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.
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.