Jonathan Roderick
Hakan Durmus
EE 348 Final Project
(Due week of 4/23)
Your task is to design a three band graphic equalizer. Each band will filter and amplify
the input signal of a CD player. The magnitude of the signal in each band will be shown
on an LED display. Here is a system diagram of the final project.
System:
Gain Adjustment
CD player
Filter
for each
band
Amplifier
for each
band

Output
Stage
Speaker
LED
display
System Specifications:
Power/Supplies: Your will be able to use 5V supplies. The power cannot exceed 25mW
through any given component. Assume the input signal coming from the CD player has a
50mV magnitude (50mV = 0dB gain).
Filter Specifications:
Three bands:
1. Center frequency 2k Hz, Bandwidth 2k Hz.
2. Center frequency 7k Hz, Bandwidth 4k Hz.
3. Center frequency 15k Hz, Bandwidth 8k Hz.
Notes:
You may want your filter to also provide some gain, but this is entirely up to you. All
filtering must be done with passive components and BJTs only. No OP-amps may be
used in this section.
Amplifiers Specifications:
Gain: Each band must have a gain that is adjustable by  10dB.
Notes: All amplification must be done with passive components and BJTs only. No OPamps may be used in this section. You will need to be able to adjust the gain of each
band, so a potentiometer may be used to do this.
Summer (): The summer should be built with the aide of the circuit shown below.
V0 Rb  V3 V4   V1 V2 

 



R0 Ra  R3 R4   R1 R2 
Ra  R1 || R2 || R0
Rb  R3 || R4 || R5
You may not need to use this exact circuit, but use it as an aide to design a summer that
suits your design needs. The output of this stage needs to have low output impedance, so
that it is capable of delivering a voltage to moderately low load resistances.
Output Stage: You will not need to design this circuit; it will be done for you. This
stage prevents any potential system instabilities that may be occur when driving a
speaker, which can have considerable inductance. Just make sure the output of your
circuit that is going to drive this section has low impedance.
LED Display: Each band should have a 5 LED read out. The LEDs should reflect the
magnitude of the signal that has been adjusted in the  10dB range. You may use OPamps as comparators for turning on the LEDs. Be careful, make sure you put adequate
series resistance in series with the LEDs or the output of the comparator will be too much
and the LEDs will burn out. You should design the LED display so each LED represents
5dB in the  10dB range. If there is no signal from the CD player, then no lights should
be on. See the examples below.
+10B
+5dB
+5dB
+5dB
+0dB
-5dB
+10dB
+10dB
+0dB
This reflects a
-10dB gain, because
just the first LED is
lit-up.
-10dB
A gain of 0 dB is shown
when all the LEDs up to
the middle one are lit-up.
When all the LEDs are
lit-up, then the
amplifiers are adjusted
to a gain of +10dB.
+0dB
-5dB
-5dB
-10dB
-10dB
Creativity and originality will be rewarded. You must also simulate the filter and
amplifier parts of your circuits in Spice. These simulations should be turned in with you
lab report. Please simulate and verify you design in Spice before you start building your
circuits, so that when debugging any problems with your project it can be assumed that
the problems are not due to math or design errors.
Extra Credit:
Problem:
The filter order of your equalizer is low, by low being an order of 3-4. When the order is
low there may not be any audible difference in the signal except for volume change. The
reason is the leakage from the sidebands through the nearby filters. (Even if you suppress
the mid-band to 30dB lets say, the leakage from sidebands will keep the signal at that
band still at 10dB).
Solution: Create an extra band for your equalizer that is a single high-pass filter with
adjustable 3dB frequency. This filter and amplifier doesn’t need an LED display, nor
does it need to be connected to the summer. It may be completely independent of your
project. Give the high pass a  10dB gain. The high pass filter should have a 3dB
frequency that may be adjusted from 300 to 1.5k Hz, above that it might be difficult to
understand the signal.