Dark Matter Replacement Power Supply for the Simmons SDS7

Alex-PML10 October 202418 November 2024

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.

Simmons SDS7 power supply PCB is not in the best place — The PSU-board isn't exactly located in the best place but in 1983, there wasn't much choice.

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.

Individual filters on Dark Matter supplies — Hidden underneath the monitor-board, are Dark Matter's filters. Each output has its own filter network ensuring quiet operation.

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.

Dark Matter installed into SDS7 — Dark Matter fits snuggly into the space previously occupied by the SDS7 power transformer.

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.

Simmons SDS7 Transformer Assembly — For those who may not be familiar with the internals of the Simmons SDS7, the transformer assembly is on the right-hand side of the box. This was where ideally, I would have liked Dark Matter to fit.

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.

Dark Matter steel mounting bracket ensures the installation is secure and safe — Dark Matter's steel mounting bracket ensures the installation is secure and safe.

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.

Secure connection between Dark Matter and SDS7 back plane — It took a little experimentation but I eventually made a secure and rather elegant, detachable power connection to the SDS7.

Like a lot of Simmons stuff from the eighties, the RESET pulse for the CPU and CE pulse for the memory, were initiated post rectifier / pre-regulator. 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.

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.

Dark Matter temperature test with eight modules — Loaded with eight modules, my test SDS7 eventually reached 47ºC and stayed there for three hours until I got bored. The saying "a watched kettle never boils" came to mind. A classic exponential curve if I ever saw one. Well, what else would you expect?

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).

Dark Matter temperature test with fully loaded SDS7 2 — Current and temperature testing Dark Matter on my test SDS7 with twelve modules and yes, that's a Simmons SDSV on the far left.

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...

Connected in parallel, two batteries means that so long as they're not totally depleted, you can change one at a time and theoretically, you're SDS7's memory should remain intact meaning that your patches won't be lost. 🙂 How cool is that?!?! 😮

Dark Matter battery board mounted on top of original SDS7 PSU board — Dark Matter's battery-board is mounted well out of the way of Dark Matter's PSU-board.

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.

Status indicators and test points on Dark Matter — Like all my power supplies, Dark Matter features visual supply line status indicators and test terminals.

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.

The Dark Matter kit contains everything you need — Dark Matter is supplied with everything you need for a complete 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 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.

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!

My Test Simmons SDS7 Is Back In One Piece — Delivering classic eighties electronic drum sounds, the Simmons SDS7 is also uniquely quirky.

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.
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.

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:

Pleiades Lithium Battery Conversion for the Simmons SDS7

Alex-PML10 October 20242 April 2025

My Pleiades lithium battery conversion for the Simmons SDS7 was initially designed to address a common problem with this legendary instrument.

Like so many instruments of its time, the Simmons SDS7 memory was maintained by a Ni-Cd battery. This type of battery requires regular charging. Unfortunately, if not regularly charged and left for a couple of years, these batteries leak and can cause irreparable damage. It saddens me deeply that there are far fewer Simmons SD7s in the world, simply due to the fact that they got forgotten about and bad batteries did their thing.

As well as making lithium battery conversion simple and straight-forward, Pleiades also provides a convenient way of implementing MIDI triggering of the SDS7's voices. Pleiades has a mounting system specifically designed for the Tubbutec uniPulse MIDI to trigger converter.

Pleiades with and without tubbutec unipulse — Pleiades provides easy lithium battery conversion for the Simmons SDS7 but also support the Tubbutec uniPulse MIDI to trigger converter (not supplied).

Included with the Pleiades kit is a tiny snap-in board, that makes interfacing the trigger signals from the uniPulse with your SDS7, a breeze. Connection between the daughterboard and the Pleiades main board is via a 26-way IDC cable. Power and MIDI also come on to the Pleiades daughterboard so everything's really easy.

Pleiades interface daughterboard snap-in — Part of the Pleiades kit, is this little board which snaps into the DIP header on the SDS7 back plane making trigger connections from the uniPulse solderless.

Occupying a spare voice module slot (preferably slot 12) in your SDS7, all you need to do is remove the SDS7's front panel to access Pleiades. This makes battery monitoring and changing simple. Pleiades also has switches and an LED which, if you have uniPulse installed, provide full access to some cool uniPulse functions.

Like all my upgrades and modifications, Pleiades doesn't require any holes to be drilled into your SDS7. You won't have to cut any PCBs or carry out any 'hacks' that will permanently change your SDS7. In fact, Pleiades can be easily removed thereby returning your Simmons SDS7 to factory. MIDI access for example, is via the pad trigger input of the voice module that's occupied by the Pleiades board. A XLR to 5-way DIN cable is included with the Pleiades kit. As I said... no holes need to be drilled! Cool, eh? 🙂

Pleiades lithium battery conversion for the Simmons SDS7 was designed at the same time as my Dark Matter replacement power supply for the SDS7 and it was my intention that the two products should be launched simultaneously. I wanted SDS7 users to have options and so I felt it important that Pleiades and Dark Matter should be compatible. Since Dark Matter has a separate battery board, it's easy to substitute this with Pleiades.

Dark Matter replacement power supply for the Simmons SDS7 — I wanted SDS7 users to have options and so Pleiades and Dark Matter are fully compatible.

You can read more about Dark Matter here.

So, you can't easily back up the memory of the SDS7 and of course, the downside of messing about with the SDS7's battery, is that you'll lose all of your patches. The only way around this problem, is if you are lucky enough to have a working original Simmons memory cassette. I say "lucky" but to be honest, they were incredibly unreliable. I therefore decided to have a go at developing a replacement.

Unlike the memory cassette clones that we've all seen over the years, I wasn't going to copy something that already was unreliable. I figured on using modern technology to produce a memory device with an edge... or two... or three (?)

Well, I bit off more than I could chew this year and regular visitors to my website will have noticed that my on-line store has quite a few new products. My dear friend Guy Wilkinson of Super Synth Products came to the rescue and took over the design of the new 'Advanced Memory Expansion Pack' or 'AMEP'.

With four times the memory of the original Simmons MEP, AMEP will allow you back up the entire contents of your Simmons SDS7 giving you peace of mind when you install Pleiades lithium battery conversion for the Simmons SDS7. You can read all about AMEP here.

If you only have a few patches in your SDS7, it might just be easier to record the parameters of each module of each patch on to a chart. To help SDS7 owners with that, I've made a Simmons SDS7 program chart which can be downloaded here.

You'll have noticed that Pleiades has a dual AA battery holder. Indeed, the batteries are connected in parallel. If not fully depleted, changing the batteries separately means that your SDS7's memory won't be scrubbed! How cool is that!?! 😮

Excluding the Tubbutec uniPulse, Pleiades is supplied with everything you need for a smooth install and a fully illustrated and detailed installation manual is available after purchase.

IMPORTANT: If you require a Tubbutec uniPulse, please order here, specifying that you require the Pleiades Simmons SDS7 kit.

!!!! WARNING !!!!

There's a post that's been on the Internet for a while in which some guy slaps a lithium battery into his SDS7, wires it up to the memory board and claims that it works. PLEASE DO NOT DO THIS.

When powered up, the SDS7 will try to charge the battery. This will not be good and could even lead to a fire. Pleiades has been designed to do the job properly. Yes, it requires a small modification to the memory board but Pleiades won't destroy your SDS7 and definitely won't catch fire.

I joined Simmons a couple of years after the SDS7 was launched but it has always been my favourite kit, next to SDX. I know that many favour the SDSV which remains an iconic analogue drum system but the SDS7 was just so much more 'me'.

The SDS7 I bought while working for the company was sold to fund the purchase of my first SDX. It was a decision I always regretted but last December, a SDS7 came up on a well-known second-hand music gear site and I just had to have it. You can read all about that here.

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

I do not keep Pleiades units in stock and each Pleiades is built to order.

And finally, my Pleiades lithium battery conversion for the Simmons SDS7 would have taken much longer to develop and might not have even happened at all, if it wasn't for the help, support and encouragement that I received from my Simmons Technical Network friends and colleagues. THANK YOU Ed Rose, aka The Simmons Guy, Michael Buchner and Patrice Jacquot.

Pleiades features extensive uniPulse integration and being a firm believer in partnerships, I'm very proud to mention the special relationship between Tobias of Tubbutec and myself. I see too many people now-a-days thinking that they can do everything themselves... and failing!

I first got to know Tobias while developing SDSM, my uniPulse adapter for the Simmons SDSV. Both Ed Rose and I had a couple of ideas for uniPulse and Tobias kindly updated the uniPulse firmware, implementing those ideas. AWSOME!

Pleiades is available to purchase here:

UPDATE - 20th March 2025

An alternative to the original Ni-Cd battery is something that SDS7 owners need now and it would have been quite impractical to see just how long the two Li-SOCI2 batteries would actually retain SDS7’s memory. After installing my prototype, I monitored the battery voltage for several weeks and there was no indication of adverse voltage loss.

It's now more than five months after this post was launched and last night I thought I'd check the battery voltage again.

Pleiades battery test after 5 months — My prototype Pleiades was fitted with a pair of SAFT 14500 batteries and as you can see, several months after installation, things look great!

The SDS7 memory comprises ten 6116 SRAM chips. These have a minimum memory retention voltage of only 2V so it would seem logical to assume that, so long as the back-up battery voltage is greater than 2V, SDS7's memory will be just fine.

In theory, that's true but there's something else going on...

IC1 (awkwardly located on the power supply PCB) is a CD4011 quad 2-input NAND gate. Used to buffer, invert and interface the RESET pulse, it's a crucial component in starting up and shutting down SDS7. It's permanently kept alive so just like the memory itself, when SDS7 is powered off, this device switches over to battery power.

IC1 CD4011 in Simmons SDS7 — The CD4011 in the SDS7 is annoyingly difficult to access.

Unfortunately, the CD4011 has a minimum operating voltage of 3V which means that the memory back-up battery has to be above this value. If it's not, then IC1 won't function, the result being that SDS7 won't boot properly, if at all. As mentioned, so long as the battery voltage is above 2V, your SDS7's memory should in theory, remain intact. In practice however, this is NOT the case and without an adequate 'chip enable' (CE) voltage applied to the 6116s, the memory will corrupt.

Of course I considered replacing IC1 but there's no pin-for-pin equivalent to the CD4011 which has a minimum operating voltage of 2V. The SN74HC00N will operate down to 2V but unfortunately gates B and D are oppositely orientated to those on the CD4011. 😡

Earlier on in this post, I touched on Dark Matter, my replacement power supply for the Simmons SDS7. Well, Dark Matter does a lot more than just supply power. When designing Dark Matter, I took the opportunity to address the short-comings of the original SDS7 RESET circuit and so completely revised the design. To maintain compatibility with the SDS7, I had to use a gate but I did so around the SN74HC14AN which is actually a hex Schmitt trigger inverter. This device does have a minimum operating voltage of 2V and so has the effect of 'extending' battery usage.

Just thought you might find that interesting. 😎

For those who don't want or unable to install Dark Matter, I'm currently working on a solution to the CD4011 problem.