DIBs Op Amp Circuit Catalog

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<CENTER>What is it?</CENTER>

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    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.

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<CENTER>What is general purpose?</CENTER>

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    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.

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<CENTER>How good are these circuits?</CENTER>

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    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.

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<CENTER>Why so many caps?</CENTER>

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    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.

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<CENTER>Output loads</CENTER>

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    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.

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