CMOS workshop part 2 - square wave fuzz

Now onto the fun stuff. :)

But first, there's a couple of important things to watch for when using CMOS.

3. CMOS usage rules

• All inputs must go somewhere. A unused gate input must be disabled by connecting it to either +9V or ground, directly or with resistor. The input of a CMOS gate is very high impedance, so a floating input can randomly determine what the circuit will do, making it change state erratically, causing noise and high current draw. Leave unused outputs unconnected.
• Inputs that comes off board should have a load resistor connected (1M resistor to ground).
• CMOS chips are sensitive to static. They have built in protection diodes, but sometimes it's not enough. It's a good idea to wear a anti-static wristband when working with CMOS.
• CMOS chips accept a wide range of supply voltages (generally from around +3v to +15v), but it's always a good idea to double check the datasheet just to sure. There are exceptions. We will only be using chips that works well with +9 volts.

The "CMOS'ifier" square wave fuzz on the breadboard

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4. Pulldown/up resistors
These resistors are often needed in CMOS circuits. They are useful when we want to force a high or low output state when the input is not changing, but still leave the input able to accept a input signal. They are typically a high value from 10K - 100K and goes to ground (pulldown) or V+ (pullup) at the input of the gate. For example, if we have a gated oscillator on the input and we want  to make sure the output always goes to ground when the oscillator is off we need a pullup resistor at the input (assuming the gate is inverting). We will use a few pratical examples later on.

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5. Lets breadboard the "CMOS'ifier", a simple square wave fuzz.

We will use this circuit as the front end for the guitar signal, so we can do other things later on, such as octave down. But it also makes a pretty cool fuzz on it's own.

For this we will use a CD4069 chip, simply because it has a easier pinout than the CD4049 and takes less space on the breadboard. First, lets take a look at the pinout of this chip. Lets connect input/output jacks, power and ground to the breadboard and the chip like this (these connections will not be shown later):

CD4069 pinout

Stage 1. Preamp
Our first stage of our circuit will be boosting the guitar signal into logic levels. This is a gainstage similar to many CMOS based distortions, such as the classic Tube Sound Fuzz by Craig Anderton. This is one of the exceptions to the digital logic and can only be done with the CD4069/CD4049. It gives a very pleasing distortion, but has the drawback of being rather noisy.

We will configure one stage of the inverter by adding negative feedback, using a resistor from the input to the output of the inverter and decoupling capacitors.

• R1: Load resistor
• R2: Sets the gain of the amplifier. Try a 1M trimmer/pot here insted for controlling the gain.
• C1 and C2: Decoupling capacitors. This is always needed at the input and output of a circuit, but it's also nessecery for the input of a linear amplifier to work properly.
• R3: voltage divider trimmer to control the output volume.
• C3: optional lowpass filter cap. Can be anything from 100pf to 47nF. How much treble it cuts depends on the value of R2 aswell. Experiment with different values!

Stage 1: linear amplifier gainstage

A CMOS inverter configured as a linear amplifier doesn't need an external voltage reference for bias, so very few parts are needed to make it work. It will self bias throught the feedback resistor thanks to the operation of the internal MOSFET output pair. This works best with the unbuffered (UBE) versions of the chips.

Lets put it up on the breadboard. Notice the green wires in the first image that are used to disable all the unused inputs. This will not be shown later, but it's a good pratice to always disable any unused input. The second image shows the same circuit with a trimmer for gain control. Put several gainstages in series for a sweet sounding distortion. I usually have two gainstages in my designs when I have one extra inverter left over.

We can use any circuit to boost the signal into logic levels, transistor-based or op amp, but the advantage with this method is that we're only used 1/6 of one chip. So the rest of the inverters can still be used for other things, oscillators, LFO, filters ect. and this will save us some space. Another easy way is to use a LM386 amplifier chip and boost the signals into a squarewave (common in many Tim Escobedo designs), or a op amp comparator.

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Stage 2. Schmitt Trigger
This stage completes the "CMOS'ifier". It will turn the signal into a pure square wave, making it compatible with other CMOS chips and eliminating noise from the guitar pickup when not playing. This part is inserted before the output capacitor.

R3 and R4 sets the upper and lower thresholds for the schmitt trigger. It's best to leave R4 at 1M and experiment with the lower threshold resistor R3. Higher values will make it more gated, but low values can result in oscillation. Here we need to find the sweetspot that depends on how strong input signal we are getting. Increasing the value of C3 (1nF - 22nF) can also help against oscillation, noise and misstriggering. Very low output pickups may sound too gated, in that case use two preamp stages in series before the schmitt trigger to increase the sustain.

If we have a schmitt trigger chip (CD40106 or CD4093) in our design we can just use one gate of that chip insted directly after our preamp stage. It's not as versatile, as it has a fixed hysteresis. In that case a pulldown resistor (100K to ground should always be used at the first schmitt trigger input).

​How does the Schmitt Trigger work exactly?
R3/R4 forms a voltage divider that offset the signal (it also attenuates it). When the schmitt trigger output goes low, R4 is effectively connected to ground (via the second inverter output). This offsets the input signal which gets pulled closer to ground, so when the signal starts to swing in the opposite direction it has to rise a bit further to cross the half supply voltage threshold (the chip threshold always stays the same) and vice versa: when the schmitt trigger output goes high, R4 is connected to your positive supply voltage via the output, so then the signal is pulled closer to V+ and needs to swing a bit lower to cross the threshold. Thus creating hysteresis. For this to work, the output needs to be non-inverting, hence the two inverter gates in series. This also helps the process as the first inverter is isolated from the feedback resistor.

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​In the next part, we'll have a look at doing something a bit more advanced - several few ways to achieve octave down!

I hope this series is interesting. I'm trying to keep it as basic as possible, but it will become more advanced in later parts. Please let me know what you think and if there is anything I can improve. Thanks!

UPDATED 2023-10-31: Fixed the schematic numbering not matching the breadboard and added a bit more info about the operation of the Schmitt Trigger/bias.

The complete "CMOS'ifier" fuzz

The complete circuit on the breadboard

What is signal offset?
Any alternating current (like a guitar signal) always have a DC bias point. It centers the signal to swing above and below a specific voltage, sometimes referred to as the DC component of the signal.

In a dual supply circuit, the signal usually swings positive and negative centered around ground (0 volts) and in a typical single supply circuit like a guitar pedal we have a Voltage Reference  (Vref) that the signal swings around. Ideally at 4.5 volts in a circuit driven by a 9V battery, to get maximum usable range.

This bias can be offset/shifted up or down which is part of how the schmitt trigger operates. It's very common to shift control voltages in synthesizers.

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Next part: CMOS workshop part 3