Cherry Audio
Cherry Audio

Care and Feeding of Cherry Audio’s Pro Soloist by Mark Barton

No one knows the new Pro Soloist virtual instrument better than award-winning developer and DSP designer Mark Barton. Join him as he takes readers beyond the initial 30 presets, lifts the lid on what makes it tick, and shares programming tips not possible 50 years ago!

Care and Feeding of Cherry Audio’s Pro Soloist

by Mark Barton

INTRODUCTION

The ARP Instruments Pro Soloist is a synthesizer tour-de-force, an absolute monster (inside), though you might not think so when you look at its front panel or take inventory of its sonic offering. It is unique among synthesizers in its feature set and electronic design, and no other synthesizer before or after (with the possible exception of a fully equipped ARP 2500, E-mu Modular, or Eurorack system) could recreate its specific tones.

I won't go into why its electronics are so different here, but interested parties can read through the service manual, revealing all. In short, the VCO is a high-frequency, double exponentially driven, frequency-to-voltage feedback closed loop design with the sawtooth derived via a Walsh-Hadamard transform. 'Nuff said. The design is absolutely brilliant and completely unconventional.

No other synthesizer of its day can shape tone color like the Pro Soloist. It has 12 filters switched in and out according to the preset, selective audio routing, and several musical performance modulation options. This "luxury vehicle" has a problem, however. You can only drive through the 30 factory-programmed destinations, most of which are attempts at imitative synthesis of orchestral instruments. It's a beast in a cage, but don't worry: we've unlocked the cage, and the monster is finally out.

The monster can now go anywhere, but the original 30 presets are always available as great jumping-off points to begin programming custom tones. There are also new additions to the architecture and slight changes that make the plugin more capable than the original hardware. However, it is very much a Pro Soloist, and we have not strayed very far.

WHAT'S DIFFERENT?

Here is what we changed and added to Cherry Audio's Pro Soloist:

  • Polyphony

  • Program and save your own sounds

  • Made it sound huge with splits, layers, and the ability to apply fine-tuning

  • Added Velocity capability to VCF and VCA.

  • Provided full polyphonic aftertouch and last-note aftertouch features

  • Added VCF routing from all resonators as opposed to just a few

  • Added the ability to tune the resonators and adjust their Qs (this is huge)

  • Added the ability to select resonators individually rather than in groups

  • Added the ability to mix SAW and PULSE into the resonator banks as opposed to Pulse only

  • Added selective audio phase inversion to the VCF

  • Added ability to apply oscillator VIBRATO as well as filter TREMOLO simultaneously

  • Made the VIBRATO / WAHWAH / PWM / TREMOLO onset DELAY variable

  • Added VCO pulse width modulation

  • Added VCA tremolo

  • Added SUPER WAVE to VCO for thick timbres

  • Added variable capability to the "dynamic pulse" feature

  • Doubled the range of the keyboard TRANSPOSE control

  • Added a Modulation Matrix 

  • Added an Arpeggiator and studio-quality Effects

These changes complete the feature set, plug a few holes in the design, and add substantial new capabilities. The instrument is not only opened up for programming, but everything is more orthogonal and intuitive than the original architecture. Even though that list may look extensive, it's still very much a Pro Soloist.

WHAT'S UP WITH ALL THOSE FILTERS?

The Pro Soloist has ten fixed 2-pole bandpass filters (the Resonator Banks), a variable highpass filter, and a 4-pole Moog-style ladder filter. It's all these filters that set this instrument apart. However, the original Pro Soloist only used a maximum of five resonators simultaneously. This is because the filters were not variable, and each different tuning required a separate resonator circuit. 

Cherry Audio's Pro Soloist has five resonators which are all tunable over the full audio range and have variable resonance. This provides the same capability to produce all the stock presets and full programmability to take these resonators anywhere you want. You can use none, one, or all the resonators and route each with a variable level to the VCF and/or directly to the VCA.

I coined a phrase quite some years back, "Doing subtractive synthesis with one filter is like doing additive synthesis with one sine wave." While not entirely true, I've always felt that subtractive synths could do way better if they had more filters to play with, so here you go. Most of the Soloist's personality is found using these resonators.

The resonators provide rich character and detail to a sound, and some are used in most presets. They provide formants and body resonances for more realistic simulations of physical instruments. The CELLO preset is an excellent example of this. You can't get that timbre from a Prophet. Some resonators are set to fairly wide bandwidth, and some are narrow and sharp sounding. You can also use them for EQ purposes. Boost bass or add sizzle to taste.

The highpass filter is a simple, variable 1-pole affair mainly used to cut the low end for realism's sake. It should not shake the room if you play the ENGLISH HORN down a couple of octaves from its normal range. The highpass filter output feeds directly into the VCF.

Early Pro Soloists caused a patent dispute with Moog because the filter directly copied their design. Consequently, ARP had to design something different, and this later filter is generally regarded as inferior. As a result, the early model is the most sought-after and the one emulated in Cherry Audio's Pro Soloist. Even though these filters are capable of self-oscillation, the Pro Soloist never takes advantage of that, and none of the presets come close to that level of resonance. Therefore, to keep within the sound-space of the instrument, the RESONANCE has been constrained to the maximum amount one experiences with the original and does not oscillate.

A BRILLIANT JUGGLING ACT

Ah, the BRILLIANCE slider. The bane of my existence. Getting this to behave correctly across the stock presets was by far the most challenging thing to get right. Just when I thought I had a preset matching the hardware perfectly, a move of the BRILLIANCE slider proved me completely wrong. It is possible to get a sound exactly the way you want it, only to find out the slider does nothing or way too much. Here's why: the BRILLIANCE slider does not apply its own control voltage to the VCF; instead, it's an envelope attenuator. The juggling act involves getting the filter cutoff, envelope amounts, keyboard tracking, and BRILLIANCE slider all to the right places. This is a four-variable problem and it's difficult. Of course, when you're programming your own sounds, there is no right or wrong, but you should always test the range of the BRILLIANCE slider to ensure it traverses a meaningful range.

As mentioned, the BRILLIANCE slider does not control the VCF directly... kind of. It is connected as an attenuator to the output of the envelope generator ADSR + AR mix. It ranges from 100% down to about 30%. With this control, you cannot attenuate the envelopes down to zero. The "kind of" refers to the fact that the envelope generators are not alone in this mix. There is also some positive voltage mixed in, which means that when the envelopes are at zero or turned down completely, there is still some voltage remaining. This enables the BRILLIANCE slider to still have some effect. Sneaky. Some presets do not use the VCF, so the BRILLIANCE control has no effect in those cases. I imagine this was a source of confusion for many users of the original Pro Soloist.  

GROWL

I know of no other synth that features GROWL. It emulates the… well… growl heard on instruments like trombones and tubas, and is featured prominently on those presets. A fairly complicated circuit produces the growl signal, runs at 32Hz, and modulates the VCF cutoff. It kind of looks like a negative half-wave rectified sine wave. It can be applied to the VCF either statically by using the GROWL knob on the VCF, or by using the GROWL TOUCH control performance switch, which gives you pressure-sensitive growl.

THE LFO

The instrument has one LFO, used as the source for VIBRATO (VCO FM), WAHWAH (VCF FM), VCO PWM, TREMELO (VCA FM), and the REPEAT function. REPEAT, when engaged, fires the ADSR envelope at the LFO rate. A variable DELAY is provided for delayed VIBRATO, PWM, WAHWAH, and TREMELO. In polyphonic mode, this delay is per-voice, even though the LFO is common to all voices.

Switching on the VIBRATO touch control removes any VIBRATO programmed into the patch. This allows you to take control of the vibrato by applying pressure. The same is true for the other effects, except REPEAT.

PROGRAMMING TIPS

- First, gain structure is critical, not because of clipping or signal-to-noise reasons, but because of timbre issues. This is mostly true when resonators go directly to the VCA, and the VCF is also enabled via the highpass filter. Use the LEVEL controls in the resonators and HPF to control the relative gain. Don't worry about making them too loud or too soft. Get the balance right for your desired tone color, and then correct the overall level with the GAIN control in the VCA. Also, don't try to balance the level between patches with the VOLUME slider. Try to keep that slider in the same spot across all your patches.

- As you will discover, the added phase inversion button on the VCF is a big deal. There are two audio paths into the VCA: one from the VCF and the other from the resonator banks. This creates all kinds of frequency selective phase reinforcement and cancellation effects. Engaging the phase button inverts the VCF and effectively doubles the number of available timbres. Relative gain, mentioned above, also affects this. If the VCF is the only source into the VCA, this button does not affect the sound.

- The Pro Soloist is all about pulse width. The starting point of most of the stock presets is a fixed pulse width. Every different width has a unique harmonic distribution evocative of a particular instrument category. 

For example, 50% sounds like a single reed, and 10% sounds like a double reed. The presets leverage this phenomenon for all it's worth. So unless you are using the sawtooth only, the selection of pulse width is a good starting point.

- If you turn up the ADSR or AR pulse width controls on the oscillator, you will engage the "dynamic pulse width" feature, especially on display in the two FUZZ GUITAR presets. How about that, fuzz guitar without using distortion! If you're going for aggression, try it out. A little goes a long way.

- Turning on the WOW touch effect turns off the RESONANCE control on the VCF because it sets it to a fixed high value.

- Looking over the stock paddle presets will give you a good idea of how various timbres are accomplished. You may notice that every stock preset has the SAW/PULSE panning control always set completely to PULSE. This is because the capability to route the sawtooth wave through the resonators does not exist on the hardware, so new timbres await!

- Regarding CPU usage, don't just leave a resonator on and turn its level down. Disable anything you're not using, including voices (set in the pop-up of the Poly button), an unused Layer, and unused Effects.

FINAL THOUGHTS

Some may claim that Cherry's Pro Soloist doesn't sound like the one they have at home, and they'd be right. That's because no two Pro Soloists sound alike. Component tolerances are such that the resonators (which create most of the unique personality) vary significantly from unit to unit. We verified this with multiple Soloists. If you have the patience, you can tweak everything to duplicate your own unit because we have opened up every parameter for programming.

Finally, there is a great temptation to bathe sounds in effects and thick reverb. In my opinion, doing so veils the Pro Soloist's true nature and can make it sound a little like everything else. This box is all about pure, clear tone color ranging from beautiful and mellow to face-peeling nasty, so please get out your expressive crayons.

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Check out Mark's custom presets for Cherry Audio's Pro Soloist, demonstrating many of these concepts and more. Just type "mrb" in the search field of the Preset Browser to display his 23 presets.