- Anirudh T
- Rahul Magesh
- Prerana Prakash
The tonal versatility of a guitar is majorly due to effect pedals, which are ideally analog circuits. They process the clean output of a guitar to generate contrasting, distorted tones. These stomp boxes are small, portable, interchangeable and are used in varied combinations to create a guitarist’s signature tone. We intend to implement one pedal (Ibanez TS808 Tube Screamer) using LTSpice. The Tube Screamer TS808 by Ibanez is the most famous overdrive guitar pedal. It is suitable for blues and all range of rock music, adding a classic standard tone characterized by the tubelike distortion, good sustain, and smooth overdrive. The frequency response is tailored to emphasize the mid frequencies, creating a hump that helps to keep the guitar sound over the general mix of the band.
The Tube Screamer circuit is divided into four main sections -
- Input buffer
- Clipping stage
- Tone/volume stage
- Output buffer
The functionality of each stage is simple - The input buffer isolates the pedal from the guitar, keeping the signal fidelity. Then, the clipping stage incorporates the distortion to the signal and the tone/volume block will add the tonal bass/treble adjustment. Finally, the output buffer will prepare the signal to be injected into another pedal or amplifier.
Input Buffer - The task of the input stage buffer is to create a high input impedance to preserve signal integrity, avoiding high-frequency signal loss. It is implemented as an emitter follower.
Clipping Stage - The clipping amplifier is the core of the circuit, and it is formed by a variable non - inverting op amp with two diodes to perform the clipping action and two filters in the feedback loop (passband filter) to shape the amount of clipping and the frequency at which it occurs.
Tone/Volume Stage - This stage is formed by a passive main low pass filter, an active tone circuit, and a passive volume circuit in order to provide tone adjustment and volume control to the pedal. The functionality is very elegant - The main passive low pass filter cuts off the harsh overtones previous to the tone circuit. Then the active tone control will boost the treble trying to flatten the response to compensate and level the effects of the previous passive filter.
Output Buffer - The task of the output buffer is to create a unit gain, lowoutput impedance path to keep the signal integrity in the guitar-pedals-amp chain. Like the input stage, it is implemented with an emitter follower.
What makes the Tube Screamer very distinctive is mainly the frequency selective distortion and the signal filtering -
- In the clipping amplifier Stage, the bass is rolled off by the active high pass filter in the feedback loop. With this approach, the frequencies above 720Hz get the full distortion, and harmonics below it get progressively less distortion, so the distortion is frequency selective.
- The treble is shaved off after the distortion stage with a passive low pass filter, generating the distinctive mid frequencies boost.
LTSpice allows the use of .wav files in voltage sources. A sample audio file was used as the input and the output voltage was exported as a .wav file. The input and output waveforms are shown below. The output sound was pleasantly distorted and it is consistent with the output one would expect with a real pedal.
Audio Files -