Ever wondered how those crunch sounds come from the guitars of modern day Rock gods. Well, the answer to this is electronics. So let us go through the history of this “rock” sound. When electric guitars were invented, amplifiers in those days were tube amplifier which unlike today’s amplifiers used triodes or tube valve instead of transistors to amplify signal. The amplifiers in those days were big and bulk. And when they were played at full volume, they produce a “distorted” sound because the guitar signal got too amplified and hence clipped. The great guitarist Jimi Hendrix was the first guitarist to recognise that guitar and the amplifier together can be used as an instrument and started using this distorted sound to produce music. Thus rock music was invented.
But tube amplifiers were unreliable, consumed lot of power and were expensive. Thus electronic engineers started to develop circuits which produced the same effect as a fully driven tube amp but were a lot smaller, effective and reliable. This gave birth to distortion pedals with transistors.
Fig. 1: Prototype of Distortion Pedal
Distortion pedals general have three stages. First stage is a buffer stage, which prevent loss of signal due to impedance mismatch between the previous stage and the pedal. This can be a simple common collector transistor. The next stage is the amplification stage where guitar signal is amplified. This is usually done using common source transistors or op amps. The third stage is the clipping stage which emulates the clipping of a fully driven tube amp. Clipping is done using diodes or mosfets. The final stage of course is tone control and output buffer stage which controls the frequencies of the output signal and helps to prevent signal loss.
Depending on what kind of electronic circuits are used inside a pedal, distortion pedal are of many kinds. For example, a distortion pedal for a bass guitar and electric guitar will have different tone control circuit as bass guitar will operate at lower frequencies as compared to an electric guitar. Also different diodes will give different clipping and hence different tone to the guitar signal.
The schematic of a simple guitar distortion pedal is given in the circuit diagram tab. This circuit uses an opamp instead of a transistor for amplification. As opamps have very high input impedance, there is no need for a dedicated input buffer. Capacitor C2 is used as a low pass filter to shunt high frequencies to ground and hence cuts high frequency noise. C3 is used for low frequency roll off. R5 in conjunction with C4 allow the gain and roll off point to be adjusted and along with the diodes etc. determine the “Character” of the pedal. C4 is very important in setting the “Character” of the distortion. Halving it raises the roll off point to 1440Hz, decreases gain and makes it more Distortion like IMHO. Doubling it lowers the roll off point to 360Hz increases gain and makes if more Fuzz. Hence it decides the tone of the pedal. R5 – sets max gain when Drive is at minimum resistance/max gain. In conjunction with C4 it sets the low freq roll off point for the op amp gain. This acts as two stage distortion. At first the diodes will clip and when the drive is high enough, the op amp will start clipping as well.
R1,R2 and R3 are used to bias the incoming signal and R3 helps improve the input impedance. R6 is reverse log potentionmeter. This changes the gain of the gain of the opamp and hence makes the output signal more muddier. R8 is a log potentiometer which controls the volume of the pedal. It acts as a voltage divider, more the wiper is away from the more the volume.
It is advisable to keep the circuit inside a metal container as it will reduce the interference from the surrounding. The capacitors values can be tweaked to change the roll of frquencies of the circuit and henc change the tone of the pedal. Some popular distortion pedals include Boss DD-1, Ibanez Tube Screamer and Big Pi Muff. Also a bypass circuit must be used otherwise the signal chain will be broken if the pedal is switched off.
Analog pedals are very expensive and hence they are getting replaced by digital equivalent which uses digital signal processing to produce the same effect. These digital units have Digital Signal Processor which run DSP algoritms to emulates analog effects. They are cheaper and can work as a replacement for multiple pedals and hence are popular.
I building it with your hands will cost you less than $10 whereas buying it will cost upwards of $150.
So if you have interest in DIY projects then go make this and keep rocking!!.
Filed Under: Electronic Projects