Dynamic Band Pass & Clipping Effects Pedal Circuit for Guitar

The original goal of this project was to build a functional wah pedal, using our basic electronics skills, with a simple and easy to understand circuit, designed for electric guitar or bass guitar. At the end of the day, however, we ended up with something that actually functions more like an equalizer, with some clipping and gain. The basic function of this pedal is to isolate certain frequency ranges of the input signal, which are then added back in variable proportion to the original input signal, with the intent of accentuating these select frequencies. Clipping is added to the filtered part of the signal for some extra fun.

The functional use of this product is straightforward. This is to be used as an effects pedal. Also, this can be used to teach effects of simple band pass filters and various electronic tools, such as basic op-amp circuits, diodes, etc. Our final result has the ability to control a bypass function, distortion gain function, mixing ratios of signal portions, and the frequency range of the band-pass filter.

All in all, the parts we used are fairly common. Resistor and capacitor values don't have to be exact, so use what you can find, and building the circuit probably shouldn't cost more than $20 at most.

Parts List:

1) For the circuit

• 2 1/4" input jacks
• 6 high-speed op-amps
  • We used LF411s because they were cheap and available, depending on how much you want to spend, you could perhaps even opt for a high-performance audio op-amp like the LME49710 or whatnot.
• 1 10nF capacitor
• 1 20nF capacitor
• 2 1uF capacitors
• 2 Germanium diodes (with forward drop voltage ~0.6 - 1.0V)
  • We personally used 1N34A diodes.
• 1 dual-ganged 100k & 100k potentiometer
• 1 50k potentiometer
• 1 10k potentiometer
• 9 3.3k resistors
• 2 1.5k resistors

2) For the power supply

• 1 triple throw, double pole switch
• 2 9V batteries, and battery holders

I've linked some sound samples below where I desecrate once respectable riffs with my unremarkable guitar-playing, so that you might have some grounds for comparison.

Step 1: The Circuit Diagram

The above relatively readable circuit diagram was generated by a great applet that can be found at http://www.falstad.com/circuit/, which is an easy-to-use circuit simulator. For simple things, I can't recommend it enough.

Step 2: Analyzing the Circuit

Although it appears complicated at first glance, this circuit is really quite simple, and can be broken down into a number of stages, which, in isolation, are fairly straightforward.

1) The first stage (colored in red, above) is our typical unity gain input buffer. We have our input signal from the guitar coming in through a 1/4" audio jack, as imitated by our 1kHz voltage source. It passes through a 1uF capacitor to shrug off any potential DC offset voltage, and then proceeds to the non-inverting terminal of our first op-amp, which is, as all of our op-amps are, wired in negative feedback. In this particular op-amp setup, the inverting terminal is connected directly to the output terminal, so we have a unity-gain output signal. The high effective input impedance and low effective output impedance of an op-amp ensures that we don't have to worry about problems with circuit loading.

2) This second stage (the section selected in pink) is an op-amp inverting band pass filter. In this configuration, we essentially have a high pass and a low pass filter coupled together. Let's call the total resistance and capacitance leading up to the inverting terminal R1 and C1, and the resistance and capacitance along the feedback loop R2 and C2. Our maximum voltage gain at the output terminal goes as -R2/R1, and the 3dB frequencies for the low-pass and high-pass filters respectively are (1/[2*pi*R1*C1]) Hz and (1/[2*pi*R2*C2]) Hz. So by adjusting R1 and R2, we can change the frequency range of our band pass filter. It just so happens that by using the pots on our dual-ganged 100k, 100k potentiometer, we can adjust these 3dB frequencies simultaneously, and the numbers work out in our favor such that when the dual-gang pot is turned up to max, the we have 3dB frequencies of ~80Hz and 160Hz, and when the pot is at minimum resistance, our 3dB frequencies are ~2400Hz and 4800Hz.

3) Since the second stage inadvertently inverted our circuit, we might as well flip it back around. The third stage is a simple unity gain inverting op-amp circuit, which accomplishes this nicely.

4) Next, we have the diode-clipping part of our circuit, which is a tad bit unusual. Our two Germanium diodes with their forward-drop voltage of ~0.68 volts lock the difference between the voltage at the output terminal and the voltage at the inverting terminal to ~0.68V. This voltage difference is originally determined by our resistance ratios--disregarding the diodes, we have a gain of 1 + (1.5k + Rpot)/3.3k. Here we decided to go with a bit more subtlety, using only our 10k potentiometer to control the gain.

5) Here we recombine our signal from Stage 4 with our original signal! Note how the top branch leading into Stage 5 is simply a wire from the output terminal of our input buffer amplifier. Doing a bit of op-amp circuit analysis on the contents of our isolated teal box give us a transfer function of:

Output Voltage = -3.3k * ([Vtop/3.3k] + [Vbottom/(1.5k + Rpot)])

where Vtop is the top branch leading in, and Vbottom is the bottom branch. So we're adding in our original signal at unity gain, and combining it with our Stage 4 output at variable gain! Note the negative sign--we've once again flipped our signal. Not sure if it matters, but our six stage will deal with that nicely. We also opted to use a 50k potentiometer here, so with the pot at minimum, our band passed signal has a gain of ~ 2, and at max, a gain of ~0.06--in other words, hopefully indiscernible. This pot is the main method of "volume control" in this pedal.

6) Finally, we draw our pedal to an end in a similar way to how it began--with another unity gain op-amp circuit, this time to serve as our output buffer. Note that this is also an inverting op-amp circuit, identical to the one used in the third stage, which has the effect of flipping our signal around again, back into its original orientation. Once again, our friend the 1uF capacitor (mislabeled in the picture, but using 10uF vs. 1uF really shouldn't make a difference) makes an appearance to once again shed any DC offset. The signal exits through another 1/4" audio jack, and is now one pedal closer to being the ideal tone. Or maybe not. Your opinion and experiences regarding the efficacy of this pedal may vary.

Alternatively, you might be able to skip this stage entirely, and simply wire Stage 5 straight to the output (perhaps keep the 1uF capacitor in there, though), if you don't mind having a flipped output signal, since the summing op-amp circuit used in the previous stage will also serve to provide a low effective output impedance.

7) We've also wired up a bypass switch to make disabling the circuit a thing of convenience. If the switch is on, the signal just jumps the entire circuit.

Step 3: Building the Circuit

If you're just looking to assemble this circuit on a breadboard, this is a pretty straightforward ordeal. Probably most useful to you will be a diagram of our power supply and the wiring on our triple throw, double pole switch. These are certainly not the best of images, for which I apologize, so allow me to explain.

For our exceedingly simple power supply: We connect the positive terminal of one battery with the negative terminal of the other. This connection forms the common ground for our circuit, so don't forget to connect it to the relevant places in the op-amps circuits, as well as to the ground terminals of both 1/4" jacks. Don't forget that 1/4" jacks also need to be connected to ground! The leftover positive terminal and negative terminal are your ~+9V and -9V sources respectively, which will power the op-amps. If you are planning on using a switch to turn the circuit on/off or enable a bypass, stay tuned for the next paragraph. (Alternatively, if our extremely suspicious power supply unit displeases you, or you wish to use only a single 9V battery, they sell little bipolar power supply chips online that will provide +9V and -9V with just one 9V battery.

For the our bypass/power switch: The switch enables the signal to bypass the circuit completely and either powers our circuit or doesn't. In our OFF state, we want the batteries to be disconnected, but we want the bypass part to be connected. Alternatively, in our ON state, we want the batteries to be connected, and the bypass to be disconnected. So in our example image above, the bypass part of the circuit shown by the gray wires, i.e. the input terminals of both 1/4" jacks, is connected to terminals 1 and 2. The +9V and -9V sources (red and black wires respectively) from our batteries are wired to 5 and 8. When the switch is ON, they'll run through 6 and 9 respectively, which then wire, respectively, to the Vcc+ and Vcc- terminals of your op amps.

But that's enough about respect. Let's talk briefly about going beyond the breadboard.

If you're looking to build your circuit in small form and put it into a fancy (or not so fancy) effects box, you'll be looking to get this circuit into printed circuit board (PCB) form and maybe do a little home engineering. For the benefit of those who may be unfamiliar with this process, this involves generating a schematic and board design, and then generating drilling files for a PCB milling machine, and either commissioning a company to do it, or finding one in your nearest electronics workshop and doing it yourself.

We've used the free version of the EAGLE software, developed by CadSoft, to make our schematics, which is available online here:

http://www.cadsoftusa.com/download-eagle/

If it all possible, I highly recommend printing a board with at least two layers. It'll cut down on size, and with all the op-amps to deal with, it'll save you from having to deal with a lot of jumper cables.

But if this is unfeasible for you, and you're stuck with a single layer board like us, you'll just have to deal with soldering on a large number of jumper cables. If you use our schematics, anyway. If you figure out a better way to do this, go for it. And let us know. But I've also attached schematics and board files for a single layer version of the PCB below. As I've earlier implied, these are less than optimal, but they'll work in a pinch. Probably. Some important things to note are that, although EAGLE only displays missing connections between the pads labeled J-D1-GN1, J-D1-GN2, and J-IC1-R6A with J-IC1-R6B, all the pads labeled VCC should also be manually connected. So in all, if you use our schematics, you'll need to use jumper wires to manually connect the following pads:

• J-D1-GN1 to J-D1-GN2

• J-IC1-R6A to J-IC1-R6B
• VCC- to VCC-23456
• VCC+ to VCC+2, VCC+3, VCC+4, and VCC+56

Finally, with regards to putting the circuit into a fancy box. We initially had the idea that the dual-ganged band pass pot would be adjustable in the manner of a wah pedal, such that the guitarist would be able to adjust, on the fly, the frequency range of the band pass. Our engineering skills not being up to snuff to develop such a mechanism in a timely manner, we ended up buying this kit. While you can have a look for yourself at how pretty the base plate is on the first page of this Instructables project, we feel compelled to warn you of its shortcomings/incompatibilities with our design:

1. This kit will only work if your relevant pot (the one to be adjusted with the rocking-pedal, the dual-ganged band pass pot in our design) has splines, or already has a gear on it.
2. This kit will only work well if your relevant pot is relatively easy to turn.
3. This kit will only work if your relevant pot is ~ < 3" tall including the shaft, and ~ < 1.5" tall without the shaft.

As you might suspect, this kit didn't really appreciate our dual-ganged 100k, 100k potentiometer. Poor thing. But if you're just looking to put this circuit into a box, sans moving parts, these sorts of difficulties probably won't plague you. In this case, the only recommendation I have to give is that if you have significant problems with noise in the circuit, it might make all the difference to electromagnetically shield your enclosure. That is, if your fancy box is made of a conducting metal, connect it the common ground of your circuit. If your box is made of some nonconducting material, you can use some copper tape (it helps to have the stuff with conductive adhesive) to swathe the inside of your box, and similarly connect that to the common ground.

But besides inhaling plenty of solder fumes, that's about all there is to building our so-called "Dynamic Bandpass & Clipping Effects Pedal". We wish you the best of luck. May your solder joints never be cold.