Analog synthesis has gone through its own unique evolution. Its primordial roots extend back as far as the late eighteenth century when people were beginning to get a grip on why feathers stuck to a glass tube after it had been rubbed on a piece of fabric. Things gathered steam in the late nineteenth and early twentieth century as vacuum tube technology was born. Through the thirties, forties and fifties, analog synthesis became something bright, shiny and new as development overtook development. New development reached its peak with the popular and commercial offerings of early synthesizers designed by the likes of Buchla, Moog and Bode in the early to late sixties.

During this period all of the basic ingredients for analog synthesis were invented. That's not to say things stopped there; it just means we have been handed a quite complete set of building blocks for implementing analog synthesis and, for anything truly different to come about, we need simply to arrange these elements in new and exciting ways.

This applies to all electronic analog synthesis equipment, including electronic analog drum instruments as well. After all, an analog drum synthesizer is really just a task-specific analog synthesizer. Even the simplest analog drum synth typically contains some form of oscillator and some variation on a method to control the amplitude of the oscillator over time to simulate both the impact and physical resonance of the particular percussive instrument being synthesized.

The simplest form of drum synthesizer is a filter at near self-resonance that is unbalanced briefly by an electrical pulse, causing it to "ring" for a short period of time before the oscillations are fully damped. Other designs use more sophisticated methods that employ some sort of noise, a function generator, a VCA, pulse processing, envelope generators, and oftentimes simple filtering to form the drum voice the design will generate. A percussion synthesizer must also have some sort of input telling it when it has been "struck". Simply put, a typical percussive voice will simulate the impact of one object striking another, and it will simulate the harmonic content of the struck object's physical vibration as a result of the impact. The analog circuitry for accomplishing these tasks have long since been in existence; in order to develop a better analog drum synthesizer, all that remains is refinement of the process.

For over thirty years Thomas Henry has designed a lot of analog synth modules and effects, the diversity of which reflects his many interests in all aspects of analog synthesis. Of course, among these interests, is the analog drum voice. In fact, he's written a thorough book on creating analog drum voices, from very simple "Twin Tee" designs through very sophisticated voltage controlled models. This book, "The Electronic Drum Cookbook", which I highly recommend, is available through Magic Smoke Electronics.

All of this blather leads to the subject of this page, a little device dubbed the "Mega Percussive Synthesizer" or "MPS" for short. The MPS is one such instrument I alluded in my preceding notes in that the elements of it are all part of the established pantheon of analog synthesizer ingredients handed down to us from the pioneers who preceded us. These carefully selected ingredients are combined and uniquely implemented in the MPS to create, literally, a monster of a percussion synthesizer. And - being designed by Thomas Henry - the elements of the MPS are derived ecomomically and with great thought to topology. This translates into a wonderfully versatile and unique percussion voice module without presenting a prohibitively complex build process for the average synth DIY enthusiast.

The MPS is truly an inspired design. One very important element of the MPS, the impact generator, was derived directly from a modular percussion patch by one of the great synthesizer masters. In fact, the MPS contains three seperate tunable oscillators. In addition to the oscillators, the MPS utilizes a ring modulator, it has a noise generator, a voltage controlled resonant filter with two selectable responses (low pass or band pass), three VCAs, and a mixer section for mixing all of the elements together to fine tune a very realistic percussive sound.

The MPS will deliver anything from a convincing snare to toms, to bass, to a sizzling high hat, to a glorious cowbell. When Christopher Walken demands "more cowbell", the MPS can deliver. Not only can the MPS deliver standard percussive sounds, it can easily be tuned to the "out of this world" percussive sounds. It can easily supply voices ranging from the electronic disco drums used in modern day hospitals to empty stomachs of their contents to the sound of an alien craft taking off with each drum beat. In fact, at the flick of a switch, the MPS will transform from a percussive synthesizer to a full-on sustained..sound..generator capable of fully rendering the Neptunian Planetary Anthem in full four part Neptunian harmony.

What does it take to build this thing? Nothing exotic or hard-to-get. In fact, it requires only a total of six ICs and a handful of other parts. And pots...lots of pots, because the MPS provides *lots* of control of the voices it will synthesize. Oh, yes, there is some amount of switches. And connectors. One thing you will save on is trimpots, because there is nary a one in the design.

The MPS is an incredibly versatile device - suffice it to say it would take an extremely large body of work to faithfully illustrate the range of percussive instruments (and other sounds) it can produce. The best I have to offer at this writing is a sort of composite of out-takes from various examples and compositions I've recorded. It can be downloaded by clicking the above link.

To help to follow along, I've written the following guide to the sample:

00:00 - The sample starts out with the first recording I made of the MPS. The MPS is being triggered by a square wave LFO and the output is run through a delay. As this portion progresses, I tweak a number of controls slightly for variation. It's a good example of the "oomph" factor of the impact circuit.

00:54 - I fade in a portion of a sample I made using the MPS in locked mode. The Shell VCO is controlled by the Klee sequencer. It starts out with the MPS noise through its filter and I use the MPS mixer to mix in the ring modulated shell output.

1:35 - This is the beginning portion of a Klee composition that used the MPS as a cowbell. Towards the end of this section I used the mixer control to bring in the noise as a sort of additional percussive voice.

2:14 - Another portion of a Klee composition. Here, the MPS is providing the sort of grainy cymbally/high hat sound.

2:30 - Yet another Klee composition fragment - here the MPS is acting as a sort of cross between a low floor tom and a bouncing basketball.

3:07 - Once again, a portion of a Klee generated sample. The MPS is providing the menacing high tom - the shell is modulated by the impact oscillator.

3:33 - In this frenetic Klee fragment, the MPS is controlled by a voltage output of the Klee to provide a range of high tom pitches. Bass drum is a UD-1.

4:00 - Hmmm....more Klee. I suppose the MPS sound could be best described as some variation of a timbale perhaps?

4:27 to end - This is a sample taken by tweaking the controls while the MPS is being triggered and all of the envelope generator decays are set to max. It's sort of like being in locked mode, only the envelope generators are still triggered (unlike like mode, the sound would eventually fade away once the triggers stopped).

Before getting into the nitty-gritty of the MPS, a bit of review of exactly which elements make a percussive voice a percussive voice is in order. This is only a light treatment placed here in order to give the sections of the MPS context. For a more in-depth treatment, I highly recommend Thomas' book "The Electronic Drum Cookbook", available at Magic Smoke Electronics.

Typically percussive sounds are very brief, lasting anywhere from a fraction of a second to a few seconds long. But, in that brief amount of time, a lot of information is transmitted to the brain. This information tells your brain if you are hearing a clave, a cowbell, a snare drum or a bass drum, for example. This information is remarkably detailed - it tells the brain more-or-less if the struck object is solid or hollow, or if it is large or small. It gives information that allows you to perceive what the object may be made of, be it wood, glass, skin or metal, and it imparts at the same time some modicum of intuition as to what object was used to strike the percussive instrument - whether it was a stick, mallet, a fist or the palm of the hand. This information allows you to discern if the object was struck forcibly or softly. That's a lot of information your brain picks up and processes in a very short amount of time.

This leaves a fair amount of work to the synthesist - just as there is a difference between simulating a basoon and a baboon, it takes just as much consideration synthesising the difference between a snare drum or a cowbell, only you have to pack that information into a tiny slice of time. As with any synthesized sound, there are certain elements that comprise the sound - pitch, loudness, and harmonic content, and the change of these three parameters over time.

So, what gives a percussive instrument its unique sound? First of all, the typical percussive instrument has to be struck by some object in order for it to make any type of sound. So, we can deduce right away the instrument must produce the sound of that intial impact - the stick striking the drum head, for example. The second part of the percussive sound is what happens after it is struck - if it's a hollow drum, the sound will resonate for a comparatively longer time than if it is more of a solid object, such as a woodblock. If the drum has a skin that is struck, like the snare or tom, that skin actually stretches for a brief time, which causes a short bend in pitch as it is struck. If the percussive instrument is metallic, it may be expected to produce some dissonant harmonics, or "pitched noise" as it rings. And, if there are snares for example, the drum will produce a wider set of frequencies more closely resembling classic white or pink noise.

So, we can break the percussive sound down to three general ingredients: Impact, Shell (the body of the instrument) and Noise. Not every percussive voice will use all three elements, but they will all use at least a couple of them.

Impact

Impact is the element of the percussive sound derived from the drum stick, hand, mallet or other object striking the percussive instrument, and therefore forms the intial portion of the sound itself. The information contained in the impact portion of the voice not only suggests the type of object striking the instrument, but also the type of surface that is being struck - a skin, wood, metal, etc. The pitch of the impact will vary from instrument to instrument. The impact envelope is generally of much shorter duration than the shell envelope. Most percussion synthesizers actually derive the impact portion of the percussive sound by filtering the pulse that is used to initiate the drum sound - the pulse may be filtered to eliminate, for example, the higher harmonics. In other words, it may be made "brighter" or "duller".

The MPS does not use this method. Instead,the MPS uses an actual tunable swept pulse wave oscillator to generate the impact portion of the percussive sound. The impact oscillator was inspired by a patch described by the great Roger Powell in which he actually dedicated a VCO to generate the impact portion of a percussive sound patch. This method creates a much more realistic impact effect, and is one aspect, if not the most important aspect, responsible for the sheer realism in percussive sounds the MPS is capable of generating.

Shell

The effect of an impact to the physical body of the percussive instrument is simulated by the shell parameter. A physical percussive instrument will typically resonate, ever so briefly, or even not-so-briefly, when it is struck. The frequency, duration and harmonic content of this resonance is determined by the size, shape, and materials that make up the body of the instrument. As an example of the electronically simulated sound of a briefly resonating object, consider the woodblock-like ticking of the atomic clock on the shortwave WWV stations - that sound is actually made up of a five millisecond burst of either a 1,000 Hz sine wave or a 1,200 Hz sine wave (depending on the location of the transmitter).

The drum heads of many percussive instruements, such as bass drums, toms, snares, etc, will actually flex as they are struck - this causes the pitch to intially bend up briefly as the head is struck. The shell section of a percussion synthesizer typically provides variable amounts of pitch bend. This bend is usually more subtle in standard physical drums, but can be exaggerated, as in the previously mentioned electronic disco drums. If this parameter is set for an excessive pitch bend, you will be transported to the land of Donna Summer at Studio 54, which you may or may not want. The MPS utilizes a voltage controlled, tunable swept oscillator as the shell oscillator.

What I sometimes think of as a sub-function of shell is something Thomas refers to as "Clank" - this is a short duration of a harmonically rich signal and is vital for the metallic, hollow percussive instruments such as cowbells. The effect of clank is to give a sort of tuned, metallic sound to a percussive voice. The MPS generates this parameter through use of a separate tunable oscillator ring-modulated with the shell oscillator.

Shell envelope durations are typically, but not necessarily, of a longer range than impact oscillations. The MPS provides a wider envelope generator time range for the shell envelope generator than the impact envelope generator.

Noise

Noise tends to get a bad rap. Very few people make it through their adolescence without at least once hearing an irritated adult loudly directing one to "Turn that noise off"; specialized headphones are manufactured to cancel out ambient noise for those unlucky enough to have to deal with traveling by aircraft on a regular basis; all types of audio devices will invariably brag of some sort of low noise specification or other. Yet, here you are considering building a device that purposely inserts noise into the sound it produces.

The subject of noise can run very deep - I've read dissertations and explanations of noise that range from the very technical to the quite philosophical, but the long and short of it is anybody who spends much time inside an anechoic chamber could tell you that your life would be close to unbearable without a bit of noise here and there. Our brains use noise at often a subliminal level to constantly process our surroundings. For example, with the introduction of digital technology to cellular phones, the cell phone providers realized that they actually had to insert what is known as "comfort noise" into the signal - the noise of analog technology had been completely removed, and the manufacturers realized that without some sort of audio cue, their customers had a difficult time determining if they were still connected to the party at the other end of the connection.

Noise plays an important role in the brain's interpretation of a percussive sound. As an example, the snares of a snare drum will emit "wideband" noise, similar to the noise a radio produces when tuned between stations, but only for a very brief time. This simple burst of information tells the brain it is, in fact, listening to a snare drum. Perhaps the burst of wideband noise is even much shorter - short enough to represent the impact phase of striking a percussive instrument. Here, the noise would be delivered in a very short, yet identifiable burst produced by a wooden stick glancing against a skin surface.

One reason the sound of striking a cymbal or high hat differs from the sound of striking a bell is that the overtones generated from striking a bell are fairly constant in frequency and "purity", whereas a cymbal will generate a series of overtones that more resemble noise. This clangorous sound can be duplicated by frequency modulating an otherwise pure tone with a noise signal. This is one method of producing "pitched noise" - a noisy signal that appears to have a general pitch, such as the noise of a vacuum cleaner or a trash can slammed to the ground at 6:00 AM on the morning you get to sleep in late.

When the "static" noise of a radio tuned between stations is passed through a bandpass or lowpass filter, the output of the filter may resemble perhaps wind, surf or thunder as the cutoff frequency of the filter is varied. When the filtered noise is not sustained, but instead is heard as only a very brief burst, it sounds instead as some sort of physical impact.

The MPS provides a white noise source that is passed through a resonant voltage controlled filter that is switchable between low pass and band pass responses. As with the static "hiss" example above, the filter serves to remove a portion of the noise in order to mold it to the purpose at hand. Through control of the filter's resonance, a specific range of specra may be emphasized. One obvious application of this section of the MPS would be to reproduce the effect of snares. Another use of the filtered noise might be to enhance the impact by providing a brief burst of noise spectra - lower noise frequencies may indicate a larger instrument whereas selectively higher spectra indicate perhaps a smaller instrument. The filtered noise envelope may be lengthened while emphasizing the higher frequencies. This signal could then be used to modulate the shell VCO to turn the "clean" shell signal into a shimmering rendition of a nice open high hat or cymbal.

The frequency content of a percussive instrument can vary over the duration of its voice. This applies to the noise component as well. When first struck, there may be a predominantly larger amount of higher spectra that diminishes as the voice reaches the end of its envelope. Therefore, both the center frequency of the filter and loudness of the VCA the filtered noise passes through are controlled by the noise envelope generator. The noise envelope generator has the same time constant as the shell envelope generator (the impact envelope generator is the only envelope generator that has a purposely shorter time constant). This arrangement allows dynamic noise frequency content over time, and also controls just how long the noise portion of the programmed instrument will last, thus helping to define the instrument.

Before plowing into the MPS circuitry, here's an opportunity to download the whole shebang in one zip file. This file contains the block diagram, schemata, and parts list in PDF format for easy printing. Click on the image above to get it.

A quick run-through of the block diagram will help to both recognize what the MPS does and, at the same time, recognize some of the elements that make it unique in the world of percussion synthesizers.

Starting at the left, note the trigger input section - it not only contains a control to set the trigger level required for triggering the MPS, but also a switch (lock) that will set all of the envelope generators high, thus opening the VCAs, which transforms the MPS from a percussion synthesizer to a sound synthesizer of alien proportion.

From there, one can see that the trigger signal is used to simultaneosly fire three envelope generators. Each envelope generator has a decay control to determine the duration of the envelopes' fall to zero level. Note that the impact generator (the center envelope generator) is purposely set to provide a shorter time constant than the shell and noise envelope generators.

The top section details the mechanics of the shell generator. First of all, it is obvious from the CV input that the Shell oscillator is a voltage controlled oscillator - the pitch can be controlled by an external voltage input. This is wonderful for creating an entire set of pitched toms, claves or (insert instrument here) out of just one MPS. For example, a sequencer can be patched to control the drum hits and pitch of each drum hit.

Notice that the CV input is normalled to a switch - the "Noise/Impact" switch. This feature allows the Shell VCO to be modulated from either the impact oscillator or the filtered noise when a plug is not inserted into the CV input. Modulating the shell VCO with noise allows one to set up some nice sizzling cymbal/open high-hat type voices, and selecting the impact oscillator allows variation of the impact timbre by briefly modulating it with the impact oscillator output.

Moving to the right is the Range control, which controls how much, if any, effect the CV, noise or impact oscillator will have on the pitch of the Shell VCO. Note that the Shell VCO has two other inputs routed to it. One is the sweep input, which allows a controllable amount of the Shell envelope generator to bend the pitch up (providing anything from realistic acoustic drum effects to space-age-disco-era electronic drum effects), and there is a pitch control to allow the initial pitch of the shell VCO to be set.

The output of the Shell VCO goes to one input of a balanced modulator, or "ring modulator", which brings us to another feature of the MPS - its ability to clank and clang is derived from ring modulating the Shell VCO with another VCO, which happens to feed the other input of the ring modulator. The ring modulator can be balanced or unbalanced, permitting more or less of each oscillator's tone to "bleed through" the ring modulated tone, allowing even more variation in timbre. This section is absolutely indispensable for creating those solid metallic sounds of the cowbell family in addition to a red shirt getting the old phaser-on-kill effect when the MPS is in locked mode.

Moving to the middle section of the block diagram unearths what really makes the MPS stand out, in my opinion - the Impact Oscillator. The Impact Oscillator is a VCO that provides a pulse wave output. It's designed, when not in lock mode, to provide the very short impact burst that is traditionally derived from the trigger pulse. As mentioned before, rather than filter the trigger pulse, this oscillator actually provides a very short tone burst that can create truly gut-wrenching impact effects. It too has controls that allow its pitch to be swept by the Impact Envelope Generator in addition to a control which allows the initial pitch to be set. The Impact VCO is then fed through a VCA which is also controlled by the Impact Envelope Generator.

At the bottom of the block diagram is the noise section. The noise is fed through a noise filter, which can be selected to have a low pass response or a band pass response. There is a control available to set the resonance of the filter. The filter can also be swept using the Noise Envelope Generator, and the initial cutoff or center frequency can be set with the center control. The filtered noise passes through a VCA which is controlled by the Noise Envelope Generator.

All three sections are then sent to a set of output jacks that allow the signal to be tapped and further externally processed. These jacks are normalled to the mixer input jacks, so that if no plug is inserted in the jacks, the signals will then go to the mixer section.

The mixer input jacks allow a signal to be inserted into the mixer, such as if the noise signal were to be routed out through a phase shifter and then be reinserted into the MPS output, for example. These signals are then mixed to produced the MPS output. The MPS output can be tapped as either a direct output at synth level, or a lower level for direct insertion into an amplifier, for example.

Here's a description of Sheet 1 in Thomas Henry's own words:

"The MPS has been designed to be fired by a trigger, following the usual synthesizer standard of 0 to +5V and 5 milliseconds wide. However, thanks to the "Sensitivity" control it is possible to accommodate a broad range of signals. The trigger is applied to jack J1, where it is differentiated by C3 and R94. IC6a is configured as a comparator and squares up the differentiated signal nicely. With "Mode" switch S1 in the "Triggered" position, R36 sets a threshold for the comparator. You can adjust this to cleanly fire the MPS on weak signals, and yet avoid false triggering on hot ones. The output of IC6a then goes to the three envelope generators in the module."

"If S1 is put into the locked position, then the comparator output is latched high. This keeps all of the envelope generators on, forcing the associated VCAs to remain open. You would employ this setting when wanting to use the MPS as a collection of sound generators to be followed by outboard analog synth modules."

"The remainder of Sheet 1 consists of the noise generator section; typically this is used to create explosions and snare sounds. The noise generator proper centers around Q1 and Q2. Q1 is configured as a back-biased diode within the amplification loop of Q2. The white noise output is AC coupled via C16. Since the noise characteristics of transistors vary somewhat, you might want to consider using a socket for Q1. This would make it easy to try out a bunch of different units for clarity and volume without all of the usual soldering and desoldering."

"The audio output at C16 is fed to a VCF configured around IC1a and IC1b. The signal applied to an OTA should be kept to a maximum of 20mV peak to peak; this is the purpose of attenuator R61/R11. If you find that your noise transistor is too hot or too weak, feel free to adjust R11 up or down accordingly and monitor pin 3 of IC1a for the 20mV value just mentioned. Of course, since this is noise we're talking about, you'll be looking for an average value on your oscilloscope."

"There's nothing too clever in the filter. This is simply a state variable unit and comes more or less straight from the LM13700 data sheet. It is moderately elegant in that it uses the internal buffers, thus cutting down on external op-amps. (Of course, this means that the signal must be AC coupled due to the negative offset created by the buffers, but here we don't care since the rest of the circuit is also AC coupled). C1 and C2 are the tuning caps for the filter. The mode can be set to either band pass or low pass by means of switch S2 which simply taps one of the two outputs. And the filter resonance is adjustable via R78. R46 keeps the unit from oscillating at the extreme, but depending on component tolerances you might still be able to make it howl, so watch the volume of your monitor amp until you get used to everything."

"The output of the VCF at S2 then is applied to the VCA, passing through C17. The VCA is built around IC2a. R66 and R9 attenuate the signal appropriately for application to the OTA, while the internal buffer manages the final output. This will pass through C18 and go to the mixer yet to be described."

"But let's back up to the envelope generator that controls both the VCF and VCA. The trigger signal, mentioned earlier, comes from pin 1 of IC6a and is steered by means of D1 to the envelope generator built around Q3. (All three envelope generators employ steering diodes to keep them from interacting). When the trigger is fired, D1 will dump a charge on timing cap C22, bringing the envelope generator high almost instantaneously. D1 prevents that charge from bleeding off backwards. The only path for the decay, then, is through potentiometer R95 which lets you set the time desired. Q3 buffers the signal and the final envelope is available on its emitter. The full strength signal (developed across potentiometer R37) goes directly to Q9 which is set up as a voltage-to-current converter. This then controls the VCA mentioned earlier. Keep in mind that all of these OTAs are current controlled devices. By the way, the VCA is linearly controlled, but the envelope generator has an exponential output, so things work out nicely sonically."

"R37 also lets you tap off a variable amount of the full envelope to modulate the VCF. We need a more precise linear control here, and that is the purpose of putting the voltage-to-current transistor Q8 in the feedback loop of IC6b. R79 lets you set the center of the filter sweep, while R37 dials in the amount of sweep desired. The output current from the collector of Q8 feeds the control current inputs of the two OTAs. Note that these are simply ganged. According to the data sheet for the LM13700 the individual units within the dual device are sufficiently well matched where we can get away with this (especially in a drum circuit). By the way, it is traditional to put in some sort of reverse voltage protection diode in the current source, but since this is a closed system none is required here. Be very careful if you start dinking with the design."

Moving on to Sheet 2, Thomas writes this:

The CMOS 4046 is employed as a current controlled square wave oscillator here. Just so you know, when using the 4046 as a voltage controlled oscillator (which is the way most people blindly follow the data sheet) the sweep range is extremely limited. But by jettisoning the usual arrangement and going current controlled we obtain several orders of magnitude more range. Here are the particulars. The usual voltage input at pin 9 is locked on +15V and left there. But pin 11 (which would normally be a resistor to set the base frequency) is now used as a current input. C14 between pins 6 and 7 is the timing cap. The square wave output appears at pin 4 of IC4, and is AC coupled before going to the VCA built around IC2b. The VCA is much the same as the one mentioned above.

The trigger signal from Sheet 1 comes here via steering diode D2, and thence to the envelope generator configured around Q4. This is virtually identical to the one in the noise section, but note that the timing cap (C19) is much smaller now; the secret to making a superior impact sound is to sweep the VCO and the VCA rapidly.

The envelope generator voltage is buffered by Q4, and then converted to a current by Q11 which then controls the VCA. Again, the response will be a musically useful exponential one since the envelope generator is exponential and the VCA linear.

A variable amount of the control voltage can be tapped off of R38; this is the "Impact Sweep" control. Potentiometer R80 sets the basic frequency of the sound. These two control voltages are combined by means of a quick-and-dirty mixer and then applied to the inverting buffer Q10. Finally, the voltage is converted to a current which feeds pin 11 of the 4046.

Not wanting to waste anything, notice how the internal XOR gate of the 4046 is pressed into service as an LED driver. The trigger signal from Sheet 1 is steered via D3 to the pulse stretcher made up of C20 and R88; this will guarantee that the LED lights long enough to be seen. By tying pin 14 of IC4 hot, the XOR gate is transformed into an inverter. Finally, when the output at pin 2 drops low, the LED will illuminate. Drawing current, as used here, is far less taxing on CMOS than supplying current.

For Sheet 3 of the schematic, Thomas describes the operation thusly:

Moving on to Sheet 3, we find the circuitry for the Shell Generator. This is essentially a VCO, control oscillator, ring modulator, envelope generator and VCA, all in one! Let's see how it works.

The control oscillator is formed around OTA IC3a. This is perhaps the simplest VCO circuit you'll ever see; I ran across it in the data sheet for the LM13700. C13 is the integrating timing cap, while D4 and D5 along with R18 and R32 set the trip points for the comparator. The control current input is at pin 1. Because this is such a simple circuit, the triangle output at pin 8 shows some pip distortion. But that is of no consequence here since we will be using this to deliberately create clangorous sounds with the ring modulator. Q12 acts as a voltage-to-current converter, and R81 sets the audio frequency of the thing.

The output of the control oscillator is AC coupled via C25, and can be tamed with R83, the "Ring Mod Depth" control. This then goes to a mixer composed of IC6c and associated components. Also mixed in is a voltage from R82 which is the "Ring Mod Balance" control. This lets you balance or deliberately unbalance the ring modulator for an immense range of weird clangorous sounds. Note too that the mixer biases the combined signals up appropriately by means of R89. We do not want any negative voltages being applied to the ring modulator input of the XR-2206 at pin 1. Again, this is a closed system and perfectly safe. But do not think about injecting external control voltages here without supplying additional reverse voltage protection.

To get maximum range, we ignore the usual arrangement of the XR-2206 (IC5) and instead configure it as a current controlled oscillator. The timing cap is C15, bridging pins 5 and 6. Pin 7 is the control current input. Pin 8, the alternate current input for FSK work, is unused. Only the triangle wave is needed here, so all of the sine wave pins, 13 through 16, are ignored as well. The three resistors and one capacitor connected to pin 3 establish the proper output bias and amplitude of the triangle wave.

The trigger from Sheet 1 makes it to D6 which then fires the envelope generator. This is identical to what we've seen earlier in the noise generator section, so nothing more need be said about it. The "Shell Sweep" potentiometer taps off a variable amount of the envelope generator and this is combined with a voltage from the "Shell Pitch" control. Note too that an external control voltage can be added in via J7. And as if that weren't enough, this is a patchover jack, and so the impact generator and noise source can also modulate the shell generator; more about that in just a moment.

The mixed control signals are then buffered by Q14 and converted to a current by Q7. The collector of the latter feeds pin 7 of the XR-2206. Observe that with the addition of J7 this is no longer a closed system. However, thanks to the way the voltage-to-current converter has been configured it is impossible for reverse currents to occur. Everything is safe, even in the hands of a knucklehead.

As alluded to earlier, the ring modulator comes along for free within the XR-2206. The output eventually winds it way to pin 2, and thence to the VCA built around IC3b and ending up at the mixer.

And, in conclusion, here are Thomas' words concerning the contents of Sheet 4 of the schematics:

Sheet 4 brings all three of the audio sources together. They are mixed by op-amp IC6d, but not before passing through a set of break-jacks. These are your typical loop send/receive sort of things that allow you to add in some external processing individually. For example, a very useful application would be to include some equalization, especially for drum work. But do remember that these are synth level signals, so don't blow up your line level equipment! The output of each generator, jacks J2 through J4, is a nominal 10V peak-to-peak maximum.

You might notice that access to the mixer bus is provided in case you wish to gang other sub-mixes here (through their own input resistors, of course). Speaking of which, R90 through R92 tame the three generators just a bit so that we don't pin the output when running all at full strength. The mixed output is available at J5 undiluted (10V peak-to-peak) and at J6 which can be brought down more closely to a level suitable for running to an amplifier, recorder or mixer.

Finally we now we see the "Shell Modulation Type" switch, S3. This neat idea from Scott Stites lets you modulate the shell resonance with either the impact generator or the noise source for even more spectacular percussive effects.

And there you have it, a whirlwind tour of the MPS. This was one of the more exhausting projects I've ever undertaken, if for no other reason than that the breadboarding and testing stage was a royal pain. Scott's aid was tremendous, from beta testing, double checking, constantly measuring things and just in general helping to keep the enthusiasm up. It took something like fourteen 12 hour days of working and reworking things, trying to hold the parts count down but without sacrificing versatility. I'd like to think that a good compromise was struck between complexity and elegance. And lastly, there were a number of unusual techniques employed throughout the MPS that perhaps had never been used together in a DIY music project before. It was a lot of work that I had hoped to make back some of my expense on (both time and money) but in the end, if other users are able to get some good out of it then I'll feel amply rewarded.

The MPS doesn't require anything drastically hard to get your mitts on. Perhaps the only mildly difficult item to find would be the XR-2206, and even they turn up with relatively little shipping.

In fact, all of the ICs can be found at Jameco.

More information on the MPS can be found in the MPS thread at electro-music.com:

The Thomas Henry Mega Percussive Synthesizer design is for personal use only and may not be published without permission of Thomas Henry or Scott Stites. Fair use only.

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