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Potentiometers are probably the most visible electrical component in an analog synth. The face of any traditional modular analog synthesizer is covered with potentiometers. Behind every knob or fader is a potentiometer or “pot” for short. Clearly, the majority of controls on modular analog synthesizers are potentiometers. You can literally see their importance in synth design.

All these pots are nothing more than a variable resistor. They come in a variety of resistance values – and a variety of sizes, shapes and even features. Here is a photo of some common potentiometers.

Common Potentiometers found in Analog Synthesizers

The type of potentiometers often seen in synthesizers look like this:

Common Synthesizer Potentiometer

This pot has a shaft coming out the top and three “leads” or “tabs” on the side. Inside the potentiometer, the two outer leads are connected to a resistive strip. The resistance between these outer leads is the pot’s resistance value.

The shaft is connected to a wiper that rotates around the resistive strip. This wiper is also connected to the center lead. You can measure the resistance between the center lead and an outer lead. When the shaft is rotated all the way to one side, the wiper will be in contact with one of the leads. At that point the resistance between the center lead and the outer lead will be 0. As you turn the shaft the resistance will gradually increase. When the shaft has rotated all the way to the opposite side, the resistance measured will be the pot’s value.

This is the schematic symbol for a potentiometer:

Schematic Symbol for a Potentiometer

In this symbol, the top and bottom connections represent the outer leads; the center lead is represented by the connection opposite the arrowhead.

Potentiometers are distinguished by what is called “taper”. In building an analog synth you will generally encounter either “linear taper” or “audio taper” (also called “log” or “logarithmic” taper) pots. The resistance of a linear taper pot increases evenly across its entire range. If the first 10% of turning a linear taper pot yields 10 k of resistance, each additional 10% will also yield 10k resistance.

As its name implies, the resistance in a logarithmic taper pot idally follows a logarithmic curve. This results in less resistance when you first start to turn the pot and more resistance as you get towards the middle. Actual logarithmic pots generally do not follow this ideal. Most log taper pots simulate a logarithmic curve by dividing the resistance into two sections. The first section takes about 1/2 to 2/3 of the turn of the pot. This first section has only about 10-20% of the pot’s total resistance. The second section delivers the remaining 80-90%.

Logarithmic taper poteniometers are also called “audio taper”. This is because humans hears a sound’s volume on a logarithmic curve. In synth design, and other audio applications, an audio taper pot would commonly be used in a volume control. An audio taper pot has audio volume rise in a relatively even manner. If a linear taper pot was used, as the volume was turned up, the sound would seem to get very loud at first, and as the pot was turned further, there would appear to be less and less change in the volume.

Finally, lets look at what is probably the most common use of a potentiometer in a synthesizer circuit – as a variable voltage divider. The circuit looks loke this:

Potentiometer Voltage Divider Circuit

We first looked at the voltage divider circuit when we looked at resistors. To configure a potentiometer as a voltage divider, one of the outside leads is connected to ground, and the other is connected to the input voltage (or signal). The wiper (middle lead) is the output. As described in the resitor article, the equation for determining the output of a voltage divider is

V out = V in x (R2/(R1 + R2))

When using a potentiometer as a voltage divider, the valus of R1 and R2 vary as the shaft on the potentiometer rotates. This causes the output voltage to vary from V out = V in to V out = 0 (or ground).

Last time I introduced op-amps and their basic characteristics. This time let’s look at some basic op-amp circuits used in building analog synthesizer modules.

The most basic op-amp circuit is a comparator. The schematic for a comparator is:
Comparator Schematic

When configured in this manner, the op-amp compares the two input voltages (V in1 and V in2). If the voltage at the inverting input is greater (V in1) the output will be roughly equal to the negative power supply voltage (if the power supply is ± 15 volts the output will be -15 v). If the voltage at the non-inverting input is greater (V in2) the output will be roughly equal to the + power supply voltage (+15 volts output for a ± 15 volt power supply).

A common use of this circuit is to determine if a constantly varying voltage rises above a specific threshold. For example, if you wanted to know when a signal was greater than 1 v, you would connect this signal to the non-inverting input, and connect the inverting input to a reference 1 volt source. As long as the signal remained below 1 volt, the output would be -15 volts (with a ± 15 volt power supply). If the signal rose above 1 v the output would jump to +15 volts (again, assuming a ± 15 volt power supply).

Here is an example of what this might look like:
Comparator Output diagram

“Ref V” is out 1 volt reference voltage. “Signal” is the signal we are measuring. You can see that as long as the signal is below the reference voltage the output remains low. As soon as the signal rises above the reference the output goes high. You can probably see how this circuit can be used to create a square wave from a triangle wave. If you can’t, don’t worry, we’ll delve into that more deeply when we get into wave shaping.

One problem with the comparator is that a change in state can be triggered by a very tiny change in voltage. In fact, if the voltage on both inputs is very close, a little noise on one input can trigger the comparator to shift back and forth between a positive and negative state. There are various ways to deal with this problem, and again we will look at some of these in the future.

Another very popular op-amp circuit used in synthesizers is the “buffer” or “follower”. With a follower, the output voltage “follows” the input voltage, or V out = V in. The follower circuit looks like this:
Op-Amp Buffer or Follower circuit

This circuit has a very high input impedance, so it takes very little to drive it. On the other hand, it has a very low output impedance, so it can easily drive another circuit or even several other circuits.

This circuit is commonly used when you would like to distribute a single signal (voltage) to several destinations, and not place a heavy drain on the signal’s source. For example, a buffer could allow a single low frequency oscillator (LFO) to drive several other modules (voltage controlled amplifiers, voltage controlled filters, voltage controlled oscillators etc…) without overloading the LFO.

The next op-amp circuit is the non-inverting amplifier. A non-inverting amplifier looks like this:

Non-Inverting Amplifier Circuit schematic

As the name implies, the output of the non-inverting amplifier is the input amplified. The amount of amplification is set by resistors R1 and R2 and is equal to (1 + R1 / R2). So, the equation for determining the output voltage is:

V out = (1 + R2 / R1) * V in

As an example, if R1 and R2 are 10k resistors, then the gain of the non-inverting amplifier would be
1 + 10k / 10k or
1 + 1 or
2

Then 1 volt in would yield 2 volts out.

Another example, if R1 = 200k and R2 = 100k then the gain would be
1 + 100k / 200k or
1 + 1/2 or
1.5

1 volt in would yield 1.5 volts out.


Finally we will look the inverting amplifier. The circuit for an inverting amplifier looks like this:

Inverting Amplifier Circuit

Again, as the name implies this circuit amplifies and inverts the input signal. As you can imagine, the amount of gain is set by R1 and R2. For the non-inverting amplifier, the gain is -R2 / R1. So, the equation for determing the output voltage is

V out = -R2 / R1 * V in

As an example, If R1 is 1k and R2 is 1k then the gain is
-1k / 1k or
-1

1 volt in would yield -1 volt out. This would be an inverting follower. Note, however, that due to the configuration, this “inverting follower” might not have the same high input impedance as the “follower” circuit discussed above. The input impedance of the inverting amplifier is just R1, thus it is possible that this circuit will not be able to drive the same number of output circuits as the “follower” circuit.

Another interesting feature of the inverting amplifier is that it can be configured to have a gain of less than 1. (Gain of less than 1 hardly seems like “gain”, but the term is still used.) If R1 = 200k and R2 = 100k then the gain equals
-100k / 200k or
-1/2 or
-0.5

So, 1 volt in equals -0.5 volts out.

From an audio and synth building perspective, one of the most valuable features of the inverting amplifier is that the inverting input can be configured as a “summing node”. When we do this, we have the basis for an active mixer. We will look at this circuit next time.

The op-amp is one of the most useful, common and versatile linear integrated circuits you will use in building an analog synthesizer.

So, What Is An Op-Amp?

This might seem a little technical so just bear with me; it will make sense…eventually.

An op-amp is essentially an difference amplifier with two inputs, inverting (+) and non-inverting (-), and an output. A signal sent to the non-inverting input will appear in-phase with the output, while a signal at the inverting input will appear 180 degrees out of phase with the output. So a positive signal at the non-inverting input will produce a positive signal at the output. A positive signal at the inverting input will produce a negative signal at the output.

The op-amp circuit amplifies the difference between the two inputs.

The inputs to the op-amp have a very high resistance, so they draw very little current from the circuit that feeds them. Conversely the output has very low resistance.

What An Op-Amp Looks Like

An op-amp can come in a variety of packages, you can even construct your own op amp from discrete components, but for DIY synth projects, you will most likely encounter op-amps in a DIP package like this:

The eight pin package on the right can contain 1 or 2 op-amps, and the 14 pin package on the left contains 4 op-amps.

On a schematic diagram, the symbol for an op-amp looks like this:

op-amp schematic symbol

The non-inverting input is indicated with the “+” symbol, and the inverting input is indicated with the “-” symbol. The Vs+ and Vs- connections indicate connections to the power supply. Often times, schematics do not show power supply connections.

Using Op-Amps

As indicated above, an op-amp will generally require a bipolar DC power supply. Most op-amps can operate with a a range of supply voltages. These voltages can range from as low as +/- 2 or 3 volts up to +/- 30 volts. Th op-amps I’ve used when building analog synthesizer modules usually require a supply voltage from +/- 5 volts to +/- 18 volts.

It is important to note that the supply voltage will set the range of the op-amps output. Generally, the op-amp output range will be a little less than the supply voltage range. For example, an op-amp with as +/- 18 volt power supply might generate a maximum output between +/- 15 volts, while a op-amp with a +/- 12 volt supply might generate a maximum output between +/- 9 volts.

In addition, you will need to decide what specific op-amps to use. The most common op-amps I’ve used when building analog synth modules are the TL07x series (TL071, TL072, TL074 – 1, 2 and 4 op-amp packages), the TL08x series (TL081, TL082, TL084), the 741 op-amp and the LF351 op-amp. If you are building from a schematic, the parts list will indicate which op-amp to use. If you are designing a circuit yourself, you can often start with whatever op-amp you have handy, and often times you can swap the op-amp with another pin compatible part number to see which sounds or works better.

So this is it for a basic introduction to op-amps. Next time we will look at some basic op-amp circuits used in analog synths.

Before getting into building analog synthesizer modules, let’s look at some basic electronic components, and some of the basic circuits that use these components. These articles will be short introductions to the components.  They will not be a thorough introduction to electronics. Hopefully it will be just enough to get started.  If you would like a GREAT, easy introduction to electronics get Forrest Mimms’ book Getting Started in Electronics. For now, let’s get started with . . . . Resistors.

Resistors are probably the most common component used in synth circuits.  They are easy to spot, they usually look like a small brown tube with stripes.  Resistors are used to limit current.  The stripes on the resistor indicate the resistor’s value.  If you do not know how to read these color codes, you can find out here. Also, this site has an easy to use resistor calculator. You select the resistor color codes, and it tells you the resistor’s value.

If you are new to resistors, analog synthesizer circuits will primarily use 5%, 1/4 watt, carbon film resistors.  These can be easily purchased from Mouser Electronics or Digi-Key Corporation.  Digi-Key has a kit containing 5 each of the most common resistor values from 1 ohm to 1 Meg Ohm. If you would like to have a selection of resistors on hand, it’s a great deal.

On a schematic diagram, a resistor is indicated with a wavy line like this:

Resistor Symbol

Resistor Symbol


One of the most common and important resistor circuits is the voltage divider. On a schematic it looks like this:

Voltage Divider Schematic

Voltage Divider Schematic

This Voltage out of this circuit is determined by a ratio of R1 and R2.  The formula is

Vout = Vin x (R2/(R1 + R2))

So if R1 and R2 are both the same value the voltage out will be half the voltage in.

You can see a voltage divider circuit working in these photos of a small test I did.

First I measured the voltage of a 9 volt battery I would use to power this test.  The voltage was 8.8v.

battery

I then configured two 10k resistors as a voltage divider in a solderless breadboard.  I connected the voltage divider In to the + terminal of the battery and I connected the voltage divider ground (the downward arrow on the drawing) to the – terminal of the battery.  I measured the voltage at the out point (where the 2 resistors meet).  It was 4.4 volts – exactly as you would expect (8.8 volts x (10k / (10k + 10k)) = 4.4 volts).

voltageDivider1

I then replaced R1 with a 20k resistor.  You can see, the output was now 2.94 volts Considering we are using resistors with a 5% tolerance, this is well within the expected 2.9333 volt output (8.8 volts x (10k / (10k + 20k)0 = 2.9333 volts).

voltageDivider2

I will end here on this introduction to resistors. Next time we will look at a very simple design for an audio mixer using only resistors. In the meantime, let me know if you have any questions or would like more details.