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Soundcraftsmen input board

Schmidlapper said:
Looks like a very well thought out board. It would pass for OEM in fact.
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Thanks for the compliment!

The future boards will look even better. I eliminated a few unnecessary parts, one visible is the unused spot for the capacitor near the edge. and the two large resistors in the corner will have a smaller footprint. They were changed to these large ones when I tapped into the high voltage power rails. The original versions tapped into the regulated +/-15V supply like Soundcraftsmen did with their input boards. But that creates a timing issue on power-down, that makes a rising chirp tone. This version I added footprints for onboard zener diode regulation. So those resistors went from 22 ohm 1/4W to 2.7K 1W. And will be a smaller case 2.7k ohm 2W resistors when I get them. The resistance is matched according to the rail voltage supplied. +/-70V in this case.
 
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Jimbo40 said:
What is the black on top of the electrolytic?
And what are the white film caps by the op-amp?
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The black is adhesive to insulate the tops of the caps from shorting out on the metal cover when installed, since electrolytic caps tend to have a charge on the case.
The white film caps are for the low pass filter.
 
PabloTincho said:
Since you guys seem to know the brand Soundcraftsmen, can you guys rank the best amps from that brand ?
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They have become so rare that nearly any of them are worth pursuing. Even the MTX versions from the '90s. My favorite is the Pro-Power Four. Many like the shoebox models from earlier. And then there is that fascination with meters! I'm sure you'll get an ear-full. I actually have more of their EQs, pre-amps, pre-EQs, and Pre-ceivers than I do the pure amps.
 
PabloTincho said:
Since you guys seem to know the brand Soundcraftsmen, can you guys rank the best amps from that brand ?
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Since you provided no criteria to go by, I would say the market is hot for the MA5002 at the moment. It looks great and is a powerful well designed amp.

Many power amps in the Soundcraftsmen lineup through out their run were basically the same topology with various features making up the model. The common schematic between multiple models bears this out.

In the case of my own, A2801, a standard class AB single rail 125 watt amp, it shares the same schematic with the A2502, RA5501, DJ600 and RA5502. What differentiated between models were various types of meters, number of output pairs, input type. It is not far off from the later MTX A200 as well, if any.

The same holds true for their dual power rail class H amps. Where it comes down to output wattage (additional MOSFETs), input type, meter type, etc, but the amp circuit topologies between them are basically the same.

The little shoebox amps are likely similar also, needing a bit different cooling to make up for lack of chassis space and their high output power.

Soundcraftsmen once they had a sound circuit design used the hell out of it.

So, pick the amp class AB, H; output wattage; type input, RCA, 1/4"; number of outputs; look, meters, no meters; and that will lead to the model you should be looking at.
 
There were 2 basic amp designs that were the basis for all the amps they made. The class H amps with BJT outputs. And a MOSFET amp that has two different power supply variants; open, and phase control regulation(PCR) with cooling fan(s). All of the MOSFET amps are basically the same except the big PM1600, PR1800, and MTX 900x2. Those are basically the standard MOSFET amp architecture on steroids. It has more output transistors and a much larger power supply. But it is also PCR. Only the 125W per channel MOSFETs are standard open power regulation, everything else is PCR. The other big amps such as the Pro Power Eight or Ten, are just multiple standard sized PCR amps in one case. The difference between many models comes down to whether they have an input board or switchable bridging. None of the shoe box sized amps had input boards. That's why I designed them. And no models had a low pass filter for bridging a subwoofer.
 
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I built a version of my input board that has switchable bridging and a switchable channel summing instead of the usual low pas filter and bass boost. And installed it in a Soundcraftsmen RA7501. I removed the stock input board. Both mine and the original are capable of balanced inputs. It was not designed for these amps, but fit perfect. input board V3.jpgIMG_2117.JPGIMG_2116.JPGIMG_2113.JPGIMG_2105.JPGIMG_2098.JPGIMG_2097.JPGIMG_2096.JPG
 
Racingh11 said:
I designed an input board for the Soundcraftsmen PCR800, PM840/860, and similar amps. It will also work on the A5002 or most amps in general with the correct power source. It uses the built in +/- 15V supply in the PCR style amps. It provides a buffer for the input to the amp and will accept balanced inputs if desired. It bridges channel A input with the push of a button. And another push switch to engage the low pass filter for a subwoofer. The low pass filter also has a potentiometer for bass boost. I have built and installed a few of them now. The first units were built with a 12dB 72hz LP filter. The most recent one requested by a customer, at 120hz,. Sounds great.

I have been testing them on an 18" , 4 ohm, 1000 watt RMS subwoofer. They put out around 900 watts in this arrangement. Definitely gets the cooling fan switching to high speed on a regular basis!

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Where can I get one of these boards? I want to add xlr inputs to mine. Thanks
 
I had someone who wanted the input board, but external to the amp. So I made an enclosure out of two side panels from a Soundcraftsmen EQ, as well as the transformer, fuse, and cord. I then made a power supply board with rectifier, caps, and +/- 15V regulators. Then setup the input board to always be in bridged mode, with push-button low pass filter and variable bass boost. There is also an internal jumper block to mono-sum A + B inputs when used with low pass filter, so one subwoofer can be used for both channels, or just bridge channel A input like many Soundcraftsmen amps do.
IMG_4500.JPGIMG_4499.JPGIMG_4498.JPGIMG_4497.JPGIMG_4495.JPGIMG_4492.JPG
 
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