The DOD FX-17 - How It Works

By Paul Marossy
Last updated 07/30/05

The DOD FX-17 is an interesting circuit that uses proprietary techniques. How it works is very clever, and thanks to Mark Hammer, cd and puretube at the diystompboxes.com forum, I can explain the basic aspects of how this circuit functions. It utilizes a pair of "4007" IC chips (which are a dual complementary pair and inverter with 3 N-channel and 3 P-channel enhancement mode MOS transistors), a "13600" (dual OTA with two current controlled transconductance amplifiers with differential inputs & push-pull outputs), a LM3080 operational transconductance amplifier, a LM358 dual opamp, a pair of J113 N-channel JFETs and a MPS4124 NPN transistor.

Each of these IC chips and transistors has a specific function. The half of the LM358 used on the input is an input buffer. The pair of J113 FETs comprise an electronic bypass switching system that is similar to the ones that Boss pedals use. To understand how this switching system works, imagine that the "FET B" has an exremely high resistance while the "FET A" has a low resistance, and vice versa. So, in "bypass" mode, the signal goes directly from the first opamp stage to the output jack. In "effect" mode, the "FET B" signal path is blocked by the very high resistance of the FET, and goes through "FET A" instead. At this point, the signal goes through the LM3080 - its gain is controlled by the foot pedal, and the wah circuit which is built around the 13600. Then the output of each FET is selected and is fed to the output jack. The classic DOD switching circuit is shown here.

The switching system is actuated by the 4007 shown on the right side of the schematic, and this also does part of the signal routing. The 4007 shown on the left side of the schematic is a HF-oscillator based "proximity control", together with the MPS4124 and the other half of the LM358 dual opamp which provides a "distance to voltage converter", that feeds a VCA (Voltage Controlled Amplifier) and a VCF (Voltage Controlled Filter). The LM3080 is the VCA and the 13600 creates two voltage controlled low pass filters which are connected in series. This comprises a state variable bandpass filter and is responsible for the wah portion of the circuit.

It is possible to do some mods on this pedal. The sweep range of the wah section can be changed by changing the value of the 0.01uF caps on Pins 5 & 12 of the 13600 IC chip. Making these caps smaller will shift the sweep range up and making them larger will shift it down. For use with a bass guitar, something like 0.022uF might be a good choice. It may also be possible to convert the pedal into a foot-controlled phaser by changing a connection for those 0.01uf caps on Pins 5 & 12 of the 13600 IC chip. Some work still has to be done to determine if this really is possible.

My DOD FX-17 Page