Review: Kilpatrick Audio Phenol

A desktop analog synth that's fun and easy to patch
Image placeholder title

Originally funded through a Kickstarter campaign, the Kilpatrick Audio Phenol presents a hybrid approach to analog modular synthesis: It provides a patchable interface like a semi-modular synthesizer, yet it doesn’t have a default synth voice connected behind the panel as you would find in such instruments as the Korg MS-20, Doepfer Dark Energy, and Roland System-1m (to name just a few). To hear sound from Phenol, you have to plug-in patch cables.

Of course, one reason people buy semi-modular synths is that they provide a relatively safe gateway to the world of fully modular instruments. They allow a musician to explore synth patching with the knowledge that they can still make sounds when they get stuck.

Similarly, Phenol provides a straightforward interface that is perfect for those getting started with modular synthesis, but who wouldn’t know where to start when it comes to assembling their first system. And while it lacks the security blanket of a hardwired patch under the hood, Phenol’s well-organized front panel is easy to figure out for the uninitiated.


Phenol is an analog synth that uses banana plugs for patching and includes standard and USB MIDI input, lowpass and bandpass filters, and scale quantization from the envelope generators. As you would expect, Phenol offers a basic complement of subtractive-synthesis tools. Its two voltage-controlled oscillators provide individual sawtooth, pulse, and triangle wave outputs, as well as a jack for the external inputs. And, it offers a pair of resonant voltage-controlled filters—one lowpass and one bandpass—that do not go into oscillation. A CV input with an attenuator is provided for modulating the cutoff frequency.

Image placeholder title

In addition to its feature-rich envelope generators (more on these in a moment), Phenol includes two DC-coupled VCAs, a digitally generated LFO with sine-wave and random-voltage outputs, a clock divider, a signal Adder (active mixer) with positive-and negative-phase outputs, an audio mixer with two inputs and pan controls, and a stereo digital delay.

All of the front-panel connections are made using banana plugs, rather than 3.5mm or 1/4" plugs as with other synth formats. (See “Going Bananas” below.) However, audio I/O and the headphone output utilize 1/4" connections.

Although the MIDI implementation is basic, it’s great having an onboard MIDI-to-CV converter, especially one that accepts both 5-pin DIN and (classcompliant) USB connectors. Phenol responds to MIDI Channel 1—monophonic, last-note priority. Note messages go to the Pitch and Gate outputs, mod wheel messages appear at the Mod Ouput, and MIDI clock signals produce 16th-note pulses at the Clock Output. The synth recognizes pitch bend, which you can adjust—from 1 to 12 semitones—using Program Change 1 through 12.

Phenol also provides a simple sequence recorder when you use MIDI. Using step or real-time input, you can add more than 500 notes/rests, as well as capture Pitch Bend, and then play the sequence as a one-shot or a loop. As the sequence plays, you can transpose it by sending it MIDI Note messages. Quick and intuitive!

LEDs are used throughout Phenol’s panel to indicate levels and signal phase, making it easy to tell what’s going on as you patch. And the metal case feels rugged enough for use onstage, without being too cumbersome for the desktop. In addition to the external power supply, Kilpatrick includes 10 banana cables of different lengths: While that’s enough to get you started, once you introduce MIDI, you’ll need more, so order another 10 and take full advantage of the patch points. It’s also worth noting that Kilpatrick’s design allows you to stack up to three patch cables going into most inputs, although the tradeoff is that stacking on outputs will sometimes cause small voltage fluctuations.


Although the feature set above may seem like standard fare to those who are used to inexpensive keyboard synths, you’d be hard pressed to assemble all of these elements into a modular system (including a case and power supply) for what Phenol costs. Moreover, a handful of Phenol’s features help it stand out at this price point.

The Adder is an active mixer for combining audio or control signals. Using its output knob, you can double the signal strength at the outputs— perfect for adding crunch or beefing up CV levels. The Adder also provides a pair of outputs of opposing phase. This allowed me to do CV-controlled panning by sending each of the Adder outputs to separate VCA amp inputs, then patching the VCA outputs to individual mixer channels that were hard-panned.

As you would expect, the Divider subdivides a clock signal or LFO input. In this case, you get discrete pulse outputs at divisions of 2, 3, 4, and 6. Furthermore, when Phenol receives a MIDI clock start message, the Divider outputs are reset.

Another useful feature is the built-in delay, which is intentionally noisy and lo-fi (especially so when you turn Delay Mix past the 12-o’clock position). You can use this section as a conventional echo effect or as a processor in its own right. The delay resonates nicely if you turn Delay Mix all the way up and Delay Time all the way off. Then, as you slowly increase the Delay Time, you will be treated to a range of metallic tones.


Where Phenol becomes really exciting is in Kilpatrick Audio’s implementation of the envelope generators (EGs). The EGs have two top-level modes—Envelope and Oscillator—with a button matrix and knob to dial-in a particular type of voltage behavior. In Envelope mode, you can create simple attack-release and attack-hold-release shapes, with knobs to control the speed of the rise and fall of the voltage. In Oscillator mode, the output voltage varies continuously and the same two knobs now set the Speed (frequency) and Level (depth) of the voltage. So, if your patch needs another pair of LFOs, here’s where to find them. But it gets even better!

The second row of buttons on each EG provides three additional modes that coordinate with the third knob (the one with the cryptic symbols printed below it). In Steps mode, the knob alters the smoothness of the output voltage: When set fully clockwise, the waveform is smooth; as you turn it counter-clockwise, you begin to hear stepping, with the steps getting larger as you continue turning. In Scale Quantize mode, you will hear arpeggiated notes, and that third knob selects one of 16 scales. The third mode is a Gate Delay, where the third knob sets the length of time before the EG is triggered.

Using the Steps and Scale Quantize modes, you can create rising/falling sequences that are quantized or not. Moreover, you’ll get fairly sophisticated rhythmic patters as you change the Up Time and Down Time settings while cycling in either mode.

The lower row of buttons control the EG’s Output mode—Normal (the output voltage goes from low to high), Inverted (high to low), and Absolute Value (a reduced-range, positive-direction voltage). To top it all off, you can use a voltage to alter the rates of the rising and falling voltages by patching a signal into the Speed jack.

That may seem like a lot to keep track of, but the feature implementation quickly becomes intuitive as you play with Phenol. Then, you’ll see how incredibly performative the instrument is. For example, with the EGs in Oscillator mode and Scale Quantize selected, all you have to do is sweep the filter cutoff and add some delay and panning to get into psychedelic territory.


In terms of sound quality, Phenol’s timbre palette ranges from mellow to gritty. It gets surprisingly crunchy when you modulate parameters at audio rate. Frequency modulation on the VCOs, in particular, yield some of the grainiest textures. If you were wondering about the product’s name, this should clue you in to why it was chosen: Phenol can cop an attitude.

And although the resonant filters don’t go into oscillation, they can still provide bubbly, glorpy sounds when needed. I liked having both lowpass and bandpass filters available, especially when processing drum loops through the External Inputs: I used the lowpass to get big, punchy bass while modulating the upper mids and highs with the bandpass.

Overall, Phenol has a unique sound and interface and is a blast to use. Experienced synth players will appreciate that it plays well with other instruments, whether or not they have banana jacks, and it can be integrated into any MIDI-based DAW or performance system. For musicians who want to get their feet wet in a modular-synth-style environment, Phenol is a great learning tool, providing a respectable feature set that is easy to wrap your brain around.

Going Bananas

The banana cable is a popular format for modular synth patching largely because it’s stackable: It allows you to send control signals to several destinations at the same time without using a mult module. Other manufacturers that use banana connections include Buchla, BugBrand Cyndustries, Fenix, and Modcan

Image placeholder title

 On Phenol, the jacks are color-coded so you can easily see their function: black is for CV and audio inputs, gray for CV or audio outputs, white for pulse/gate inputs, and red for pulse/gate outputs.

One downside of using banana connectors is that they do not include a ground wire like 3.5mm or 1/4" cables do. Consequently, if you want to patch Phenol to another modular synth, you have to make a ground connection between the two.

Phenol and other banana-based synths provide a jack specifically for this: Simply connect a patch cable from the ground jack of one synth to the other and you’re all set. On the other hand, if you want to patch between Phenol and modules with 3.5mm and 1/4” jacks, run a cable from your Eurorack or 5U system to the Phenol’s External input jack. No additional grounding is needed if you are simply connecting Phenol’s audio outputs to a mixer.


Extensive EG features. Stereo digital delay. MIDI-to-CV converter. Bandpass and lowpass filters. Adder and clock divider.


Resonant filters do not oscillate. Too few patch cables included. Voltage fluctuations when stacking cables on outputs.


Gino Robair is the editor-in-chief of Keyboard magazine as well as Electronic Musician’s technical editor.