- This is how I worked my way to a solution
for the audio equalizer challenge.
I started by thinking about what each part
of the equalizer needed to do,
and then picking op-amp circuits
as building blocks to accomplish those tasks.
For the two filter blocks, I decided to use
a basic op-amp filter circuit
that consists of an RC low-pass or high-pass filter
connected to an op-amp unity gain buffer.
And for combining the two equalizer signals
back together at the end,
that's a perfect application
for an op-amp summing amplifier.
With those two blocks in place,
I just needed to figure out how and where
I wanted to include the ability to control
the gain for each equalizer band.
One possibility that I considered was using rheostats,
which are manually controllable variable resistors,
as each of the input resistors
on the final summing amplifier stage.
While that seemed like a viable solution at first glance,
I quickly realized that I would run into problems
when I lowered either input resistance too much.
For example, if I turned the rheostat
for the bass signal all the way down to zero ohms,
mathematically, that would make the voltage
to resistance ratio for the bass input channel infinity,
which would make the output of the summing amplifier
go down towards negative infinity.
Now, in reality, the op-amp can't produce infinite voltages
because its output is limited by the supply voltage.
But that means if the op-amp is trying
to output negative infinite voltage,
it'll be saturated by the supply voltage,
and that still doesn't do me much good.
If I wanted to use the summing amplifier stage
to control the gain of each signal,
I would need to add some additional limiting resistors
in series with those rheostats
to make sure the circuit gain stayed
within a reasonable range.
At this point in the design process,
I realized that the summing amplifier solution
was going to be a bit more complicated
than I originally thought.
So I decided to look for another easier way
to control the signal gains.
The other option I considered,
was to replace the unity gain buffer
that follows each of the RC filter circuits
with a variable voltage amplifier.
I decided to go with this route
because I was already familiar with using
variable gain op-amp filters.
So it'd be a lot easier for me to design and build.
To figure out the component values
that I would need for this circuit,
I focused on the RC low-pass filter first,
which has a target cutoff frequency
of 500 hertz plus or minus 50.
After some trial and error,
based on the different components
I had available in my parts kit,
I settled on using a 3.3 kiloohm resistor
and a 100 nanofarad capacitor.
According to the equation
for an RC filter's cutoff frequency,
those component values would give my filter
a cutoff frequency of about 482 hertz,
which was well within the required
plus or minus 50 hertz range.
Shifting my focus over to the variable amplifier circuit,
the challenge requirements did not specify the gain range
for the bass and treble signals,
that was left up to me.
I like when my amps go up to 11,
so I decided to set that as my target for the maximum gain.
I had a few extra potentiometers in my part kit
that I could use as variable resistors
and adjust anywhere from zero to 10 kiloohms.
Since that variable resistor
had a maximum resistance of 10 kiloohms,
I needed to use one kiloohm for the other resistor value
to achieve my target gain.
With those component values,
when the variable resistor is lowered
all the way down to zero ohms,
the amplifier will act like a unity gain buffer
with a gain of one.
And when the variable resistor is cranked up to 10 kiloohms,
the amplifier's output will be cranked up to 11.
That completed the design process
for the low-pass filter block
for the bass equalizer channel.
And since the high-pass filter block
had the same target cutoff frequency,
I simply swapped around the location
of the 3.3 kiloohm resistor and 100 nanofarad capacitor
in the low-pass filter design
to turn it into a high-pass filter.
This RC high-pass filter will have the same cutoff frequency
as its low-pass counterpart of 482 hertz,
which falls within the required range for the challenge.
With both of the filter blocks planned out,
all I had left to do was design the summing op amp circuit
and then connect the three blocks together.
For the summing amplifier stage,
I wanted the two input signals to be added together equally.
So I started by making all of the resistors
the same value, 10 kiloohms.
By using the same resistor values
for both of the input resistors and the feedback resistor,
the equation for the output voltage of the summing amplifier
simplified down to being just
the negated sum of the two input voltages.
Since audio signals use alternating current
that's constantly changing,
at any point in time, the voltage
at either of the input terminals
will fall somewhere within a certain
minimum and maximum range.
For example, the AC signal might have
a peak-to-peak voltage range
from negative three volts to positive three volts.
Since those two signals are being added together,
the output voltage can end up being
anywhere within the range twice that size,
going from negative six volts to positive six volts.
When I was testing my circuit,
that began to cause some problems.
I was using a nine-volt battery
to generate the positive and negative 4.5-volt supply rails
to power the op amps in my equalizer circuit.
That meant the op amps would only be able to produce at max
output signals that were within plus or minus 4.5 volts.
Since I was already amplifying my bass and treble signals
by a factor of 11, I found that sometimes
when my input signals were at full volume,
the sum of the bass and treble signals
would exceed that plus or minus 4.5-volt range
on the power rails and caused clipping.
To prevent that, I needed to reduce
the amplitude of the signal
coming out of the summing amplifier.
I reduced the feedback resistor
from 10 kiloohms to five kiloohms.
Doing that cut the amplifier signal gain in half
so that the possible range of the output signal
would be the same as the input signal ranges.
I don't have to worry about the bass
and treble signals adding together
to create a signal that's larger than either one of them.
For practicality, since five kiloohms
is not a standard resistor value,
I used a 4.7-kiloohm resistor instead,
which is the next smaller common resistor value
that I had in my parts kit
and close enough for this purpose.
After I finished designing
each of the subsections in my equalizer circuit,
I built a prototype with the design on my breadboard,
but before using my new equalizer circuit
with an actual audio signal, I decided to test it first
to see how it responded to different frequencies.
Before I use my newly built equalizer circuit
with an actual audio signal,
I want to test it out first
to see how it responds to different frequencies.
So I've connected my function generator
to produce a 200-millivolt peak-to-peak sine wave
as the input signal,
and I have that input signal displayed on my oscilloscope
in yellow on channel one,
and the output from the mixer
is displayed in blue on channel two.
Right now I have the function generator
producing a 50-hertz sine wave,
which is well within the range of bass frequencies.
When I turn the gain knob for the low-pass bass filter,
I can see that the amplitude of the output signal
changes quite a bit.
But if I turn the treble knob,
the output doesn't change very much.
There's not much high-frequency content in this signal
to go through the high-pass filter.
Now, to test my circuit at a high frequency,
I'll adjust the input frequency
to be around five kilohertz,
which is well within the treble range.
And I'll adjust the horizontal scale on the oscilloscope
to get a better view of the signal.
Now, when I turn the treble knob,
it has a significant effect on the signal,
and I am getting some distortion in it due to the circuit.
But turning the bass knob doesn't have
much effect on the output,
and allows some DC offset to get through,
but the overall amplitude is basically the same.
Now that I've tested my equalizer circuit
with individual frequencies,
it's time to see how well it performs with some real music.
I'll use a three-and-a-half-millimeter headphone jack
to connect to the input of the circuit,
and that's connected to my MP3 player on my cellphone.
And I'll connect the output of the circuit to this speaker
which has a built-in amplifier.
If I want to focus on bass frequencies,
I can adjust the bass knob to emphasize those.
And if I want to hear treble frequencies,
I can turn the other knob.
I can hear a difference between the treble and bass,
so I know my equalizer is working,
but frankly, it sounds bad.
There's static and noise and popping sounds in the music,
it's getting some source of unwanted noise.
And I realize now
that I forgot to include decoupling capacitors
on the breadboard's power bus.
The noise on those power lines is affecting the op amps
and creating those pops and static sound in the output.
Fortunately, that's an easy fix.
I'll just add a few 100-nanofarad capacitors
to the power bus,
and I'll put them between power and ground
as close to the power lines
supplying power for the op amps as I can.
For safety, I disconnect power to my circuit
before making any changes.
Now I'll reconnect power.
And I can hear a difference.
No matter how well I think I've designed a circuit,
when I actually build and test it,
I usually find new unexpected problems to overcome.
But those challenges are part of the fun
when working with electronics.