Building a Stomp Box

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For ages I have been curious about what goes into making guitar effects pedals. I’ve had so many stomp boxes over the years, seen articles on people building them but stayed away from trying it myself until this past year when I picked up a few books on the topic:
Craig Anderton’s Electronic Projects for Musicians
Brian Wampler’s How to Modify Guitar Pedals

Wampler’s book in particular was helpful in getting me started – turning my run-of-the-mill Boss Overdrive pedal into a classic Ibanez Tube Screamer by switching out only two components. That was a six months ago and now I’ve build 12 pedals and I can’t tell you how strangely satisfying it is to build and play your own pedals.

In this posting I’d like to take the time to explain some pedal building basics and show you how to get into it using the last pedal as a case study. I’ll give you deep explanation of the infamous Fulltone’s OCD overdrive circuit and how I created a clone.

The schematic, or blueprint of the OCD is easily found online at any number of Pedal building forums like https://www.diystompboxes.com/, http://pedalpcb.com and https://www.madbeanpedals.com. As far as schematics it’s not too complicated and makes a good starter pedal. If you don’t have a lot of experience with reading electronic schematics I’ll do my best to explain how they work. First, we’re take a higher level look at the overall circuit, breaking it down into sections and their functions.`

Simplified OCD Schematic

Essentially in all pedals the guitar signal is amplified then modified, in the case of the OCD there is a clipping section that distorts the signal, then amplified and given a tonal shape. In the OCD the clipping section uses diodes to drop the voltage (more on this later). This signal flow is typical of many overdrive or distortion pedals with the sonic differences primarily caused by variations in equalization in both amps and clipping circuit.

Power Section

As the schematic shows, both the amplifiers are created using one double operational amplifier (TL082 – Dual Opamp). Normally op amps has double power supplies with central point, but pedals commonly use single 9V power supply, so the voltage needs to be split into two.

Power Section – splits voltage into two 4.5v

This power section is quite common for all pedals and uses capacitors C1 and C2 to filter and store the power supply voltage. These caps will supply current to the rest of the pedal. C1 is an electrolytic capacitor which are good for storing current but not particularly good at filtering high frequencies. That’s why we’ve used a smaller value film capacitor C2.

D1 is protection diode used in case power supply with wrong polarity (reverse) is plugged into the pedal. The diode prevents the current from flowing to and damaging other pedal components.

Converting a single power supply into a lower one is done using voltage divider circuit , see the two resistors (R1, R2). This produces a new virtual ground (white triangle in the diagram). Current flows through these resistors (I=U/2R) from power source and voltage drop on each resistor is equal to half of power source  (U1=U2=I*R=U/2R*R=U/2). Beside the current which goes from R1 to R2 we have current which will go to our virtual ground. This mean that voltage drop will be not equal on both resistors. This effect could be reduced by decreasing value of R1 and R2. This will increase current going through the resistors and virtual ground current will be smaller comparing to it. Ground current in average should be equal to 0 as we do not have any DC voltage on the input of the op-amp. This means that we can reduce variation of voltage drop by adding capacitor parallel to R2. Here we can use higher values then original (OCD) to increase dynamic range of the op-amp.

Preamp section

The Preamp or Gain stage is a simple negative feedback circuit. Here’s a breakdown of the components:

Preamp section – op-amp with negative feedback loop

The guitar signal comes from the left and is labeled “in”. As we move to the right we can trace the signal flow, we see the R5 resistor which links capacitor C5 to ground. C5 removes DC (direct current) from the main signal. Without this resistor when true bypass is on the left connector of the capacitor C5 will be ungrounded. Voltage can arise on this side and result in a “bump” sound when you turn on the switch. The “bump” you hear is the voltage on the capacitor going to 0. Because of input capacity of the op-amp, some current will go to this capacitor. To reduce this current after the capacitor there is resistor R3. Then resistor R4 links input to our virtual ground.

Amplification coefficient of op-amp can be calculated using simple formula for a negative feedback amplifier ( k=1+(R6+R7) / R17 ). Using this formula we can calculate maximum and minimum possible gain of this stage.

Max: k = 1 + (1M+18k) / 2.2k = 464 or 53db.

That’s crazy! so much gain

This means that even without diode clipping stage we will have a fair bit of overdrive. Now let’s imagine that in the input we have 0.5V amplitude signal coming from a guitar. Then in the out we will have 464*0.5=232V. With our +-4.5V power supply we are not able to get more then about +-4V output. In such extreme cases the sound of the pedal depends on the op-amp model and power supply. Nothing is stopping us from increasing the voltage of power supply for the op-amp, in fact, that’s exactly what the Klon Centaur does, doubling the 9 volts to 18 volts in the final output stage to increase the headroom. For more info on the legendary Klon Centaur pedal, check out ElectroSmash’s in-depth analysis.

Capacitor C6 is widely used with op-amps. This capacitor cuts very high frequencies which can lead to trigger self generation in op-amp. Capacitor C7 plays double duty, first, we can not use virtual ground and the cap decreases current passing through. Second, C7 lets us cut some low frequencies. Value of this capacitor varies from revision to revision and is a commonly modded part of the OCD pedal.

Clipping Section

The section is probably the most atypical part of the OCD schematic.

You might wonder why it has MOSFET transistors instead of diodes here? And yes, we are actually using a diode too. The 2n7000 transistor has its own protection diode which prevents reversed polarity voltage in the transistor. Protection diodes have a voltage drop of about 0.7V. In some versions of the OCD the diode is Germanium. Germanium diodes have a smaller voltage drop then silicon ones. This diode adds asymmetry to the clipping. Symmetrical clipping (exact same diodes in reverse polarity) create a soft smooth distortion while asymmetrical clipping sounds more like a single-ended tube amp’s overdrive, which is what the OCD is going for. The resistor R8 is an important one, with the diodes it makes a “nonlinear” voltage divider. The resistor has dual purpose here, first, it prevents the op-amp from high-output current. Secondly, it makes the clipping shape smoother.

Tone and Amp Section

Finally, there is one more amp with tone and volume control. Much like the preamp section, we have the second part of dual operational amplifier (op-amp). Resistors R10 and R11 gives us amplification factor a bit smaller then 5. This mean that maximum amplitude of the signal is 0.7 times 5 (3.5).

After the op-amp we have to connect to ground. For this reason we’ll need a coupling capacitor C11 to change our signal’s impedance from low to high. The end of the signal path is similar to how a guitar tone and volume controls work. Resistors R12 and R13 together with C12 and R14  are high frequency voltage dividers. When switch is in on position total resistance of parallel R12 and R13 is smaller and  and voltage drop on high frequencies is smaller resulting in a brighter sound.

This concludes my epic rundown of the OCD clone pedal that I’ve now built twice. There are so many variations in components that result in subtle sound differences. I’ll try to round up some of the most used modifications to this pedal circuit next time.

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