “Flame Linear” or “Blaze Linear” due to a random tendency to deliver the full power-supply voltage (+/-100 Volts DC) to the speakers when a latch-up occurred in the front end of the circuitry. Many sets of expensive speakers went to the junk pile in the process.
Now that I am older I can afford one of these amplifiers but they have been used hard in most cases and some of them are now in sad shape. However, this gives me an opportunity to practice my Navy electronic skills and a chance to rebuild one of the best amplifiers ever designed by Bob Carver, who left Phase Linear in 1979 when Pioneer bought Phase Linear. Bob went on to design amplifiers for Carver and Sunfire.
Sad was the condition of my “new” amplifier, a Phase Linear 700B, when I took it out of the box. Another $100.00 Ebay broken box disaster. Fortunately, all the parts that were missing were found in the bottom of the box including the meter brackets and several chassis screws. One of the power supply capacitors was bouncing around and the overall condition of the inside was dirty. The amplifier actually smelled like beer. This was typical because many of these amplifiers had been damaged and put away after years of powering disco and DJ speakers. After all, they are at least 30 years old.
I had finally obtained my dream amplifier but I could not get it to work because one channel was dead and another had some distortion, plus, none of the lamps worked. I replaced all the transistors and capacitors on the control board and I still had problems. Just about that time, I learned that White Oak Audio (WOA) had developed a replacement Control Board for the Phase Linear power amps after purchasing a replacement light board that substituted, the obsolete and no longer available incandescent light bulbs, with LEDs. I volunteered my amplifier for testing and received the kit from Joseph King, the design engineer and builder for the new board. I waited patiently for the new Control Board to arrive.
The Phase Linear 700B amplifier had several issues discovered during the initial inspection. First, most of the components were original and likely the electrolytic capacitors were suspect simply based on age (circa 1974). In fact, three capacitors on the control board (47 mF, and both of the 2.2 mF caps) showed 0 on my capacitance meter and two others (100 mF/16 Volts and 100 mF/6.3 Volts) were out of tolerance, probably due to heat exposure. Second, the AC power switch had been replaced by a small switch rated at only 2 Amps/125 Volts AC. Third, the input pair of transistors were KA1778 instead of the TIS 97 called out on the schematic and both of the Q3 transistors were MPS A55 instead of 2N5401 as originally used (or the approved Phase Linear replacement MPS A93). Fourth, the top surface of the control board was very dirty and several of the solder connections on the bottom of the control board and at the lower wiring points were loose. Finally, the Phase Linear 17B meter lighting board was badly burned where R57 (1800 Ohms) is connected to the left output. This meter lighting board was to be replaced by the White Oak Audio Design Upgrade LED board that I recently purchased.
When the WOA Control Board (Rev. A) arrived in about one week, I did a thorough inventory inspection. The FR-4 (woven glass and epoxy) board was professionally made in terms of double thick construction (93 mils or 2.325 mm or 3/32”) with all through-holes soldered, a heavy insulation mask, appropriate width for the solder runs, low-level signal runs right next to the ground paths, and high-quality silk-screened artwork for circuit designations. There was a static-free bag of semi-conductor parts containing transistors and diodes. Joe was nice enough to pre-bend the leads of the resistors and identify them in a Styrofoam block for ease of selection. One more thing, these are top of the line components – the resistors are Xicon 1% metal film low-noise devices, the electrolytic capacitors are Nichicon Muse audio grade, and the hardware is the best money can buy.
This was a First-Class kit and I had serial number two.
I assembled the board and installed it in my PL700B. I connected the amplifier to a Hafler CD player and selected some appropriate test music. Humble Pie’s live and subsequent gold recording, “Performance - Rockin’ The Fillmore” recorded in May 1971 at Fillmore East in Manhattan seemed appropriate for this test and the young Peter Frampton’s guitar licks would certainly flex the muscles of the newly rebuilt Phase Linear 700B, now White Oak Phase Linear (WOPL) amplifier. During the song, I Don’t Need No Doctor, I actually thought I WOULD need a doctor because the music was so loud that the CD started to skip from the acoustic waves in the room and that is almost too loud. I recorded an average voltage of 61.5 Volts RMS on the Channel A output and using Ohms law (Vsquared / R = Power in Watts) 61.5 Volts x 61.5 Volts or 3782.25 Volts / 8 Ohms = 472.78 Watts per channel or 945.56 Watts total, both channels driven. I was driving JBL PA speakers that measured 5 Ohms DC and were probably closer to 6 Ohms at musical frequencies. At full power, those JBL speakers were handling about 1260 Watts RMS. The two JBL PA speakers each consist of a JBL 15” Model 2226 Woofer and a JBL Model 2426J Horn Driver behind a 2370A Bi-Radial Horn and driven by a JBL Model 3110 passive 800 Hz Crossover. The cabinets are my own design and built from glued, nailed, and braced ¾” MDF and finished in truck bed liner coating with all stainless fasteners. The cabinets are connected with Neutrik Speakon connectors and twin 12 gauge copper wires. This setup provides extremely clean sound to painful thresholds. The amount of reserve power in the WOPL amplifiers is truly incredible.
I later tested the amplifier at low (1/2), medium (3/4), and maximum (Full) power for 30 minutes each time. During the maximum power test (I was outside of the house because the sound was deafening) after about 12 minutes the amplifier shut down due to overheating (there are thermostats inside the amplifier on the wall of transistors set at about 160 degrees). This is the same as someone pulling the wall plug at full power!!! I turned down the amplifier and it came back on as it cooled off (at reduced volume) with no ill effects.
As far as I was concerned, when it was introduced a 700 was the ultimate amplifer. None that I was aware of at that time had as much power and that's all I cared about.
When I had the opportunity to get a used 700B 30 years ago, I did. After it destroyed our PA speakers for the third time, it had a lengthy hibernation. I found this forum when I decided to wake it up and was talked into not mearly restoring it but to make it better.
I grew up close to Ann Arbor Michigan. My four years of high school I was captain of the debate team & spent a LOT of time at the U of M Law library. When I'd take a break I'd go annoy the staff at a stereo shop near the diag. They carried Mac & PL. My senior year 72, I first saw the series 1 700. & lusted for one ever since.
Joined up here a couple years ago & Lee sold me a nice 700b. Perry made it special & I'm wringing it out daily. Love it.
I do have a minty original 700 now also.
When I was 18 ,together my father , I listened to a pair of K Horn ( powered by PL 700B and PL 4000 ) in the shop where my hifi guru worked .
Flashed on Phase Linear ' s road . Nothing else .
Summer of 1974 in New Orleans, between freshman and sophomore years at University of Colorado at Boulder, I did renovation work for a local contractor. All the shit jobs. Sanding off exterior lead paint with power sanders, installing fiberglass insulation. All in high heat and humidity.
Got together enough money to get a PL 400. Brought it back to school that fall and ROCKED! Still have it. WOPL'd of course.
Somewhere around 1977 I was at a Peaches record store. They had a 700B driving a pile of JBL L26's through 70 volt transformers. It looked so cool. In 1982 I went too work for a pro sound company that outfitted rock & roll bands with racks of 400's. I learned how to fix them real fast. After leaving the sound company I applied for and was granted status as an authorized repair station. I got a box with all of the manuals and a note book of part numbers.