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.