What’s New Never Became Old: Cherry Audio PS-20

Jim Aikin reviews Cherry Audio’s 2021 software take on the famous Korg MS-20 synth. (Hint: don’t miss the videos!)

1978 was a big year in synthesizer history. Leafing through my archive of back issues of Keyboard, I found the first ads for the Sequential Prophet-5 and the Oberheim Four-Voice. And then, in November, the full-page ad for the Korg MS-20 that you’ll see here. The MS-20 was a bit of an odd duck, and as a non-programmable monophonic synth it faced an uphill battle against the Prophet-5 — but then, everything did. Still, the MS-20 had some strong features, notably its patch panel, and a distinctive sound.

Korg has revived the design several times, notably as part of the software Legacy Collection, as a component in the Kronos hardware synth, and as an inexpensive iPad app. But it was left to Cherry Audio to take the MS-20 concept and run with it. Their PS-20 software expands the MS-20 profile in some important ways. I don’t have those other versions, much less an original hardware MS-20, with which to do A/B listening tests, but I’ll go out on a limb and say the PS-20 is every MS-20 fan’s dream come true. The sound is rich, and the feature set goes the extra mile while remaining true to the unusual configuration of the original.

What’s new and different? Up to 16-note polyphony. MIDI compatibility. Effects (distortion, delay/modulation, and reverb). Significantly more patch points than the original. An 8-step, 3-lane sequencer. The standard sync-to-host option for the LFO, delay, and sequencer. A one-finger chord mode. Also a substantial library of great-sounding presets.

What you won’t find in the PS-20 are the features we’ve come to expect on modern instruments. The envelope generators have no adjustable curves or attack time modulation, the oscillators don’t sport dozens of wavetables, the filters are not multimode, nothing like that. What you get is a powerful sound and the sense that you’re reliving those thrilling days of yesteryear, when Human League was at the top of the pops.

Overview

The PS-20 sports two analog-type oscillators and a noise source, two filters in an unusual dual configuration, two envelope generators, an LFO, a step sequencer, and some basic effects. You can create your own presets using just the knobs, because all of the signal routings are normalled, but when you attach a software “patch cord” to an input jack in the patch panel, the normal routing is overridden. This opens up a lot more possibilities for sound design.

The patch panel doesn’t give you access to every signal routing you might crave. There are no inputs to control the filters’ Peak (resonance) values, for instance, nor for modulating envelope times. But if you’re using the PS-20 in a DAW, you can easily automate these parameters from the host software.

The user interface is mostly quite easy to navigate. Cherry Audio’s standard Zoom and Focus features allow you to resize the plug-in or zoom in on a specific part of the panel. There are no hidden modes or pop-up menus, so you can always see exactly what’s going on. Plus, pop-up tooltips give you the current value of each parameter, which is a great aid in programming. The one area that you’ll have to learn is the patch panel. Unlike the panel on the ARP 2600, which is the most obvious ’70s-era precursor to the MS-20, the jacks here are laid out separately from the modules and knobs. The flow-chart lines on the patch panel help quite a bit, but in this case authenticity won out over ergonomics.

Filters

The dual filters in the PS-20 make no effort to give you a modern multimode design. One is highpass, one is lowpass, and there’s no user access to the rolloff slope. But don’t despair. They can be run in series, which allows for bandpass filtering. In split mode, with oscillator 1 routed to the highpass and oscillator 2 to the lowpass, you can get a sort of home-brew notch filter.

Each has both Peak and Drive knobs. Crank up the Peak and Drive, add a bit of detuning between the two oscillators, and you’ll have a good fat sound. Keyboard tracking for each filter is an on/off switch, but with the patch panel you can easily set up fractional tracking if you need it.

Oscillators

The oscillators are not fancy, but they do the job. Each has triangle, sawtooth, and variable-width pulse waves. Oscillator 2 can be synced to oscillator 1, there’s a ring modulator, and oscillator 2 can be detached from the keyboard pitch input and also used as a second LFO. The portamento (glide) operates only in monophonic mode, and it’s always on; there’s no switching for when two notes do or don’t overlap. But this qualifies as an authentic detail, not a defect.

Sequencer

The step sequencer is a little fancier than you might expect. It’s only eight steps (actually, from one to eight steps, though why you would want a one-step sequence is a bit hard to imagine), but there are three rows of knobs and three CV outputs. The CV outs can be either quantized to normal pitches or left unquantized, and the range of the outputs can be set to a maximum of from one to three octaves. You can set it up so that it only plays when you’re holding a key on the keyboard.

At first I was confused by the fact that each sequencer row has its own gate output. They’re all going to output the same continuous series of gates, right? Not true. Each of the knobs can be set below 0V to “off.” So by setting some of the knobs to “off,” you can create various rhythms. When a knob is at “off,” the CV output will retain whatever the previous knob value was, exactly as it should.

Patching

Explaining the PS-20’s patching capabilities in a quick summary would be all but impossible, and listing the inputs and outputs would be mind-numbing and largely pointless. But let’s give it a try. There are five output jacks that give you direct access to incoming keyboard notes (CV, gate, velocity, mod wheel, and pitch-bend). In case it isn’t obvious, all of the signals run at audio rate, so you can do tricks like modulating filter cutoff from oscillator 2.

There’s a sample-and-hold, a CV mixer/attenuator, and a separate VCA with in, out, and control jacks. With the separate VCA you can do tricks like adding tremolo to just one of the two oscillators. Each filter has a cutoff frequency input and an audio output, but no audio input. The extra audio input to the filters is via the “external” signal routing, which can either accept an external signal or be patched up to receive an internal signal.

The LFO has two outputs, one for triangle/saw waves and another for square waves. Both envelope generators have both positive and inverted output jacks. They also have gate inputs, allowing one or both envelopes to be gated from the sequencer or the LFO. External audio can be processed to produce an envelope, a gate, or even a pitch CV. The audio-to-pitch-CV tracking is quite good with simple input signals, but of course you can mess with it deliberately by sending it a complex signal.

There are certainly things the patch panel won’t do — modulating the effects parameters, for instance. But you’ll have to look hard to find them. The universe of things it will do is quite large. Want to set up an inverted keyboard the way Josef Zawinul did with his 2600? No problem.

About the Audio Clips

This quick demo (ps_riff.mp3) uses 11 instances of PS-20, and nothing else. The snare and kick are not that great, but I think the character of the instrument comes through. It’s a bit vintage, but that’s the point. The other three clips (ps_sequences, ps_pads, and ps_basses) each show four different unedited factory presets.

Summing Up

Cherry Audio has introduced a series of vintage synth clones in software — the Eight-Voice and the Memorymode as well as the PS-20. They’re all good, but the PS-20 is my personal favorite. Maybe that’s just because I love patch cords, but I think it’s more to do with what the patch cords can achieve in the way of fresh and unexpected sounds. Now that I’ve taken the time to learn the ins and outs of the not very intuitive patch panel, I’m sure I’ll be using this instrument in my tracks.