Phase Linear/White Oak driver boards

Michael F

Active Member
For those unfamiliar with White Oak Audio, they specialize in Phase Linear amplifier modifications, one of which being an updated driver board that addresses certain original design and component shortcomings and subsequently bring the 400 and 700 series specifications up to a whole new level.

The performance gains are well documented however, a first hand account of before and after changes is what interests me. I`d love to hear about the sonic differences perceived after performing this mod on either a 400 or 700.

They also sell meter and panel LED lighting kits which are of very high quality,
have a look at my 700B :yes:

TIA
 

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I'm (once again) seriously considering this for my 700B now that I have a use for an amp with big power. It's either restore back to stock spec or (bite the bullet and) go with the White Oak board. It would be nice to hear what I'll be getting in return for my investment.
 
Is it currently in running shape? Rebuilding the stock board wouldn't take much. There just isn't all that much in there. About all that would cost any appreciable amount are the "beer can" capacitors. Might be worth a couple bucks to refresh it and listen to it in stock form to get a baseline at least.
 
I've been using it for the past several months to power a pair of bucket subs through a Rane active crossover. Seems to be working fine, though I do have to crank the output levels up pretty high to get the subs' levels matched to the other pair of subs I use in a distributed bass rig. I need to try it with some normal speakers to know for sure that it's operating normally. I have noticed it doesn't seem to get very hot.

Do you know if there's much truth to what I've been reading about the stock driver board being sufficiently antiquated that sonic performance suffers when compared to the more modern WO board? The few post-WO mod testimonials I've managed to dig up claim improved clarity and dynamics, greater detail, and lower distortion at high output levels. I've also read the amp becomes more stable with challenging loads and power jumps up to around 500 wpc at 8 and IIRC 700 at 4. I'd love to have an amp with sledgehammer power, but I've been enjoying a pair of restored and modified Threshold s/300's and a Pass First Watt F5 clone so my ears are maybe a little spoiled. I'd very much like that sledgehammer amp to be able to keep company with the aforementioned designs. Maybe offer the same caliber of performance, but perhaps just a different "flavor".
 
There are many discussions of this on AK and elsewhere. AK member laatsch55 is a major PL amp upgrader and expert. AK member dimlay3 is another and he sells a protection relay kit for the PL amps. There's a lot of information in this non-AK forum.
 
All I can really say is that the stock amp isn't really supposed to get that hot. It runs in class B, which doesn't generate nearly the heat that an AB amplifier does. I've got this one sitting here barely working, usually peaking about 3.5 watts on the front meter, and its room temperature. I think more heat comes off the driver board's small transistors than from the big outputs.

It also is not extremely sensitive. I want to say it needs close to 2 volts to hit full rated output. I didn't take particular note of the level, but I had to bump my audio generator up from 1v max to 3v max to be able to drive it to full power for some of the tests in the service manual.

No clue about the rest of it. This is well outside my realm of comfort. I'm a dozen watts per channel out of small glass bottles kind of guy. I will tell you that these JBL L-40's are producing bass that I've never heard from them before. If I were going to own one of these, at an absolute minimum it would get a protection relay board though. Having absolutely nothing between the power supply and the speakers except for transistors that could fail in a bad way makes me uncomfortable.
 
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I`ve since replaced the driver board in my 700B, more out of necessity than anything else. Unfortunately, I didn't spend enough time with the amp in stock form to be able to accurately comment on the sonics. It sounded decent enough, especially in Direct input mode (bypassing the capacitor).
The two things that really made an impression were the massive bass slam and control and the seemingly endless amount of power on tap, this amplifier is a true monster!
That upgrade wasn`t limited to the driver board alone. Output transistors, emitter resistors were replaced with modern components, the grounding scheme was also re worked. The only thing that wasn't replaced interestingly enough were the filter capacitors.
Again, I cannot comment on the sonics because the amplifier has only seen subwoofer duty since but I`m pleased to report that the previous attributes have been maintained, if not surpassed, such are the pitfalls of having too many amplifiers in the arsenal. That, and I still haven't gotten around to building a load protection circuit to protect the precious JBLs. In the short time spent driving full range speakers, there is no doubt that the amp is more transparent.
The amplifier is quieter and far more stable than before. Gone are the turn on thumps and the dreaded Blaze Linear syndrome.
I would strongly recommend spending some time on the Phoenix website. You`ll find a lot of good information there along with some pretty hard core Phase Linear dudes, great guys too.
 
I've been using it for the past several months to power a pair of bucket subs through a Rane active crossover. Seems to be working fine, though I do have to crank the output levels up pretty high to get the subs' levels matched to the other pair of subs I use in a distributed bass rig. I need to try it with some normal speakers to know for sure that it's operating normally. I have noticed it doesn't seem to get very hot.

Do you know if there's much truth to what I've been reading about the stock driver board being sufficiently antiquated that sonic performance suffers when compared to the more modern WO board? The few post-WO mod testimonials I've managed to dig up claim improved clarity and dynamics, greater detail, and lower distortion at high output levels. I've also read the amp becomes more stable with challenging loads and power jumps up to around 500 wpc at 8 and IIRC 700 at 4. I'd love to have an amp with sledgehammer power, but I've been enjoying a pair of restored and modified Threshold s/300's and a Pass First Watt F5 clone so my ears are maybe a little spoiled. I'd very much like that sledgehammer amp to be able to keep company with the aforementioned designs. Maybe offer the same caliber of performance, but perhaps just a different "flavor".

As Lex has linked....the PL topology is greatly improved by Joe at White Oak Audio. He now has come out with boards that replace the sockets on the backplane which also allow for the option of finishing the amp either as a quasi comp or fully comp. The result is a quieter amp that now tests at UNDER 300 MICROVOLTS in residual npoise and signal-to-noise ratios approaching -120db.
I recently had the opportunity to do a side by side comparison between a Fully Comp WOPL (white oaked phase linear) and a Carver PT2400. In stereo mode the TP 2400 tested at 838 watts per channel into 8 ohms , with both channels being loaded and driven, with the aforementioned WOPL testing at 512 watts per channel into 8 ohms with both loaded and driven. Both amps were tested using my Audio Precision ATS-1DD audio analyzer.

The speakers were pair of Soniphase S6's with a 1000 watt EVX-155 15" woofer. The Carver was trying to clip and go into protect mode before max output was reached. The WOPL drove the Soni's above and beyond what the Carver did. We pushed the Soni's with the WOPL so hard the crossover section that dealt with the mids and tweets actually melted the solder on the connections. The Carver never got close to it. Why would an amp that tested at 838 not outperform an amp that tested at 512?? Good question..my guess as I have no definitive test to tell me is that the design of the original PL's allows for the delivery of massive amounts of current. Yes I am aware of Ohm's law. but there is something about Bob's design that still shines today that after Joe tweaked and improved it, now puts them in my estimation, as a world class amp. Flat response down to 10Hz....ok not flat, but it measures down .(point) 13db at 10 hz. Gentlemen that's a bad ass amp. The max output nimber on the WOPL's can vary from 480-527 watts per depending on secondary transformer voltage. The said WOPL also was tested to max out on 4 ohms. That figure was 775 watts , both loaded and driven. SRail voltage had drooped to 79 volts. With a stiffer power supply it would have doubled into 4 from the 8 figure, a flaw I am in the process of curing by building 2-outboard power supplies (one for each channel) and raising rail voltage to 120 +/- DC. Max out should approach 700 into 8 when I'm done, AND DOUBLE into 4....that enough juice for ya??
 
I also run a WOPL 700 on my severly modded K-Horns and it is a phenomenal combination.....I can vibrate your eyeballs if you sit in the sweet spot. My room really isn't big enough for the K's being only 24 by 13, but I've been to a lot of concerts, and lately a few with the new Nexo arrays........my basement sounds better..

BTW...subs with this combo would only muddy the waters. The K's produce some of the tightest bass I've heard using the Crites 1526s woofs.
 
All I can really say is that the stock amp isn't really supposed to get that hot. It runs in class B, which doesn't generate nearly the heat that an AB amplifier does. I've got this one sitting here barely working, usually peaking about 3.5 watts on the front meter, and its room temperature. I think more heat comes off the driver board's small transistors than from the big outputs.

It also is not extremely sensitive. I want to say it needs close to 2 volts to hit full rated output. I didn't take particular note of the level, but I had to bump my audio generator up from 1v max to 3v max to be able to drive it to full power for some of the tests in the service manual.

No clue about the rest of it. This is well outside my realm of comfort. I'm a dozen watts per channel out of small glass bottles kind of guy. I will tell you that these JBL L-40's are producing bass that I've never heard from them before. If I were going to own one of these, at an absolute minimum it would get a protection relay board though. Having absolutely nothing between the power supply and the speakers except for transistors that could fail in a bad way makes me uncomfortable.

I have had an output fail at full tilt boogie on a 700 hooked to my K's. I had Don's board installed. The relay contacts were useless for audio after that, but a 15.00 relay to protect my 8K horns was a bargain. Don's DC protect board works AND WORKS WELL.
 
Great info, Laatsch, and thanks for sharing. I'm guessing you're just repeating a lot of stuff you've said before and I appreciate you taking the time to spell it out again for the umpteenth time.

Here's where I'm coming from. I'm a drummer and on-again/off-again working musician. Live venue db is not my thing when I'm in the listening chair as I've been exposed to more than my fair share of sonic onslaught after more than two decades of gigging/practicing and will likely have plenty more ahead of me....God willing. So sonic quality matters a lot to me, sheer output capability not so much. In the research I've done on the WO modded 700B's I get a lot of "unbelievable headroom', "effortless gobs of unlimited power", "face melting output" type of stuff. Comments on resolution, transients, sound stage reproduction, realism, jump/blink factor and the like aren't nearly as easy to find. Don't get me wrong, I can appreciate the hell out of the sheer sonic power the WO 700B can generate, but since I don't listen at stupid high volume levels that performance characteristic is of much less concern to me than making the magic happen well south of 100 db. That's what I'm hoping to hear more about.

- Michael
 
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I don't listen at stupid high volume levels that performance characteristic is of much less concern to me than making the magic happen well south of 100 db. That's what I'm hoping to hear more about.

- Michael

Even with moderately efficient speakers, the 700B is way overkill for your application. At those levels, the meters would barely deflect even when set to -20db position, it would essentially be idling.
Whether you keep your amp stock or modify it, I would be considering a protection circuit in the mean time, cheap insurance.
 
Even with moderately efficient speakers, the 700B is way overkill for your application. At those levels, the meters would barely deflect even when set to -20db position, it would essentially be idling.

Well, I do have some very hard to drive designs in the collection as well as plans to build some OB's that will use four 15" Acoustic Elegance IB15's ...so I could use the power. But if brute power were my only concern I'd use the two Crown amps I use in my personal PA rig. I was hoping the WO PL700B could bring considerably improved fidelity to the table...along the lines of my Thresholds, Pass or Aragon I once owned.
 
OK, for the sonic qualities, I have retired my Spec 2's, My Denon POA 2400, my SAE 2400, and my Bryston 4B. I recently did a 400 for a fella on Phoenix, Az. He sold his Mark Rowland and Parasound after a week with that little sweetie. The soundstage is outrageous now in the fully comp versions. Ya know Bob had it right 40 years ago and now the modern components have cayght up to the design and with Joe's constant improvement in the general layout and tweaking of the original components they truly are state of the art now. They have to be heard to be believed. The power is a nice plus as there is no substitute for headroom, and these are clean headroom watts, a totally different world from stock. Joe is a world class engineer and in his day job heads up 4 engineering departments with approximately 400 engineers under him in a global data storage company. Audio is his hobby. He's in the development stage of redesiging the board to do away with the germanium protect transistors and with the obsolescence of the 2N3439's and 2n55416's he's working modern subs in for them. He hasn't decided what to do with the front diffs , but with Joe ot'll be something that vastly improves performance ands reliability. He's a perfectionist and one of his motto's is--" Anything can be fixed Lee".....
 
Here's how well Joe understands this. This post was in answer to why the P{L's would latch up to a rail....


Description of latch up tendency in Phase Linear Amplifiers
Published this document earlier in one of the long threads and it is now buried somewhere in those archives. Bringing it to its own thread to make it easier to find.

Description of the latch up possibility.
When power is applied to the PL amp, the +/- bulk caps charge rapidly to provide the main power for the amp. During this start up phase, the critical ‘front end’ long-tailed pair formed by the differential amp (Q1 and Q2) is not in control yet because it is not provided sufficient voltage to properly bias itself and operate as an op amp. In addition, the positive and negative main rails do not rise at exactly the same rate leading to bias imbalance in this front end during this critical start up period.

Before the front end can control the amp output using global negative feedback provided from the output through R15, R13 and C6, the tendency is for the amp to come up with a positive bias on the output. This is because the positive rail feeds directly through R17 then through R18 then through the base emitter junction of Q10, turning it on quite hard on power up which in turn drives Q11, Q13, Q15, Q17 to turn on quite hard. This DC voltage is fed back through R15, R13 to begin charging C6 in the positive direction. During this startup phase, Q5 is not active yet (off) because the front end is not biased properly and cannot control the second stage current amplifier formed by Q3 and Q4 to provide base current to control Q5.

If the amp starts properly and the front end comes into regulation fast enough, the global negative feedback compensates for this and through the action of the differential front end turns on Q4 and in turn Q5 to bring the positive turn on bump back to ground again and all is well.

In the latch up condition, this start up process begins but then runs away as follows. Q10, Q11, Q13, Q15, Q17 all turn on hard due to the bias from R17 and R18. This positive bump produces a current in R15 and R13 starting to slowly charge C6. The rapid voltage rise at the nodal junction of R15 and R13 starts a current flowing through R10 and C4 into the base of Q2. This base current due to both +/- rails starting up at similar rates will flow through both the base-emitter junction (into R6) as well as the base-collector junction (into R7). The split of current flow is somewhat indeterminant but the base-collector junction is typically a few millivolts lower than the base-emitter junction in a TIS97 transistor. As a result, the tendency will be for this feedback current to flow through the path of least resistance, the base-collector junction and into R7. This is not the intent by design. This action produces a cut off of Q4, the MPSA93 transistor which continues to hold off Q5 which reinforces the continued rise of the output voltage in the positive direction. Essentially negative feedback turns into positive feedback by accident, the amp latches up and the output rail goes to the positive rail. In the case of the PL20 board with the +/-20V regulators for the front end, once the amp is latched up, the collector of Q2 rises to approximately 29V turning off Q4 very hard, giving the amp no chance of coming back into regulation. This collector node of Q2 is nominally supposed to operate at 10V. If the amp is latched up and then power cycled rapidly again, C6 will have retained much of this ~29V charge causing the amp to almost guarantee to latch up again.

How do the back to back diodes around C6 fix this?
It is mainly the diode with its anode connected to the positive side of C6 and its cathode connected to the negative side of C6 that does the trick. The other diode is added for symmetry even though the amp has no propensity to latch up in the negative direction. This first diode clamps the node at the junction of R15 and R13 to a maximum of +/- 2.36V for the PL400 or +/- 2.85V for the PL700B. This allows the front end to become properly powered and stabilized and keep the negative feedback signal within the allowable common mode voltage range of this front end stage. Any tendency to latch up will be quickly remedied once the +/- 20V supplies come into regulation. These diodes do not come into play when the amp is amplifying audible frequencies as the impedance of C6 is so low (for example, at 20Hz its impedance is only 79 ohms and will only experience 0.49V peak when the amp is producing a 80V peak signal output which is less than the Vf of the diodes involved, at 40Hz there is only 0.25V peak across C6 and so on). This is an elegant and effective fix and allows the amp to be power cycled rapidly with no concern of latch up.

What about turn on and turn off thump? These are actually artifacts of the front end not being in control during turn on and turn off. If these thumps exist, it is evidence that the amp is starting to latch up and then recover. This fix allows the negative feedback signal to more rapidly work with the front end to stabilize and regulate the output by staying well within the common mode range of the front end

Originaly posted by Joe at Phoenix Audio Community Forum. Since that post the "Gepetto Mod" as it's known as has now become integral with the release of the Rev "C" board. The Rev "D" board has been out awhile now and allows for fully comp operation with his new backplane boards...
 
Well, I do have some very hard to drive designs in the collection as well as plans to build some OB's that will use four 15" Acoustic Elegance IB15's ...so I could use the power. But if brute power were my only concern I'd use the two Crown amps I use in my personal PA rig. I was hoping the WO PL700B could bring considerably improved fidelity to the table...along the lines of my Thresholds, Pass or Aragon I once owned.

Power aside, I doubt the stock PL could compete with those amps. It is after all, a 40year old amp. Design aside, the components that were available at the time have probably drifted from spec. Modern equivalents have been greatly improved upon, so it would stand to reason that a full blown WOPL (complementary output) would make for a worthy adversary to many a SOTA amplifier.
 
:D
Here's how well Joe understands this. This post was in answer to why the P{L's would latch up to a rail....


Description of latch up tendency in Phase Linear Amplifiers
Published this document earlier in one of the long threads and it is now buried somewhere in those archives. Bringing it to its own thread to make it easier to find.

Description of the latch up possibility.
When power is applied to the PL amp, the +/- bulk caps charge rapidly to provide the main power for the amp. During this start up phase, the critical ‘front end’ long-tailed pair formed by the differential amp (Q1 and Q2) is not in control yet because it is not provided sufficient voltage to properly bias itself and operate as an op amp. In addition, the positive and negative main rails do not rise at exactly the same rate leading to bias imbalance in this front end during this critical start up period.

Before the front end can control the amp output using global negative feedback provided from the output through R15, R13 and C6, the tendency is for the amp to come up with a positive bias on the output. This is because the positive rail feeds directly through R17 then through R18 then through the base emitter junction of Q10, turning it on quite hard on power up which in turn drives Q11, Q13, Q15, Q17 to turn on quite hard. This DC voltage is fed back through R15, R13 to begin charging C6 in the positive direction. During this startup phase, Q5 is not active yet (off) because the front end is not biased properly and cannot control the second stage current amplifier formed by Q3 and Q4 to provide base current to control Q5.

If the amp starts properly and the front end comes into regulation fast enough, the global negative feedback compensates for this and through the action of the differential front end turns on Q4 and in turn Q5 to bring the positive turn on bump back to ground again and all is well.

In the latch up condition, this start up process begins but then runs away as follows. Q10, Q11, Q13, Q15, Q17 all turn on hard due to the bias from R17 and R18. This positive bump produces a current in R15 and R13 starting to slowly charge C6. The rapid voltage rise at the nodal junction of R15 and R13 starts a current flowing through R10 and C4 into the base of Q2. This base current due to both +/- rails starting up at similar rates will flow through both the base-emitter junction (into R6) as well as the base-collector junction (into R7). The split of current flow is somewhat indeterminant but the base-collector junction is typically a few millivolts lower than the base-emitter junction in a TIS97 transistor. As a result, the tendency will be for this feedback current to flow through the path of least resistance, the base-collector junction and into R7. This is not the intent by design. This action produces a cut off of Q4, the MPSA93 transistor which continues to hold off Q5 which reinforces the continued rise of the output voltage in the positive direction. Essentially negative feedback turns into positive feedback by accident, the amp latches up and the output rail goes to the positive rail. In the case of the PL20 board with the +/-20V regulators for the front end, once the amp is latched up, the collector of Q2 rises to approximately 29V turning off Q4 very hard, giving the amp no chance of coming back into regulation. This collector node of Q2 is nominally supposed to operate at 10V. If the amp is latched up and then power cycled rapidly again, C6 will have retained much of this ~29V charge causing the amp to almost guarantee to latch up again.

How do the back to back diodes around C6 fix this?
It is mainly the diode with its anode connected to the positive side of C6 and its cathode connected to the negative side of C6 that does the trick. The other diode is added for symmetry even though the amp has no propensity to latch up in the negative direction. This first diode clamps the node at the junction of R15 and R13 to a maximum of +/- 2.36V for the PL400 or +/- 2.85V for the PL700B. This allows the front end to become properly powered and stabilized and keep the negative feedback signal within the allowable common mode voltage range of this front end stage. Any tendency to latch up will be quickly remedied once the +/- 20V supplies come into regulation. These diodes do not come into play when the amp is amplifying audible frequencies as the impedance of C6 is so low (for example, at 20Hz its impedance is only 79 ohms and will only experience 0.49V peak when the amp is producing a 80V peak signal output which is less than the Vf of the diodes involved, at 40Hz there is only 0.25V peak across C6 and so on). This is an elegant and effective fix and allows the amp to be power cycled rapidly with no concern of latch up.

What about turn on and turn off thump? These are actually artifacts of the front end not being in control during turn on and turn off. If these thumps exist, it is evidence that the amp is starting to latch up and then recover. This fix allows the negative feedback signal to more rapidly work with the front end to stabilize and regulate the output by staying well within the common mode range of the front end

Originaly posted by Joe at Phoenix Audio Community Forum. Since that post the "Gepetto Mod" as it's known as has now become integral with the release of the Rev "C" board. The Rev "D" board has been out awhile now and allows for fully comp operation with his new backplane boards...

Lee, do you have an English translation of that?:D
 
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