Circuit Analysis

Footswitch and Expression Pedal Circuit Analysis

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Recently, I bought the Qu-Bit Stardust, which is a “cosmic” tape looper. It’s actually a really cool looper and I have not used loopers very much. My reason for adding this to my collection is to have a way to layer recordings from non eurorack synthesizers (traditional synths). Immediately, I found a need for a way to punch in and out of recording easily so that I could play along with the loop, but then punch in to record a new part without breaking my “flow”. A footswitch seemed to be the obvious answer, but the implementation in my eurorack system was not so obvious. I had never really considered how a footswitch (or expression pedal) works, so my first course of action was to figure that out.

The Setup

For this example, I am using a Boss FS-6 Dual Footswitch and a Moog EP-3 Expression Pedal. The Boss FS-6 contains a 9V battery and there are switches on the back to allow each of the footswitches to reverse polarity and work in either “Latch” mode or “Momentary” mode. The Moog EP-3 also has a switch on the bottom to switch between “Standard” or “Other”. In addition the EP-3 also has a trimmer knob on the side that “scales the output” according to the Moog writeup about it. I will get into what these do shortly.

What are we even talking about?

First off, a footswitch is just a switch. By engaging / stepping on the switch, you are just closing (or opening) a circuit. This is the first important concept to grasp. The second thing to note is that the footswitch may be opening or closing the circuit. The FS-6 has a polarity switch that allows you to set it either way. The last thing is that the FS-6 can be latched or momentary. For this example, we are going to have the polarity set to normally closed and the latching on. This way, when the footswitch is “off”, the circuit is actually completed through the footswitch. When the footswitch is “on”, the circuit is broken within the footswitch and it remains broken until the footswitch is clicked again.

So, let’s say for an example that we want to send a gate signal to that Stardust looper that I mentioned earlier. We would have to route a constant voltage through our footswitch and when we step on the footswitch, it passes that voltage to the looper. If this voltage remains high, the looper records. Stepping on the footswitch again would open the circuit and the looper stops recording. Let’s get into how this actually works.

The Dual Footswitch Circuit

A dual footswitch connects to a device with a typical 1/4″ TRS cable. The tip and ring are connected to switch 1 and switch 2 of the dual footswitch and the sleeve is a common return for both. So on the receiving module in the eurorack system, we have a 1/4″ TRS jack. The positive 12V connects to the tip and the ring of this jack through 2 10k resistors. When I started to research this simple circuit, I immediately found that Doepfer solved this for us a long time ago with the A-177-2 module. It’s such a simple module, that I was able to reverse engineer it from photos alone.

Imagine the dual footswitch plugged into that circuit with the first switch connected to the ring and sleeve of the TRS cable. When the switch is “off” (closed), 12V flows through a 10k resistor and gets shorted to ground through the switch. You press the switch and it latches “on”, which opens the switch and now that 12V does not short to ground any longer, instead it becomes present on the Switch 1 Output jack, giving us approximately a 10V gate output. The same thing happens for Switch 2.

Express Yourself

Now on to the expression pedal. A 1/4″ TRS cable is used for it as well. All the expression pedal really is, is just a potentiometer. The wiring may vary, but the standard pedal connects the tip to one side of the pot with the sleeve on the other side. The wiper of the pot is connected to the ring of the TRS cable. I think in the alternate wiring, the tip and ring may be reversed. That’s what the switch on the Moog EP-3 changes between. In addition, many expression pedals have an attenuator knob on the side. This just changes the maximum voltage at the output. You could also run the output through another module to attenuate, offset, or invert it.

Back to our eurorack receiving module, we have a 1/4″ TRS jack with the tip connected to 12V through a 1k resistor and the sleeve connected to ground. The ring connects directly to our output. This way, when we sweep the expression pedal, we are generating a voltage from 0V to approximately 10V.

Conclusion

That’s really all there is to it. You could tailor these circuits to your exact needs by adding attenuators or voltage dividers to obtain voltages in other ranges. It’s a pretty simple circuit to assemble, but if you don’t have the spare parts, time, and curiosity, you are likely better off to just buy the Doepfer A-177-2 and be done with it.

Until next time…

DJTW Dual LPG

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Versatility or Confusing?

The DJ Thomas White Dual LPG is a nice sounding resonant filter to be sure. I think that the quality of the vactrols makes a pretty big impact on the sound. This can be a problem since vactrols are harder and harder to find.

This module is very well laid out for a variety of uses. The ability to easily customize the inputs and outputs, as well as the power connection is pretty cool. This is also one of its biggest problems. If you just jump straight in without doing a lot of research first, you are bound to make mistakes.

The version I was working on was the Dual LPG, so I had two of the boards at the top of this post. Each of these boards contains circuits for the filter / LPG channel, as well as an audio mixer circuit and a CV mixer circuit. If you are building a dual or quad version, you would need to wire things differently. Every component is not even used on every board. You configure it for your own purposes. The lack of documentation is frustrating to say the least.

The most useful thing I found during all of my searching was a zip file containing docs gathered from modwiggler. I am including a few diagrams that are contained in that collection.

As you can see, this first image is a pretty clear illustration of how to wire two of these boards together. Some of the most notable things to pay attention to are the “Audio” and “CV-In” jumpers, as well as the “Res” jumpers. In addition to those, which are the same on both boards, the “Audio Mixer” is being used on “Board 2” and if you look carefully, the “Mix Out” jack is wired to a pad that is underneath the “Audio” jumper. The way this works is that the header at the bottom left corner of the board is connected to the TL074 mixer circuit and the output of that circuit is the pad near the “Audio” jumper. There is also a header (not shown in diagram) that allows multiple CV jacks to be input and a similar CV mixer that utilizes the other half of the TL074. For this Synthcube style Dual LPG panel and PCB kit, there is only one CV input and one Audio input, but you could potentially have 3 inputs for each and mix them on each channel. It opens up some interesting possibilities for the more extreme DIY enthusiast. On the example above, “Board 1” is not using the mixer section of that PCB at all, so you can actually leave several components off of that board. See below.

Feel free to comment below any questions you might have. This has only been a basic overview of the challenges I encountered while attempting this build. At some point in the future, I may attempt a redesign of this, making it a eurorack exclusive build and fix the other glaring problem it has, which is that it is about 5 inches deep when assembled.

Until next time…

Feedback Drive of Erica Synths DIY BBD/Flanger Analysis

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Today, I would like to go over the feedback drive circuit of the Erica Synths BBD/Flanger. Let’s jump right into it.

The Switch

SW3 is a double pole, double throw switch. In the center position, there is no “drive” and at up or down, it switches between 2 pairs of diodes (or LEDs) and 2 resistors or different values simultaneously. The resistors balance the gain between the two drive flavors and you will need to choose these resistors carefully for your specific module. As you can see below, there are 6 diodes shown on the schematic, however, only 4 are to be populated.

LEDs or Diodes?

You may use either LEDs or diodes in two of the VD6, VD7, VD10, and VD11 spots. VD4 and VD5 are meant to be 1N4148 silicon diodes. If you choose to use LEDs in the other drive spot, you would populate them in VD7 and VD10. You could use a different pair of diodes, such as BAT41 schottky diodes in the VD6 and VD11 spaces instead. The feedback drive switch, it simultaneously switches the diode pair and the resistor in the gain circuit (R71 or R72). This is where confusion begins to creep in for the builder. The BOM shows R71 as NU (not used) and R72 is shown as 10k. The schematic shows R71 as “Drive Gain” and R72 as 10k once again.

Mixed Signals in the Documents Leads to Confusion

The third source of information, the “Guide To Assembly” contains the following text at step 4 of the instructions.

“Next to the FEEDBACK DRIVE switch you’ll find solder pads for overdrive diodes. You can use any diodes of your taste for pairs VD4/VD5 and VD6/VD11 or VD7/VD10 which are LEDs. The kit contains two red LEDs (if you use LEDs, do not populate VD6/VD11) and 1N4148 for VD4/VD5. You may experiment here with different diodes (try germanium diodes for softer overdrive) of LEDs of various colours it will result in different overdrive flavour. I used 33k for R71 and 100k for R72 in the overdrive configuration supplied with a kit.”1

As you can see, in the build guide, Erica Synths calls out that they used 33k for R71 and 100k for R72. If you make this mistake, you will have a module that has a severe volume change when switching between those drive flavors. I’ll be honest, it has been a month or so since I worked on this module and I don’t remember the exact resistor values I used, but that is also not important. The important part is that you figure out the correct value for the module you are building. In step 7 of Erica’s build document they state this: R71 sets the gain for VD4/VD5 and R72 sets the gain for LEDs (VD6/VD11 or VD7/VD10). You need to adjust the gain of the feedback signal so that the module goes into self-oscillation at approximately the same settings for all switch positions.

Resource for Diode / LED Clipping

As I studied this part of the build, I found a webpage that does a fantastic job of describing saturation flavors from a variety of diodes and LEDs.

https://www.guitarpedalx.com/news/gpx-blog/a-brief-hobbyist-primer-on-clipping-diodes

Several circuits listed there, as well as comparisons between the diodes and LEDs. It is a great resource if you are building this module or anything else where diode clipping is involved. At some point in the future, I have a plan to build a diode clipper circuit from scratch. Distortion and saturation is one of my favorite things, so stay tuned for that.

Footnotes
  1. https://www.ericasynths.lv/media/BBD_Delay_manual_assembly1.pdf ↩︎