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op-amps – An Introduction For Synth Design

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.

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