Designing the Sputnik: A Space Odyssey

or: How I learned to stop worrying and love solder fumes.

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The Sputnik started on the premise of designing a pitch modulated reverb pedal that wouldn’t be based on DSP, algorithms and microprocessors but on a dedicated reverb module. This on one hand removes the countless possibilities available with software design but at the same time sets a very interesting challenge on circuit design. 

With microprocessors out of the equation there are three more common solutions for building a reverb: the Spin FV-1 chip, a classic spring reverb tank and finally the Belton brick reverb module. The FV-1 is a programmable multi-fx DSP chip which is very versatile but still in the microprocessor “family” so it was ruled out as cheating.
Spring reverb tanks are typically used in guitar amplifiers where their large size is not an issue. Smaller spring tanks have been succesfully integrated in pedals but again the enclosure would have to be uncomfortably large and expensive.
The third option, Belton bricks, are compact modules with stereo outputs that somewhat try to emulate a spring reverb by using three cascading PT2399 delay chips, all enclosed in a black epoxy brick. The PT2399 are of course digital ics but by today’s standards any current digital ic would laugh at the primitive nature of the PT2399.

The Belton BTDR-2H brick has been used in many well known pedals and seemed like the better option. It’s by no means a studio grade reverb but its limitations are what made this premise interesting and challenging. Much more challenging than I thought!

There is some inherent pitch modulation on the BTDR and smart people from DIY pedal forums(Madbean, Diystompboxes)  have cut open and studied the circuit suggesting that it’s caused by modulation of the power supply or from an internal oscillator. Sure enough I tried both ideas.
First, modulating the power on pin 1 of the brick did cause the internal PT2399 chips to jump around and in turn result in pitch modulated reverb. Problem was that sometimes the brick would not get enough power and shut down resulting in all kinds of
 loud bleeps and pops.

Next I took an exacto knife and carefully cut open the epoxy cover, revealing the circuit board. I removed the onboard modulation and connected an external LFO to one of the delay chips. Again, pitch modulated reverb was happening until some point I damaged one of the delay chips and miserably failed to get the brick working again. This method worked better but was very time consuming and included the risk of damaging the tiny smd components of the brick. 

The next logical idea was of course this: why not send the reverb output through an external delay circuit and modulate that instead of the brick itself? That seemed more promising. I set up a simple PT2399 circuit on the breadboard, added an LFO to modulate its time control pin and sent the reverb through the delay circuit.

Instantly this prototype circuit sounded very spacey and the modulation circuit could get the reverb extremely detuned. This was a great base still though much work had to be done.
For example there was a whole delay circuit just serving as an extra reverb stage when it could essentially be a fully functional delay circuit like the Mocking Bird Delay. The second BTDR output could become another reverb channel with a separate mix control, a different modulation type and even be used in stereo. Also I had decided that a random modulation mode was mandatory for this pedal. After about a year of experimentation and breadboard abuse I had a circuit that worked pretty well. The prototype circuit board was designed and sent for fabrication.

The pedal had grown quite a bit in size and many knobs had been added. In this prototype version, the Sputnik featured two reverb channels: a more standard one with tremolo modulation and the delay-verb hybrid channel with pitch modulation. Random waveforms were emulated by combining three oscillators and four overall waveforms as described in this great article by R.G. Keen:
It also featured stereo output jacks, a tails switch and a feedback adding momentary switch similar to the Mocking Bird Delay. When the prototype boards arrived I immediately started building the first Sputnik anxious to see how it worked or didn’t. Much later that night I finally turned it on and the sounds coming out of it were out of this world. Lush ambient stereo reverb that could get stuttery with random tremolo or jump around in pitch on the delay-verb channel. Its weaknesses though were obvious too. 

The noise level and minor bugs that I believed were caused by the messy breadboard layout had followed the pedal on the circuit board version. Also it’s on a actual pedal format that you really understand if your pedal’s design makes sense, if the controls are intuitive and if the pedal is easy to use while actually playing music.
The most major issue was the signal to noise ratio from the BTDR reverb brick which from its nature is not too great. It’s not really obvious while playing but for a pedal with a tails function where the brick’s noisy output can be 
always connected to the pedal’s output this was a big drawback. Studying noise gate circuits along with some very helpful input from a friend who was much more familiar with them I eventually came up with a jfet controlled noise gate that worked like a charm: opened up as soon as the reverb was producing sound and closed smoothly as the reverbarations trailed off. And then if one wanted to completely shut off any output from the reverb when bypassing the pedal, the tails on-off switch was moved to a footswitch for easy access.

Next, having 4 rotary switches and 3 Rate controls to dial in some random waveforms now seemed counterintuitive and overcomplicated. A different method had to be used. I had tried some sample and hold circuits in the past without much success but it seemed I had to dig deeper into them. A sample and hold circuit consists of a random noise source which gets sampled in periodical intervals and then that random noise level or voltage is stored(hold) until the next pulse from the LFO comes in and samples a new voltage. After experimentation, using 2 transistors as the noise source, an electronic switch and an opamp as the sampling circuit being controlled by the LFO, I had a sample and hold circuit that was working great and used a much simpler interface.

So after almost a year of fixes, tweaks and additions the heavily modified prototype board was working much, much better in a pedal format. I now had to translate all the chaotic, spaghetti wiring into a new schematic and circuit board layout!

While waiting for the new boards, it was time to finalize the artwork and adjust it to the new board layout. The original astronaut drawing was a black & white ink sketch I had done a few years back and fit in with the concept of the Sputnik. In a pedal design, artwork has to cooperate with the control layout and vice versa: it’s a balancing act between a well laid out interface and enough space for the artwork to breathe. More tweaking and moving around until it all fit into place and the final artwork was ready to transfer on a silk screen for printing on the pedals. For the Sputnik I wanted to use a two color scheme to make the controls easier to understand: black lettering and knobs for the delay-verb channel and white for the standard reverb channel. 

The second version boards arrived and worked perfectly with a couple minor mods, the screen printed black and white artwork really popped on the raw aluminum enclosure and the pedal looked as atmospheric as it sounded. 
So after almost three years of trial & error and hair pulling electronic compexities the Sputnik was finally coming together. It now features two channels which can be mixed together in parallel or sent out to different amps for huge stereo ambience. Via a toggle switch, the black delay-verb channel can be set to work as a standard delay, a second reverb or both in the center position. Each channel has its own fx loop where you can hook up external effects for even more experimental reverb scapes and since the pedal seemed very suitable for soundscape design I added a dry kill switch that leaves only the wet signal. Also I updated LFO to cover a huge range, from super slow pulses to ring mod territory.

After a couple of changes for the final production boards the circuit was complete and the Sputnik almost ready for release. This also meant that for my part, the most interesting chapter was over and it was now time for the more tedious tasks of sourcing parts, designing a manual, building the first batches and advertising the pedal. At the same time though, another interesting process begins, outside the workshop and the bench, where the Sputnik actually gets out to the hands and feet of music players.
I am curious to see how you will use it, what sounds you’ll come up with and if(hopefully) its adventurous nature will inspire and reflect on your music.

Watch the demo below, follow the links for the Reverb shop: 
and Sputnik’s webpage:           


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