AMEP Advanced Memory Expansion Pack for the Simmons SDS7

Alex-PML10 October 202417 December 2024

Over forty years and a couple of clones later, I'm delighted to announce the all new AMEP Advanced Memory Expansion Pack for the Simmons SDS7.

It can take days, weeks or even months to program a bunch of patches into your Simmons SDS7. After all your hard work, there's only ever been one way to back it all up and that's with a memory cassette via the expansion port. Well, not anymore! 🙂

The original Simmons memory cassette is exceedingly rare, more so than the SDS7 itself and finding one that hasn't been eaten by the batteries is virtually impossible. Unfortunately, the same can be said for the clones that have appeared over the years.

Also using 6116 SRAM chips, the original MEP was very large and just like the memory-board in the SDS7, it had built-in NiCad batteries that required the memory cassette to be regularly plugged into a powered up SDS7, so that the batteries could be recharged.

Original Simmons MEP — The original Simmons memory cassette was large, used a NiCad battery that required regular charging and was annoyingly unreliable.

Being a large device, connecting it to the SDS7 via the rear DIN 41612 socket that wasn't really designed to take that kind of thing, caused its own problems. A lot of people kept a box of matches with their memory cassette. Perhaps you can work out why.

Over the years, clones have been developed but strangely, they're all copies of the original Simmons memory cassette. Hence the term 'clone', I guess. Despite huge changes in technology since 1983 however, this means that all the issues associated with the original Simmons memory cassette, have also been copied.

This year, while developing a couple of peripherals for the Simmons SDS7 which were intended to enhance performance and reliability and hopefully extend life span, I realised a need for a 'better' external data storage solution, especially since two of the peripherals I've developed, involve disconnecting the SDS7's memory back-up battery. Of course doing so, will mean losing all the data.

A good starting point for something like this was to define objectives so here's a sneak peak at the design brief I gave myself:

Small and compact design
More memory than the original MEP
Retain the 'Data' Protect' function
Remove dependency associated with rechargeable batteries
Battery should last for ten years minimum
Include a mechanism so that when the battery is changed, data is retained for a minimum of ten minutes
The new device must be affordable.

So all that was great but how was I going to achieve it?

The first thing was to get a concept together and then take a look at the Simmons SDS7 to see if something radical could actually work.

Well, I couldn't do all that by myself, not in any realistic timeframe, anyway. Fortunately, I've got a friend. In fact, I think I can safely say, that Guy Wilkinson is my best friend! 🙂

Guy and I got to know each other back in 2020, through the work he's done on the Roland Super-JX and my first product Aurora, a modular switched-mode power supply for the Roland MKS-80.

In June 2024, Guy and I took a day out and went to the 'Synthesised' exhibition at the National Computer Centre in Cambridge. We arrived early so went for a coffee and it was then that I mentioned my idea of AMEP Advanced Memory Expansion pack for the Simmons SDS7. Guy was surprisingly excited and quite keen to help.

Alex and Guy at Synthesised 2024 — Me and Guy at a coffee shop somewhere in Cambridge, UK.

As has already been pointed out, the original Simmons memory cassette was very large, exerting undue torque on the SDS7's DIN 41612 expansion port. Hence, the box of matches which people would put underneath the cassette! So, not wanting to copy (clone) the original device and using newer, more compact technology instead, my idea was to make AMEP such that the body would sit perpendicularly to deck and parallel to the back panel of the SDS7. Instead of sticking out from the SDS7, it would run up the back of the SDS7. To keep costs down, Guy and I also explored the potential for a 'caseless' design.

To achieve this, AMEP began as a sandwiched, 2-board idea which both Guy and I were really happy with and which allowed us some versatility with regards layout.

Oh and there's one more thing...

When hooked up to the SDS7, the original memory cassette obstructed access to the SDS7's sequencer input 1 jack socket. SERIOUSLY?!?!? It should be straight-forward enough to design AMEP so it doesn't do the same!

When engaged, the original Simmons memory cassette blocked access to the SDS7 sequencer input 1 — When engaged, the original Simmons memory cassette compromised access to sequencer port 1. AMEP does not!

Within a very short period, Guy had knocked up the concept and contacted me to come over and rip apart my test SDS7. I was developing my Dark Matter modular switched-mode power supply for the SDS7 at the time, as well as Pleiades, my lithium battery converter and Tubbutec uniPulse adapter, so I was a bit apprehensive. On the other hand, this had to be done and I really wanted all three products to go out at roughly the same time.

I had some other plans for the new storage device and so we checked out a lot stuff on the SDS7 than we actually needed to. Unfortunately, the 'keep these free for future expansion' lines as referenced on the original SDS7 schematics, didn't go anywhere and my idea of including MIDI for example, went straight out of the window. 🙁

8th September 2024 and we had our first prototype. Guy came over and plugged it in. I performed a bank dump. I removed AMEP, switched off my test SDS7, pulled the battery and then we tried to reload from the AMEP. Well, we were delighted. AMEP seemed to work. We tried this several times, checking the memory protect function and writing / reading from all four banks on the AMEP.

First prototype AMEP was a bit of mess but we had to make sure that the SDS7 would see it as a friend — The first AMEP prototype worked straight-away. Well, after I'd remembered how to save to an external device! You also get a glimpse of my Dark Matter prototype in this picture and if you know what you're looking for, you can even catch a bit of Pleiades.

Yes, AMEP prototype 01 looked a bit 'Heath Robinson' but Guy and I didn't know for sure if SDS7 would even talk with our new gadget so knocking up something to see if SDS7 would recognise a friend, was a necessary first step. Anyway, the test results were very encouraging. In fact, we were positively thrilled and so we continued.

The next couple of weeks were busy. Guy poured a considerable number of hours into finalising a pre-production prototype while I focused on further testing, graphics, enclosure and packaging.

You'll notice the very small LR44 type battery on Prototype 01 in the image above. Well, one criteria missing off the original list was that AMEP should use a standard and readily available battery and so the design had to be changed to accommodate a much larger CR2032. An appropriate battery holder has to make battery changing easy but should also be as discrete as possible.

I continued to test AMEP on my studio SDS7, my test SDS7 and a customer unit (thanks, Peter). Initially, AMEP didn't work on the customer's machine and I quickly discovered a problem with the 40-way IDC ribbon cable between the PSU board and the back plane. You need to remember that these cables are forty years old! One new IDC cable later and everything was fine. Hey, I'm not even sure if Simmons tested the SDS7 expansion port during the production process.

Replacement IDC cables for the Simmons SDS7 — After forty years, every SDS7 could benefit from new IDC cables.

One problem that Guy and I have is that we use different electronic design software. This makes exchanging files very difficult and with something as involved as a new data storage device for a forty year old synthesiser, we had to come up with workarounds, which was a headache in itself.

By 24th September 2024, we were ready to submit an order for the first batch of PCBs.

AMEP Advanced Memory Expansion pack for the Simmons SDS7 pre-production prototype PCBs arrived — 1st October 2024 and AMEP first production PCBs turned up.

Friday 4th October 2024, Guy came over and we made three AMEPs. It was an exciting and fun-packed afternoon. We even had Duran Duran playing in the background. Hey, all three AMEPs worked perfectly! 😮

Alex and Guy making the first batch of AMEP Advanced Memory Expansion pack for the Simmons SDS7 — 4th October 2024 and Guy and I are making the first AMEPs. By the way... THANK YOU Ed Rose for the awesome SDSV T-shirt! I can't believe you made this, Dude!!!

WRITE VERIFICATION

There's no mention in the Simmons SDS7 manual about any routine to verify that data has correctly been written to an external data storage device. Guy and I discovered however, that if AMEP's memory protection switch is ON and one attempts to write data to an AMEP memory location that already has that data, then the SDS7 display will flash up 'AC' meaning that everything's fine. We couldn't see any reason why SDS7 would respond differently, with an original Simmons memory cassette as this feature doesn't have anything to do with AMEP. We're definitely not claiming any credit for that one but it's a cool hidden feature.

Simmons SDS7 AMEP Write Fail Write Pass — The left image shows what is displayed on SDS7 when a write command fails. The right image shows what SDS7 displays when a write command is either successful OR with the AMEP memory protect switch ON and attempting to write data to an AMEP location that already has that data (verification).

I couldn't wait to tell my Simmons Vintage Technical Network colleagues the good news. With confidence high, Michael Buckner asked a question which was going to come sooner or later; "Would AMEP Advanced Memory Expansion pack for the Simmons SDS7 also work on the SDS6?"

Well, I couldn't see why it wouldn't but having confirmation would be a big deal so I decided to send Michael Buchner and Ed Rose, AMEP samples and await their response.

At last! Decades after its release, there's now a cost-effective and reliable way to save your valuable SDS7 and indeed, your SDS6 data to an external device.

AMEP 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...

I really wanted to do AMEP justice so AMEP is supplied in it's own bespoke tin which has a transparent lid section.

I designed an insert, originally to be made from ESD foam. The cost of this small item was insanely prohibitive, however. In fact, I figured that I could buy a 3D printer and make these myself, for the prices I was quoted, all be it from plastic.

Well, my graphics guy Tony and I have been meaning to get into 3D printing for a long time and we thought this would be a good excuse to make a start. Hence, the insert is 3D printed by me.

AMEP Insert in design stage — Bambu Studio is a nice bit of software.

Tony observed that everything about AMEP except the tin, is made by Plasma Music Limited! How cool is that?

If you're keen to keep all your Simmons SDS7 data safe, you can buy AMEP here:

And finally, AMEP advanced memory expansion pack for the Simmons SDS7 would have taken much longer to develop and perhaps might not have happened at all, if it wasn't for the help, support and encouragement I received from my Simmons Vintage Technical Network friends and colleagues. THANK YOU Ed Rose, aka The Simmons Guy, Michael Buchner and Patrice Jacquot.

In particular, special thanks go to my dear friend Guy Wilkinson of Super Synth Projects who poured in hours and hours of work into this one. Guy, THANK YOU for doing so much to get AMEP finished and working. What you did is just brilliant! 🙂

UPDATE - 16th October 2024

Wow! I've already received the big question pertaining to the weird SDS7 Bank Anomaly. Whilst this hasn't got anything to do with AMEP, it's an important question and so I thought I would add my answer to this post.

According to the original owner’s manual, the SDS7 has two banks: HI and LO. The HI bank includes patches 40 to 79 and the LO bank includes patches 01 to 39.

Well, it doesn’t take a rocket scientist to notice that the HI bank has forty patches and the LO bank has thirty-nine patches! 🤔

THE SDS7 BANKS ARE NOT THE SAME SIZE 😡

So how the hell does this work? A good question and I don’t have a working original Simmons memory cassette to test that.

In theory, backing up the HI bank and then writing it to the LO bank will mean that you’ll lose a patch. Going the other way, backing up the LO bank and writing it to the HI bank will mean that you’ll have one patch unaffected (patch 40).

So here’s how it works:

Backing up the HI bank (patches 40 – 79) and writing to LO will result in patch 40 being lost. Patch 41 is written to patch 01 instead and patch 79 is written to patch 39.

Backing up the LO bank (patches 01 – 39) and writing to HI will result in patch 40 not being written to. Patch 01 is written to patch 41 instead and patch 39 is written to patch 79.

Yeah, I know. WTH?!?!?!?

Whilst AMEP is an incredibly cool piece of kit, it’s not ‘intelligent’. AMEP doesn’t have an on-board processor to negotiate the SDS7 bank anomaly. If it did, AMEP would have been prohibitively expensive and physically, much larger. ☹

BUT…. There’s a workaround:

!!! DON’T USE PATCH 40 !!!

Of course, if you save the LO bank and write it to the LO bank and if you save the HI bank and write it to the HI bank, everything will be fine. If however, you want to swap banks, then you'll have to use the workaround. I'd imagine it's the same if you have an original Simmons memory cassette.

UPDATE - 1st November 2024 - AMEP IS NOW ARM'D

Manufacturing tolerances have changed considerably over the past forty years and while AMEP will click in nicely into a brand new female DIN 41612 connector, that's not always the case with the same connector on the back of the SDS7. After four decades, things can feel a little loose.

To cater for a loose fit, AMEP now has an Anti Rock Mechanism (ARM) which securely keeps it in place. Not rocket science, just some PCB spacers added to AMEP's connector end but which work just great.

AMEP now has Anti Rock Mechanism — AMEP now features an Anti Rock Mechanism.

Unlike the original Simmons memory cassette, AMEP is small and light but the new ARM makes things feel even more solid and will hopefully reduce wear of the SDS7 expansion slot, over coming decades.

AMEP's anti rock mechanism keeps it securely in place — AMEP's ARM keeps it securely in place and prevents rocking when engaged with the DIN 41612 connector in the back of the SDS7.

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:

Simmons SDS7 Program Chart

Alex-PML8 October 20247 January 2025

Programming the Simmons SDS7 is a laborious task and to get a collection of patches or kits, can take days, weeks or even months. Hey, it might be an idea if you had a record of all your hard work, right? Well, here's something to help you with that. My Simmons SDS7 program chart is an easy way to make sure that all your SDS7 patch data is safely recorded.

There are several reasons why a record of your patch data may come in useful. When you tweak settings for example, you may need to refer to a reference, you may accidentally overwrite one of your favourite snares, or you may just want to create something new and a set starting point would certainly help.

Designed to reside on your computer, you don't need any special software to fill in these charts. Adobe Acrobat Reader on Mac or Windows will work just fine. If you'd prefer a hard copy, then just print them out.

My Simmons SDS7 program chart allows you to record the following information:

Date
Patch (kit) number
Your own notes
Sample name of each module for up to twelve modules (limited to nine characters)
All module parameters for up to twelve modules (numbers between 0 and 255 only, go figure)

All you need to do is enter your information, parameter values and save. It's printable too, just in case you'd prefer a hardcopy.

You can try out my Simmons SDS7 program chart online here or just download it here.

Having a written record of your SDS7 patches is one thing but what if you want to store your patches somewhere safe, somewhere other than your SDS7's memory? Well, now you can! AMEP or Advanced Memory Expansion Pack for the Simmons SD7, will do just that.

With four times the memory capacity of the original Simmons memory cassette, 20 - 25 year battery life, an on-board super-capacitor meaning that you won't lose your data when you do change your battery and a fraction of the size of the old memory cassette, AMEP is the ultimate memory back-up system for the Simmons SDS7.

Oh yeah and AMEP also works on the Simmons SDS6! 😎 In fact, you can save both SDS6 and SDS7 data on to a single AMEP. How cool is that?

You can read more about AMEP here.

My Test Simmons SDS7 Is Back In One Piece

Alex-PML1 October 202423 October 2024

Today is a BIG day. Today my test Simmons SDS7 is back in one piece! SDS7 serial number 70320 was finally put back together after several months of doing exactly what I bought it for… to host the development of a small range of peripherals for one of my favourite electronic drum systems. 😊

Back in December last year, I found a really nice SDS7 up for sale and having wanted one of these ever since I sold mine back in 1987 to help fund the purchase of my first SDX, I just wanted to get another one.

My early Christmas present to myself got me all smitten again and I had ideas to design stuff that would help keep this legendary instrument going for another forty something years. The only problem was that I didn't really want to develop stuff on the SDS7 I'd just bought, as it was immaculate.

Then in May 2024, a friend pointed me in the direction of a SDS7 that was up for sale relatively locally (London). It had some faults but having an idea what the problems were, I grabbed it for a really good price.

Subsequently, SDS7 #70320 became the platform on which I developed Pleiades lithium battery conversion and uniPulse MIDI to trigger adapter, Dark Matter modular switched-mode power supply, AMEP advanced memory expansion pack and NC-7 noise cancelling snap-in, all of which will be launching very soon. Indeed, I’m delighted to announce that SDS7 #70320 is actually running Pleiades, Dark Matter and NC-7. AMEP will be here very shortly. In fact, the pre-production PCBs should arrive today.

Using paper cut-outs to make sure that Dark Matter would fit into SDS7.

You may have noticed that the top-case of my test SDS7 looks very 'black' and seems to have an acute absence of age-related scratches, dents and rust. Well, that's because I got it sand blasted and then re-powder-coated. I actually had a customer's SDS7 in at the same time so I got both top-cases done together.

Simmons SDS7 Top Case Refinished — I was absolutely delighted with the results of sand blasting and powder coating this old Simmons SDS7 top case. The guys even managed to reproduce that original eighties leatherette texture.

It's been a heavy few months and there were a couple of points during that period when I thought I’d been a bit too ambitious. Ed Rose (aka The Simmons Guy), Michael Buchner, Patrice Jacquot and my close friend Guy Wilkinson (of Super Synth Projects) have however, been so incredibly supportive. Without their help and encouragement, things would have taken so much longer and perhaps might not even have happened.

My test Simmons SDS7 is back in one piece and #70320 looks and sounds amazing. I’m just so pleased that it all worked out. Pleiades, Dark Matter and AMEP will be launched at the same time and very soon, like within the next couple of weeks, hopefully so stay tuned. 🙂

UPDATE - 16:00, 1st October 2024

This is my very first post update that has the same date as the post itself! Yes, something happened a few hours after I published this post. My prototype AMEP PCBs arrived. 😮

AMEP pre production prototype PCBs — Oh! Look what just turned up! AMEP pre-production prototype PCBs. Oh WOW!!

NC-7 Noise Cancelling Mod for the Simmons SDS7

Alex-PML1 October 20244 October 2024

My NC-7 noise cancelling mod for the Simmons SDS7 is for those SDS7s that didn't get this mod factory fitted.

"Surely that can't be" I hear you ask.

Pictured below, is part of the Simmons SDS7 showing the quad op-amp, part of which is used to buffer the stereo outputs. Due to the mish-mosh of analogue, digital and power tracks on the back plane, Simmons was forced to come up with an easy retrofit solution to reduce specifically, CPU noise picked up by the stereo buss. Able to take advantage of a spare op-amp stage in the TL084 and dropping an 'aerial' on the rear of the back plane, inverted noise was mixed on to the stereo buss, thereby cancelling out the 'in-phase' noise. Highlighted in red is the pre-set pot and the two 10kΩ resistors which were part of that mod.

There are however, a few SDS7s out there that didn't get the mod! Pictured below, is my test SDS7 which I used to develop Pleiades, Dark Matter and AMEP and as you can see, it didn't get the mod fitted back in the day.

You'll also notice the absence of a small green capacitor, highlighted in yellow on the second image. This 47pF capacitor which is connected between pins 10 and 12 of the TL084, is sometimes soldered on the back of the back plane PCB. More about this later.

Well, I was going to implement the mod as per the original service bulletin but as it turned out, the track layout of the back plane version in my test SDS7 was different to later revisions and I wasn't able to drill the three holes for the pre-set without busting a track.

Then I had a thought; "Why don't I knock up a little snap-in PCB to do the job?" That way, anyone with a SDS7 that didn't have the noise cancelling mod installed, could also benefit. 🙂

Snaping into the IC socket that the TL084 is in and taping the aerial on to the rear of the back plane, means that NC-7 can easily be installed in a few minutes.

Of course it's not quite that straight-forward. The noise cancelling circuit has to be 'tuned', hence the pre-set pot. Instructions on how to do that are in the installation manual and that process also takes only a few minutes.

The two through-hole resistors of the original mod, are replaced by SMD 0805 resistors that are situated underneath the IC on NC-7. That 47pF capacitor I mentioned earlier, is also superfluous as on the underside of the NC-7 PCB is a SMD 2010 47pF capacitor. Hence, if the original capacitor is indeed soldered to the rear of the TL084, it must be removed.

47pF capacitor on back of TL084 — Irrespective of where the 47pF is located, it should be removed as NC-7 has a 47pF on-board.

NC-7's aerial wire can't simply flip over the back plane PCB. That's because one of three guides guide in the SDS7 top-case secures the back plane PCB exactly where the aerial wire needs to be. The wire I chose for the NC-7 aerial wire is CAT5/6 cable strand so it's solid. If it catches into the guide of the top-case, it'll break. Hence, the aerial wire has to go to one side.

Rear view of SDS7 back plane showing NC-7 aerial glued in place — In a similar manner as the original mod, the aerial from NC-7 is hot-glued to the rear of the back plane in between the edge connectors for modules 9 and 10 and across the stereo outputs busses.

So while looking into the infamous SDS7 noise and testing my NC-7 noise cancelling mod for the Simmons SDS7, I thought it might be an idea to actually record this annoying phenomenon.

The video below shows the effect on overall noise level, as the NC-7 noise cancelling mod is gradually tuned in. Check out the VU meter on the right.

WARNING!!!! The signals have been amplified so TURN DOWN YOUR SPEAKERS

Below is a spectral capture of the noise and the effect of the NC-7 noise canceller. As can be seen, tuning in the NC-7 noise cancelling mod, reduces noise between 3kHz and about 11kHz but doesn't do anything for lower frequency noise.

There's one other thing... In fact, you may already have noticed, that instead of using a TL084, NC-7 is fitted with a newer NJM2060. This device is of a much higher specification than the humble TL084. Since two stages of this quad op-amp are used to buffer the stereo buss, having a quieter device will no doubt improve things a little. 🙂 Having said that, I'm not sure the difference will be too noticeable with all of that other stuff!

NC-7 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 NC-7 units in stock and each NC-7 is built to order.

Simmons SDS7 Battery Leak

Alex-PML30 April 202430 September 2024

This Simmons SDS7 battery leak occurred because the battery wasn’t charged regularly.

Recently in, was a Simmons SDS7.This is my second most favourite electronic drum systems of all time, after the Simmons SDX. In fact, you can read how much I love this instrument here.

Anyway, this SDS7 was apparently just fine a couple of years ago but when the owner recently got it out of storage, there was no sound. Powering up but otherwise completely dead, only the voice-card name illumination seemed to be coming on with nothing showing in the programmer display. Well, I had a pretty good idea what was wrong with it.

Like a lot of music technology at the time, the pre-MIDI Simmons SDS7 used NiCd (nickel-cadmium) batteries to back up its memory. Some manufacturers like Cheetah, continued to use NiCads all the way up to the end of the eighties, perhaps even a little later. Unlike lithium batteries which soon took over, NiCads need to be regularly charged. If left uncharged for an extended period, NiCd batteries die and WILL LEAK.

Fortunately, the Simmons SDS7 was built with vertically mounted PCBs. Despite the fact however, that there was also a version (like this one) which, instead of having a single battery soldered directly to the memory-board, used a triple AA battery holder (3 x 1.2V in series = 3.6V), battery acid managed to leak out over those already old solder joints, tracks and components. I soon noticed for example, that the C̅E̅ line was down and in fact there were several other issues preventing code from running and therefore not allowing the machine to boot.

There was also, a fair bit of damage to tracks on the rear buss-board (marked up as 'SDS7 BACK PLANE'), which couldn't be resolved by cleaning alone and which required some hot-wiring. Annoying but to be expected.

Battery acid damage on Simmons SDS7 rear buss PCB — Although the memory-board was okay, some tracks were badly damaged on the rear buss PCB. The one highlighted in red had completely perished and only the white trace was left.

Above you can see that I'd already jumped one track but there were another eight which although buzzed out okay, I really didn't like the look of. One in particular looked as though it had completely succumbed to battery acid.

Technical literature on really old machines like the SDS7 is thin on the ground. Techs often have to make do with copies that were scanned many years ago on what would now be classed as, low-resolution scanners. I therefore, decided to make myself a little reference diagram.

Unlike the image above which is a standard 1920 x 1280, the original pdf document is pretty big but without going to that kind of trouble, I'd be shooting in the dark. In the end, I actually decided to install my wires around the back of the back plane PCB.

You'll notice in the two images above, that the polarising key in the CPU card edge connector (P13) is somewhere where it shouldn't be and that the key in the edge connector to the right (that takes the memory-board) is missing altogether. In fact, there was a polarising key on one of the voice-cards that was also out of place and so the card didn't work properly.

Anyway, while I was buzzing things out on the back plane, I carried on cleaned up the memory-board.

Battery acid damaged Simmons SDS7 battery holder — The battery holder can't be salvaged but I have other plans anyway. I've made a start on cleaning up the memory-board. You may notice that I changed the two grey battery wires. They are now colour-coded!

When I say "cleaning up the memory-board", it actually took several days to buzz out connections and hot-wire tracks that had been damaged by battery acid. After all that, this SDS7 finally managed to consistently boot but there was another problem; the data encoder was unresponsive. Grr... The problem turned out to be a simple but very annoying, dry joint under a buffer that reads the signal off one of the optical receivers. Phew...

My own SDS7 is serial number 71722. The serial number on the back of this one was #70320. If sequential, that's 1,402 units difference. More than just a batch or two!

Indeed, as I went over the insides of this SDS7, it became apparent that it was quite an early version. It had the optical encoder as opposed to the electrical contact type. The polarising keys in the edge connectors on the back plane were the small bits of black plastic type, instead of the yellow clip types used later. There were also various wire jumpers on the CPU-board that aren't on later revisions.

On a side note... There was also some work to do around the back of the unit as the socket pins although not corroded (because they can't), were very black and so needed a good clean.

Shiny socket pins on Simmons SDS7 — The XLR pins on the audio outputs were black so needed a good clean. I think I'm also going to have to do the XLR receptacles on the pad inputs.

The lid was smacked in at the front and was also badly scratched and lots of areas that showed corrosion.

Straightening the top-case on Simmons SDS7 SN70320 — Removing the creases on a piece of metal like this is a time consuming task but it'll be worth it. This SDS7 is otherwise in pretty good cosmetic condition.

After several days (or perhaps two weeks) of patient straightening with a combination of heat, G-clamps and weights, I took it to a local company to be sand blasted and powder coated. They said that they didn't think they'd be able to match up the leatherette finish but when I collected the lid a week later, I was blown away. This top case looks like it just came from the factory! WOW!!!! 😮

So, back to batteries... Amongst instruments like the afore mentioned Cheetah MS6 and true vintage gems like the Sequential Circuits Prophet V, a Simmons SDS7 battery leak is something I've seen quite a few times over the years, or should I say decades. Don't think that everything can be fixed. It took me two years to concede that the last Prophet V I had in with a leaked battery wasn't salvageable. It's incredibly sad and something that would potentially 'brick' an iconic piece of vintage music tech' so please take note: If you have a piece of old gear like this, do check what kind of battery it has. If it's got a NiCd battery, then make sure it's charged regularly.

! ! ! WARNING ! ! !

Floating around on the Internet, is a post of someone who's substituted a NiCd battery with a lithium battery in (coincidentally), a SDS7. PLEASE DO NOT DO THIS!

Memory retention is designed for specific batteries. The chemistry of a lithium battery is very different to that of a NiCd battery. For a start, the former can't be charged and if you try to do so, you may end up with getting more than your fingers burnt... literally.

For over thirty-five years now, equipment has been designed to take batteries like the CR2032 which we're all familiar with and which outputs 3.0V. Older NiCd batteries are rated at 3.6V. That 16.7% difference can sometimes be significant.

You should also double check when buying batteries as many unscrupulous or perhaps just ignorant retailers advertise NiMH (nickel metal hydride) as NiCd and lithium-ion as lithium. In all fairness, sometimes it's just not clear. As we all know, a Google search for example, will often throw up results that are close to your search term so please check before you buy. Yes, I know it's frustrating, especially when NiCd batteries are becoming increasingly more difficult to procure but please don't be tempted to take the easy option. Batteries are all different. Even those that can be charged, have different charge rates, depending on their respective chemistry. Charging circuits for NiMH batteries for example, are usually designed to measure the status of the battery, thereby dynamically reducing the current that's being fed into them. This is NOT the case with NiCd charging circuits. If you substitute NiMH into a system that normally takes NiCd, then you stand the risk of overcharging the battery. This might not be disastrous but will reduce the lifespan of the battery.

APLOGIES TO SELLERS

If you're considering buying a Simmons SDS7, please do also consider asking the seller to show you a picture of the memory-board. My apologies in advance to anyone selling a Simmons SDS7.

AND FINALLY...

It's quite difficult to find information on the Simmons SDS7 and so I thought it prudent to offer some useful links for those interested.

For a start, if you're in the US, you need The Simmons Guy on your side. Ed is not just knowledgeable and experienced, he's methodical, tidy and VERY helpful. Just one thing; because of all that, he's amazingly busy!

Ed (The Simmons Guy) is also very active on social media and it's worth checking out his Facebook page and his YouTube channel. In fact, in amongst his videos, you'll see my own SDS7 #71722 before I bought it from him, LOL. Ed, where on earth do you get the time?

This site has a lot of really useful SDS7 information and may be of help to those who just need a few facts.

And of course, there's the Simmons Virtual Museum which also has a few useful resources.

UPDATE - 30th May 2024

Since writing this article, I'm delighted to advise that the SDS7 featured above, is now fully up and running! 😀

It took a while and as I focused my attention on the memory-board, actually there were a couple of tracks on the back plane that slipped my eye. Grr...

Since posting however, I've had a few more older items in with completely leaked out batteries.

Three very old NiCAD batteries

If you think they look bad, then you wouldn't want to see the insides of the machines that these all came out of! One of them had to be scrapped as it was totally unsalvageable.

My New Simmons SDS7

Alex-PML20 December 202323 September 2024

I often get asked, which part of my carrier was my favourite. Well, to be honest, if you’re into music, it’s a bit like someone asking which is your favourite band or song. The seventies and the eighties were very exciting times. As an electronics engineer wanting to leave defence and branch out into music technology however, the eighties were truly magic. Indeed, my time at Simmons Electronics Plc in particular, well "kid in a candy store" comes to mind! Fast, hi-tec and an amazing bunch of people, Simmons was one of the many cool electronics companies around, uniquely being the only British electronics manufacturer at the time, to have an office in Tokyo.

What used to be the old Simmons factory in St. Albans (picture taken September 2021). Above the main entrance / reception area and at the top of the stairs (to the left of where it says 'Marshall Volkswagen') there used to be a massive Simmons drum kit.

Within a few months of starting with the company, I was very familiar with machines past, present and future and working on them daily wasn’t always enough. I ended up buying a SDS9, a SDS7, an EPB, a couple of SPM 8:2s and I even bought a Silicon Mallet. And then, in 1987, 'we' released SDX and I just had to have one! Fully expanded (and costing a small fortune), SDX was capable of holding a whopping 8MB of RAM. Well stupid me, I decided that 8MB wasn't enough for drums and cymbals so I ended up buying two fully loaded SDXs, both complete with pads.

My two Simmons SDXs — Here are my two Simmons SDXs from many years ago, in a previous (life) studio. I still have them and hope I can get them back into my studio soon.

Simmons ZI pads for the SDX used FSR technology — And here's an old snap of my 'small' kit. This expanded to double-kick, a few SDX ZI cymbals and a lot more toms. Geesh... What was I thinking? I'm not even a drummer!

Young and stupid (I think I've confessed to such already), I sold my SDS7. I can’t remember why but it might have had something to do with buying the two SDXs which if memory serves me correctly, started at 13,000 GBP each for like the basic version which only had 2MB of memory and fewer pads. Little did I know that years later, selling my lovely SDS7 would be a decision I'd regret and so my long quest to find a really nice replacement began.

In recent years, I'd regularly check out the usual auction sites and scour the Internet looking for a nice SDS7. Well, over thirty years after selling the one I bought while at the company, I finally found one that looked too good to be true.

I'd heard about a Simmons tech in the US called The Simmons Guy and the unit I found was one that he was selling. That in itself, immediately made me feel comfortable as from what I've read, this dude seriously knows his stuff. The thing is that apart from being totally functional, this particular example, looked in really good condition.

So, on 11^th December I bit the bullet and today (20^th December) my new Simmons SDS7 serial number 71722 arrived.

I was recovering from my first bought of COVID but I simply couldn’t contain my excitement. It took me a while to get through the superb packaging. Of course #71722 is a US unit and therefore the transformer is wired for 120V operation. In such incredible condition, I reluctantly opened the unit and converted it to UK 240V, which included stepping down the fuse.

Simmons SDS7 arrived just as I came out of COVID — My new Simmons SDS7 on the bench and powered up for the first time and after its conversion to UK 240V.

The Simmons Guy is 'Ed Rose'. We got to know each other pretty well, while I waited for my new Simmons SDS7 to arrive. Such a lovely dude, he threw in so much with this unit including a beautifully discrete Tubbetec uniPulse MIDI conversion on a 3.5mm socket, loads of EPROMs, a patch-changer and many internal tweaks such as EPROM size selectors on each voice card and the hi-hat fast decay mod'.

Three Simmons SDS9 pads — These lonely SDS9 pads have been on the landing of the new studio for months. It's like they've been waiting for something...

Launched in 1983, the SDS7 didn't have MIDI. In fact, the first MIDI keyboards, the Roland Jupiter 6 and the Sequential Circuits Prophet 600 only appeared that year and even the MIDI Manufacturers Association wasn't established 'till 1985. Indeed this was a very exciting time.

Fitted with a Tubbutec uniPulse MIDI to CV converter which I have to say, has been done quite discreetly, #71722 can now be triggered via MIDI IN. Even the hi-hat open / hi-hat closed can be controlled via MIDI controller 18. Tubbutec supplies a configurator for Windows and OS X. With no MIDI Out though, things are a bit 'one-sided'. Having said that, the uniPulse hardware is fitted with a momentary switch which allows the system to 'learn' (and memorise) the incoming MIDI channel. A very convenient touch.

1983 Simmons SDS7 with Tubbutec UniPulse MIDI to CV.

All trigger inputs and audio outputs are on 3-pole XLRs. In the case of the audio outputs, one might be forgiven for thinking that they're all balanced. Unfortunately, that's not the case. In fact, being high-impedance as well, I've wired things to accommodate a couple of active unbalanced to balanced converters between the SDS7 and my Yamaha DM-2000 mixing desk.

My new Simmons SDS7 in the control room — I would normally have got this racked up but #1722 is in such amazing condition that I decided it should have pride of place.

I've also hooked up the SDS7 to a TP-Link Kasa smart power outlet and a schedule has been created which powers on the SDS7 every Monday at 12:00 mid-day for a couple of hours. That way, the NiCd battery receives regular charging.

My new Simmons SDS7 in its new home — Simmons SDS7 #1722 in its new home.

At some point, I might consider modifying the memory back-up battery circuitry so as to take lithium batteries, similar to the modification I offer as part of my external power supply package for the Cheetah MS6.

What the google hook-up also means however, is that I can say "Hey Google, switch on my SDS7" and guess what happens! 😮

LAST WORDS

How practical will my new Simmons SDS7 be for contemporary music production? Who know! Who cares!! What I do know is that I will be using it and I'll absolutely love using it. Yes, it's a huge chunk of mega nostalgia for me but I'm a firm believer in keeping the creative process fun and that's totally subjective.

UPDATE - 28th December 2023

I'm totally over COVID and #71722 is now MIDI'd up to the rest of my studio. For the time-being, stereo outputs are going directly to my Yamaha DM2000 and I AM SO HAPPY!!!!!

My new Simmons SDS7 responded immediately to MIDI and sounded just as I remember back in 1986 (ish). Oh boy! I'm gonna have some fun with this. 😀

NOTE: While I wait for my active unbalanced to balanced converters to arrive, the stereo outputs from the SDS7 are running unbalanced to my DM2000 and I have to admit that I can't believe how quite this box is. While I don't recall having any particular noise issues with my original SDS7 back in the day, it's a well documented fact that some SDS7s did / do suffer from what was referred to as CPU noise. In fact, Simmons developed a rather ingenious 'noise cancelling mod' which took advantage of a spare op-amp. A piece of wire strapped to the back of the 'back plane' PCB acts as an aerial, picking up the so-called CPU noise, which is then inverted and mixed back on to the stereo buss. This particular SDS7 however, is really very quiet. The DM2000 meters will show up failing chorus chips in something like a Roland JX-8P which might not be immediately apparent on the monitors and yet the unbalanced audio outputs from my new Simmons SDS7, aren't showing anything when not played. To be brutally honest, I didn't expect that, especially from a machine that's forty years old.