Audio Applications for Op Amps, Part II
by Bruce Carter
Advanced Analog Products, Op amp Applications
Texas Instruments, Incorporated
Introduction
This is the second in a series of articles on single-supply audio circuits. The reader is encouraged to review the introductory material in the first article which concentrated on low-pass and high-pass filters. This article will concentrate on audio notch filter applications and on curve-fitting filters.
Audio Notch Filters
There are applications where a single audio frequency was not part of
the original audio, is undesirable and annoying to the listener, and needs
to be rejected. Frequencies on either side of the rejected frequency, however,
contain useful audio content and if they are rejected as well there will
be an equally annoying "hole" in the audio. Notch filters are
used to reject very narrow frequency bands with minimal attenuation on either
side of the notch, but compared to low-pass and high-pass filters they are
hard to implement. Components that would only cause a slight ripple or washout
in a low-pass or high-pass filter often have a dramatic effect on the depth
of the notch; a slight mistuning of a low-pass or high-pass filter is inaudible,
but mistuning a notch filter may cause it to miss the interfering frequency
altogether. This section will give some hints for implementing a notch with
a reasonable degree of confidence.
60-Hz Hum Filter
One of the most common problems with audio is the presence of 60 Hz. 60
Hz is one of the most prevalent interference sources in much of the world
due to the fact it (with 50 Hz) are the frequencies used for ac power distribution.
Usually, 60-Hz hum is the result of poor grounding practices and it is far better to attack the 60 Hz problem at the source, rather than filter the audio. Nevertheless, there may be situations where the grounding of a particular system may not be accessible and in that case an add-on filter may be appropriate.
The 60-Hz hum filter shown (Fig. 1) is based on a Twin-T configuration. This topology is very effective, but it can be temperamental. The circuit response is very dependent on the absolute values of R1, R2, R3, C1, C2, and C3. All these tuning components should be 1% parts, but that may not be tight enough: They should all be taken from the same manufacturing lot when parts tend to have the same characteristics.
If the parts are matched properly performance can be very good, but mismatched
components will seriously degrade the response. In the Twin-T configuration,
R3 is half of R1 and R2 and the best way of making the resistance value
for R3 is to use two of the resistors used for R1 and R2 in parallel. Similarly,
taking two capacitors of the same value as C1 and C2 in parallel forms C3.
This increases the component count of the circuit by two (one additional
resistor and capacitor), but greatly benefits the matching -- because the
designer can take easy steps to ensure that the parts are from the same
batch (off the same reel -- out of the same box, etc.).
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