Stompbox Studies – Class 1

Want to build your own effects pedals? A good place to start is understanding how transistors work. Transistors are those three legged devices seen populating many stompbox circuits.

Transistors are are a core building block in electronics. Integrated circuits, like op-amps, are made up of many transistors! Transistors are used in all of the circuits you’ve probably played or heard on recordings: Rangemaster, Fuzz Face, Tonebender, Big Muff, Tube Screamer, and too many more to list here.

The goal of this series of blog posts will be to present concepts you can use to understand how guitar effects pedals work, apply these ideas to debug, and make your own pedals.

Why I am I doing this? I have been building Stompboxes as a hobby for years now and want to build a deeper understanding of the electronic concepts underneath it all. I have been building pedals for years but it has always been sort of paint by numbers. I had some friends get into the hobby recently and they had lots of questions. Here are the answers I could come up with.

This class will be broken into three parts: a discussion in this blog post, a lab where your goal is to build the Dallas Rangemaster, and a reading assignment and study guide.

Discussion: Dallas Rangemaster

The Dallas Rangemaster is classic guitar pedal. Created by Dallas Muscial Ltd. in the 1960s. With a name like “Dallas” you’d think they were from Texas but in fact the company was located in London England!

The Rangemaster is a booster. Basically it’s a single transistor amplifier that filters the lower frequencies. It takes your guitar signal and shapes it and amplifies it.

The Rangemaster was used by so almost every guitar player you can think of from the 60s. Here’s a short list of notables:

  • Eric Clapton
  • Tony Iommi
  • Marc Bolan
  • Rory Gallagher
  • … too many more to list

Many more used other pedals similar to or derivatives of the Rangemaster. Many companies sell boosters that are direct copies of the Rangemaster or are built from a single transistor like the Rangemaster for the same purpose. It’s definitely a pedal worth study.

  • Zvex Super Hard On
  • Electro Harmonix LB1
  • Hornby Skewes Treble Booster
  • Brian May Treble Booster
  • … and the list goes on

The Rangemaster circuit represents a basic building block that can be used in many guitar pedals. Understanding this circuit is a gateway to explaining how lots of classic effects work. It’s also a building block you can apply to effects of your own invention.

The circuit

Electronic circuits are expressed with schematics that have their own symbols. This is similar to the way music is expressed in notation or way a roadmap may be drawn.

Usually we give each part a number and a value. The lines represent an electrical connection between parts. Here is a list of the part numbers and values for the Rangemaster.

PartValue
R1470K
R268K
R33K9
C10.005µ
C20.01µ
C347µ
POT1A10K
Q1PNP
Rangemaster Parts

The parts here are four types: Resistors, Capacitors, Potentiometers, and transistors. lets look at each of these.

Resistors

Resistors are all listed with the prefix R in the parts list. There are three resistors R1, R2, and R3. Resistors impede the flow of electrons. We can use resistors to limit how much current is at any point in the circuit.

POT1 is a potentiometer, which is an adjustable resistor. Use a “pot” when you need a variable amount of voltage or current at a point in your circuit. For example a pot is often used as a volume control.

Capacitors

Capacitors are like little electrical reservoirs. They act as a gap that DC can’t pass but they allow and AC signal to pass! Capacitors used in the Rangemaster are C1, C2, and C3.

Capacitors are used to affect how audio is passed through your circuits. Capacitors are used to create filters and control the frequency of AC.

Some capacitors are polarized! One leg is marked with a + and that leg should go to the more electrically positive side of the circuit with the other leg going to the more electrically negative side of the circuit. Notice C3 has a + on the bottom that connects to the +9V in the schematic.

The transistor

In the original Rangemaster the transistor was OC44 or OC71. This was a germanium transistor. Transistors come a few varieties. Two of the most common are PNP and NPN. The OC44 is a PNP transistor.

You can build Rangemaster type circuits with almost any transistor. This is great because the OC44 and OC71 are hard to get and expensive these days. Some common transistors that would work as replacements are: 2N3906 or 2N5087. These are silicon transistors rather than germanium.

Typical transistors

The symbol for PNP and NPN transistors look like this. These are almost the same. Note the arrow! You can remember which is which with this: NPN = Not Pointing iN, PNP = Pointing iN.

The original Rangemaster was built with a PNP transistor. You could also build a Rangemaster type circuit with a NPN transistor. Some common part numbers for NPN transistors are 2N3904 and 2N5089.

NPN Rangemaster

Notice the changes to the circuit. (1) the transistor Q1 is NPN. (2) the +9v and -9v are swapped. This is where your battery would connect. (3) the capacitor C3 has been flipped around so that the + terminal is pointing towards the more positive side of the circuit and it’s other terminal connects to the -9v the more negative side of the circuit.

OHMs Law

OHMs law is a basic principle of electronics. OHMs law determines the flow of electrons through a circuit. If you thought of electrons like cars and the circuit diagrams above like a roadmap OHMs law could be used to predict the flow of traffic!

OHMS has three ideas:

  • Voltage (abbreviated E) is the electromotive force (measured in Volts, millivolts, and microvolts)
  • Current (abbreviated I) is the number of electrons in the flow of “traffic” (measured in amps, milliamps, and microamps)
  • Resistance (abbreviated R) is the opposition to the flow of electrons (measured in OHMs)

We can use math to calculate the flow of electrons through our circuit. Use the diagram above to help you remember the formulas.

  • E = I * R (find the voltage from the current and resistance)
  • I = E / R (find the current from voltage and resistance)
  • R = E / I (find the resistance from voltage and current)

The numbers you use need to be in the same realm. If you are working with Volts (E) then Current (I) needs to be amps. If you have voltage in millivolts then current needs to milliamps. If the units are mixed you can multiply or divide by 1000.

Volts VAmps A
Millivolts mV (V / 1000)Milliamps mA (A / 1000)
Microvolts µV (mV / 1000)Microamps µA (mA / 1000)

What does this mean? Take a look at the images below. You can use OHMs law to calculate the current and voltage.

In the first image we have 9 volts going through R8 which has a resistances of 2000 ohms (or 2K ohms). Whats the current? Using OHMs law we can figure it out!

  • I = E / R
  • I = 9 / 2000
  • 0.0045 Amps or 4.5 milliamps or 4500 microamps

What about the second image. Image we don’t know the voltage but we do know we have 2 milliamps going through 1000 ohms resistance.

  • E = I * R
  • E = 0.002 * 1000 (Notice I converted milliamps to amps)
  • 2v = 0.002 * 1000

Voltage Divider

Traffic moving down a road divides when it reaches an intersection some traffic will go one direction and some traffic will take the other route. The same is true of electrons moving through a circuit. If you were watching the road from a helicopter you’d see more traffic take the larger roads and less traffic take the narrow roadways. Think of the size of roads as resistors. Larger roads have less resistance and narrow winding roads create more resistance that slows traffic.

Look at the first example. There is 9 volts going through R1 and R2. What is the voltage at the intersection ?1. You can solve this if you understand voltage dividers. The formula is:

  • ?1 = 9V * ( R2 / (R1 + R2))
  • ?1 = 9V * ( 1K / (1K + 1K))
  • ?1 = 9V * 1000 / (1000 + 1000)
  • ?1 = 9V * 1000 / 2000
  • ?1 = 9V * 0.5
  • ?1 = 4.5V

Here I walked through the steps to solve the problem. Notice I converted 1K to 1000 ohms. Then finished up from there.

The formula is: V * R2 / (R1 + R2)

The shortcut here is if both resistors are the same value then voltage is divided in half. This would be true for any values for R1 and R2. Try it yourself. Imagine R1 and R2 are 10K. Then try R1 and R2 at 47K and 100K.

What happens when the values are not equal? What’s the voltage at the intersection: ?2.

  • ?2 = 9V * 100K / (100K + 20K)
  • ?2 = 9V * 20K / 120K
  • ?2 = 9V * 20,000 / 120,000
  • 1.5V = 9V * 0.166

Solve number 3 on your own!

To solve ?4 we have to know that resistors in series are added together. That means that we have a total resistance of 16.7K. To solve ?4:

  • ?4 = 9V * (10K + 4.7K) / (2K + 4.7K + 10K)
  • ?4 = 9V * 14.7K / 16.7K
  • ?4 = 9V * 0.88
  • ?4 = 7.92

Solve ?5 on your own…

With this knowledge you can start examining the Rangemaster circuit. Notice R1 and R2 for a voltage divider!

POT1 is also a voltage divider. Let’s take a moment to look at potentiometers.

Potentiometers

These are adjustable resistors. You’ll use potentiometers or “pots” when your circuits need to be able to adjust the resistance at a point in the circuit. A pot has three legs. Imagine the center leg is located between two resistors. The resistances change as you turn the pot!

We draw the pot like the image on the left. Internally it acts like the images on the right.

Imagine we have a 10K pot. With the knob in the center the resistance is divided equally with 5K on the top and 5K on the bottom. This is a voltage divider! In this case you would see have the input voltage at the center leg!

If you rotated the pot clockwise the resistor on the top (R3) might be 0 ohms and the all of the resistance 10K might appear at the bottom.

If we went counter clockwise it would be reversed.

If you rotated the almost to the end you might have 2K resistance at the top and 8K at the bottom. Imagine there was 9V at the top, use the voltage divider to calculate the voltage at the center.

Transistors modeled as a current controlled resistor

With these ideas we can now look at the transistor, which is the heart of the Rangemaster. The Rangemaster is an amplifier. It takes a small audio input and produces a louder audio output. The transistor is used to amplify the signal.

A transistor has three legs. For NPN and PNP transistors the legs are called: Emitter, Base, and Collector. The Emitter is leg with the arrow. The Base is in the middle.

To explain how the transistor works we will model it as a current controlled resistor. For this discussion we will use the NPN version of the Rangemaster schematic and the transistor will be NPN.

Take a look at the diagram above. Imagine the transistor drawn on the left is in your Rangemaster circuit. Imagine that inside the device is a resistor. The value of this resistor is controlled by the amount of current present at the base. Outside the transistor the Rangemaster has a 10K resistor (POT1) at the collector and a 3K9 resistor at the emitter.

With no current applied to the base there is so much resistance you can image that there is no connection at all. We would see about 9V at the collector.

When a small amount of current is applied to the base the resistance between the collector and emitter starts to decrease. Imagine the collector/emitter path is now showing 100K resistance. Now we have a voltage divider! You could use the formula above to calculate the voltage at the collector!

With more current applied to the base the C/E path might go down to 10K resistance. Using the voltage divider formula the voltage present at the collector goes down as more of the electron traffic goes through the 3K9 resistor.

With a lot of current at the base the C/E path might go as low as 100 ohms and the voltage at the collector would go even lower!

Did you notice when the current at the base goes up the voltage at the collector goes down. And conversely, when the current at the base is low the voltage at the collector goes up. A transistor amplifies because it allows out circuit to turn small changes in current to larger swings in voltage.

Notice the diagram above. When the current is high the voltage is low! This is an inverting amplifier. The signal at the output will be a mirror or opposite of the input signal.

Biasing the Transistor

There is a little more needed to make this work. Imagine the incoming signal might swing between +1v and -1v. When the input is above 0V the transistor starts working. When it’s 0V or less the CE path is closed off.

Look at the diagram above. The signal going in is a sine wave but the signal coming out is getting chopped. This would cause some distortion! In this case it probably wouldn’t sound too good. It also explains why your transistor circuits sometimes don’t sound right!

To make the amplifier work we need to bias the transistor so with no input the output settles at a point somewhere between the extremes.

Take a look at the diagram above. Here I’ve added R1 and R2 from the original schematic. These provide an amount of current to the base so that it sits around 7V. With this arrangement positive input pushes more current to the base opening the CE path and lowering the output. When the input signal is negative this pulls current from the base which increases the resistance of the CE path and the voltage at the output goes up.

Biasing is the term that describes applying a DC current to the base of a transistor such that it is it near the middle of it’s range. It needs bias so the output can go up and down from the bias point.

All 4 of the resistors play a part of the amplifier. R1 (470K) and R2 (68K) provide the current to bias the transistor. POT1 (10K) and R3 (3K9) also determine where the bias should be since the lowest voltage output is determined by the voltage divider created by these two resistances. Imagine the CE path had 0 ohms resistance.

  • ? = 9V * 3.9 / (10 + 3.9)
  • ? = 9V * 3.9 / 13.9
  • 2.5 = 9V * 0.28

This concludes the discussion of the Rangemaster. From here the goal is to build your own! Along the way think of this discussion.

Building the Rangemaster

For this part of the lesson it’s up to you to figure out which build method and which circuit you want to build.

There are several approaches you can take to building the Rangemaster:

  • Kit
  • PCB
  • Strip/Vero-board
  • Perf-board
  • Terminal strip
  • Solderless Breadboard

Kit

Buying a kit will cost more but you’ll get everything you need in one package!

PCB

A PCB makes for an easier neat clean build but you’ll have to source parts yourself.

Strip/vero board

This is perforated boards with strips of copper. Use it for prototyping.

Perfboard

Like vero board but has one pad per hole.

Terminal Strip

The original Rangemaster was assembled on a terminal strip. This is for the artists and historians.

Solderless bread board

Use a solder less bread to experiment with the design.

Study guide

Here is some reading material to follow up the discussion above.

This guide to Breadboarding the Rangemaster at Small Bear Electronics is a great guide to circuit.

http://diy.smallbearelec.com/HowTos/BreadboardRMs/BreadboardRMs.htm

Stompboxology was a news letter put out in the early days of DIY when the internet was young. It never caught on and it and it’s creator mysteriously vanished. That said I found this issue to contain one of the best discussions of transistors for stompbox usage I have read. It also contains a discussion of core concepts for electrical engineering that you will need to know to build effects pedals.

http://moosapotamus.net/files/stompboxology-going-discrete.pdf

This article provides an in-depth analysis of the Rangemaster. Most of it is over my head but it’s still good reading.

https://www.electrosmash.com/dallas-rangemaster

One Reply to “Stompbox Studies – Class 1”

Leave a Reply

Your email address will not be published. Required fields are marked *