Phase Linear and Pink Floyd

Lee,
John Robison put two fans on the back, just like we did! I wonder if that is what he meant by saying, "more rugged output stages" because we know the amplifiers are capable of 500 WPC if you can keep them cool enough. I wonder what he meant by, "internal limiting which upped the power 50% over their civilian counterparts while retaining the sweet sound"? Maybe different current limiting transistors?

I see that he used sliding pots on the inputs instead of the rotary pots, in the music biz they are easier to see what the level is backstage because you are not looking for a skinny black line on the edge of a volume knob. The LEDs are a no-brainer because the meters are too delicate and hart to read from 10 feet away. Lee, I like the idea of blown DC fuse indicators although I don't thing they are necessary for a home amp, for a road amp they would be necessary and useful. I also wonder how he wired up a "Malfunction" light? What would you consider a malfunction in the circuit? Loss of bias? Loss of board voltage? Maybe I'll contact him and see if he has ay schematics laying around or still in his head.

He is in Amherst, MA. Isn't that close to Joe, sort of...

Mark
 
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Once he knows a bunch of PL nuts hang at the place, he won't be able to resist.,.....
 
Great thread, fun reading. reminds about the picture of Pink Floyd with their gear on the back cover of the Ummagumma LP. Here's one i found from that photo shoot:

pfply.jpg


I remember they used the PL 700 amp in the showroom at the Speakerlab Roosevelt store in the early/mid 70s. A few years ago I met Bob Carver when I brought a friend's PL 700 into a audio repair shop that he was involved with. It was kind of cool to talk with him especially about the Andromeda and Carver Amazing speakers which I've enjoyed and which he is clearly still excited to talk about.

IMAGE_01273-4.jpg
 
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Good afternoon, folks.

I had no idea there was a Phase Linear community online! I got a Facebook message asking about this thread and the work I did on the Pink Floyd amplifiers. I was the engineer for Britannia Row in Long Island, when the Floyd first set their sound company up in the United States. That was in the mid to late 1970s.

When we got the amps they had already been ruggedized by a process that was briefly described earlier in the thread. The Brits had made cubes of angle iron with trays to hold two amps. The power transformers were removed from the chassis and mounted to the heavy trays. That made the remaining amplifiers light enough that they did not flex and break apart on the road. Then the inputs and outputs were hardwired to the standard connectors we used for input and speakers. Power was supplied through one twist lock and fans were fitted to the backs of each heat sink, and the back of the whole enclosure, which was itself slid into an anvil road case.

The amps had various custom faceplates, some of which have been shown in stories on my blog over the years (it's at http://jerobison.blogspot.com )

However, they still were not reliable and I attacked that problem here in the USA with some success. These are some of the things I did (there were others I don't remember; it's been 35+ years)

1 - We changed to higher speed Super Boxer fans and had two speeds for them. The higher speed was noisy but moved a lot of air. I fitted front panel warning lights that illuminated when the fans stepped up to high speed in response to sustained high temps.
2 - We experimented with different output transistors, modified driver circuits, and used heavier resistors on the driver boards.
3 - We combined the power supplies of the two amps and doubled the filter capacitor bank. I used a relay bridging a resistor bank to bring the capacitor banks online in steps so as not to overload the rectifiers. What that did was increase the peak power output because the supply voltage no longer sagged under load. That change made a significant difference in potential peak power.
4 - I also added MOV and diode transient suppression and I added clamps on the output lines for the same purpose. It turned out that poor quality power and cable induced transients had been responsible for some failures and that helped with that problem.
5 - I developed a limiter circuit that used a comparator to detect audio levels approaching the clip level as defined by power supply level at that moment and lowered output in advance. A companion circuit used a preset, time variable signal level limiter to prevent clipping and resultant destruction. The first circuit addressed the problem of driving amps to destruction when they clipped because the mains voltage had sagged and not enough power was available. The second circuit would allow maximum power at first but cut back to about 90% of potential after about 1 second to keep the outputs from overloading. With those circuits running the amps would be almost immune to clipping, even on powerful bass passages.
6 - Earlier in this thread someone wrote about three-way amplification, but by 1978 I had build Pink Floyd five-way crossovers so we had separate low bass, bass, low mid, high mid, and high amps and speakers. The great thing was, when the limiters were engaged for the low bass the mids and highs were generally still running free and you did not even hear the limiter effect except that the sound stayed clear.

After my time there I worked on many other high power amplifier designs, many of which were used in government and commercial power applications. My interest in transients and how they destroyed circuits led me to design the power conditioning for the ground zero test gear for our country's last big underground nuclear tests in the 1980s Some of those things were shown on the Discovery Science Channel's Ingenious Minds show a couple years ago. You can find that online I believe.

I never wrote much about my audio designs because I moved on to other things but I still loved that work.

John Elder Robison
 
Mark, I just posted a reply to this but it it a bit up the thread. The limiters I referred to were twofold. One used a comparator to keep audio output from attempting to exceed the DC supply voltage (the significance of that is that the DC supply was not regulated and sagged if the AC mains sagged, so a level that was OK with 125v mains would lead to destructive clipping if the mains dropped to, say, 105v. That was am important improvement. The second used Raytheon CK1123 photocells to create a slow-moving (by the speed of the incandescent drive lamp) main signal limiter.
 
Well John, I hope you stick around and post a wee bit as there are still some gaps in the PL legacy. Glad to see you posting...
 
Were you aware of the new White Oak Audio driver board, if not you are in for a treat, and some reading to get caught up.
 
John,
I am so glad you are here. We are just novices compared to you but with Joe doing the heavy lifting in the engineering department (and the White Oak Audio design of the new PL driver board) we manage to not blow up too much stuff. PM me (click on my name on the left side of the page) if you want me to send you one of the new White Oak Audio driver boards.
 
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I had no idea people were doing things like making new driver boards, etc. It makes me want to get the Pink Floyd amp I have here running and put it to use
 
Ed's dying to get his hands on it, or at least get some pics and descriptions!! John, you'll find a lot of stuff happens in this small forum........


You did say you had just one.....??
 
Was the Recording Studio Design 800 (a British amplifier which is basically a PL 400)_ ever used by Pink Floyd? Somewhere on this forum there is a thread about this amp. I was lucky enough to snag one about a year ago and with the help of folks here, was able to breathe a little new life into it...If needed, I'll locate the thread and post a link here, along with some photos.It is a really nicely engineered piece of gear which I really love. I'd likie to have a stack of them to put in my collection. So far, I think I have 4 700B's and 5 or 6 400's....most of them rebuilt using Joe's White Oak PCB, capacitors, LED's, new driver and output transistors, and the DC protection circuit designed by our famous Don Imlay (wattsabundant).This is the best forum on the internet for all things Phase Linear. Not to mention the NICEST people you will ever meet in your life. This IS a real community....with friends....not some common forum with a bunch of fools running their mouths..
 
The main PA in my time used Phase Linear 700, 400, Quad 303 and some mongrel amplifiers. I don't remember Recording Studio Design but that doesn't mean they weren't there. There were literally hundreds racked up for a big show
 
The big show I remember was the original California Jam in April of 1974. They used Tycobrahe-branded BFA 2000 amplifiers and the power supplies were separate from the electronics, just like you describe for the Pink Floyd PL amps, maybe a copy of your technique. The power supplies used 75VDC rails, so the rated power output might be into 2 Ohm loads or just peak power. They reportedly had 54,000 watts of power for the show, that would mean 27 amplifiers, not counting the monitor amps. They discussed the supply voltage sagging down to 90VAC during the heaviest bass portion of the sound and we know that the original PL amplifiers start to clip when that happens. The White Oak Audio driver board compensates for much of that sagging, but the output suffers when the rail voltage drops. The regulation of that voltage is a nice modification, we'll have to see if we can make it work in our upgraded PL (WOPL) amps.
 
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