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SMS Planet 7 System

Modular Analogue Rack Synthesizer By Gordon Reid
Published June 2007

SMS Planet 7 System

Synthetic Music Systems have a unique approach to designing modular synths that are both high in quality and, wait for it, low in price. Let's investigate...

I first met Andy Pledger, the man behind Synthetic Music Systems, in 2001 when he demonstrated the original MARS (Modular Analogue Rack Synthesizer) to me. This was a 1U rackmount that proved to be much more flexible than its single VCO, VCF, VCA, LFO and EG architecture might have suggested. Unlike the other 1U synths of yesteryear (the Doepfer MS404 and a rash of TB303 clones spring to mind), the MARS offered a comprehensive set of signal and CV inputs and outputs, making it a cost-effective expander as well as a small, semi-modular synthesizer in its own right.

At the time, the MARS suffered from one of my pet hates — I/O sockets on the rear panel — so I was pleased to hear that it was to be joined by a breakout box (BOB) and an expander that would link two MARS racks, add extra modules, and be replete with a full set of front-panel connections. But after just a handful of sales everything went quiet, and it seemed that SMS had gone the way of many other British synthesizer companies, disappearing without trace. Nevertheless, Pledger was still active in the industry, and some of the MARS technology was incorporated in the Spawn synthesizer from Analogue Systems. With a higher specification that included extensive MIDI routing, the Spawn was always intended to be a limited-edition model, and you'll rarely see one for sale; the people who own them seem to feel no temptation to sell them.

Now SMS are back and, with a powerful configuration in front of me that includes no fewer than three MARS synthesizers, a Planet 7 Expander and a BOB, I'm ready for some serious twiddling.


The first thing to say about the SMS modules is that they're blue. Very blue. A thick, powder-coated blue that I suspect many people will prefer to the brushed aluminium of other synthesizer modules. The second is that they are well laid out and well made, with good-quality 3.5mm sockets and smooth, positive-feeling knobs. Little touches such as the flat-topped LEDs mounted flush with the front panel are also indicative of the overall care taken when designing and assembling them. The third is that they're presented as 19-inch rackmount units. This is not trivial: I know of no other commercial, 19-inch modular synths. Integrated analogue synths in 19-inch form? Yes, plenty. But not modular synths.

Before moving on, we need to address the question of what constitutes a modular synthesizer, because the MARS somewhat redefines the term. To start with, you can approach it as an integrated synth with which you can create sounds without a patch lead in sight. This is because there are internal signal paths. To be precise, the output from the VCO is connected to the input of the VCF, and thence onward to the input of the VCA. Similarly, the LFO can modulate the oscillator waveform and filter, and the envelope is directed to the waveshape, VCA and VCF cutoff frequency. But if you want to break these connections and turn it into a true modular synth, you need only turn the appropriate level controls to 'zero', and no signals flow. You are then free to patch the MARS as you wish. Alternatively, you can use the patch points to augment the internal signal paths, and the unit then acts as a semi-modular. That's neat.

The Audio Oscillator & LFO

Inspecting the oscillator, I found that, despite the limitations imposed by squeezing everything into a 1U rack, the amount of control was quite impressive. There are three pitch ranges, with normal and minus two octave settings, both with fine tuning ranges of plus or minus one octave, and a wide setting that allows you to use the tuning knob to sweep the frequency across a massive range; from subsonic to around 6.3kHz with no CVs applied.

Here you can see three MARS synths, a Planet 7 and a BOB bolted into a 19-inch rack cabinet and wired together.Here you can see three MARS synths, a Planet 7 and a BOB bolted into a 19-inch rack cabinet and wired together.Photo: Mike Cameron

The Wave knob allows you to select between a sawtooth (fully anti-clockwise) and a square wave (12 o'clock), with any mix of the two between. If you then continue clockwise from 12 o'clock, the square wave narrows to a two-percent pulse.

Next to this, the Mod knob allows you to apply LFO modulation (anti-clockwise from 12 o'clock) or envelope modulation (clockwise from 12 o'clock) to the wave shape. Using the pre-patched triangle wave from the LFO to create pulse-width modulation results in a warm chorus when the waveform is a square or pulse, and an interesting mix of waves when the starting point lies between the sawtooth and square positions.

Turning to the CV inputs, I found a Sync input, a pair of 1V/octave inputs for both playing and modulation duties, and a waveshaping input for use with external modulators.

Given the modularity of the MARS, I was able to take the output from the oscillator and insert it directly into my mixer. I liked what I heard. The unfiltered sawtooth wave was bright and buzzy, the square was hollow, and the pulse waves offered the distinct nasal quality that they should. Checking these results on an oscilloscope demonstrated that the shapes were reasonable approximations to their ideals, albeit with a 'peaky' overshoot at the cusps.

Alongside the oscillator lies what looks like a very basic LFO. It has just one knob, for rate, and an LED that indicates the frequency. But when you look at the back panel of the MARS or at the BOB, you find that the LFO offers independent outputs for all four of its waveforms — sawtooth, ramp, square and triangle — and a CV input that allows you to 'play' the LFO from a keyboard, or affect its rate using another modulator of your choice. With no CV applied, I found that it varies from around one cycle every 18 seconds (slow enough for gentle sweeps) to a little over 30Hz, as specified. However, there was a disappointment here. Given the CV input, I had expected to be able to use external CVs to shift the LFO well into the audio frequencies and use it for basic FM synthesis, but I found that 30-something Hz appeared to be its upper limit.


The VCF is a 24dB/octave ladder filter, which is many manufacturers' code for 'designed to sound something like a Moog'. It offers five controls: the input signal level, which provides overdrive at settings above 70 percent or thereabouts; the expected cutoff frequency and resonance knobs; LFO modulation (triangular to the left, square to the right); and envelope modulation with either polarity. The four patch points provide two CV inputs for 1V/octave modulation, plus an audio signal input and an audio signal output.

The filter's response is more than reasonable, ranging from 16Hz to 15.5kHz with no CVs applied, and it will go higher if asked to do so. As you would expect, it oscillates when the resonance is cranked up, and does so across its full range of cutoff frequencies. This means that the MARS' filter is never going to respond quite like a Minimoog's (which does not oscillate at low frequencies), despite the use of a similar architecture.

Next to the filter lies the VCA. Like the LFO, this appears fairly basic, with just a level control that again offers overdrive from around 70 percent upward, plus a knob that allows you to apply the envelope in either linear or logarithmic form. (The ability to apply the envelope in these two forms is important: the characteristics of many vintage synths are in part defined by the choice of one or other response.) The I/O sockets echo these capabilities, with an audio input and output, and two control inputs; one logarithmic, the other linear.

I'm happy to see that the ADSR envelope generator responds to both Gate and Trigger, which means that you can use the MARS as a multi-trigger synthesizer if your controller can provide the required signals. But, looking at the response times on the 'scope, these are not the snappiest I've ever encountered. At eight milliseconds, the attack is definitely on the squelchy side, although the decay, at 2ms or so, is much more lively. Given that SMS claims a fastest attack time of 5ms, I contacted Andy Pledger, who was surprised to find that his stock units confirmed my results. He then discovered that a simple modification would make it possible to obtain percussive sounds with attacks of less than 2ms duration, which is in line with many other analogue synthesizers. I understand that he has now modified all stock units so that they are capable of generating the improved envelope. But before leaving this topic, I should also note that the MARS is also not the most languorous of synths. The longest envelope times are around eight seconds (attack) and 15 seconds (decay and release), thus precluding very slow contours.

Products & Pricing

  • MARS: A 1U rackmount modular synthesizer with independent LFO, VCO, VCF, VCA and EG sections, pre-patched internally but isolatable using the signal and modulation level knobs. £250.
  • BOB (Break Out Box): A 1U rackmount module that relocates the 21 patch points of a MARS to the front of a rack. Two versions are available: one with a red, pink and white noise generator, the other with an AC-coupled ring modulator. It includes patch and power leads to connect one MARS to one BOB. £100.
  • Planet 7 Expander: A 2U rackmount expander that links two MARS synthesizers and brings all 42 patch points to the front of a rack. It adds 12 further synthesis modules to the system, and includes patch and power leads to connect two MARS units to one Planet 7. £250.


So what can you do with a single MARS module and no ready access to the patch points on its rear panel? The answer is, about as much (or as little) as you might think. Single-oscillator monosynths with a fixed signal path and a single LFO and envelope generator have to work hard to offer interesting results. So, while traditional imitative sounds such as brass and flutes leap forth readily, as do simple bass and lead sounds, I wanted to see whether the MARS could do more.

I discovered that choosing between the two envelope responses in the VCA made it possible for the single envelope to control the VCF and VCA differently. It wasn't the same as having two EGs, but was still more flexible than applying an identical contour to both devices. I also used a little slow PWM to eliminate the sounds' otherwise static character, and a touch of overdrive to thicken them up.

I patched a lead sound using all of these tricks, and the results, played from my SH101, worked well (normal pitch on the MARS and 'high' transpose on the SH101), but proved to be remarkable as a super-deep bass (minus two octaves on the MARS and 'low' transpose on the SH101). I created a simple bass sequence on the Roland and used this to control the MARS while I modulated the filter using good ol' fashioned fingers. The results were magic! I was deep into Tangerine territory, and was getting far more depth from the MARS than I had expected.

Of course, most of the powerful and exotic 'synth' patches we all love will forever be beyond the reach of a single-everything architecture such as the MARS alone, so let's now add the BOB. You connect the two together using the supplied pack of 21 patch cables that loop the outputs on the synth's rear panel to the inputs on the rear of the BOB, which are themselves hard-wired to the sockets on the front. Setting this up takes just a couple of minutes: quicker than connecting the power and screwing the equivalent number of Eurorack modules into cases.

In truth, I doubt that many people will attempt to get the best from a MARS without a BOB because, while it adds to the price, it makes the synth hugely more useable. Granted, it doesn't overcome the basic limitations of the architecture, but it adds some important facilities. These are the ability to re-patch the modules in the MARS; the ability to inject and treat external audio and control signals wherever you choose, isolating (if you wish) the five synthesis modules within the MARS; and... the choice of a noise generator or an AC-coupled ring modulator!

Hang on a second: how can a simple breakout box offer a signal generator or a signal processor that needs a power source? The answer is that there's a power output on the back of each MARS, and you connect this to the power input on the BOB using the dedicated three-pin cable supplied. It's another neat solution.

Nonetheless, there are still problems with the expanded system. One obvious one is the bane of many analogue synthesizers: the amount of LFO modulation applied to the VCO pitch modulation knob is far too great. While some people might enjoy sweeping the pitch over more than two octaves using the LFO, creating a musical vibrato is an exercise in patience and very gentle adjustment. More significantly, and in common with many other modular synths, the MARS and BOB combination offers an inadequate number of multiples and mixers. To be precise, it has none of either. This means, for example, that you can't direct the pitch CV from your controller synth to both the pitch CV input on the oscillator and one of the modulation inputs on the filter (ie. no filter tracking). That's not good. Similarly, the lack of mixers means that you can't combine sources to create exotic modulation signals. Nevertheless, it would be churlish to belabour these points because, for the price, MARS and BOB offer more than you might expect. It's just that the system needs to go further...

Using The MARS With MIDI

It goes without saying (but I'll say it anyway) that the MARS, BOB and Planet 7 are analogue systems, with not a microprocessor to be seen anywhere among them. This means that if you want to use any combination of these modules in a contemporary studio environment, you'll have to supply your own MIDI-CV converter.

Adding The Planet 7

If your budget extends beyond a single MARS and BOB, the combination of two MARS racks and a Planet 7 moves you deeper into interesting, modular territory.

Each of the MARS synthesizers connects to the Planet 7 using the supplied packs of 21 patch cables and, as before, a three-pin connector powers the expander from one of the synths. The system then offers dual oscillators, dual LFOs, dual filters, dual VCAs and dual contour generators, and to these the Planet 7 adds two rows of patch points, one for each synthesizer, plus... (take a deep breath) a multiple, a CV buffer and a mixer with positive and negative polarity outputs, a sample and hold, a DC shifter and a slew generator, four attenuator/inverters, a ring modulator, a noise generator, and two quarter-inch buses for connecting the outputs to conventional audio equipment. Phew!

A comprehensive SMS system with three MARS synths, a BOB and a Planet 7 expander moduleA comprehensive SMS system with three MARS synths, a BOB and a Planet 7 expander modulePhoto: Mike Cameron

Many of the additional modules are very welcome. I was pleased to see the slew generator (what would a synth solo be with portamento?), sample and hold, and the noise generator (which simultaneously offers red, pink and white noise). In addition, the attenuator/inverters are a godsend, because they overcome problems such as the aforementioned over-modulation. Just direct the output from the LFO to an attenuator, chastise it severely with the knob, and then send it back where you want it to go. On the other hand, there's only a single one-into-two multiple, and if you've used this to overcome the filter-tracking deficiency, you can't direct the tamed LFO to more than one destination unless you use the single one-into-two CV buffer. Or, if you use the buffer and multiple to control the two synths from a single CV and Gate, you can't overcome the filter-tracking deficiency or direct the tamed LFO to more than one destination. Arghh!

Now, as anyone who owns a Korg 800DV (an unusual monosynth you can read about at will tell you, having two independent monosynths responding to a single CV and Gate is very different from having a dual-oscillator instrument with a single signal path. To demonstrate this, I patched the pitch CV output from my SH101 to the Planet 7's CV buffer in, and then directed the output to the VCO CV inputs on each of the synthesizers. Likewise, I took the Gate from the SH101 to the Planet 7's multiple and directed the output to each of the MARS' Gate inputs. I could now patch the two synths independently to create composite sounds. For example, I created a fairly uninspiring bass patch on MARS number one and enlivened it beyond description by adding a filter-swept sawtooth patch tuned a fifth higher on the MARS number two. You can't do that on most integrated synths! I also liked layering similar timbres to create warm solo sounds and patches that would be ideal fodder for sampling into rich, polyphonic pads and ensembles. As for what I started to achieve when I began cross-patching between the MARS synths... well, I'll leave that to your imagination.

The Complete System

Finally, I hooked up the complete system with all three MARS modules, the BOB and the Planet 7. To say that this was capable of a huge range of sounds would be an understatement, as it would be for any patchable three-oscillator synthesizer with three independent low-pass filters, three independent VCAs, three independent... well, you get the picture. I liked using one MARS as a master generator in sync'd patches, whereupon I could obtain different 'sync' characters by varying the amplitude and waveform of the master wave. I also liked using two envelope generators, one connected to the logarithmic CV input and one to the linear input of a single VCA, to create more complex contours than one could obtain on any non-modular analogue synthesizer.

However, it was while creating sounds in this fashion that the lack of multiples and mixers became most apparent. SMS needs a module that provides a selection of these, or the system will never reach its full potential. To overcome this, I placed the review system alongside my RS8000 Integrator and used its complement of multiples and mixers to augment the patching on the SMS products, and everything leapt to life, with complex modulation, filter tracking, and all the other tricks that you would want now available.


If I had the ability to improve these SMS modules with just a click of my fingers, I would make three changes that would not alter their characters one iota. Firstly, I would make the internal modulation routing controls much less sensitive. Secondly, I would add a centre detent to all the bi-polar knobs because, currently, it can be quite difficult to find the 'zero' position. Finally, I would find a way to move the on/off switches to the front panel. I realise that there is no room for them, and that squeezing them in would destroy the careful balance of the control layout, but I hate back-panel mains switches on rackmount equipment.

These relatively minor criticisms notwithstanding, the MARS, at £250, is a bit of a bargain. At £750 for two MARS synths and a Planet 7, the combined system is a lot of a bargain. OK, it's not fully comprehensive, but since I carried out this review the company have announced a further five modules that extend its functions considerably. Unfortunately, there's no mixer/multiple rack, and I still feel that this is a necessity if the existing (and the new) modules are to reach their full potential. With any luck, this will appear in the not-too-far distant future.

In conclusion, I like these products. They are far from run-of-the-mill, the MARS synth offers a unique form factor, and the whole modular/semi-modular/integrated approach is an excellent concept. Finally, I like the care that has gone into the designs and (hand-built!) manufacturing. Andy Pledger is clearly devoted to his synths, and it shows. 


  • Well engineered, sturdy and nicely presented.
  • They can be used as integrated synths, semi-modular synths, or 'pure' modular synths.
  • A unique offering as far as I am aware.


  • A lack of sufficient mixers and multiples.
  • The patch points are on rear without a BOB or Planet 7.
  • The on/off switches are always on the rear!


We should congratulate SMS on bringing something different to our rather jaded analogue world. The technologies in these products are not new, but the idea of placing this type of system in 19-inch rackmount units is a good one, and the ability to isolate the modules within each synthesizer to allow them to be used as 'pure' modular synths, or hybrids of modular and semi-modular, is excellent. At these modest prices, they deserve success.


See 'Products & Pricing' box.

+44 (0)1767 681369.

+44 (0)1767 681369.