Soundcraftsmen PR1800 Specs?Info?

[wingzam]

Well,there's a Soundcraftsmen PR1800 amp up for auction and I just have this thing for amps that you can't find any info for...and who did it better than Soundcraftsmen?Anyway,can anyone give me some specs or info on this thing?The seller knows nothing (not even if it actually works!).If it goes cheap enough I may not be able to resist....

[cyclecamper]

My first post.

If it was eBay I'm too late, but I know the answer to this one.

They're rated at 900 watts a channel driving some low impedance load. This is the full-rack 19" wide big brother of the PCR800 PM860 etc. half-rack line, which (in addition to the hi-fi market) catered to the semi-pro PA and instrument market with 1/4 inch inputs. The PR1800 is the pro model of their PCM1800, it is about the same but the PR1800 also has balanced XLR inputs and slots in the front plate. The exact same amp was relabelled the MTX 900X2 after MTX bought Soundcraftsmen. Bridging switch, volume controls on the back panel. Power and clipping LEDs on front.

These things are oldschool/newschool. Switching power supply, single rail voltage, mosfet output. So it's a fancy modern power supply but the amp section is not class D or H or G or anything new-school. But the power supply is small for its incredible stiff-regulation high-current output. One power supply for both channels. It uses a large power transformer mounted intelligently near the front panel (I think the primary is wall outlet 110 voltage) that's still small for the power rating because it's a switching-mode pulse width modulation regulated power supply. It has good regulation, and the electrolytic caps are big enough and small enough to work with such good regulation. Each output section is a LOT of power MOSFETs in some kind of parallel config. The heat sinks are an ingenious cheap effective stamping of heavy-guage aluminum plate to form offset finss. Fan cooling, and the speed of the fans depends on the demands on the amp (I'm not sure whether it's temp or current demand that determine the fan speed??). Slow and quiet most of the time, you know the volume is getting serious when the fans go on high.

The early production had high in-rush current when you turned them on. Not really a big problem, but when you turn them on they would draw a LOT of current for a very short time as the regulated power supply tried to charge the power supply caps VERY suddenly. Probalby not good for the caps if they're old. In venues where the wiring was in conduit pipe you could often hear the wiring SNAP inside the conduit from the high current and earth's magnetic field, no lie. Almost all the early production ones soon had their power switch contacts welded permanetly on. Later production added a soft-start circuit which charged the power supply caps more gradually.

The big high-power MOSFETs in these high-power transistor cases are just not available anymore as far as I know. Screwed down with 2 screws they handle a lot more heat than some 3-pin case with one screw thru a tab. So if you buy one any of these mosfet amps used, make sure they still have all the output transistors and that some repairman didn't just unsolder a few of the parallel array, leaving the amp functional but somewhat neutered. Some repairmen scavenge the mosfets out of lesser amps in the same series to keep these monsters in service.

In real-world use they are pretty nice. The regulated power supply is nice when your house mains wall voltage at some gig is questionable (like somebody else's extension cords off a rented generator with unstable speed). And the FET output doesn't seem to go into thermal runaway and self-destruct when the load is low-impedance. If you short the output terminals the house lights dim as the amp sucks a ton of current, the fans go on high and blow 5 years accumulation of dust out the grate, the shorted wire starts to glow red, but they don't go into thermal runaway and generally don't blow out.

They sound pretty darn good and are pretty darn reliable. Made in the USA and built like a tank. Even the fans are reasonably reliable.

I've got 4 in my kitchen.

[rickedd46]

If anyone needs schematics for any of the PCR based Soundcraftsmen shoot me a pm and I will be glad to share it with you.

I have schematics for the following...

PCR800 (both versions), PM840, PM860, PRO-POWER 1, THREE & FOUR, DJ600, 450X2, 300X4, PRO-POWER 10, PM1600, PRO-POWER 8, PR1800, 900X2, A900PRO, A400, A400PRO, A200, A200PRO, A2502, DJ600, RA5501, RA5502, PRO-REFERENCE 2 and PA200X2.

I might have the Class H schematics somewhere. I have to look.

Cheers.

Ricardo.

[cyclecamper]

Oh yeah, I definitely need schematics. I have 4 of these PR1800 amps now (the most recent one is the same amp badged as an MTX 900X2).

[cyclecamper]

I'm collecting these. If anyone sells theirs or finds one, please let me know.