Download Circuit Analysis of Overdrive Tube Amplifier Circuits

Circuit Analysis of Overdrive Tube Amplifier Circuits
AbhijeetSutrave
[email protected]
Physics 193: Physics of Music
University of Illinois at Urbana-Champaign
1. Introduction
This paper covers a simple and relatively non-technical analysis of the overdrive tube
amplifier circuits. The specific model I analyzed was the BYOC Overdrive II circuit, which itself
was a clone of the famous Ibanez TS (Tube Screamer) 808. The circuit is meant to emulate the
sound effects of tube amplifiers in general but using solid state silicon components instead of the
notoriously difficult and cumbersome vacuum tubes. Since the Two circuits are overwhelmingly
similar, I was able to analyze the BYOC box using the information for the TS 808 circuit along
with the schematic. Also because there is so little variation between the models I found that it
was also possible to compare it to studies done on other tube amplifiers and overdrive circuits.
2. Sound in Electronics
But before we understand what an overdrive circuit is, it is necessary to understand what
sound means in terms of electronics and conventionally. When a guitar string is plucked on an
acoustic guitar, it sets the body of the guitar into a mode of oscillation. The body in turn produces
sound by causing pressure oscillations in the air. This is what our ear perceives as sound. An
electric guitar however works quite differently. When the string on an electric guitar is plucked,
it causes a current and voltage to be generated in the magnetic coils at the end of the string.
These currents and voltages are oscillating because the string generating them is oscillating.
These voltage waves, hereafter referred to as signals, can be broken down into a sum of pure sine
and cosine waves of particular frequencies by Fourier analysis. It is these voltages and
frequencies that are modified by the circuit in the effects box. The modified signal is then passed
to a speaker where it is used to drive a magnetic coil into vibrating and causes sound. Electrical
signals are interesting because they can be modified much more easily and in much more
interesting ways than conventional acoustic signals. The tube screamer circuit is an excellent
case in point
3. Effects on Sound
The tube amplifier was made famous by legendary American blues singer and guitarist
Stevie Ray Vaughan. The circuit has long been associated with the smooth blues feel that Vaugn
was famous for. It lends an air of mellowness to the sound of the guitar by reshaping the
frequency distribution away from harsh lower and higher frequencies and emphasizing the center
frequencies. It also distorts the signal by using diodes to cut off the signal past a certain voltage.
This emulates the push-pull effect of the vacuum tube in the original tube amplifiers.
4. Analysis Technique
I analyzed the circuit using a technique known as block association. By breaking the
circuit up into pieces I was able to analyze each part as a block and assume that it was mostly
unaffected by the rest of the circuit. The circuit itself consists of four main parts and an optional
bypass switch.
5. Input Buffer
Input Buffer
The first piece I looked at was the input buffer. The input buffer consists of two low pass
capacitors that filter noise and harsh unwanted low frequencies on either end. The two resistor
provided high input impedance which is important for keeping the signal in an acceptable range
and prevents the output of the box from overloading the amplifiers. The transistor is arranged in
a simple emitter follower configuration that doesn't do much except buffer the signal. The
arrangement has a gain of 1.
6. Clipping Amplifier
Clipping Stage
The next piece of the circuit was the most important part of the circuit: the clipping stage.
The clipping stage is what is responsible for the characteristic sound of the overdrive circuit. The
input is again filtered through a capacitor, draining away harsh low frequencies. The op-amp is
used as a high gain amplifier. The circuit filters off particular frequencies by taking advantage of
the fact that the impedance of a resistance-capacitor (or RC) combination is inversely
proportional to the the frequency of the signal. The actual clipping occurs when the voltage
across the diodes is greater than 0.6 V. When this occurs the diode turns on and forces the voltage
across the diode to remain at a constant value of 0.6 V. This gives rise to the “clipping” of the
signal above certain voltages. And since there are two diodes pointing in opposite directions, the
clipping occurs at both sides of the sine wave (i.e. at both polarities, positive as well as negative).
The 51 pF capacitor softens this sharp clipping by charging and discharging when the diodes turn
on and off respectively. Since this was the most important part of the circuit we simulated it in a
fair amount of detail. The results are covered later in the paper.
7. Tone and Volume Control
Tone and Volume Control
The next stage was the tone and volume control portion of the circuit. This part of the
circuit is fairly simple and uninteresting. The volume is increased by increasing or decreasing the
impedance with the help of the variable resistor. The tone control is slightly more complicated. It
boosts certain frequencies by taking advantage of the selective frequency filtering of the RC
potentiometer. It is usually used to boost either the bass or the treble depending on what is
required.
7. Optional Bypass Circuit
Bypass Circuit
The next stage of the circuit is the optional bypass circuit. It uses a FET (field effect
transistor) to turn the box on and off and uses a simple flip-flop to memorize the state of the
circuit. There isn't much that is very interesting or complex about this phase of the circuit so I
ignored it almost entirely.
8. Output Buffer
Output Buffer
The final stage of this circuit is the output buffer. It, like the input buffer is a standard
emitter-follower configuration. The properties are basically the same as that of the input buffer
and it servers more or less the same purpose: to prevent overloading of the speaker.
9. Waveform and Frequency Analysis
The next part of this paper analyzes the output wave forms and frequencies generated by
this circuit. In all the wave cases the input waveform was a pure sine wave. In the case of the
waveform analysis it was a 100 Hz sine wave with a 4 V peak-to-peak amplitude. We decided to
simulate only the main clipping amplifier part of the circuit, since the buffers and controls don't
change the signal in any significant way. Also note that the scale on the frequency analysis
graphs is logarithmic and that the scale on the waveform analysis is 1V per division on the Y-axis
and 1 us per division on the X-axis The circuit that we simulated is shown above. The values of
the variable resistances are shown below.
Resistor State
R8 (100k Ohm)
R9 (500k Ohm)
R10 (10k Ohm)
1
60.00%
100.00%
5.00%
2
60.00%
100.00%
100.00%
3
5.00%
5.00%
100.00%
4
5.00%
5.00%
5.00%
10. Results of the Simulation
Frequency Analysis for Resistor State 1
Waveform Analysis of Resistor State 1
Frequency Analysis for Resistor State 2
Waveform Analysis of Resistor State 2
Frequency Analysis for Resistor State 3
Waveform Analysis for Resistor State 3
Frequency Analysis for Resistor State 4
Waveform Analysis for Resistor State 4
11. Conclusion
It is easy to see how the stage deforms the wave, especially when the resistance of the
variable resistors is low as in State 4. You can also see how frequency peaks around the middle
of the frequency spectrum, resulting in the mellow blues for which the tube screamer is so
famous. All in all, I think I learned quite a bit from writing this paper and analyzing this circuit.
And while it was a fair amount of work, I think it was well worth the effort.
12. Bibliography
1. R.G. Keen (1998). Technology of the Tube Screamer. Retrieved from
http://www.geofex.com/article_folders/TStech/tsxfram.htm
2. K. Vonderhulls. Build Your Own Clone Overdrive 2 Kit Instructions. Retrieved from
http://www.buildyourownclone.com/overdrive2.html
3. Yeh, D., Abel, J., Smith J., (2007, September 10). Simplified, Physically-Informed Models
of Distortion and Overdrive Guitar Effects Pedals. Retrieved from Scopus.