Phase Linear 400 Re-work

jdjohn

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:


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


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!

Clipdat

Nice job on the refurb! Does it run hot?

george daniel

Attaboy!

mhardy6647

The White Oak do-overs have a very good reputation -- that's for sure.

jdjohn

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

jdjohn

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.

erniejade

Very cool! I have always like the looks of those amps and the 4000 preamp.

mhardy6647

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.





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

jdjohn

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.

mhardy6647

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.

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.

tophatjohnny

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 😀

invalid

Quasi comp can be direct coupled

dromunds

This is really impressive work. I remember lusting after that amp back in the day but of course no way I could afford it.

jdjohn

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

Trumpetguy

I can't say enough good about these amps. I'm running 2 PL 400s, with White Oak upgrades -- all circuit boards, full comp (=cleaner sound, deeper bass), bigger power caps. The sound is clear, detailed, and massive when you turn it up! Do they run hot? Not at normal listening levels. At full volume? Yes, of course -- they're putting out 400+ watts! For me, the build is fun and satisfying. Make sure you have a high-wattage soldering iron with good temperature control.

jdjohn

Hi, @Trumpetguy , and welcome to the club Polk forum! For reference, what is the rest of your gear?

It's been about 18 months since I made the original post about my PL-400 with the WOPL mod/upgrade, but I still feel the same. Just today, I pulled my modded PL-400 out of the chain (after a rather long stint), in order to try a new power cord (with a different power amp), so we'll have to see how things go. IMO, there are big shoes to fill when the PL-400 is put in the bull-pen.

P.S. Speaking of power cords: the stock PL-400 has a very old-school, non-polar, two-prong power cord. I truly wonder if a polarized power cord (with grounding) could improve things even more.

audioluvr

Or it could introduce a ground hum.

Trumpetguy

Hi @jdjohn. I just saw your comment from July 11! In answer to your question, I have a setup that is, for the most part, miscellaneous and vintage. The heart of it is a McIntosh C20 preamp (fully recapped, of course), driving the two WOPL Phase Linear 400s. For speakers I have 4 AR-11s. They put out massive sound, and create a nice soundstage. My room is roughly 15' x 30' with an 8' ceiling. Inputs are a Marantz CD6005, and a Pioneer PL-500 turntable with an Ortophon Blue cartridge. I have a very old, custom phono preamp that improves on the McIntosh phono section. I stream via bluetooth with an iFi Zen Blue. It isn't up to the level of the rest of my system, but the price/performance ratio was right. That goes for the whole system -- I would say I have invested about $5000 in this configuration over the years, and it is comparable to systems at 4 times the price. The AR-11s are nice, but I often wonder what more modern speakers would sound like.

I also wonder about the power cords. Some people claim that $100 power cord upgrades make an audible difference.