Being ex-Simmons, I guess it was only a matter of time before I'd end up designing peripherals for my favourite electronic drum systems. My Dark Matter replacement power supply for the Simmons SDS7 is the first in a small range of products, specifically for this legendary instrument.

Selling my own SDS7 while I was still at Simmons to help fund the purchase of my first SDX would end up being one of my biggest regrets. Decades later, I was able to buy a SDS7 in lovely condition and you can read all about that here.

Well, I got smitten all over again. Indeed it was like falling in love with an ex-partner who you hadn't seen for decades, LOL. As it transpired, a friend of mine contacted me a couple of months later, informing me of an on-line advert for another SDS7 which was for sale in London. Well, the SDS7 I'd recently purchased was in remarkable condition and keen to develop peripherals for this instrument, I decided to buy yes, another SDS7 but specifically for R & D purposes. I picked up the second machine for a really good price, as it had several issues all highlighted by the seller.

With lots of ideas in mind, Dark Matter replacement power supply for the Simmons SDS7 was going to be my first project.

So why bother?

• At the time of writing, the Simmons SDS7 is forty years old. Back in 1983, power supplies were perhaps 15% efficient. Modern power supplies can be nearer 95% efficient, thereby generating far less heat than the old linear supplies. Less heat means less thermal stress on other components.

• The Simmons SDS7 can accommodate twelve sound modules. The problem is that loaded with more than say eight, the power supply starts to stress out. The regulators on the +15V and -5V supplies are 78 / 79 series devices which are only rated at 1A. Even the massive 3A rated LM309 Steel regulator on the +5V was seriously pushed. Rated at 75W as opposed to the original 45W power supply, Dark Matter was definitely intended to address this issue.
• All power supplies generate noise. The original SDS7 power supply is situated between the rear sockets PCB at the back of the instrument and the so-called back plane. The latter is a vertically mounted PCB that runs the width of the SDS7 chassis and into which, are connected all the (also vertically mounted) voice boards, CPU-board and memory-board. All analogue audio, digital and power are distributed through the back plane so having a power supply nearby isn't ideal and power supply generated noise was readily picked up by the rear sockets PCB, the back plane and an IDC ribbon cable which connected the two.

• My initial idea was to see if I could squeeze Dark Matter into the space occupied by the SDS7 power transformer. If I could do that, the power supply would be far away from any audio carrying conductors and would therefore potentially make SDS7 a little less noisy. Well, several layouts into the project and I finally ended up with a paper cut-out that suggested this was feasible.

• As was quite common at the time, the original memory back-up battery in the Simmons SDS7 was a Ni-Cd device. This required regular charging. Unfortunately, if not regularly charged, these batteries are prone to leaking and it is a sad fact that today, there are far fewer SDS7s in the world than there should be, simply because they've been forgotten about and damage caused by battery leakage has been too extensive to even consider repair. Check out my post here and you'll appreciate what I'm saying. As such and like my other designs, Dark Matter incorporates a memory back-up battery. It's lithium-based and so, doesn't require charging. It's also located far away from being able to cause any damage!

• And there's one more thing... Dark Matter is a modular switched-mode design which means that it can be used on virtually any domestic supply. Wow! No need to change transformer taps or fuses.

On a personal note, like Aurora for the Roland MKS-80, Supernova for the Roland Juno-106, etc, I simply wanted my Dark Matter replacement power supply for the Simmons SDS7, to keep this rather famous instrument reliably powered for at least another forty years. 🙂

So let's have a closer look at Dark matter...

Dark matter doesn't simply comprise a bunch of AC / DC converts. Obscured by the monitor-board are carefully designed filters which reside on the back end of each supply. When designing power supplies for audio applications, noise and hum need to be kept to a minimum and that's exactly what the filters do.

Considering installing Dark Matter into a space other than that occupied by the original power supply was challenging but there was more to come. Indeed my next problem was getting power from Dark Matter to the SDS7.

To start with however, Dark Matter needed to be safe and secure so once I was confident I could fit my new power supply into the original transformer location, I decided to design a robust steel bracket to keep my baby solidly in place. Although weighing considerably less than the old transformer assembly, Dark Matter mounts on to a substantial 2mm thick plate which also offers improved screening.

The plate uses the same holes which secured the old transformer assembly so no drilling or anything like that is required, to get Dark Matter physically in place.  Supplied with a full set of M5 fixing hardware, you don't even have to bother negotiating the Imperial stuff that Simmons used back in 1983.

I've already mentioned that the original power supply was situated between the rear socket PCB and the back plane. Voltage supplies from the original power supply PCB to the back plane are via four wires, one of which is  earth. Those wires are simply soldered to points on the rear of the back plane. If removed however, the solder points reveal four through-hole solder pads. Being designed in 1983, the solder pads adhere to the standard 0.1" spacing of the time. Oh!! That could be convenient.

Experimenting with a couple of connectors, I soon settled on a Molex system that I've used a lot and am quite familiar with. The header / housing system provided a reliable, non-exposed detachable connection between Dark Matter and the SDS7. Perfect! Just what I wanted.

Like a lot of Simmons stuff from the eighties, the RESET pulse for the CPU was initiated post rectifier / pre-regulator. The same circuit was used to generate the CE level for the memory. The rising voltage on power-up would be conditioned by a simple R C network, prior to being fed into a quad 2-input NAND gate array. The CE level for the 6116 SRAM chips needs to be active all the time, even when the power to the SDS7 is switched off, otherwise the 6116s will be disabled. This suggests that the back-up battery also supplies this circuit or at least a component in it and that's exactly right!

To get things up and running, I simply copied the original RESET circuitry on to the first Dark Matter prototype. Things worked fine but I was doubtful regarding reliability. Dark Matter is a modular switched-mode design so there's no easy way to access 'pre' converter (AC)  power.

So, on my second prototype, I replaced the front end of the circuit that generates the RESET pulse, with a modern type supervisory IC. The entire pulse width of the new circuit was only a little more than just the time constant of the original circuit and so my test SDS7 booted noticeably quicker.

Each gate on the  CD4011 quad 2-input NAND is actually configured and used as an inverter. The device has a minimum operating voltage of 3V which is annoying when the important 6116 SRAM chips can run down to 2V. So, to deliver extended battery usage, I eventually replaced the CD4011 with a SN74HC14 hex inverter which can also operate down to 2V.

So what do I mean by extended battery usage?

To me, the term 'extended battery life' suggests that the current demand on the battery has been reduced, thereby allowing the battery to maintain it's desired output voltage for longer.

I've coined the term 'extended battery usage', to mean something different, however. By lowering the minimum voltage at which all battery backed-up devices will operate at, the battery can run down to a much lower voltage, in this case, 2V instead of 3V. I hope that makes sense.

Dark Matter was unique in as much that it was going into a host with variable current demands on each of the voltage supplies; the more sound modules you have in your SDS7, the more will be the current requirements. Anyone with a fully loaded Simmons SDS7 will know that it can struggle with twelve sound modules.

I was confident that Dark Matter would be able to supply sufficient current for any SDS7 but that wasn't what I was worried about. Remember that Dark Matter is in a relatively confined space with restricted air-flow and I had no idea how hot things would get. 🙁

Attaching a thermocouple to the +5V converter and temporarily fitting the top-case back on to my test SDS7, I observed that after half an hour or so, the temperature reached 40°C (104ºF) and that after ninety minutes, things stabilised at 47ºC (117ºF). Three hours later and there was no further rise in temperature.

I switched off and quickly removed the top-case from my test SDS7. The converters were only warm to touch.  WOW! 😮 This was seriously good news.

Thanks to some lovely customers, I was also able to test Dark Matter with a fully loaded SDS7. Running the test again for three hours, the results were similar to my previous test using eight modules but this time, the temperature flatlining a little higher at 55°C (131ºF).

The results were very encouraging but at the same time, the tests had revealed a problem...

My first couple of prototypes had the memory back-up battery mounted on Dark Matter's monitor board but of course while 55°C is fine for Dark Matter, it's not a good idea to have batteries exposed to that kind of temperature. So, I decided that a complete relocation of the battery was necessary. A slight headache and even more cost for yet another batch of prototype PCBs but it actually turned out to be a blessing in disguise and led to a more beneficial configuration.

Prototype version 3 (or was it 4, I can' remember) and Dark Matter's battery-board now mounted on top and to the far left of SDS7's original power supply PCB. Located here and having more space, I was able to accommodate a dual battery holder. Why is that cool? Here's why...

The memory in the ten 6116 SRAM chips in the SDS7 can be retained for several decades with a single AA 3.6V LiSOCI2 battery. Having two battery holders however, means that when the time comes, so long as the battery isn't depleted, dropping in the new battery before removing the old one, means that your SDS7's kits won't be lost. 🙂 How cool is that?!?! 😮

Each battery holder has its own protection diode and resistor so there's no chance of the new battery inadvertently rush-charging the old battery, for the few seconds that the two batteries are in situ.

So, the downside of putting the batteries virtually anywhere in a SDS7, is that the top-case needs to be removed when it's time to change them.

But wait... that's not the case with Pleiades! Although you will need a spare module slot to fit Pleiades, being accessible from the front panel means that changing the batteries is much easier and of course, Pleiades conveniently plugs into Dark Matter's monitor-board, in place of Dark Matter's battery-board. Indeed, for battery back-up, you can either use Dark Matter's battery-board or Pleiades.

Situated on Dark Matter's monitor-board, are three status LEDs which confirm that a voltage is present on each of the three supplies. Just beneath the LEDs, are conveniently located test terminals so that the supply lines and indeed the SRAM voltage can be measured.

Dark Matter gives the Simmons SDS7 a significant performance boost, increases reliability and will ensure that this machine will be around for a long time.

With a detailed and fully illustrated installation manual that's available after purchase, Dark Matter is supplied will all the parts you need for a hassle-free installation.

You may have noticed that the AC / DC converters on Dark Matter are British-made Vigortronix. Consistent with all of Dark Matter's components, I want the best.

Dark Matter's installation does require some original components to be removed but no holes need to be drilled, no PCBs need to be cut and there are no other permanent changes that need to be performed, Dark Matter has a fully reversible installation meaning that you can uninstall Dark Matter and return your Simmons SDS7 back to factory at any time. Why you would want to do that, I don't know but the option is always there.

Although quite involved, the installation of Dark Matter isn't particularly difficult, especially if you're patient and prepared to take your time. You should however, be familiar with SDS7's unique and quirky personality.

Simmons was no doubt, an innovator and its products certainly inspired a whole branch of music technology as well as music itself but the company also had its failings. One of them was a severe lack of production engineering and another was an equal lack of quality control. Some aspects of the SDS7 design are seriously frustrating and appreciating an inconsistency in manufacturing and quality, small differences between individual instruments, can be really annoying.

So, if you're considering Dark Matter, then please be warned. This is an installation for experienced Simmons techs only!

At the beginning of this post, I mentioned that Dark Matter is one of several peripherals that I've developed for the Simmons SDS7 so I rather hope that you're asking "what are the others?" Okay, here you go:

• Pleiades is a lithium battery adapter but it's also a convenient mount for the Tubbutec uniPulse MIDI to trigger converter. Featuring a special clip-in secondary PCB, Pleiades makes MIDI triggering for the Simmons SDS7 a breeze. Pleiades is fully compatible with Dark Matter and there's more about that later.
• AMEP or Advanced Memory Expansion Pack is a modern replacement of the original Simmons memory expansion pack. With no way to back up SDS7's memory, installing either Dark Matter or Pleiades and hence, removing your SDS7's battery, would mean losing your patches! I didn't see the point of copying the original Simmons MEP, like those who developed clones for example and so AMEP is a truly radical design. AMEP is reliable, it doesn't need to be charged, it's a fraction of the size of the Simmons MEP and subsequent clones, it features four times the memory of anything previous and has a facility whereby changing the battery won't wipe the data! I have no idea why no one has made one of these as it's so desperately needed. Anyway, AMEP gives you the option to back up your SDS7 prior to doing anything heavy that'll involve disconnecting the battery on SDS7's memory-board.
• NC-7 Noise Cancelling Mod for the Simmons SDS7 is for the few SDS7s that didn't have this mod factory fitted.
• RESET-7 replaces the CD4011 quad 2-input NAND gate in the SDS7's power-on-reset circuit, with a modern hex inverter with Schmitt trigger inputs. The replacement chip  has a minimum supply voltage of 2V. This has the effect of extending battery usage.
• Simmons SDS7 Program Chart is an editable pdf which allowing you a written record of a single patch with all the parameters of up to twelve modules.

I've tried to synchronise the launch of Pleiades, Dark Matter and AMEP as they can all work together and with each other. That in itself, has been a challenge and has made summer 2024 particularly busy for me.

Dark Matter is handmade in Hemel Hempstead, Hertfordshire, United Kingdom, only a few miles from St. Albans where Simmons used to be based and where I used to work.

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And finally, Dark matter would have taken much longer to develop, if it wasn't for the help, support and encouragement of my Simmons Vintage Technical Network friends and colleagues. THANK YOU Ed Rose, aka The Simmons Guy, Michael Buchner and Patrice Jacquot.

There's also one other person that I need to thank and that's my good friend Guy Wilkinson of Super Synth Projects. Guy popped over one afternoon and I just happened to have my first Dark Matter prototype on the bench. I had a really annoying problem and Guy was quick to point out something that really wasn't right. It was one of those "Seriously?" moments!  Thanks, Guy. 🙂

Dark Matter is available to purchase here: