of the Tube Screamer (called Overdrives: the OD-808 and OD-9 as opposed to Tube Screamer, TS). TS9B: Released around 2011, it was a bass overdrive pedal designed for bass players, in a dark green stompbox with five knobs : Drive, Mix, Bass, Treble and Level controls. The Mix and 2-band Eq. controls allow bassists to produce the sound they want while still maintaining the feel of the original Tubescreamer and keep their low-end original signal. Design According to of Maxon, the designer of the Tube Screamer, the initial design concept was to create something to compete with BOSS OD-1 and MXR Distortion+. In doing so, he used an innovative circuit, using the monolithic operational amplifier device, introduced in early 70s, to create a pedal sound different from the "discrete" transistorized 60's fuzzes. The overdrive is produced using two silicon diodes in anti-parallel arrangement into the negative feedback circuit of an operational amplifier ("op-amp") circuit, to produce soft, symmetrical distortion of the input waveform. When the output exceeds the forward volt drop of the diodes the amplifier gain is much lower, effectively limiting the output to + and - one diode volt-drop, although due to the exponential I-V curve of the diodes this is not a hard limit. A "drive" potentiomenter in the feedback path provides variable gain. The original 4558 based circuit uses transistor buffers at both the input and the output, to improve impedance matching. This is mathematically equivalent to mixing the input signal with a clipped version of itself, however. Thought of this way, it is said that this "preserves the original dynamics of the input signal which otherwise would get lost at the threshold of clipping" and "avoids muddiness and vastly improves clarity and responsiveness." Characteristic of the distortion is the symmetrical nature, which produces mainly odd-order harmonics for a sine wave input. All this justifies the "vacuum tube" sound, and the "screaming" tone. As well, Tamura added a simple but effective post-distortion equalization circuit with a first-order high-pass shelving filter that "is linearly dependent on its gain", an approach called "progressivity". The overdrive stage is followed by a simple low-pass filter and active tone control circuit and volume control, using the second op-amp available on-chip. The circuit uses transistor buffers at both the input and the output, and a modern electronic field-effect transistor (FET) "noiseless" bypass switching to turn the effect on and off. The TS7