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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

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 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 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

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.

a very Basic Audio Mixer
by Joseph Fosco

Using resistors (which we discussed last time) we can build a very basic audio mixer. The schematic looks like this:

Basic Audio Mixer Schematic

Basic Audio Mixer Schematic

All resistors should be the same value. You can generally use any value from 1k to 5k.

Theoretically this mixer can be extended to as many channels as you would like by adding additional inputs and resistors. The problem is that each additional channel reduces the volume of the output. If all resistors are the same value, the formula for determining the loss in dB is

loss in dB = log(n) * 20

Where n is the number of inputs.

A  2 input mixer has a loss of 6.02059991 dB, a 4 input mixer has a loss of 12.0411998 dB, an 8 input mixer has a loss of 18.0617997 dB etc…. You can compensate for some of this loss by raising the input signal levels.  You can also raise the level of the output.  However, the resistors add a small amount of noise to the signal – this is called thermal noise.  Raising the level of the output, will also amplify thermal noise.

Another limitation of this mixer is that you cannot change the relative volume level of the inputs.  All inputs are mixed together at the same relative level.  This is the same as using a traditional mixer with all faders set to the same level.

The faders on a traditional mixer are called potentiometers. Potentiometers are variable resistors. You cannot replace the resistors in the above circuit with variable resistors.  Because of the way this circuit works, if you use potentiometers, changing the level of any one channel will affect the volume levels of the other channels.  We will look at ways around this later when we discuss potentiometers.

So that is it, a very simple audio mixer. Next time we will look at building this circuit.