Keith Richards, the lead guitarist of the Rolling Stones, is an extremely brilliant and talented guitar player and all guitar players aspire to play like him. This article began because I received an email from a visitor who had seen the Philips Compact Cassette Recorder EL-3302 article and wanted to know if it could be used to overpower the audio so that it would clip the sound. Electronic engineers usually go to great lengths to ensure that clipping does not occur in their amplifier circuits because it is an undesirable effect. After some research, it transpired that Keith Richards might have used a Philips Cassette Tape recorder as a preamplifier front-end to his guitar to generate a clipping effect.
Overdriving EL-3302 Cassette Recorder
There is no conclusive proof of any superstar over-driving a cassette recorder by using it as a preamplifier to a guitar, however I remember seeing an old photograph where Mick was holding one. The Philips EL3302 was a great gadget of that era which many musicians would have had. It was a portable voice recorder, so almost all the singers of that era would have had one.
After studying the circuit diagram, I can see that it has some unique features that will not be found today in modern gadgets. This was the first widely sold portable tape recorder and the engineers designed it with the minimum of components. Being a new and expensive component, the transistor count was minimal. In this model, there are nine transistors. Two transistors for the motor regulator, which keeps the tape, speed constant. One transistor is for the level meter. Three transistors are to drive the small speaker. Therefore, there are only four transistors for the audio amplifier circuit.
There is no automatic gain control circuitry because that would require at least three more transistors. Thus if the input signal level was too high then clipping distortion is the first thing you would notice. There are plenty of transistor biasing resistors, and inter-stage decoupling capacitors keeping the circuit very stable even when it is clipping. The amplifier transistors are AC125 in a metal tube style package and would have been mid-range specification of that era.
After some extensive testing of the circuitry, I discovered that it was only the first transistor in the input section that is responsible for the clipping. This is understandable as its purpose is to amplify the signal from a microphone. However, the interesting thing to note was that the clipping is soft and fuzzy because the transistor recovers quickly.
The AC125 documentation indicates that it has a minimum hFE of 50, and Ic=2 mA. None of the parameters are remarkable except that its frequency handling is very narrow, which is to be expected of these old transistors. I wonder if it is that which provides a useful filtering property.
Overdriving an amplifier
Overdriving an amplifier creates a wonderful effect that guitarists love. There are many things happening here. The most obvious of which is the clipping distortion in the sound. Many people including musicians have not fully understood the exact nature of this phenomenon but have latched onto the obvious fact that there is a clipping distortion and somehow related to this amazing sound. However, the clipping is the side effect to the phenomena.
To understand what this phenomena is, a person with mathematics, electronics, computer, physics, and music skill is required. However, where can you find them these days? Most adults leave college with very little useful education. You get these rich kids with doctorates and over-inflated grades that cannot string a sentence or tie their show-laces.
Clipping Diode Amp
Clipping by increasing the gain is very different to clipping by using diodes on the final output. The former method amplifies the harmonics and clipping is an undesired after effect, whilst the latter method simply introduces the clipping distortion without amplifying the harmonics.
Many people have not understood the true principle behind the secret of this phenomena and designed circuits that simply created the distortion effect. This seems plain crazy to me because this is like creating the side effect without the main useful effect.
It is easy to create the clipping distortion. Two general-purpose 1N1418 diodes in parallel (pointing in opposite directions) between the signal and ground ought to do it. You do not need any special op-amp circuits either; the audio output from a preamplifier clips easily this way. However, that is not going to amplify the overtones or produce that unique sound.
"I can't get no satisfaction"
On the Internet, you will see countless amplifier circuits with clipping diodes at the final output stage to create the clipping distortion effect. That is exactly what it does create - the distortion. However, none of these circuits will produce that unique Keith Richards sound.
A visitor emailed me to say that he had tried nearly all of the clipping diode circuits and none of them produced that unique sound he had heard when Mr Richards was playing. He could not obtain any measure of satisfaction. :-)
That is interesting; I suppose everyone tends to jump onto the 'clipping amplifier' bandwagon. It is the one obvious and simple idea people can understand and hear.
The Secret to Keith Richards' Unique Sound
The secret is very simple in practice but requires some experience to achieve. By increasing the gain of the preamplifier, or overdriving it, there will be clipping occurring however, that is not all that will be happening.
The gain increase is very critical. You need it turned up at the point where it just starts to clip the sound. The clipping distortion is not the desired effect or the goal though. The secret is to amplify only the overtones without the fundamental frequency. It just so happens that the fundamental will be the main one with the highest amplitude and therefore increasing the gain to the point where it starts to clip limits its audibility.
However, since the gain is high, another important thing happens. The overtones, which are usually lower in amplitude and not easily heard appear louder, and brought out into the front. When that occurs, one hears that amazing sound that is breath taking.
Even numbered overtones are the most pleasing to human ears however unfortunately; overtones are generally lower in amplitude energy because the fundamental frequency usually overpowers them.
If one can block the fundamentals, by clipping or any other means, then the overtones will be easier to hear. In addition, if the overtone gain increases then you should hear that unique warm glowing guitar sound that all guitarists would walk the ends of the Earth to create.
Overtone filtering is difficult to achieve even today with our digital processor technology and Fourier Transform algorithms, however not impossible if you understand the principle.
I think it is remarkable that Mr Keith Richards made use of this phenomena because the physics and the mathematics are rather involved and worthy of a PhD degree. Essentially, it boils down to creating a filter, which will filter out the fundamental frequency of any note.
This is achievable using FFT on ATMega32 - Fourier Analysis and digital processing techniques. Sample all the data in the time domain, and then transform them into the frequency domain. Delete the fundamental frequency and the odd harmonics, and then transform the data back to the time domain. It is that simple, and there would be no clipping distortion only a perfect guitar sound that all guitarists would fall on their knees when they heard it.
There is also an additional important phenomenon occurring that nobody has discovered or understood yet. I do not know how important this Holy Grail to the Keith Richards guitar sound really is. However, depending upon the hits this article gets I might expand on this and make a circuit of my own that everyone can follow and make. It will be in the Electronic Engineering section of this site, with the mathematics in the Digital Signal Processing DSP Tutorials section.
Currently, I have more important things that need doing such as getting my winter fuel bills paid, and feed my starving cats.