There
are different kinds of loudspeakers like Tweeters, Mid-range Speakers,
Sub-Woofers and Woofers which can reproduce the voice in their own
particular frequency bands only. In an audio play back device, the
entire audio signals are separated into different bands and are applied
to the corresponding type of loudspeaker. The Tweeters are normally fed
with frequencies above 5 kHz, Mid-range speakers are fed with
frequencies in the range of 300 Hz to 5 kHz, and Sub-Woofers with 300 Hz
to 40 Hz and Woofers are fed with frequencies below 40 Hz.
The
entire audible voice spectrum stretches from around 20Hz to 20 KHz and
there is not loudspeaker design which can reproduce all these
frequencies with the same effect. The Woofers are made to produce
subsonic sounds (below 20 Hz) and there are musical instruments which
can produce frequencies above 18 KHz. To reproduce all these sounds
different types of loudspeakers are fed with their own band of
frequencies extracted from the music.
The filter circuits that are used at the output side of the audio device which filters out different band of frequencies and use them to drive different type of loudspeakers are called Audio Crossover Circuits. Three-way cross over circuits
are very common in output side of the audio devices which filters out
frequency bands for Tweeters, Mid-range Speakers and Sub-Woofers. This
tutorial discusses the design and implementation of Two-way audio cross-over circuit using Active filters for quality filtering.
The Two-way audio cross-over circuits
are used to drive the Mid-range Speakers and Sub-Woofers separately.
Mid-range speakers are fed with frequencies in the range of 300 Hz to 5
kHz, and Sub-Woofers with 300 Hz to 40 Hz. Since the musical sound
normally falls around the maximum frequency of 5 to 8 KHz, for driving
the Mid-range Speakers a high pass filter of cut-in frequency around 300
Hz is enough. Bass-beats of the songs appear in the Sub-Woofer range
and a Band-pass filter can be used to separate out these frequencies
from the entire audio signals. The Two-way cross-over circuit can be
considered as a combination of HPF (High Pass Filter) and BPF (Band Pass
Filter) as shown in the following block diagram;
The
HPF and BPF can be realized using the passive components like inductors
and capacitors only, but here Active Filter circuits are used to
improve the quality of the filters. Again for HPF an active synthetic
inductor based filter is used to avoid a bulky inductor from the
circuit. The BPF is designed based on a MFB filter for a good quality
yet simple circuit.
The basic concept of the Synthetic Inductor circuit
is to use a capacitor and inverse its properties so that it behaves
like an inductor. The advantages of this circuit over the actual
inductors are the very low internal resistance, easily varying the
inductance value in wide ranges, possible to design high quality filter
circuits etc.The circuit diagram of a Synthetic Inductor circuit is given below;
Here
the property of the capacitor ‘C’ in the above circuit has been
inverted with the help of the unity gain op-amp circuit. The value of
the inductance depends on the values of the resistors R1, R2 and the
capacitor C also. The inductance of the Synthetic inductor circuit is
given by the following equation;
L = R1 * R2 * C
The
high frequency filter can be realized using a single capacitor and a
synthetic inductor connected in series in which the one end of the
inductor is grounded, input is fed from the free end of the capacitor
and the filtered output is taken from the point where the capacitor and
the inductor are connected together in series. The circuit diagram and
the equivalent circuit of the Synthetic inductor based High Pass filter
is given in the following diagram;
The
circuit contains a capacitor in series ‘Cf’ which forms a High Pass
filter with the Synthetic inductor circuit. If the inductance of the
Synthetic inductor is say ‘L’, then the pass band of the High Pass
filter starts from the frequency given by the following equation;
Most
of the audio frequencies other than the Bass frequencies appear above
an average frequency of 700 Hz and hence the High pass filter is
designed for a cut-in frequency of 700 Hz.
The
MFB filters are very commonly used in circuits due to the fact that
they provide reasonable performance with the simplest circuit. They can
be designed to obtain narrow bandwidth and high gain. They are suitable
for the design of Band Pass filters since the bandwidth, mid-band
frequency can be easily adjusted or varied. These circuits has an
amplifier with more than one feedback and hence the name. The circuit diagram of a MFB band pass circuit using a single resistor and capacitor feedback is given below;
The equations relating the value of the components to the gain, Q factor, bandwidth and mid-band frequency are given below;
Since
this filter will be used to drive a loudspeaker a buffer circuit needs
to be added at the output of the MBF band pass filter which will drive
the loudspeaker without affecting the filter characteristics. The buffer
can also be made easily with another op-amp. An MFB band pass filter
with Fm = 70, Q = 15, Am = 100 has been designed and the complete
circuit is given below;
A
synthetic inductor based HPF and a MFB based BPF has been designed
separately and to make an Audio Cross-over circuit one has to connect
the input of both the circuits together and fed the audio to that common
input point of both the circuits. The complete circuit diagram of the Audio Cross-over circuit is given below;
The
audio input is fed from a PC and the filtered audio in this experiment
is demonstrated in the video using a normal headset, since the headset
speakers are designed for the good quality reproduction of both the high
frequency sound and low frequency bass.
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