CR-1615 Analog multi-effects processor

Overview

This project is a multi-effects processor for guitar, synthesizer, audio, or any kind of sound that you can plug in. There are 5 separate stages when sending audio through this circuit. First, we have a gain stage which simply is used to up the amplitude of the audio signal coming in if need be. This is usually used for guitars to get them up to line level. Next is the distortion stage with 2 knobs: one for moving between “dry”(no effect) and “wet”(effected) signal, and one for changing the amount of distortion on the signal. Next we have 2 similar stages, the filters. Both filters have a knob and a photocell to control them, plus a toggle switch to change between either manipulating the filter with the knob or the photocell. Next, we have the tremolo effect with knobs for the frequency of the tremolo, mix between dry and wet signal, and a mix between square and triangle wave to drive the effect. Lastly, there is a true bypass switch that can be used to bypass the effect entirely.

Technical Description

Starting from the beginning of the circuit, we first have a “safety circuit”; basically just a capacitor to eliminate DC bias and a resistor to keep the signal level at a manageable level (In this safety circuit there is also a resistor after the capacitor to ground because without it, the capacitor will make the signal “float away” and DC bias, the resistor releases that DC bias to ground).

Next we have the primary gain stage. This is basically just a non-inverting op amp circuit with a maximum gain of 50X using a 500k Ohm potentiometer in the feedback loop and a 10k Ohm resistor to ground. This is followed by a voltage follower to separate it from the next stage.

After that is the distortion stage; this consists of a potentiometer-controlled non-inverting op-amp circuit with a max gain of 250X (this controls the amount of distortion applied) and 2 diodes going to ground, one in reverse bias and one in forward bias. After this there is a potentiometer that is used as a cross-fader between the signal before and after the distortion circuit so the user can choose how much of each signal they would like in their mix. This is, like all stages, followed by a voltage follower.

The signal is then split between 2 paths, both which are low-pass filters: one controlled by a potentiometer and the other controlled by a light sensitive resistor (photocell). On the potentiometer side the capacitors add up to 20nF with a 500k potentiometer making its sweep roughly 15Hz (if the potentiometer actually went up to 500k, it actually only went up to about 400k) to 79kHz. Both filters had this sweep for the potentiometers. The photoresistors go from roughly 350Hz to 5000Hz without the use of a light.

Both the lowpass filter and highpass filter work the same and have voltage followers between each to make sure that the impedance from one, doesn’t affect the rest of the circuit.

Last in the chain of effects is the tremolo effect. This uses an oscillator below the frequency of hearing to control an LED going on and off. This LED then affects a photocell that’s coupled to it and changes the amplitude of the signal. In my circuit there are 3 controllers for the tremolo: an A50K pot for the crossfade between dry and wet signal, an A100k pot for the frequency knob, and a B5k pot for the crossfade between square and triangle waves in the oscillator.

Next we have a B5k pot with one leg going to ground in order to make it a volume knob with gain of 0-1. After that is another safety circuit of a capacitor to take away DC offset and resistor to damper the amplitude and then a voltage follower so headphones can be used to listen to the circuit.