5-BAND GRAPHIC EQUALISER
This equaliser uses low-cost op-amps. Good-quality opamps powered by a single voltage supply are readily available in the market. The op-amp should have a noise density of less than 24nV/√Hz, slew rate of more than 5V/μs and gain bandwidth product greater than 3MHz. The NE5532 or LM833 used in this circuit meets these requirements Equaliser circuits typically divide the audio spectrum into separate frequency bands and have independent gain control for each band. The output of each band is mixed at IC4(A) and then fed to an
audio power
amplifier Proper quality factor (Q) needs
to be selected to avoid overlap in adjacent
bands as this introduces
colouration into the audio signal.
We have used the multiple-feedback
bandpass filter topology shown
in left-most corner at the bottom of the
figure. This is a circuit for single-channel
bandpass filter. If the capacitors areof the same value, the calculations are
fairly simple. For calculating the component
values, use the following formulae:
Centre frequency (fo) : 1/2πC√(Ra||Rb)Rc
Bandwidth (B) : 1/πCRc
Quality factor (Q) : fo/B = πfoCRc
Gain (A) : –Rc/2Ra
These can be combined to give the
following formulae:
Ra = Q/2πfoAC
Rb = Q/2πfoC (2Q2–A)
Rc = Q/πfoC
Begin the calculations by choosing
a large value of capacitance (~0.1F) and
smaller value of resistances. Increasing
the capacitance decreases resistances
(Ra, Rb and Rc). Care must be taken to
avoid overloading on the input buffer
op-amp. Note that stray capacitances
on the board reduces the value of ‘C.’
The bandwidth and gain do not depend
on Rb. Hence, Rb can be used to
modify the mid-frequency without affecting
the bandwidth and gain.
For equalisers, there are standard
mid-frequencies that are normallythe octave division, application and
some degree of manufacturers’ preference,
but nearly all share the basic octave
boundaries that are based on a
centre frequency of 1000 Hz.
A balance between the number of
filters and bandwidth need to be observed.
It is possible to use a wider
bandwidth and fewer filters, or narrower
bandwidth and more filters.
Anything narrower than 1/3 octave is
rare, since the complexity of the filters
increases for higher values of ‘Q.’ This
can get rather expensive and in reality
is of limited use for most applications
in audio systems.
National Semiconductor lists the
following mid-frequencies for a 10-
band graphic equaliser: 32, 64, 125, 250,
500, 1k, 2k, 4k, 8k and 16k. It also recommends
a ‘Q’ of 1.7 for equalisers.
The table lists the component values
for different centre frequencies of
the equaliser. We used ‘Q’ of 1.7 and
gain (A) of 4.
The circuit for the 5-band equaliser
uses IC1 (A) LM833 as the buffer stage
for the equaliser. It is a non-invertingamplifier with a gain of ‘2.’ The input
signal is divided by ‘2’ by the resistive
network comprising R3 and R4. Hence
the net gain of this amplifier is unity.
Two 100k resistors (R1 and R2) are
used as a voltage divider and the junction
voltage is fed to its
positive input through R6. This
divider has enough power to feed all
other op-amps directly. Resistor Ro
(R8=R12=R16=R20=R24=R28=R30=100Ω)
has the dual function of noise reduction
and resistive isolation of capacitive
load. It may be varied between 50
and 150 ohms depending on the noise
in the circuit. the pot metrs [VR1 through VR5] are in the signal path hence should be of the
best quality possible. wrap the body of the pots with bare copper wire and solder the other end
of the wire to the ground.since the filters are very sensitive,allresistance should be metal film
type and the capacitor should be polyester type.
Each stage of the op-amp needs to be capacitively coupled to the next stage so that the DC does not get propagated and amplified. For a good low-frequency response, this coupling capacitor should be greater than 1 μF. A 10μF, 16V capacitor is used in each stage of the circuit here. The circuit is powered by a 12V DC regulated supply. A well-regulated supply using 7812 is recommended. Ground the Vcc pin of each op-amp with a 0.1μF ceramic disk capacitor to
This equaliser uses low-cost op-amps. Good-quality opamps powered by a single voltage supply are readily available in the market. The op-amp should have a noise density of less than 24nV/√Hz, slew rate of more than 5V/μs and gain bandwidth product greater than 3MHz. The NE5532 or LM833 used in this circuit meets these requirements Equaliser circuits typically divide the audio spectrum into separate frequency bands and have independent gain control for each band. The output of each band is mixed at IC4(A) and then fed to an
audio poweramplifier Proper quality factor (Q) needs
to be selected to avoid overlap in adjacent
bands as this introduces
colouration into the audio signal.
We have used the multiple-feedback
bandpass filter topology shown
in left-most corner at the bottom of the
figure. This is a circuit for single-channel
bandpass filter. If the capacitors areof the same value, the calculations are
fairly simple. For calculating the component
values, use the following formulae:
Centre frequency (fo) : 1/2πC√(Ra||Rb)Rc
Bandwidth (B) : 1/πCRc
Quality factor (Q) : fo/B = πfoCRc
Gain (A) : –Rc/2Ra
These can be combined to give the
following formulae:
Ra = Q/2πfoAC
Rb = Q/2πfoC (2Q2–A)
Rc = Q/πfoC
Begin the calculations by choosing
a large value of capacitance (~0.1F) and
smaller value of resistances. Increasing
the capacitance decreases resistances
(Ra, Rb and Rc). Care must be taken to
avoid overloading on the input buffer
op-amp. Note that stray capacitances
on the board reduces the value of ‘C.’
The bandwidth and gain do not depend
on Rb. Hence, Rb can be used to
modify the mid-frequency without affecting
the bandwidth and gain.
For equalisers, there are standard
mid-frequencies that are normallythe octave division, application and
some degree of manufacturers’ preference,
but nearly all share the basic octave
boundaries that are based on a
centre frequency of 1000 Hz.
A balance between the number of
filters and bandwidth need to be observed.
It is possible to use a wider
bandwidth and fewer filters, or narrower
bandwidth and more filters.
Anything narrower than 1/3 octave is
rare, since the complexity of the filters
increases for higher values of ‘Q.’ This
can get rather expensive and in reality
is of limited use for most applications
in audio systems.
National Semiconductor lists the
following mid-frequencies for a 10-
band graphic equaliser: 32, 64, 125, 250,
500, 1k, 2k, 4k, 8k and 16k. It also recommends
a ‘Q’ of 1.7 for equalisers.
The table lists the component values
for different centre frequencies of
the equaliser. We used ‘Q’ of 1.7 and
gain (A) of 4.
The circuit for the 5-band equaliser
uses IC1 (A) LM833 as the buffer stage
for the equaliser. It is a non-invertingamplifier with a gain of ‘2.’ The input
signal is divided by ‘2’ by the resistive
network comprising R3 and R4. Hence
the net gain of this amplifier is unity.
Two 100k resistors (R1 and R2) are
used as a voltage divider and the junction
voltage is fed to its
positive input through R6. This
divider has enough power to feed all
other op-amps directly. Resistor Ro
(R8=R12=R16=R20=R24=R28=R30=100Ω)
has the dual function of noise reduction
and resistive isolation of capacitive
load. It may be varied between 50
and 150 ohms depending on the noise
in the circuit. the pot metrs [VR1 through VR5] are in the signal path hence should be of the
best quality possible. wrap the body of the pots with bare copper wire and solder the other end
of the wire to the ground.since the filters are very sensitive,allresistance should be metal film
type and the capacitor should be polyester type.
Each stage of the op-amp needs to be capacitively coupled to the next stage so that the DC does not get propagated and amplified. For a good low-frequency response, this coupling capacitor should be greater than 1 μF. A 10μF, 16V capacitor is used in each stage of the circuit here. The circuit is powered by a 12V DC regulated supply. A well-regulated supply using 7812 is recommended. Ground the Vcc pin of each op-amp with a 0.1μF ceramic disk capacitor to
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