Review: Kilpatrick Audio Phenol Patchable Analog Synth

Kilpatrick Audio’s Phenol Patchable Analog Synthesizer was originally a crowd funded project. The Kickstarter project raised almost 3 times the initial funding goal. Needless to say, this was a highly successful project. 

Synthesis Architecture 

The design of this synth is pretty interesting. It is a completely modular system in a desktop synth format. We already know this particular segment of the market is expanding quite well, with synths like the Moog Mother 32, Korg MS-20M, Dreadbox Erebus & the highly anticipated Make Noise 0-Coast to name a few. Unlike these other desktop synths, Phenol is not semi-modular, meaning you will have to make patch connections to hear a sound. Semi-modular synths will usually have a ‘normalled’ connection that does not require any patching to get sounds going. The drawback to semi-modular synths is that some patching possibilities may not be available. With a fully modular system like the Phenol, you have almost endless possibilities.  

 

Oscillator Section

Phenol has 2 identical oscillators. You get a triangle, sawtooth and pulse wave shapes. There is also an audio input section on each oscillator (EXT IN) so you can replace the oscillator sound with any other signal via the audio input jacks. The pulse shape’s pulse width can be varied with the PULSE WIDTH dial or you can use the PWM IN jack to modulate the pulse width with any other CV signal. Besides the two COARSE TUNE and FINE TUNE controls, you also get an FM IN for frequency modulation. Oscillator hard sync can be achieved with the SYNC IN jack. 

Filter Section

There are two filters in Phenol. A low pass and a band pass filter. Both filters have CUTOFF & RESONANCE controls. There’s also a FM IN jack for filter cutoff modulation with any signal.

Amp Section

There are two identical VCAs with AMP control as well as amp modulation via the AMP IN jack. 

Envelope Section

Probably the most interesting section on this synth is the Envelope section. Again you get rwo identical envelopes but you won’t see the usual Attack, Decay, Sustain  and Release controls. The envelope section is more like a function generator that can change modes. You get three modes of operation, Attack, Hold, Release mode, Attack Release mode and Oscillator mode. You get a UP TIME & DOWN TIME dial to control the attack and release time. In Oscillator mode the two dials control SPEED & LEVEL of oscillation respectively. So essentially you get a triangle shaped LFO. The speed can be modulated with any CV signal using the SPEED jack. The Envelope can be positive, negative/inverted or even rectified to work in the positive range only. The Envelope can be triggered with the GATE IN or a tiny switch right next to the jack. 

There is a third dial that can change the way the envelope works. This too has three modes. In the first mode, you get a delay to the onset of the envelope whose delay time can be controlled by the third dial. This is useful for when you want one envelope to trigger a bit later compared to the other. Let’s say the first envelope triggers the amp and the delayed envelope modulates filter cutoff. In the second mode, the envelope is quantized to scale steps. So when you use the envelope to modulate the pitch of an oscillator, you can get interesting musical arpeggios that go up and down depending on the UP TIME & DOWN TIME controls. The third dial here controls the different scale types.The last mode is similar to the scale mode but you get evenly spaced steps. The third dial controls how many of those steps you get. When the dial is all the way to the left you get the maximum stepping for the entire range of the envelope, and when the dial is all the way to the right you get no steps or a smooth ramp shape. The dial set anywhere in between gives a variety of stepped envelope shapes. For example, when you use this as a modulator for the pitch of an oscillator, you can create whole tone type scales. 

LFO Section

The LFO section is relatively simple. You get a RAND OUT which gives you a noise Sample & Hold type shape & a SINE OUT. There is just one control for the SPEED of the LFO. This seems quite underwhelming but a lot of the LFO type functions can also be achieved by the envelope section so you may find yourself using the envelopes more than the LFO. It is a bit odd that there is just one LFO when the synth seems to be following a pattern of two of each.  

Adder Section

The ADDER section is useful for combining different signals before sending them to the next stage in the synth. For example, if you want to send both oscillators to one filter, you can connect the two oscillator outputs to the IN 1 & IN 2 on the adder and connect the OUT + to the IN on the filter. There is a OUT - which outputs an identical but inverted signal. You also get a LEVEL control so this section can also be used as an attenuator or even to add gain to soft signals.

Divider Section

The DIVIDER section can be used like a clock divider. You get one IN and 4 OUTs. So any signal going into the IN of the DIVIDER will then create a toggle ON or OFF type signal at a division of 2,3 4 & 6 at each of the outputs respectively. 

Mixer Section

The MIXER section is quite comprehensive. You get two INs with a LEVEL control as well as a PAN control. There is a built-in Delay with controls for DELAY TIME and  DELAY MIX and an overall MASTER level control. The delay is a bit noisy but that may also be because of ground loop issues I had with the unit.  

MIDI to CV Section

The MIDI to CV section is great for when you want to play this synth with a MIDI controller. Once a MIDI signal in received via the 5 pin DINN connector or USB, that MIDI signal will be converted to CV and output via the PITCH OUT, MOD OUT, GATE OUT & CLOCK OUT. The MOD OUT will send CC1(Mod wheel) data out the jack. The CLOCK OUT will send 16th note pulses when MIDI clock information is received.    

There is also a simple sequencer built-in that can record and loop playback a sequence of incoming MIDI notes. Unfortunately, the pattern cannot be stored in the synth and clears out when the synth is powered off. 

Patching

Phenol is a modular synth so to create sound you need to patch. All the inputs and outputs on Phenol are color coded making it easier to understand the patching capabilities. All audio/control voltage inputs are black, audio/control voltage outputs are grey, pulse & gate outputs are red and pulse & gate inputs are white. Just following this color coding makes it a lot less intimidating to work on a fully modular system like this.

Unlike the Eurorack modular synths standard of using 1/8 inch patch cables, Phenol uses banana patch cables. 10 of these cables are included in the box. The advantage of banana cables is that they are stackable. Meaning you can combine multiple outputs to one input just using the cables. For example, if you want to send the LFO output to two or more places, you can just stack connect the output from the LFO to the various inputs. Having said that, it is possible to find 1/8 inch stackable cables.

Compatibility with Eurorack modules

Most people who are into the modular synthesis world probably own a setup with Eurorack modules. The biggest problem of compatibility with these systems is that Phenol uses banana patch cords and Eurorack modules use 1/8 inch cables. You can solder your own cables that use banana connectors on one side and 1/8th inch on the other but if you don’t want to go through that hassle, it can be hard to find such cables for purchase. Another issue with interconnecting the Phenol with Eurorack modules is that you need to setup a ground connection between the devices. Not doing so may lead to damaging both devices. Phenol has a ground connection in the rear that can be connected to your other devices but that again uses banana patch cables. I did notice some ground loop hum issues when connecting to a computer via USB. 

In terms of voltage range for CV signals, Phenol uses a range of -5V to +5V, gates or pulse signals jump between 0V & 5V. Oscillators are tracked using the 1V/oct system. All this is pretty standard but the envelopes are a bit unique, as in they use the full range of -5V to +5V. So if an envelope is at , it's at -5V not 0V. If you were to compare this with the Moog Mother 32, that synth has envelopes that work in the range of 0V to +5V, which is a lot more common. So envelopes are bipolar in Phenol. The implication of this is quite important. Think about assigning an envelope to a filter cutoff. Normally the envelope will only modulate the filter cutoff in the positive range. If the envelope has an invert function it will exclusively work in the negative range. In Phenol, the envelope is doing both simultaneously. So for example, if you have the filter cutoff set to approx. 1000 Hz and then apply an envelope to modulate it. The modulation is not going to start at 1000 Hz and go up depending on the envelope shape. It’s going to start much lower and then go up depending on the envelope shape. I’m not sure if this is a good or bad thing, but it’s great that the envelope has the option of being rectified to only work in the positive range so it works like most other envelopes in other synths. So you get the best of both worlds. Unfortunately the invert function of the envelope only works in the full -5V to +5V range. So you can’t get an inverted envelope that only works in the negative range, but then the Adder section has an inverted out jack so maybe this could be used as a workaround. 

Connectivity

On the front panel there is a MIDI IN via the 5-Pin DINN connector, and on the rear you have a bunch of additional ins and outs. There are 2 EXT IN 1/4 inch jacks for replacing the signal on the two oscillators with any external audio signal. There is the GROUND jack that I previously mentioned. There a DC in for power. The adapter is provided. There is a small POWER switch to turn ON/OFF the unit.  There are two 1/4 inch LINE OUTs (Left & Right). Since the master section of Phenol has a panner for each of the two inputs, you can create stereo signals on this synth. There’s a 1/4 inch headphone jack & lastly a USB in for MIDI signals.

Final Thoughts

The Kilpatrick Audio Phenol Patchable Analog Synthesizer is a great all in modular system. It has all the essentials to get going with modular synthesis or even learning the basics of modular and subtractive synthesis. The only drawback for me is in its expandability. Unlike a Eurorack system, you can’t add in more modules to extend the sonic possibilities with the synth. You can integrate it with other modular systems that use CV, but you will have to make banana to 1/8 inch or 1/4 inch cables to integrate it with other systems and you will also have to figure out grounding between the devices to avoid damaging your equipment.