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CMOS workshop part 2 - square wave fuzz

8/21/2015

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

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.

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.

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.

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.

Next part: CMOS workshop part 3

30 Comments

CMOS workshop part 1 - basics

8/19/2015

7 Comments

Many of my designs are based around CMOS chips and a few people has asked me if I know any good resource for stompbox designs based on CMOS. So I thought it might be fun to share alittle of my knowledge of the CMOS basics and hopefully you will learn something and will be able to experiment with your own CMOS based circuits. :)

We will use the breadboard later, but first, lets start with the basics (the boring stuff...)

CMOS chips are based on MOSFET transistors, unlike the earlier TTL logic chips that are based on BJT transistors. I won't go into detail on how they work on the insides. For that I really recommend reading the book: The CMOS Cookbook. It's been a huge help and inspiration for me.

1. CMOS LOGIC
The first thing one must understand is that CMOS works with digital logic:
The output will be either high or low, depending on the voltage at the input(s).

These input/output states are often also called "true or false", "on or off", "1 or 0". The CMOS chips are basically acting like digital switches that can either source or sink current.

High = positive supply rail (v+)
Low = ground

There are some CMOS chips that doesn't follow this digital logic, for more specialized functions (like a Phase Locked Loop). Some chips can also have a third disconnected output state (floating/undefined state).

This means that no matter which kind of waveshape we put into a typical CMOS logic chip, the output will always be a square wave. There is no in-between, no dynamic range, just 1's and 0's. The output logic changes at about halfway up the power-supply voltage, so to be able to use a CMOS chip with a guitar signal, the signal needs to be boosted into logic levels, then preferably made into a square wave: digitalizing the signal (in a crude way).

This is the reason you don't see a gain control on many of my designs and why it often sounds totally fuzzy. A square wave is as distorted as it can get, as the nature of the square wave introduces alot of odd harmonics into the signal. This is also the reason why the output of a CMOS based guitar effect have a gated sound with a decay that dies abruptly (as the input signal falls below the schmitt trigger threshold).

More about schmitt triggers later.

The Arcadiator uses 3 CMOS chips

Don Lancaster's CMOS Cookbook

Keep in mind that there's a limit on how much current the outputs can source and sink which can vary alot between different CMOS chips, so check the datasheet for the chip you are using.

Especially if you are driving LED's directly, then you need to use a current limiting resistor that is big enough to not draw more current than the gate can handle.

Here's a short video of my oscilloscope that displays a guitar signal that is boosted to reach logic levels, then later fed through a CMOS schmitt trigger to make it a square wave so it will play along with other CMOS chips. No audio output is connected in this video, it's for the visuals only, so you can get a idea of how the waveshapes look when it's getting "CMOS'ified". :)

Later we will breadboard the circuit I used, a very basic square wave fuzz, so you can listen yourself to how it sounds.

But first, lets iron out alittle more of the basics...

2. GATES
A single CMOS input/output is called a "Logic Gate". Sometimes each gate can have several inputs. A chip usually consists of several identical gates that can be used independently. In the datasheets we will find something called a "Truth Table". It tells us how the gates operates based on their logic type. Lets take a look at a few different chips and their logic.

Buffer (CD4050)
This chip is rarely used in stompbox applications. But I wanted to use it as an example. As you can see by the Truth Table, if the input is high the output is high, and if the input is low the output is low. So this chip is simply a buffer (mostly used for interfacing to TLL logic).

Buffer schematic symbol and truth table

Inverter (CD4069/CD4049)
This chips, also sometimes called a NOT gate (or hex inverter because the chip has 6 gates), inverts the input. If the input if low, the output is high and vice versa. The output is 180 degress out of phase with the input. Notice the small circle (knot) at the output on the schematic symbol. This means that the gate is inverting.

This is probably one of my favourite CMOS chips since they also can be configured as linear amplifiers aswell. More about that later.

Now the logic gets alittle trickier...

AND GATE (CD4081)
This chip has four AND gates. Each gate has two inputs. With either or both inputs low, the output will be low; with both inputs high, the output will be high.

Both input A and input B needs to be high for the output to be high, hence the name "AND gate".

NAND gate (CD4011)
This is basically the same as a AND gate, except that it inverts the output. It can be useful when doing gated oscillators. I used a NAND schmitt trigger chip (the CD4093) for that application in my Flawed Logic Fuzz. We will breadboard a gated oscillator later on.

NAND stands for "NOT AND". Remember that a NOT gate is an inverter.

OR GATE (CD4071)
This chip has four OR gates. Each gate has two inputs. With either or both inputs high, the output will be high; With both inputs low, the output will be low.

Input A or Input B needs to be high for the output to be high, hence the name "OR gate".

NOR GATE (CD4001)
You can probably guess by now that the NOR gate (NOT OR) is an inverting OR gate, so I will not add the NOR gate schematic symbol and truth table.

NAND and NOR gate are sometimes called the universal gates, since they both can be used to form all the other basic logic gates.
Is it starting to make sense? Let have a look at a couple of more...

XOR GATE (CD4070)
This chip has four Exclusive OR gates. It also has two inputs per gate. It's a very versatile chip, because one of the inputs decides if the gate is inverting or non-inverting by tieing the input to either +V (non-inverting) or ground (inverting). In the Arcadiator I use this feature to make the alternating octaves - more on this later. This chip can also be used as a comparator as identical inputs gives a low output and different inputs gives a high output. So basically, if two different signals is put into the two inputs the output is the sum of the differences of those two (hence the name "exclusive or gate"). This is patrially how my ring mod The Corruptor works.

SCHMITT TRIGGER (CD40106)
Here's another very versatile chip. It's an inverter, but it has something called hysteresis built into the chip which makes it a schmitt trigger. Hysteresis is basically a "noise immunity zone". For example, if we use a 5 volt supply, the input signal will have to reach above 2.9 volts for the output to go low and it will have to drop down to belop 2.3 volts for the output to go high. That's a 0,6 volt hysteresis that will keep out any noise at the input that is below the threshold. The built in hysteresis also lets us configure this chip with diodes and resistors to do other logic funtions: multiple inputs NAND or NOR gates, pulse width modulation and much more.

Schmitt trigger schematic symbol

Those are a few basic CMOS logic chips. We'll use them later to breadboard stuff or analyze a few of my designs.

I mentioned before that not all CMOS chips follows this logic. We have many exceptions, switches, linear amplifiers, MOSFET cmos chips ect. I will cover a few of those later.

Lets stop there for a while. This post turned out alot bigger than I imagined and it can be a bit overwhelming! I had a hard time wrapping my head around this when I started out. If you are confused by the logic inputs and outputs, don't worry. In the next part we'll actually make something useful, a simple squarewave fuzz. The breadboard is a great learning platform, and by making a few small circuits I hope you will begin to understand these logic alittle better.

Please let me know what you think of this post and if you find anything that is incorrect.
/ Fredrik

UPDATED with some additional info and minor clarifications 2023-10-31.

Next part: CMOS workshop part 2

7 Comments

Flying Guillotine Fuzz

8/19/2015

4 Comments

EDIT 2014-08-19
I added a single layer PCB layout to the bottom of the post that fits better in a 1590B. Effects layouts also made an excellent perf and PCB layout, check it out at: http://effectslayouts.blogspot.se/2015/07/flying-guillotine-fuzz.html

Here's a fuzz circuit I made up about a year ago that turned out pretty cool. The front end, tonestack and the output stage are all borrowed from the Big Muff, but the two Muff gainstages is replaced with a discrete op amp (by Joe Davission) modified with a odd feedback transistor clipping configuration.

In the soundclip the gain is maxed, tone at noon and volume at about 9 o clock.

I got the name from one of my favourite movies. :)

flying_guillotine_pdf.pdf
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Arcadiator release

7/24/2015

2 Comments

Update 2015-07-24
The Arcadiator is now released and avaliable in the shop!

The first batch is limited to 30 pedals, 27 sold in the shop (25 red and two white).

These are sold at a special introductory price. At first I was going to sell the first 10 pedals only at this price. But I have had some problems with the production of the faceplate, so I decided to sell them all for this price. The faceplate have minor beauty flaws, but don't be alarmed, it's just barely noticable when you look at it from a certain angle. All in all i'm very satisfied with how these pedals turned out.

I'm going on a vacation trip soon (31/7) and will be away for a about one week. So order now and I will have it shipped out before leaving for vacation. I will start on the second batch when I get home from my vacation. I need to check up on alternative means of producing the faceplate, so the next batch may take up to 3 months before they are avaliable.

2 Comments

Flawed Logic Fuzz MKII

6/25/2015

19 Comments

I'm proud to present the Flawed Logic Fuzz MKII.

The original Flawed Logic Fuzz was a fun circuit, but it suffered from too short sustain and the modulation part could have been better. The MKII is improved in these areas and also has a couple of new features such as a depth control and a waveshape switch for the LFO. I'm especially happy with the new triangle LFO waveshape that makes a sort of phaser'ish sound.

This circuit is made up of a square wave fuzz followed by two gated oscillators in series (NAND gates). The LFO modulates the frequency of the second oscillator. This produces a very synthy, throaty sound that is unlike any other fuzz. I hope you like it. :)

PCB avaliable in the shop

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19 Comments

The Sidescroller

6/16/2015

10 Comments

Here is a new original design - The Sidescroller Fuzz.

It started out as a derivative of the Silver Fox Octaver, but it turned out quite different. Soundwise It's basically "the arcadiator light" since It also does octave down, octave up and pulse width modulation. Compared to The Arcadiator it has tighter controls, fits in a 1590B and has a unique feature: An LFO to modulate the Pulse Width. All this only using a couple of CD4069 inverter chips. :)

PCB avaliable in the shop

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10 Comments

Eagle Claw Octave Fuzz

4/29/2015

9 Comments

Here is a new original design - The Eagle Claw octave fuzz

It's a very simple CMOS based circuit with a full wave rectifier for the octave up. Makes a fairly strong octave, but usual caveats apply for best results - humbuckers, neck pickup and tone rolled off.

For fun I threw together a short demo song for this one insted of a regular soundclip. This it how it sounds in context with drums and bass (I mixed the bass low). It has some minor eq for touch-up.

My completed pedal. It turned out ok, but the etch could have been alittle better. My toner is running out so the transfer wasn't that good to start with.

For this build I used a prototype dual layer PCB from OSH Park. PCB's will be avaliable for purchase when I get my webshop up and running. The last image shows the microphone placement in the demo. I only used one microphone on the drums. Why make it more complicated then it has to be? :)

The name is after the kung fu style. Here is one of my favourite movies that features the eagle claw style.

PCB avaliable in the shop

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Sonic Reducer

4/23/2015

40 Comments

Here is my taken on the Analog Bit Crusher by Colin Raffel. It's a cool circuit that uses sample and hold to degrade the signal. I guess a more accurate description would be aliaser / sample rate reducer.

I replaced the oscillator with a better one that has a wider range and an additional pulse width control. I also added a much needed gate for the carrier oscillator - a envelope follower that turns off the oscillator when not playing. No more carrier bleedthrough. :)

The controls are:
Resolution - controls the pulse width of the oscillator. Wider pulses lets more clean signal through
Sample rate - control the frequency of the oscillator
Gate (trimmer) - this sets the gate threshold for the oscillator in relation to the input signal

I etched the enclosure for this one. I'm very happy with the result. :)
This tutorial is very good: http://diy-guitar-effects.tumblr.com/etching

The name and text on the enclosure comes from the lyrics from this song. But I bet you knew that already, otherwise - shame on you! :o)

PCB avaliable in the shop

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Green Currant tremolo

4/10/2015

5 Comments

edit 2016-03-08: Added a demo video

Here is a new original design. I'm very happy with how this one turned out. :)
It's a very percussive tremolo based around the TDA7052A amplifier chip.

It continues with the idea I had when I designed the Black Current Vibrato, that two signals out of phase is summed together. One side is static while the other side is being modulated and filtered to leave some frequencies unaffected. This results in what sounds almost like a light phaser and vibrato'ish effect. So in a sense this is a continuation of the Black Current Vibrato (hence the play on words) but without the stutter mode and otherwise improved in every way; more neutral sounding, less noisy, more control, more versatile LFO and no ticking noise.

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The controls are:
Rate - Controls the speed of the LFO
Range - Controls the range of freqencies effected by the modulation.
Attack switch - Sets the waveshape to hard (sawtooth) or soft (ramp).

I will post a video-demo later, but not until I have boxed this one using a fabbed PCB (just because it will be alot smaller and easier to fit inside a 1590B - the vero is verified and perfectly fine). So that might take a while.

PCB avaliable in the shop

5 Comments

DIY or DIE

3/30/2015

1 Comment

For me DIY is about sharing info and knowledge and this blog is my way of contributing to the DIY community. I love seeing other people build by humble creations. Here is a few builds done by other people :)

I hope it's alright that I snatched these pictures of the web. With enough pictures I might do a gallery section on the blog, so please send me your pictures if you got any and want me to share. :)

On a more serious sidenote.. There are a couple of unwritten rules within the DIY scene. 1. Never use DIY schematics or layouts for profit without permission. I think it's ok to build a couple of pedals to sell to friends, but nothing more. 2. Always give credit to the original creator and never take credit for someone elses work. I always give credit for every vero layout I make ect. I only ask that you do the same for my stuff and respect these rules. Thank you!

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