Sunday, 16 December 2012

Tube Screamer goes digital

Having enjoyed playing about with the "Tube Screamer" guitar overdrive effect, I decided to experiment with a digital version. Purists need not panic - I don't mean anything vulgar like an embodiment of the effect in digital signal processing. Rather, I mean a digitally controlled analog Tube Screamer. This way, I get to keep the architecture and (hopefully) sound of the original "urban legend", but add flexibility of control, easily recalled presets and - one day perhaps - external control by MIDI.

 Here's my initial experimental version, which I completed today...


The breadboard includes a standard Tube Screamer circuit (here implemented using two single (NE5534) op-amps to spread things out a bit) with the potentiometers replaced by digitally controlled resistor networks.

The whole shooting match is under the control of a big fat PIC 16F887 micro-controller - chosen because it has LOTS of IO lines.

 Instead of the "knobs" there is a user interface comprised of a simple "keypad" and a 16*2 LCD Display. The keypad allows the user to move around a menu system, giving access to the "Drive", "Tone" and "Level" controls. One day (though I haven't implemented it yet) there will also be some default settings.

Here's some (poorly photographed) views of the menu structure...


The keypad and menu system were borrowed from that I developed for the m0xpd multi-mode beacon project.

At the moment, I'm experimenting with two alternative means of implementing the digitally controlled resistors, both of which emerged from the m0xpd "junk box". First, for the "drive" control, I'm using a 4066 quad bilateral switch, the elements of which can short a ladder of resistors in a "base 2" sequence of magnitudes (in so far as E12 resistors can approximate such a sequence), giving 16 different drive levels.

Here's the bones of the schematic...


The tone and level controls use pukka AD5220 digital potentiometers (the junk box does have some pretty high-class junk), with the level control made to have a log-like characteristic using Rod Elliott's "Better Volume Control" trick.

The whole thing works surprisingly well - more reports later when I add more sophistication!

...-.- de m0xpd

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