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Soundcraftsmen input board

Racingh11

Racingh11

Super Member
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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Nicely done. You are certainly a much more advanced Soundcraftsmen modder than I. I've added an off the shelf soft start/shutoff board, star grounding and a custom transformer snubber to my A2801. Is that a DX4200 in the background? I use the DX4000, also modded.
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Schmidlapper said:
Nicely done. You are certainly a much more advanced Soundcraftsmen modder than I. I've added an off the shelf soft start/shutoff board, star grounding and a custom transformer snubber to my A2801. Is that a DX4200 in the background? I use the DX4000, also modded.
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Thank you. That's nice work there too. That is a DX4200 in the background. That's the one I'm keeping, along with the AE2000 on top of it. There is also a freshly serviced DX4000 on the shelf as well as the serviced Pro Control Four I'm using at the moment.
 
This is good stuff.
It's to bad S.C. doesn't have its own forum.
It really is some excellent performing equipment.
I really only have alot of experience with the ma5002 and sp4002 pre.
But I've run it up against some good gear and it out performs stuff that supposedly is better.
Huge fan of soundcraftsmen.
Racingh11 your several steps ahead of anyone I know in this field.
Nice work.
 
I purchased my first Soundcraftsmen (PE2217) back in 1974, or so. Still have it. My main system now uses Pro-Control Four, Pro-EQ 44, Pro-Tuner Four. I have enough SC stuff to make several systems including backups for the Pro-Control Four, a Pro-Power-Four and several ProPT5 and 3, a couple of DX4400, etc.

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Racingh11 said:
Thank you. That's nice work there too. That is a DX4200 in the background. That's the one I'm keeping, along with the AE2000 on top of it. There is also a freshly serviced DX4000 on the shelf as well as the serviced Pro Control Four I'm using at the moment.
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I purchased this A2801 new in 1988, in the early 1990s I separated the power supply into a second chassis, also removed the clipping led wiring and thermal shutdown wiring. This was my first foray into modding. Recently, after being tired of juggling two cases I decided to put it all back into just the one chassis. Over the decades I lost the original power switch, so it uses a heavy duty military toggle with circuit breaker built in.

I’m not sure if you are familiar with power transformer snubbers, but the purpose is to eliminate the inherent ringing stimulated by diode switching. This really cleans up the power in the amp and gives it a nice dark background. I use a jig designed by Mark Johnson over on DIYaudio forum. It allows me to stimulate the transformer into ringing while placing a test snubber across it that can be adjusted for any transformer. Here the results for the A2801. I have one in my DX4000 and my tuner.

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Schmidlapper said:
This is the DX4000 snubbed result.

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Is there any measurable difference in distortion or S/N ratio? It is interesting. You've told me about this before, I just didn't get around to trying it myself. I may design a board for one and see what happens.

I also added a muting circuit to the input board. Should give a turn-on delay and mute when the supply voltage drops a few volts or more. That way it mutes during shut-down while the caps are draining. Will be testing soon.
 
Racingh11 said:
Is there any measurable difference in distortion or S/N ratio? It is interesting. You've told me about this before, I just didn't get around to trying it myself. I may design a board for one and see what happens.

I also added a muting circuit to the input board. Should give a turn-on delay and mute when the supply voltage drops a few volts or more. That way it mutes during shut-down while the caps are draining. Will be testing soon.
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I have not measured the S/N before and after. Unfortunately my capabilities for measuring distortion are limited to an old external sound card and software I use when setting AC balance in my tube amps. I'm not sure I could measure a solid state amp in a meaningful way using it. I had heard much talk of the noise associated with diode cut off and saw this as an effective means of eliminating it in the power supply. The results were noticeable enough in the scope, and also what I perceive when listening that I have made it a standard practice in all my audio equipment. If you were to measure before and after S/N, I would certainly be interested if it is measurable, around 60 and 120 Hz. Regardless optimizing the power supply is worth the effort.

I found the delay board to be a welcome improvement in convenience with these, I can now just turn it off without worrying about shutting down everything in the correct order.

Bill
 
Looks like I have a new project to get in to. lol
That snubber might make a big difference if I can get one to work with these PCR amps and the ringing that must be caused by the SCRs in the phase control. Or even to further blend the class H control in the A5002. The PCR amps have a rated S/N of 105dB relative to full rated power. I'm sure it would show improvement if the switching was snubbed a bit. Do you have schematics you could post or send me, or a link to the information?
 
Here are links to the DIYAudio forum post that started it. https://www.diyaudio.com/community/...rmer-snubber-using-quasimodo-test-jig.243100/ https://www.diyaudio.com/community/...ty-transformer-snubber-bellringer-jig.250582/

Mark Johnson is the engineer behind the device, he provides excellent theory, documentation, test results, free Gerber files for the boards and even designed a cheap version called the CheapModo. He is a no BS type guy, and may come off a bit rough, but he knows what he is talking about and offers this with the best intentions. I imagine he has had to explain and defend against no end of amateur experts over the years, and just gets to the point. I purchased a kit and built a cheap modo myself, but many bread board them using the provided schematics. The documentation explains how to test and covers multiple styles of power transformer installation. The forum thread delves deep into it everything. Have fun.
 
Schmidlapper said:
Here are links to the DIYAudio forum post that started it. https://www.diyaudio.com/community/...rmer-snubber-using-quasimodo-test-jig.243100/ https://www.diyaudio.com/community/...ty-transformer-snubber-bellringer-jig.250582/

Mark Johnson is the engineer behind the device, he provides excellent theory, documentation, test results, free Gerber files for the boards and even designed a cheap version called the CheapModo. He is a no BS type guy, and may come off a bit rough, but he knows what he is talking about and offers this with the best intentions. I imagine he has had to explain and defend against no end of amateur experts over the years, and just gets to the point. I purchased a kit and built a cheap modo myself, but many bread board them using the provided schematics. The documentation explains how to test and covers multiple styles of power transformer installation. The forum thread delves deep into it everything. Have fun.
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Great, thanks.
 
Racingh11 said:
Looks like I have a new project to get in to. lol
That snubber might make a big difference if I can get one to work with these PCR amps and the ringing that must be caused by the SCRs in the phase control. Or even to further blend the class H control in the A5002. The PCR amps have a rated S/N of 105dB relative to full rated power. I'm sure it would show improvement if the switching was snubbed a bit. Do you have schematics you could post or send me, or a link to the information?
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I forgot, the A5002 has one already. It's on the primary/triac side of the circuit. The PCR amp has one for the cooling fan rectifier, but nothing for the power transformer.
 
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