This started out as a way to add oscillation/vibrato effects to my Earl's Court PT2399 delay circuit. The EC is based upon the Sewer Pipes (1), a ring-verb effect from Earthscum that makes for a great tank reverb sound. For my design I added a basic dual op-amp circuit with the Sewer Pipes blended in à la the EQD Disaster Transport Jr. (2), which helps keep things at unity when the effect is engaged. Adding a B50K pot to ground from pin 6 of the PT2399 chip expands the circuit to a delay and something more like the Keeley Magnetic Echo or the 1776 Multiplex Echo Machine circuits (3). Figure 1 shows the B50K from pin 6 in the Magnetic Echo (below). You can also see an LFO in the mix, which we will get into later.
SCHEMATIC
The schematic is fairly simple, as the left hand side with the JRC4558 IC comprises the LFO and drives two LEDs. D2 is an indicator LED while D1, when coupled with the LDR, makes up a vactrol-like control. The SPEED pot does just that, allowing you to adjust the frequency of the LFO and how fast the LEDs flash. Other LFOs circuits also have a WAVE pot that replaces the 100K R7 and changes the shape of the waveform. I'm already running high on potentiometers for the EC build (6 at this point), so I just left this resistor value static.
The far side of the schematic works right-to-left, as the OUT signal connects in between the pin 6-linked B50K TIME pot/1K resistor in the PT2399 and runs more of the signal to ground. The DEPTH pot provides resistance to signal, as 500K can pretty much dulls the vibrato, but this pot can also be increased to 1M to completely dampen the vibrato effect. The 1K R11 can be increased (in some cases, as high as 180K) but doing so also lengthens your delay time. The most important portion here is the LDR photoresistor being manipulated by D1, with highest resistance in the dark and a rapid decrease when activated by light from the LED. The GL5516 LDR I used has an average resistance of 500K in the dark and 5-10K in light, so you're talking about rapidly cycling through 10x changes in resistance when you have it coupled to a flashing LED. There are other LFO mod circuits for PT2399 delays that don't use a vactrol or LED/LDR, so the vibrato mod can be done without the optical parts. The Keeley Magnetic Echo is one example of this and there are a ton of schematics out there for something similar. This little mod circuit is ripe for the breadboard when you want to customize for your own design.
Just a note: for my EC build, I will be using a DPDT switch to toggle this mod on/off. Having just the B50K/1K combo running from pin 6 gives a nice control for the delay time and I don't plan on using the vibrato effect all of the time. Using a DPDT switch allows you to also regulate power to the board so that your LEDs aren't on and flashing the entire time the pedal is powered up. A momentary DPDT foot switch could be a cool thing to use here if you wanted a highly interactive pedal build to play with.
The layout is small and has mounted potentiometers, but depending on needs you can always wire these off-board. I socketed D2 in order to eventually try out different colored indicator LEDs in an enclosure. Keep in mind the forward voltage of colors other than red (˜1.8V) can vary, but even when I used a white LED in D2 things worked fine and those have forward voltages higher than 3V.
Figure 4. EC LFO Mod Board BOM
PERFBOARD BUILD
DEMO VIDEO
Just a little noodling and varying the B50K TIME control on the EC to show how it interacts with the mod board. The initial sound is the tank reverb setting followed by a slow-speed, high-depth LFO mod kicking in at 12 seconds. The tank verb setting with a fast LFO speed gives a nice vibrato, but dropping the LFO speed and increasing the TIME resistance gets you some really weird sounds.
