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Friday, July 14, 2017

Op Amp Frequency Response for Bass Guitar Effects

This is a comparison of the frequency response of the OPA2132 and LM358N operational amplifiers.  These are two ICs that vary widly in price.  The OPA2132 is $6 while the LM358N is $0.40.  Both chips each contain a pair of op amps.  A few screenshots visually showing some differences is followed up by a pair of bass guitar effect circuits that highlight some audio application differences.

Test Circuit

The input is a sine wave AC of 10kHz and 500mv.  Both chips are powered using a split supply with V+ = 4.5V and V- = -4.5V.


At 10kHz input, the difference isn't that significant.  With the LM358N, there is a slight lag as indicated in the following scope screenshot at 0 degrees.

LM358N (Blue) Amplifying a Sine Wave (Yellow)
The OPA2132 is a little smoother.

OPA2132 (Blue) Amplifying a Sine Wave (Yellow)

At 100kHz input, the difference is more interesting.  Ths LM358N can't keep up with the changes.

LM358N (Blue) Not Really Following Input (Yellow)
The OPA2132 handles it.

At a lower frequency, say below 10kHz, the difference in performance isn't as significant, though there is some at the start of a cycle.  However, if you need better high frequency performance, then the higher price of the OPA2132 is justified.

Practical Applications

Usually, you don't want your ops amps to be overloaded.  However, this application can produce nice effects in music.  The bass guitar frequencies are far below 10kHz let alone 100kHz, but the op amp response is "tighter" with the more expensive OPA2132.

The LM358, because it breaks apart, makes a good dirty distortion circuit.  This reminds me of the bass break at the end of Rage Against the Machines "Know your Enemy" or the Beastie Boys "Sabotage".

The OPA2132 tried to handle the input better but will also break apart when pushed.  This circuit reminds me of bass fuzz as in Sly & the Family Stone's "I Want to Take You Higher" or the Edgar Winter Group's "Frankenstein".

This was recorded on an iPhone with a Jazz Bass + MarkBass amp.

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