Phase Linear 400 Re-work

jdjohnjdjohn Posts: 1,878
These power amps, designed by Bob Carver, have a bit of a reputation, including references such as 'Flame Linear' due to a few random mishaps. But I can tell you with certainty, when re-worked with modern modifications and upgrades, these amps have the capability to supplant many others.

White Oak Audio https://www.whiteoakaudio.com/ is the go-to place for mods and upgrades for PL400 and PL700 amps. Joseph (Joe) King was the guy I corresponded with along the way during my project. He was very responsive, and their documentation and knowledge base on these amps is quite impressive...second to none. They have an eBay presence for their upgrade kits, but of course going directly through their website saves on fees.

My subject amp was a tired and broken PL400 from my dad's collection. I stumbled across the White Oak Audio re-build kits on eBay - re-designed main board with new components - and decided to take a chance on it to try and get the amp working again. The new PCB with all-new passive components arrived quickly, along with a myriad of PDF docs via email. Cost was $150 plus taxes and shipping. I also purchased White Oak's power supply capacitor upgrade ($70), which uses 15,000uF caps, replacing the stock 5,900uF caps.

I dove right in, assembling the replacement board according to the instructions, without incident. It was actually quite fun, but don't tell my wife that ;) Of course having the right tools is essential, including a variety of tips for your soldering iron.

Here's the front of the amp for reference:
qzhovyq3jxoc.jpg

This is the newly-built board, along with the upgraded caps:
ukdknme0lrq9.jpg

Based on the results, I cannot recommend this upgrade enough! These were powerful amps as-is, with 200WPC into 8ohms, but power alone does not mean much, as 'loud' does not equate to 'good'. However, I can report that with the 'WOPL' upgrade, this amp really impresses.

I was not expecting much for the (relatively) small investment in the kits, but wow, the results were a bit jaw-dropping. Full disclosure: I also replaced all 16 (yes, you ready that right) output transistors. In the end, this rivals power amps costing in excess of $2,000...maybe more, IMO. Headroom, clarity, separation, imaging, soundstage, bass control...it's all there in spades. I'd say the only 'weakness' in the design is having no power switch. When you plug it in, it's on...literally...and with just a simple non-polar two-prong plug.

Honestly, I'd thought after performing the upgrade, I would try to sell it and still make a little profit. But after listening to it, I will be hanging on to it awhile longer!
"This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
"Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon

Comments

  • ClipdatClipdat Posts: 9,445
    Nice job on the refurb! Does it run hot?
  • george danielgeorge daniel Posts: 12,110
    Attaboy!
    JC approves....he told me so. (F-1 nut)
  • mhardy6647mhardy6647 Posts: 27,276
    The White Oak do-overs have a very good reputation -- that's for sure.
  • jdjohnjdjohn Posts: 1,878
    Clipdat wrote: »
    Nice job on the refurb! Does it run hot?
    Thank you! I was curious about the operating temp as well, but it does not run hot IME. Perhaps I need to drive it harder >:)
    "This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
    "Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
    Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
  • jdjohnjdjohn Posts: 1,878
    mhardy6647 wrote: »
    The White Oak do-overs have a very good reputation -- that's for sure.
    Yes, and I found there's a group of guys in a Phoenix audio forum who are real enthusiasts for the 'WOPL' builds. Joe King from White Oak Audio actually hangs out there. One of the differentiators with the Phase Linear amps is/was 'quasi-comp' vs 'full-comp'. I'm not going to pretend I actually know the difference, but it was some kind of change in circuitry that occurred after a few years of production.
    "This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
    "Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
    Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
  • erniejadeerniejade Posts: 5,510
    Very cool! I have always like the looks of those amps and the 4000 preamp.
    Den: Lumin D1,Wireworld Silver Eclipse RCA, KEF LS50 Wireless, Velodyne SPL1200, Technics 1200, Denon DL160, Jolida D9,

    Living Room:,T+A PA 1530R,Wireworld Silver Eclipse SC, Tyler Highland H2, High Fidelity Reveal RCA, Innuos Zen MK3 , W4S recovery, LKS MH-da004,

    Have but haven't used in a while: LH Labs VI Dac, Cayin SCD50T, Grant Fidelity Tube Dac11,Aries Mini
  • mhardy6647mhardy6647 Posts: 27,276
    jdjohn wrote: »
    mhardy6647 wrote: »
    The White Oak do-overs have a very good reputation -- that's for sure.
    Yes, and I found there's a group of guys in a Phoenix audio forum who are real enthusiasts for the 'WOPL' builds. Joe King from White Oak Audio actually hangs out there. One of the differentiators with the Phase Linear amps is/was 'quasi-comp' vs 'full-comp'. I'm not going to pretend I actually know the difference, but it was some kind of change in circuitry that occurred after a few years of production.

    Quasi-complemetary amplifiers don't have separate + and - DC power supplies; thus both halves of the push-pull signal are amplified at (AC) voltages above zero (i.e., there's an "offset" voltage for the signal after amplification). This requires some sort of "offset" blocking device, typically a capacitor, between the output transistors and the speakers (or the next stage in the chain, if it's a preamp that's quasi-complementary). Early solid state amplifiers used quasi-complementary amplifier stages because early transistors were "all" (at least, mostly) of the "PNP" type -- there weren't "NPN" transistors, or they were very expensive. Full-complementary topologies require pairs of PNP and NPN transistors. :)

    That's what I know -- and all that I know!
    Transistors are of the Devil, as all devout hifi enthusiasts know. That's why those old PLs are associated with flames. B)

    9fnx6vcf3sbr.png

    B)

    https://www.pitt.edu/~qiw4/Academic/ME2082/Transistor Basics.pdf

  • jdjohnjdjohn Posts: 1,878
    PL actually used all NPN in their quasi design. Here's a blog excerpt from Joe King:
    What is the difference between Quasi-Complementary and Full-Complementary?
    With fully-comp circuit topology, the positive half and negative half of the signal are amplified through the final current gain sections with symmetrical circuit design through the use of NPN drivers in the plus half of the signal, and PNP drivers in the minus half of the signal.

    The quasi-comp used NPN on the plus half, like the full-comp version, but the minus half had the PNP driver emulated by a combination of a small PNP transistor coupled with (2) NPN [as] pre-driver and driver instead of using PNP devices. When the Phase Linear was initially introduced, this is what the designers had available to design with, so they went with it.

    In the last of the PL400 and PL700 production, PL did convert to fully-comp on a small number [of units] before end-of-life of the product.

    Some benefits, amp gain is more equal on the positive and negative half of the signal. In the quasi-comp version, the gain of the negative side was always higher than the positive half. The global feedback compensated for this difference, however, and quite effectively nulled it out.

    The bias settings are now extremely close for both [the] positive and negative half. Part of this comes from the use of precision 1% resistors in the bias resistor locations, the balance comes from the symmetrical topology.

    The current limit protection circuits are also now fully symmetrical, leading to a more balanced onset of protection for each of the plus and minus sides.

    Of course with this PL400 Backplane board, the configuration of full-comp or quasi-comp is easily selected by several components, jumpers, and of course the choice of output drivers.

    In the full-comp versions, the MJ21196 (NPN) is used in the pre-driver and driver positions on the plus half, and the MJ21195 (PNP) is likewise used in the pre-driver and driver positions on the minus half.

    The use of these outstanding drivers everywhere leads to a more robust output stage as a side benefit.

    So I think by changing a few components in the board assembly, and changing half of the output transistors, one can convert a PL quasi-comp version to a full-comp version.

    "This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
    "Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
    Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
  • mhardy6647mhardy6647 Posts: 27,276
    I had a 50/50 chance of being right! ;)
    I thought PNP transistors came first; but I could have it bass-ackwards, as they say. :p

    Not that these PL products were all that early, but they (Bob) may have simply wanted to avoid the challenges & pitfalls of ensuring truly balanced operation. If something else is in line to take out any DC offset, it's one less thing to worry about in terms of immolating the loudspeakers.
  • tophatjohnnytophatjohnny Posts: 3,201
    Reminds of of the good ol 70’s bar to bar gigs! Our PA had a few of those in the racks but never knew they’d be used for home stereos too! I know our main singer in the group loved the amps but my mind was always elsewhere 😀
    "if it's not fun, it's not worth it & remember folks, "It's All About The Music"!!
    *****************************
  • invalidinvalid Posts: 455
    Quasi comp can be direct coupled
  • dromundsdromunds Posts: 9,166
    This is really impressive work. I remember lusting after that amp back in the day but of course no way I could afford it.
  • jdjohnjdjohn Posts: 1,878
    Thank you, Don. It's not terribly difficult with decent soldering equipment and patience. The kit from White Oak Audio is exceptional in both quality of components, and instructions.

    I pulled the amp out of service while making some other recent changes, but now need to put it back into circuit for more listening :)
    "This may not matter to you, but it does to me for various reasons, many of them illogical or irrational, but the vinyl hobby is not really logical or rational..." - member on Vinyl Engine
    "Sometimes I do what I want to do. The rest of the time, I do what I have to." - Cicero, in Gladiator
    Regarding collectibles: "It's not who gets it. It's who gets stuck with it." - Jimmy Fallon
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