The Tube Screamer’s Secret....
Just because I find this stuff interesting.....
Tube Screamer®’s Secret
This work is dedicated to Mr. S. Tamura, the designer1 of the Tube Screamer®
TUBE SCREAMER® is a registered trademark of Hoshino Gakki Co. Ltd. It is used solely to identify the product whose tones and sounds were studied during the research and discovery of the information provided in this article. This information is provided “AS IS” without warranty of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose. The author specifically disclaims responsibility for any loss of profit or any consequential, incidental, or other damages resulting from the use or misuse of the information provided in this article. VST is a trademark of Steinberg Media Technologies GmbH.
Although the circuit design details of the legendary Tube Screamer® overdrive pedal have been extensively analyzed and documented2 in the past, the key aspect of its design, which is primarily responsible for the signature overdrive sound, has remained unrevealed. This key aspect consists of a subtlety in the clipping circuit employed by the legend.
Operational Amplifier based inverting amplifiers with back-to-back diodes in the negative feedback path are common clipping circuits which are regularly utilized in overdrive and distortion effect designs3. Figure 1 shows the typical arrangement.
Figure 1. Operational Amplifier based Inverting Clipper
When the output voltage exceeds the forward-voltage drop of the diodes (about 0.3 V for germanium diodes and 0.5 V for silicon diodes), the diodes turn on gradually and softly clip the output waveform symmetrically as shown in Figure 2. This is exactly what is expected from this circuit.
Figure 2. Input and Output waveforms of the inverting clipper (R1: 4k7 - R2: 51K - D1, D2: 1N914)
However, when the same circuit is modified so that the input voltage is applied to the noninverting input of the operational amplifier (i.e. the circuit is converted to operational amplifier based noninverting amplifier) something strange happens. Figure 3 shows the new arrangement and Figure 4 shows the resulting output waveform.
Figure 3. Operational Amplifier based Noninverting Clipper
Figure 4. Input and Output waveforms of the noninverting clipper (R1: 4k7 - R2: 51K - D1, D2: 1N914)
It turned out that the output waveform of the noninverting clipper consists of two components: (1) the amplified and clipped version of the input waveform (2) plus the unamplified input waveform. That is, the noninverting clipper adds (or mixes) the original input signal to the amplified and clipped input signal. The amplified component is softly clipped at the forward-voltage drop of the diodes, whereas the combined components are clipped hardly at positive and negative supply rails in rail-to-rail operational amplifiers or at a level lower than the positive and negative supply rails in non-rail-to-rail operational amplifiers.
Inserting a 47nF capacitor in series with the resistor R1 in Figure 3 forms a pre-clipping first-order high-pass filter with a cutoff frequency of 720.484 Hz (R1= 4.7 KΩ). This filter causes phase shift between the unamplified input waveform and its amplified and clipped version, which results in the output waveform shown in Figure 5.
Figure 5. Effect of the high-pass filter’s phase shift (R1: 4k7 - R2: 51K - D1, D2: 1N914)
Mixing the input signal with the output signal of the clipper preserves the original dynamics of the input signal which otherwise would get lost at the threshold of clipping. Preserving original dynamics of the input signal avoids muddiness and vastly improves clarity and responsiveness. This subtle feature constitutes the heart of Tube Screamer®’s legendary sound and feel.
Block diagram representations of Tube Screamer®’s clipping circuit and post-clipping equalization circuit are shown in Figure 6 and Figure 7 respectively.
Figure 6. Block diagram of Tube Screamer®’s clipping circuit
Post-clipping equalization circuit contains another subtle detail: the exact cutoff frequency of the first-order high-pass shelving filter is linearly dependent on its gain (i.e. boost/cut gain in dB) parameter (with slightly different slopes for boost and cut segments). This is called progressivity of the parameters 4 and it is an inherent feature of almost all great sounding analog equalization gear.
Figure 7. Block diagram of Tube Screamer®’s post-clipping equalization circuit
 T. Hughes, Analog Man’s Guide to Vintage Effects, For Musicians Only Publishing, 2004, page 179.
 R.G. Keen, “The Technology of the Tube Screamer”, 1998.
 S. Franco, Design with Operational Amplifiers and Analog Integrated Circuits, 3rd edition, McGraw-Hill, 2002.
 S. Barbati and T. Serafini, “A perceptual approach on equalization”, 2002.
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