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Pittsburgh Modular Patch Box

Modular Synthesizer
By Gordon Reid

Pittsburgh Modular Patch Box

What do you get when you cross a modular synth with a stompbox?

Originally conceived as a ‘small format’ laboratory and industrial equipment standard, the Eurorack has enjoyed a surge in popularity in synthesizer circles that would have seemed unlikely when Dieter Doepfer and Bob Williams first brought it to the attention of a wider audience of electronic music enthusiasts in the 1990s. But nowadays, you can buy modules not only from Doepfer Musikelektronik and Analogue Systems, but numerous others manufactures worldwide such as AJH Synth (UK), Cwejman (Sweden), MFB (Germany), TipTop Audio (USA), Sound Machines (Italy), and XAOC Devices (Poland). Even established synth manufacturers seem to be becoming interested in the format, with the first two modules from DSI now available.

Many of these manufacturers supply mounting cases for their modules. Some of these are utilitarian aluminium frames just a step or two away from the laboratory while others, housed in large slabs of expensive and lovingly polished woods, are pieces of furniture that wouldn’t look out of place in Lady Whatsername’s drawing room. A third group, designed to accommodate just a handful of modules, is sometimes used to host smaller modular synths and signal processors. These have the benefit of lightness and portability, although obviously at the expense of power and flexibility when compared with their larger brethren. Manufactured by relative newcomers Pittsburgh Modular Synthesizers, the Patch Box falls into the last of these three categories although, unusually, it’s presented as a large and rather attractive stompbox. Furthermore, even before you start to insert your choice of modules, it offers many valuable facilities as standard.

The Technology

Starting at the top left, you’ll find a knob that controls the gain of the preamp that sits between the quarter-inch unbalanced input on the rear panel and the dual, buffered 3.5mm input sockets on the top panel. Unfortunately, the documentation gives no information about this and tells you only that the, “signal begins to overdrive after 12 o’clock”, which isn’t very helpful. So I measured it. Using an unfiltered sawtooth wave of 0.6VAC as the input, the gain when fully anticlockwise was approximately unity and, when fully clockwise, the output was around 5.4V, which is a maximum gain of around 19dB for this signal. Anyway, once you’ve determined the gain, you can then direct the resulting signal — whether clean or overdriven — to other sockets running along the top of the Patch Box or to the modules installed within it.

Alongside the top–panel inputs, you’ll find two 3.5mm sockets marked Exp 1 and Exp 2. These carry the control voltages determined by the expression pedals connected to the quarter-inch Exp 1 and Exp 2 sockets on the rear panel. You may think that this arrangement is a bit wasteful, after all, why not connect the pedals directly to the modules? The answer lies in the fact that the pedals determine control voltages (CVs), so the Patch Box supplies a 5V DC signal via each of the rear-panel sockets, and the physical position of the pedal then determines the voltage (in the range 0V to 5V) available on the top panel. This means that you have to use suitable pedals such as the Moog EP3 supplied with the review unit but, once you’ve got everything up and running, it’s a great way to control the CVs that can themselves control the modules.

Next (scanning left to right) comes a passive three-way multiple used for splitting a single audio signal or CV into two identical signals. In common with all multiples, you shouldn’t use this as a mixer, but no doubt some people will try.

To the right of this, the Switch sockets are a real bonus. Technically, the large Switch 1 and Switch 2 footswitches at the bottom of the unit connect either the left-and-centre or the right-and-centre pairs of their associated sockets, which, when I first looked at them, seemed straightforward and not particularly exciting. But I didn’t fully appreciate the benefit of this architecture until I started to experiment with the Patch Box, after which I began to realise how flexible the switches make it. Imagine that you have a modulation source connected to the centre socket of Switch 1. If you tread on the switch so that the associated LED is off, that modulator will be directed to any destination connected to the socket on the left. Tread on it a second time so that the LED is lit, and the modulation will be directed to any destination connected to the socket on the right. Alternatively, imagine that you have two modulation sources, one connected to the left-hand socket and the other to the right-hand socket. Now, if the centre socket is connected to a destination, the first modulator controls that destination when the LED is off, and the second does so when the LED is lit. Of course, if you patch just one modulation source and one destination to either switch’s sockets, it also acts as an on/off switch. This is all rather neat, and particularly appropriate for a stompbox.

The Patch Box’s rear panel is a simple affair, with audio input and output and the two Exp sockets on quarter-inch jacks, and a socket for the external 15V power supply.The Patch Box’s rear panel is a simple affair, with audio input and output and the two Exp sockets on quarter-inch jacks, and a socket for the external 15V power supply.The final top-panel socket is marked Out, and any signal presented to this will be amplified by the Output level control (which, the company says, will provide anything from guitar-level to line-level signals) before being passed to the quarter-inch output on the rear panel. Hmm... that’s not quite true. If you press the Bypass switch, the whole of the Patch Box’s signal path is bypassed, and the signal presented to the rear-panel input is passed directly to the rear-panel output.

It’s all rather well conceived and, at this stage, there are only two things that strike me as a bit odd. Firstly, the rear–panel’s quarter-inch input lies behind the top panel’s out socket, while the rear panel’s quarter-inch output lies behind the top panel’s input socket. Given that most people will be connecting and patching from the top, not the rear, this could lead to confusion. Secondly, the Bypass is ‘on’ when the associated LED is off, and ‘off’ when the LED is on. Either the operation of the LED should be swapped, or the switch should be renamed ‘Active’ or something similar, because the current arrangement is confusing.

The Supplied Modules

Although the review unit was supplied fully populated, installing modules should be straightforward. You need to lie the Patch Box on its front and remove the base plate to expose the internal power buses that lie under the top-panel switches and sockets. Then, after connecting the required number of 16-pin power cables (up to a maximum of six) you reattach the base, flip the unit over, decide how you’re going to configure your modules, and install them up to a limit of 42HP. All of the usual caveats apply: specifically, connecting modules and power cables incorrectly will almost always result in dead stuff. But with care, there’s no reason why you should have any difficulty. Then, once everything is screwed into position and you’ve reconnected the external PSU, you should be ready to have some fun.

The first of the installed modules was an envelope follower called Detect, and this appeared to be a new product, if only because it wasn’t on the company’s web site at the time of writing. It’s simple, well laid out, and it does what you would expect so, unless you’re a modular virgin, you should have no difficulty using it. Unfortunately, there doesn’t seem to be anything that tells us what the maximum and minimum Attack and Decay times are, or even what the generated gate voltage is. It’s not a problem but, unfortunately, this lack of information turns out to be a common thread running through Pittsburgh’s product descriptions and manuals.

Next came the dual LFO. The upper device generates triangular and pulse waves simultaneously and allows you to shape these from sawtooth to ramp waves (in the former case) or to determine the duty cycle (in the latter). There’s also a high/low range switch and a knob to control the frequency. The lower device is even simpler; it outputs just triangle and square waves and again offers a range switch and frequency knob. None of these facilities can be controlled using CVs, so dynamic wave shaping and frequency sweeps are only possible if you grab a knob and twist it. If you need a more flexible LFO, you’ll have to replace this one with something more advanced.

This was followed by a dual four-way passive multiple, but we need say no more about this and can now jump directly to the Filter module. This generates four signals simultaneously: low-pass filtered, band-pass filtered, high-pass filtered, and the output from a state-variable filter that, in SEM fashion, sweeps from low-pass through notch to high-pass. Happily, the cutoff/centre frequency and resonance of the filter can be controlled using CVs, with both positive and negative polarity. However, the most novel facilities in this device lie behind its two switches. The first is marked Gain Of 1/Gain Of Q. In its ‘1’ setting, this tries to ensure that the output level remains roughly constant at any Q so, if you wind the resonance up, rather than the filter ringing or screaming at high volume, it attempts to do so at the same loudness as sounds having low resonance. I’ve never seen that before, and it suggests some interesting possibilities. The ‘Gain Of Q’ setting restores the normal response. The second switch is marked Filter Mode/Osc Mode, and switches between non-oscillation at high Qs (filter mode), and lots of oscillation at high Qs (osc mode). Unfortunately, Pittsburgh’s documentation again seems to want to keep everything mysterious, because there are no explanations or specifications regarding any of this.

The final module was called the Analog Replicator, which, to you and me, is a delay. I know that it’s trendy to think up exotic names for modules, but this module doesn’t need to be aggrandised in this fashion — it’s interesting whatever you call it, although whether it’s “cleaner, dirtier, brighter, darker, and more versatile than any other analogue BBD delay-line effects unit available,” as the company claim, is a moot point. Nonetheless, its use of dual 4096-stage BBDs is highly unusual, as is its maximum delay time of 2.6s. With three CV inputs (time, feedback and mix), each with their own dual-polarity attenuators, an Invert Polarity switch that inverts the feedback signal within the module, and dual (mix or delay-only) outputs, the module is very flexible and, with appropriate settings, can generate a wide range of simple chorus effects, analogue reverbs and echoes. It also excels at special effects when the gain of the feedback loop is equal to or greater than unity. Bear in mind, however, that in common with all such devices, slowing the clock to obtain the longest delays will result in a high-pitched whine as it drops into the audio range. Two further switches are marked Bypass and Spillover. The first of these defeats the effect immediately in the usual fashion, while the second defeats the input but allows any signal already within the delay line and feedback loop to decay naturally or, at high feedback values, to loop indefinitely.

The Patch Box measures 244 x 229 mm and gives you 42hp of Eurorack accommodation.The Patch Box measures 244 x 229 mm and gives you 42hp of Eurorack accommodation.

In Use

Since Pittsburgh Modular have designed the Patch Box as a stompbox, that’s how we should judge it. On that basis, I found that it acquitted itself rather well. To start, I tested the preamp by taking one of the braided 3.5mm patch cables provided with the unit (which are supplied both in white and black so that you can differentiate between audio and control signals if you wish) and patched one of the input sockets directly to the output. Playing a simple tone generated by a Roland SH101 through this and winding up the Drive certainly added an edge to the sound, and did so without making it muddied or cloudy. Doing the same thing with a guitar plugged directly into the Patch Box allowed anything from clean signals at the lowest settings to serious distortion at the highest.

Next, I added the filter into the signal path. Starting with the SH101, I took its CV out and directed this to the frequency CV input of the filter so that I could make it track my playing in a variety of ways, and then experimented with the various combinations of modes, both at low and high resonance settings. There was no question about it — I was hearing a huge range of sounds that no-one could have wrung from the synth alone. Turning again to the guitar, I took advantage of the dual outputs from the preamp, directing one to the envelope follower and the other to the audio input of the filter. Appropriately patched, this made all sorts of auto-wah effects possible, as well as some truly out-of-control sounds when the filter mode was set to Osc and the self-oscillation was allowed to interact with the signal from the guitar. Adding the delay line into the signal path generated yet more extreme effects, and I particularly liked routing one of the Exp pedals via a four-way multiple to the CV inputs of the filter cutoff frequency, the filter resonance and the delay time, either with the same polarity, or with one increasing while the others decreased (and vice-versa). The range and quality of the effects I obtained from the Patch Box was excellent although, if I were specifying one for my own purposes, I probably wouldn’t have populated it with this combination of modules: a more flexible modulation source, a contour generator and a CV mixer would be high on my list of alternatives. But that’s the beauty of any modular system; I can insert clean, precise modules, and you can install nasty, unpredictable ones if you prefer.

Inevitably, the Patch Box isn’t perfect and, in addition to my comments about the I/O sockets and the Bypass LED, there are four further issues that I would address if it were in my power to do so. The first is operational: the Bypass switch generates a small thump when pressed, which could be embarrassing at high amplification. The second concerns the use of an external PSU. This will be fine in the studio, but stompboxes live on the floor where they can be kicked around, and an internal power supply and an IEC cable would give me more confidence in any live situation. I realise that there may not be enough room in the current design to incorporate this, but I still think that it’s something that the company should consider. Thirdly, there’s the on/off switch — or, rather, the lack of one. The Patch Box was designed so that connecting a cable to its quarter-inch output acts as the on/off switch, but I discovered that inserting a TS (tip and sleeve) plug into either of the Exp pedal inputs — which creates a short between the ring and sleeve of the TRS socket — also does so. I’m sure that this can’t be doing the unit any good. Finally, there’s the question of the documentation, which offers no hard information regarding the operation of the Patch Box or the modules. The gains of the amplifiers? Don’t know. The attack and decay times of the envelope follower? Don’t know. The frequency ranges of the LFOs? Don’t know. The bandwidth of the filter? The slopes of its various modes? The signal levels? Don’t know. And so it continues. Of course, I could measure all of these, but it would take a few hours, and I would prefer to spend that time playing music.


I don’t know whether anyone has ever created a product quite like the Patch Box before. It’s possible, of course, but I don’t know of it, which means that it may be truly ground-breaking. It’s not perfect, but none of my criticisms are deal-breakers, and if I found myself needing a small modular synth or modular signal-processing unit for live work, I think that I might be very tempted by the Patch Box provided that I could fit the modules I wanted. In a world replete with ‘me-too!’ modular wotsits of every shape, size, form and function, it’s to Pittsburgh Modular’s great credit that they have designed something innovative, attractive and useful.

The Eurorack Format

Although it’s easy to assume otherwise, the Eurorack format isn’t precisely specified, which makes it all the more surprising that disparate manufacturers’ modules can generally reside side-by-side in a single case. It wasn’t always like this; in the 1990s, Analogue Systems and Doepfer Musikelektronik manufactured modules with their mounting holes in different places, meaning that there were gaps if you tried to mount RS Integrator and A100 series modules adjacent to one another on the same screw-strip. More significantly, their power supplies used different connectors, which soon led Analogue Systems to upgrade their original PSUs and offer dual-format systems. Even today, there’s much about the format that isn’t common to all products: there’s no enforced geometry for the mounting holes, no standard depth, no signal connection standard (3.5mm TS sockets have been widely adopted, but some systems use banana plugs and quarter-inch connectors for some purposes), and there’s no single standard for the power–supply cables. On some systems, the PSU headers can even carry electronic signals such as CVs, triggers and gates. Consequently, we shouldn’t really talk in terms of a Eurorack standard, and it’s always worth checking whether your chosen host can support and fully utilise the modules that you might want to buy for it.

This is particularly pertinent in the case of the Patch Box (oops... no pun intended) because, despite the company’s statement that it’s “ready to customize with a huge selection of Pittsburgh Modular, Dwarfcraft, or other compatible Eurorack format modules”, its internal depth is just 42mm, which means that there are many modules that will not fit. If your ambition is to configure a Patch Box using your own selection of third-party modules, it’s very important that you check their depths and power supply requirements before splashing any cash.

The Patch Box FX1 With ‘Crush’

In common with several other modular synth manufacturers, Pittsburgh Systems can be (let’s be polite) a little flamboyant in their descriptions of some of their products. Nowhere is this more prevalent than in the web page that describes the Crush ‘Analog Waveform Decimator’ module that would have been installed had I reviewed the standard FX1 version of the Patch Box. If you look closely, you’ll find that Crush is another BBD-based delay line, but one whose sampling rate, reconstruction filter and output mix can be controlled using CVs. That’s rather clever, and it suggests all manner of possibilities. What Crush is not, however, is “a sonic laser designed to shatter waveforms into a million pieces” or a device that takes advantage of “unlimited bit- and sample-rate resolutions” that can “downsample at infinite resolution”. It’s a shame that this flowery language has crept in because, once you’ve discarded the hype and worked out what’s happening, Crush sounds rather interesting, and I would very much like to hear what happens when you install one in the Patch Box and control it using a footpedal.


  • It’s rare to discover a product that adds something new in the field of modular synthesis.
  • It’s solidly built and attractive.
  • It performed faultlessly during the review.


  • There are some design details to be addressed.
  • The manual lacks hard information.
  • The external power supply and on/off system should be reviewed.


The Patch Box is not just an empty Eurorack case, and it offers a number of facilities that prove to be very useful when you start to populate it. I would like to commend Pittsburgh Modular on designing something that appears to be genuinely innovative as well as genuinely useful.


Patch Box Case £299 including VAT. Modules available separately.

Patch Box case $349. Modules available separately.

Published March 2016