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Bastl Softpop SP2

Semi-modular Synthesizer By Simon Sherbourne
Published February 2023

Bastl Softpop SP2

Bastl’s Softpop SP2 reins in the chaos but is still a far from conventional synth.

The Bastl Softpop SP2 is a hyper‑patchable desktop semi‑modular which shares some of the crazed cross‑modulating DNA of the original Softpop, but is way more versatile, tameable and usable. Its defining features include highly performative sequencing, customisable pitch quantising and modulation sourced from sample & hold and the oscillators. Sonically it can serve up classic monophonic bass lines and acid, but also veers off into chip‑tune and generative modular madness.

The SP2 is very recognisably a Bastl instrument (it’s actually a collaboration with Casper Electronics). It has the ice‑cream‑sandwich‑shaped enclosure that looks like it’s slotted together from black, silver‑etched PCBs; however, it feels a lot more grown up than its predecessor — solid, with clicky buttons, throw switches, a chunky metal knob and a dense Eurorack‑compatible jack field. The six main faders are joined by five mini sliders, and the built‑in speaker has been bumped out in favour of a MIDI in port. In photos, the MIDI port serves as a visual reference for how small this thing is. Unlike some previous Bastl boxes, though, it’s not at all fiddly to use on a desktop, and it’s playable when held in your hands like a game controller, twiddling the faders with your thumbs. Power is via USB so you can go mains‑free with a charger pack.

Pop Art

The front edge panel of the SP2 shows an overview of the default patch, and there’s a more detailed version on the bottom showing patch points. The audio path starts with a pulse oscillator whose shape is defined by a ‘triangle’ oscillator running into a comparator (more on why triangle is in quotes in a minute). This feeds into a state‑variable filter, then through a VCA (something the SP1 didn’t have) and out into the world.

Every surface of the SP2 is covered in diagrams and helpful reminders for the many button combos.Every surface of the SP2 is covered in diagrams and helpful reminders for the many button combos.

Other than the intriguing business that generates that initial tone, you couldn’t get a more conventional subtractive synthesis scheme. With modulation controls minimised you can achieve a range of ‘classic’ monosynth sounds, and very nice they are too. Looking at the diagram, though, you’ll see that there’s a bunch of modulation in the picture that spices things up considerably, even before you pick up any patch cables.

From left to right the main faders work in pairs, controlling pitch, filter and envelope. The first is the main pitch control, and the second sets pitch mod depth. This mod is derived from a sample & hold circuit which is triggered (by default) by the sequencer. All pitch control and modulation on the SP2 other than the fine‑tune slider is quantised, so the effect of the pitch mod slider is to randomly change notes in your sequence, but always constrained to the current scale.

The next two faders control the filter cutoff and envelope mod amount. Above these, a horizontal slider sets the resonance. The filter has a lovely dusty quality, and can squelch and whistle. It self resonates, which you can draft in as another tone source if you feed it CV from the sequencer. There are low‑, band‑ and high‑pass modes, and the band‑pass gets a parallel output.

The final pair of main sliders set the time and shape of the SP2’s single envelope. The envelope has a simple attack‑release contour, with exponentially curved slopes for a percussive response. The shape control adjusts the proportion of attack and release: fader down gives instant attack, in the middle results in an even rise and fall, etc. The envelope is triggered by the sequencer (unless overpatched) and also has a Cycle mode where it becomes an LFO.

Bubble & Squeak

Bastl have set up the Softpop so that it doesn’t take much to steer off the beaten track. The pitch mod is not the only prewired, self‑influencing source of interest/chaos. Above the envelope controls is the Pop slider which introduces audio‑rate mod to the filter from the pulse oscillator. This has a fairly wide range of outcomes depending on the starting tone, the oscillator pitch and also the resonance. For mid to higher notes you get a progressive change as you move the slider, from the natural ‘soft’ sound of the filter, through a varying formant‑type tone, to classic Commodore 64 era pseudo‑noise at the ‘pixel’ end. With resonance up, and especially on the band‑pass, Pop can add acidic stickiness to the filter.

There’s also fixed modulation from the triangle‑wave oscillator to the envelope time. This is subtle and when you’re doing normal synthy things you don’t really hear it. But the oscillator can go down to LFO rates, at which point the envelope time wows noticeably. In a normal synth you’d never know because the oscillator is subsonic at this point, but fiddling with the SP2’s controls to add a little pitch mod, Pop and filter self‑resonance brings forth various bursts, drips and bubbling bleeps, and sets off Krell‑like circular interactions. This is more like the default position for the original Softpop.

Digital Switchover

When the SP2 was released its main oscillator was an analogue triangle generator. A subsequent update procedure was revealed that gives you the option to convert to a multi‑mode digital oscillator. Not only does this provide eight different tonal starting points with manually sweepable ranges, it offers stable tuning compared to the somewhat drifty original. This option was presumably planned all along, as there’s a labelled jumper on the board that you move in addition to changing the firmware. My test unit came with the digital option already enabled and I saw no reason to change it: there’s plenty of wonkiness elsewhere in the SP2 to satisfy the analogue urge.

Remember, the default audio source in the SP2 is actually a pulse wave generated by an analogue comparator, driven by the triangle oscillator. This can be pulse‑width modulated from a patch point. However, if you want to hear the digital oscillator undiluted you can patch it directly to the filter’s input. Alternatively you can patch it into the SP2’s input point to hear them both. This option is great for adding a load of nicely saturated bass to any sound.

The digital triangle’s modes include Fractal Triangle, Super Triangle, Super Saw, Triangle to Noise and Glitch Radio. Morphing each mode between its two endpoints is achieved by holding the Slide button and adjusting the Fine Tune slider. You can change the slider to default to this function and relegate tuning to a shifted option, which I did. There isn’t a patch point on the panel for voltage modulation of the wave shape, perhaps because it wasn’t even a thing when the unit was launched. When I asked about this, Bastl mischievously hinted that there may be a way but it’s an as yet undiscovered Easter egg. I couldn’t find it, but I did learn that mod‑wheel data (CC1) coming into the MIDI port does control it. Perhaps this will require a user hardware mod, something that Bastl have left the door open to, even though it will void your warranty (see the ‘Rack & Hack’ box).

A Step To The Left

The left‑hand inch or so of the Softpop SP2 panel is dedicated to sequencing. Twelve unassuming buttons hide a sequencer of pretty astounding scope and playability. Essentially it’s an 8‑step sequencer, with gates set by the array of smaller buttons. You can store eight banks of eight patterns, recalled via the Pattern button.

There are a few ways to enter pitch values into steps. The simplest is to hold the record combo (Pattern and Slide) while the sequencer is playing and wiggle the Pitch fader. The results of this real‑time capture are not far from random but it’s a fast way to get started. In Stop you can also enter values step‑by‑step by holding a gate and moving the pitch slider, or tapping the up/down arrows. Slide steps can be entered with the slide button, providing auto‑glide between steps in classic 303 fashion.

Everything goes through the quantiser, so even a rough real‑time record pass is transformed into something more‑or‑less musical. The Scale button lets you pick your scale from eight presets or you can even define your own. The sequence can be transposed in semitones as an edit operation, or in real time via CV.

But we’re only scratching the surface. Patterns can be chained together easily by tapping them in succession. Recalled patterns play back with their original absolute notes, overriding the current pitch fader setting, but subsequent movement of the fader will transpose the sequence. As well as patterns, you can chain scales. You could, for example, set the same pattern to play twice in a major scale, then twice in minor. Add to this the play modes, where you can choose to play patterns in various orders or shorter chunks, and you have a lot of very easily accessible variations and longer sequences at hand.

My favourite thing, though: with the Trigger button held, the step buttons become a palette of real‑time effects, Pocket Operator style. Boy I love a real‑time performance effect. You can fire off variations and fills such as fast repeats, triplet‑speed ratchets, noise bursts or temporarily apply 50 percent probability to the gates. The effects can be combined.

37 Patch Points

With 37 patch points, it’s hard to know where to start an exploration of the Softpop’s routing and modulation possibilities. Audio‑wise you have output points for both the triangle oscillator and the pulse wave that derives from it. By patching from the triangle into the filter you break the normalled connection from the pulse and get to hear the triangle in its raw form. If you wanted to mix the pulse back in you could connect it to the audio input.

The audio input is also great for integrating or processing external audio. By switching the sequencer to Absolute CV output mode, you can feed the sequencer with all its mods to another synth and loop its voice back in. I linked up the SP2 with the trusty Make Noise 0‑Coast, bringing in audio and also borrowing its more complex contour generator. (The Softpop seemed perfectly happy to work with Eurorack audio and CV, even though its USB power means it works at slightly lower voltages than a typical modular.) The audio input adds saturation to anything you bring in. There’s also an envelope follower circuit connected to the input, which is fun for processing loops or adding side‑chain pumping, etc.

The Pop modulation’s patch input works in an interesting way: when you make a connection, the Pop slider turns into an attenuator for the input voltage. This is handy for setting a modulation level but also great for using a trigger source to accentuate particular notes. In fact a cool trick is to subvert the slide gate output to create accents that don’t coincide with every note. Another option is to use the regular gate output and trigger the envelope from the clock.

Along the top row of the jack field you’ll find a generous and welcome utility section that adds hugely to the potential and usability of the matrix. There’s a group of four mults for duplicating or mixing any sources. You can actually sever some of the connections between these as a user mod if you like (see the ‘Rack & Hack’ box for more). Then there’s the X‑Y section, which comprises two inputs (X and Y), one output (XY) and the knob in the top‑left corner. This is incredibly versatile, with different functionality depending on what’s patched.

If you plug something into the Y input, the module operates as an attenuverter, allowing you to tame and invert any modulation via the knob. With both inputs connected, the unit becomes a mixer with the knob acting as a crossfader: running the two oscillator sources through here is an obvious and effective use for this. With just the X input connected the knob produces a voltage offset, and with nothing connected it just outputs a controllable voltage source, great for taking hands‑on control of parameters like PWM.

It’s an addictive and loveable little beast, with real character...


A synth review can sometimes seem like one of those movie trailers that gives away all the good bits, but this time I feel like I’ve only scratched the surface. The Softpop SP2 is a micro modular with enough patching permutations to keep you exploring. But unlike the Softpop 1, which was chaotic and feral by default, the SP2 is a more domesticated analogue groovebox which can provide instant musical results, until you let it off the leash and encourage its wild side. It’s an addictive and loveable little beast, with real character thanks to its multi‑mode oscillators, quantiser and self‑modulation, and the interactive sequencer makes it a fantastic performance box.


The SP2’s MIDI implementation is interesting. There’s a full‑size MIDI In port, but no MIDI Out (although see the ‘Rack & Hack’ box; it is possible to mod one in). There’s no MIDI over USB — it would have been nice to be able to both power the synth and clock it with one cable from a laptop or MPC. MIDI Clock from other devices worked as expected (as did modular clock) and it’s easy to choose from various tempo subdivisions for the sequencer on the fly.

There are two modes for how the SP2 treats incoming MIDI notes. You can set it to play notes coming in like a regular synth. The patchbay has a MIDI gate out port that you can use to trigger the envelope, but there’s no MIDI‑to‑CV source. Instead, if you’re in MIDI Pitch mode notes get taken care of automatically. (I think the MIDI is playing the quantiser rather than oscillator frequency.) You can also choose to have notes trigger the envelope automatically.

The interesting (and default) mode is MIDI Scale, which is designed to work with the sequencer. In this mode, the MIDI notes you hold define the scale for the quantiser. This works like a kind of note‑filtering arpeggiator: the current sequence gets pulled into the chords you play.

There’s also a generous set of other MIDI mappings to control the SP2. Oscillator shape tracks the mod wheel, and various other CC assignments control things like the pitch and pitch mod, pattern, scale and waveform selection. You can also use MIDI notes above C7 to trigger the performance effects. For live use it would be awesome to set this up with a Novation Launchpad.

Rack & Hack

Bastl have left a clear trail for anyone with enough know‑how to mod their Softpop SP2.Bastl have left a clear trail for anyone with enough know‑how to mod their Softpop SP2.

Although the Softpop SP2 is a self‑contained instrument, Bastl have built in many opportunities for customisation and circuit‑bending. If you pop the back off, the whole assembly comes apart, leaving just the main panel. With an optional adaptor you can mount this in a Eurorack case and hook up power with the built‑in port that you can see in the picture. Closer inspection of the board reveals that many solder points are labelled, giving directions to anyone with the expertise and nerve for more invasive hacking. Hidden on the inside of the rear panel is more information about this, as well as a friendly reminder that you’ll be voiding your warranty.


  • Sonic range from fat to freaky.
  • The sequencer is brilliant.
  • Hugely patchable — with grown‑up patch points.


  • Lots of button combos to learn.


Engaging, bendable, performable sequencing synth with mad patching potential.


£549 including VAT.