DIBs Op Amp Circuit Catalog

What is it?

This is a catalog of general purpose opamp circuits that are known to actually work with general purpose op amps. Most of these circuits are commonly used for audio applications. And it is a catalog in the sense that I have all of these circuits on the computer so that I can use them to design products.

What is general purpose?

General purpose here means easily available parts such as 5% resistors, common electrolytics, and disc capacitors. General purpose op amps means the RC4558 and the NE5532. A lot of other common op amps will work in these circuits. Many will be restricted to medium impedance loads (>2K). In particular, if you must use Bifet's like the LF353 and the TL072, they will not drive 600 ohm loads with any quality at all.

How good are these circuits?

In this case, general purpose means medium quality. There are many things that can improve these circuits. The first thing is better parts such as 1% metal film resistors instead of 5% carbon film. Also replace the electolytic coupling caps with bipolar or non-polar electrolytics or film capacitors and use film or silver mica caps instead of ceramic disc caps. Note that better often means more expensive and harder to get parts. Better also means better construction such as paying attention to ground and signal paths, parts layout, and power supply bypassing. And, of course, better op amps ($) for the particular application.

Why so many caps?

Caps are used for AC signal coupling between stages so that DC offsets can generally be ignored. They are also used to control the frequency response, both low and high, and ensure stability. Are they all needed? Not necessarily, but if you start leaving them out, you have to start paying more attention to the details and the circuits are no longer 'general purpose'. Also, see the paragraph above about improvements.

Output loads

Many op amps require a minimum 2K resistive load with virtually no capacitance so they can't be used to drive outputs. The RC4558 and the NE5532 op amps will drive 600 ohm loads with some restrictions. If the ouput goes to an output connector, they require output 'build-out' resistors to isolate them from the capacitance of any cables that you use. The 100 ohms shown on the schematics is a compromise value that works most of the time. When you are driving a transformer with an op amp, ideally you should be connected directly to the output with no resistor. If you have problems, you may have to put more than 100 ohms between the op amp and the transformer. This is because transformers windings have a low DC resistance that can draw too much current from your op amp if the DC output voltage isn't very close to zero. This will problably make it sound worse (higher distortion), but it will prevent the op amp from blowing up.