Misc: LFO Modulation Board

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.

 

 

Figure 1. Magnetic Echo, pin 6 to GND "time" effect (pic borrowed from PedalPCB)

 

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. 

 Figure 2. LFO Modulation Patch Effect for the Earl's Court Delay

 

PERFBOARD LAYOUT

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 3. Perfboard layout (top) and reversed underside traces

 

 

Figure 4. EC LFO Mod Board BOM

PERFBOARD BUILD

 

 Figure 5. Perfboard LFO Mod Build

 

 Figure 6. LFO Mod Board integrated into the Earl's Court via breadboard

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.

Synth: Coisa Linda (Square Wave Section)

This project started out with a schematic I found in the LM386 datasheet (1) for a square wave oscillator. I was a little surprised since I was more familiar with the LM386 as a power amp in circuits like the Earthquaker Acapulco Gold (2) but once I got a working breadboard it was interesting. Eventually I found out you could make an electronic piano with an LM386 (3) and worked the tuning pot/switch combination the the design, which gives you about 1 octave of range that you can change with a B10K potentiometer. The square wave section is then followed by a gain recovery stage using an LM741-based op-amp that is very similar to the MXR Distortion + (4). There are five controls in this basic design:

1. VOLUME - A100K, controls master volume out

2. GAIN - B1K, connects pins 1 & 8 of the LM386 and affects the gain/grit of the square wave

3. BASS - B100K, usually fixed at 1K, but by adding resistance with a potentiometer you can vary the pitch dramatically and get sub-bass tones

4/5. TUNE 1/2 - B10K, adjust pitch of the square wave

This module by itself produces a tunable sound but also has the option of blending in another effect (at the two pins flanking the 20K R2 in the schematic below). The idea was to make a useful synth that people can customize however they like. For my first try I used the PT2399 section of Mad Professor's Deep Blue Delay (5) from a PCB and patching in/out at the right spots on the board. You can see some results in the video below. Since the square wave is already pretty fuzzy, you might have best success with modulation effects like phaser, chorus or other delays. Additionally, one could swap out R2 with an LDR and  incorporate an LFO-driven LED to make a tremolo-like effect.

Fig 1. schematic  

 

PERFBOARD LAYOUT

 

Fig 2. Perfboard layout with hardware and BOM

 

PERFBOARD BUILD 

Fig 3. Perfboard build

DEMO VID

This video was made with the full build but I have the delay switched off (using an SPDT) and blended out so that the "clean" square wave signal can be heard at the beginning. The delay mod is turned on at 1:35 and you can see what it adds to the signal.

Fig 4. Coisa Linda bass line rinse out sesh

 

 

 

 

WIP: Buffered Dual FX Blend Board

This one's more of a utility circuit, it combines two signals by blending the output and uses an gain recovery amp to reduce loss of volume. It's the expansion of a schematic included in an older Geofex PDF about panning circuits for use with guitar effects (1). The inputs are buffered to help with variations in output impedance from the circuits being blended. There was an extra opamp left on the IC so I buffered the power (2) and used a TL074 IC. Right now this is designed as a standalone circuit but can easily be integrated into a circuit that internally blends signals, like the MXR Blue Box (3)...actually, more on the Blue Box some other time.

Fig 1, schematic

1. The TL074 can be substituted with two TL072s. If you don't want the power buffered, you can do a TL072/TL071 combination as well.

2. The non-inverting amp (IC2_C in the schematic) has a gain of 11 dB (4), but can be adjusted according to your taste. There's a loss of volume after the B10K pot and this is what got my setup to unity volume. 

3. This circuit isn't supposed to color the sound too much and there are easy changes for mods. C1 and C3 have a big effect by filtering respective input, increasing the value will eventually dampen the sound while going lower (˜100pF) could result in more ground hiss. The R9-C12 R-C filter (R13-C13 as well) after the buffer can also be adjusted. R22 also has an impact on the output filtering and can be modified. 

 

BREADBOARD BUILD

It took some work but I got this circuit to fit on a 400pt breadboard. Both inputs work (can hear signal in output) when a guitar preamp is connected and the B10K pans nicely between the two sides.

 

 

Fig. 2, breadboard build

 

PERFBOARD LAYOUT

It took three versions to get this laid out on perfboard but I have not verified it yet. The Volume pot can be wired on-board to help you secure the circuit in the pedal build. I'll update this project when I get the build up and going.

 

Fig. 3, perfboard layout and B.O.M.

Guitar: Sound to Light Fuzz

This fuzz started out as as adaptation of a circuit I saw online that used a peizo mic for input to convert sound into light flashes. The schematic looked like it could easily be adapted to a guitar pedal with volume control, jacks, power filtering and polarity protection.

1. You can also use a 1N5817/100uF power filtering combo and will have a lower voltage drop across the 1N5817 in series (1). Be sure to not just sub the 5817 for the 4001 in this schematic, it won't work that way.

2. This fuzz is high gain, so one mod could be to add resistance to ground from Q2. This could roll back some of the harsh distortion with these high gain transistors. 330R-1K could be an option to start and you could also try adding a cap to ground - like at Q2 in the ToneBender - for extra crunch with the resistor in place(2). I am going to try this on a breadboard soon and will report back. 

3. Changing to input cap (C1) to a higher value like 470nF or 1uF will allow for more bass frequencies to come through. C2 and C3 are adjustable as well. 

4. Q1-3 can be almost anything with a higher gain - 2N5088, BC549C, etc. 

5. POT1 can be adjusted to taste, a B500K might roll off some of the treble if the circuit is too "bright" with the A100K (no pun intended).

 

Fig. 1, schematic

STRIPBOARD LAYOUT

This one is fairly simple, but depending on what enclosure you box it up in you may want to wire the LEDs off board. For my build I socketed them so I can just wire them up however I want in the end. 

 

 

Fig. 2, stripboard layout and BOM

 

STRIPBOARD BUILD

 

Fig. 3, stripboard build, unboxed

 

DEMO VID

Fig. 4, demo vid

Bass: GBS Fuzz

This is the GBS, it developed from a chat with JSA Effects (1) about adding some buffering in front of the Bazz Fuss, a simple but powerful-sounding fuzz circuit. I ended up adding a passive Muff-style tone control plus gain recovery stage for some EQ as I had an unused half of an opamp. Fatter coupling caps also make it more appropriate for bass frequencies.

1. The buffer is standard Klon-style (2) but the gain stage uses a BC549C instead of the 2N5088 seen in many Bazz Fuss builds and is biased by the 3.3M R5 resistor. This gives the fuzz more "growl" and saturation to my ear. I borrowed this from a Brazilian company called Epider Sound and their Heavy Metal pedal (3). The addition of the R5 doesn't seem to work with BC549Bs, so make sure you have the right transistor.

2. D1 can be swapped out with other types of diodes for different textures - other silicon (1S1588) or BAT41/46 are good starting points for more open vs compressed gain.

3. Right now, the gain recovery stage is set at 10 dB (470K/47K for the non-inverted second amp). A B500K pot could replace the 470K (R11) if you wanted the option to dial it back. I will definitely try this on the breadboard again, since this fuzz can get really saturated.  

4. The tone control has corner frequencies of 1540 and 800 Hz, but these are easily modified with standard BMP tone stack values (4).   

5. You may want to socket the 47nF cap C12, it's what worked best for a bass fuzz but can be customized for your own tastes. This cap has a big impact on the amount of background hiss.

Fig. 1, GBS Schematic

 

The perfboard layout is pretty straightforward and I managed to get it all together without any jumpers. As of right now, this exact layout is unverified. The one I used for the pedal build needed a few fixes so I worked in changes to clean up the wiring for this layout.

 

Fig. 2, Perf Layout and Mask (Reversed)

 

Here's a few pics of my build, since I used a DPDT switch there is a daughter board with the Millenium 2 bypass for the indicator LED. A 3PDT switch means you don't need this tiny board. 

 

Fig. 3, GBS Build Apr 2023

 

Demo vid:

 

Fig. 4, It's only 1 minute