Final Project: Adjustable Treble-Mid-Bass EQ
This project allowed me to dive into a new aspect of this particular radio circuit without a guided lab manual to follow. This being said, much researching and simulating was done in order to better grasp the response of my circuit with the added EQ. In the end, some major issues were found with the design, and simple mistakes were able to hide in plain sight, taking hours to find.
What is an EQ?
An EQ or equalizer allows different frequency ranges to be adjusted for the listening pleasure of the user. It is very common for these to be found alongside music applications and circuits. The EQ that I have decided to create has three adjustable ranges: Trebble (~2kHz-20kHz), Mid (~200Hz-2kHz) and Bass(~20Hz-200Hz) as shown in Figure 1. These three ranges correspond to different pitches that the human ear hears while enjoying music, and allows a user to tweak the output sound according to their liking.
How to Adjust Frequencies
There are two basic ways to manipulate these frequencies: Low Pass Filters (Figure 2) and High Pass Filters(Figure 3). A low pass filter allows low frequencies to pass through, and essentially shorts high frequencies to ground. A high pass filter allows higher frequencies to pass, and shorts low frequencies to ground. These filter's cutoff frequency (also known as the -3dB frequency) is the boundary frequeny that the particular filter operates at. The equation to calculate the -3dB frequency is found in Equation 1.
\[f_{-3dB} = \frac{1}{2 \pi RC} \]
The Equalizer Circuit
After some extensive research into different EQ types and arrangements, it was decided to use a Fender TMB EQ circuit as show in Figure 4. This particular circuit is within many of Fender's guitar amplifiers in several different forms, but the base circuit is shown below. Something to note about this particular choice of EQ ciruicts is that this is a passive EQ. This means that there is no amplification of the input signal, only a reduction.
In creating this in LTSpice, a work-around was found to implement the adjustable potentiometers as parameters. In this network, there are three main filters present, one to control the treble, mid and the bass. Depending on the value of the potentiometer parameters adjusts what level the particular frequency range is adjused to.
Frequency Response
The ideal and actual frequency responses are shown in Figures 5 and 6. It does not take a professional RF Engineer to notice that these responses are not quite the same. After careful examination, it is inferred that this is largely due to lower frequencies being filtered out much earlier in the audio amplifier circuit. Nearly every stage of different amplifiers have a high pass filter incorperated into it, filtering out much of the low frequencies before they can reach the EQ circuit. The mid and treble frequency ranges do provide comperable ranges and curves!
Conclusion
This project required much independent SPICE work and research. In the past, this was done by being thoroughly guided by a Lab Manual. This time around, there was no lab manual, as well as no specific end criteria. It was very interesting to dive into different avenues of frequency response and its applications in audio equlizers.