Phase Linear PL 400 with LDX Technologies UPGRADE KIT Installed (by Lou Dorren)

[Dan Bower]

Hey Flamers, say, working on a PL400 for a friend, it has this LDX transistor kit installed by Lou Dorren (LDX Technologies). Apparently, a kit available to "upgrade" this unit. Here is my question, I have RATFW'd the installation instructions, and nowhere does it mention pulling out the DC protect transistors Q8 and Q9. They are Germanium 2n1304 and 2n1305. I think those transistors only work to protect the driver board incase there is a short happening upstream on the output transistors, but will not protect the speakers. Someone has installed what appears to be a Speaker Protect kit on it, (not a Watts Abundant) unit, so no schematics, or documentation on it. I think, it's one of those EBay units, where you have to feed some AC into a separate transformer to get 12vac to drive the board. I am going to hand draw this unit into a schematic. It's interface to the output is rather odd.

If someone has run into this kit installed, I would be grateful for some experiential insight, especially regarding the DC protect transistors missing Q8 and Q9. they are near unobtainable these days.

thanks
DB916GT

 

[gadget73]

The protection transistors are more of an over-current limiter. They cannot break DC to the speakers if the output transistor(s) short. I'd have to study the schematic to get my head back around how it works but somehow they are used to limit current to the drivers so that the output transistors are less inclined to melt down from working into too low of a load.

One of the tests in the PL service manuals is to run the amp up to full output and dead short the speaker terminals, then basically verify it didn't explode. Pretty sure it would fail that one if those transistors were gone. Me, I'd put them back in and make sure nothing else in that part of the circuit was modified when they were pulled. Make sure the carbon resistors are still OK too, or just replace them. I did some work on a 700-II for a friend some years ago and most of them were way off value.

The Watts Abundant board is a DC protection board and will kick the speakers out if the output stage shorts. In theory a generic one should too but probably worth confirming it actually functions as intended. Not sure how its wired but I'd expect any of those boards to just sit between the output of the amp and the speaker terminals. Unless maybe it also kills the rail voltage or something?

For some reason I'm thinking there is a low voltage winding on that big transformer to run the meter board. Don't know what it provides offhand but you might be able to tap that for the power to it and get rid of the extra transformer if you feel the need to.

Probably worth asking which driver board this has. The original PL400 and the Series II are different. PC-14 vs PC-36 or something? I forget exactly but its a different board either way.

no idea who or what an LDX Tech upgrade involves. I'm not deeply familiar with PL stuff, I've worked on my own Pro 700 and a 700 Series 2 for a friend, along with his 4000 preamp. Anything I know about them comes from whatever I remember from when I was info gathering on how to fix his when it melted down. Never had my hands on one of the earlier models with the other driver board.

 

[mbz]

Someone has installed what appears to be a Speaker Protect kit on it, (not a Watts Abundant)

I happily use those chi-protection boards on smaller (30-50W) amps. I'd be concerned about using such a kit on the PL400, betting that the relay would not interrupt the shorting current so spkrs would be toast. Go for the Watts Abundant as it's purpose built or better still use power fets rather than relays. Guys at diyaudio did some nice work on this option.

As a first step for protection you should install the back to back diodes across C6 as described by the good guys over at pheonix

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