Radio Enhancement Project - Bass and Treble Control

The main focus of this project is to add bass and treble control to the radio. The necessity of this improvement was brought to light when the radio first became operational, and the bass was overpowering. It was then decided that augmenting the circuit to allow for manual equalization of the output signal could allow for a more pleasant listening experience. The basic concept we were building upon was the implementation of parallel linear filters. Our idea was to split the signal in two halves based on frequency and run each half through a separate potentiometer. Upon actually researching the subject, we found that there is already an established circuit which can achieve this effect. From this realization, we arrived at our decision to implement the Baxandall Filter.

Tone control enables the listener to tailor the sound to his/her own preferred listening experience. Also, due to the crude nature of our circuit, not all frequencies will be received/amplified equally. The 8Ω speaker used by the radio simply isn’t capable of producing a quality tone for the complete range of frequencies it receives. By adding independent controls for bass and treble, the user is allowed to make adjustments to compensate for these limitations.

The Baxandall Filter is the parallel combination of a high pass filter and a low pass filter. It allows for independent attenuation for the high and low frequency components of the audio signal. Its placement in the circuit comes after the AM detector but before the audio amplifier. Each filter is passive in nature, and they operate by “cutting” the signal they pass through the use of a potentiometer. We can better understand the nature of the Baxandall Filter by analyzing the bass control first and adding the treble control later.

Figure 1, Baxandall Circuit Schematic

Figure 2, Bass Control Minimum Cut

When the potentiometer is at the top of its resistance track as in Figure 2, C1 is shorted by the potentiometer. In this configuration, the bass control circuit is acting as a low pass filter. The low frequency signal will see C2 as an open circuit and will pass to the output. Also in this configuration, most of the mid and high frequency signals pass to ground, with only a small fraction of the original high frequency signals being passed to the output. In this configuration, the bass control circuit is passing the largest bass signal possible.

Figure 3, Bass Control Maximum Cut

When the potentiometer is at the bottom of its resistance track as in Figure 3, C2 is shorted by the potentiometer. In this configuration, the bass control circuit is technically acting as a high pass filter even though it is still sending the majority of the high frequency signal to ground. The input resistance seen by the low frequency signal is very high. As a result, we can treat both the potentiometer and C1 as an open circuit in regards to the low frequency signal. In the maximum cut configuration, the smallest possible bass signal is being passed to the output.

These two cases are the extremes for the bass control circuit. The majority of the time it is operating, the wiper on the potentiometer will be somewhere inbetween the two extrema. In this range, as the wiper progresses downward on the resistance track, there will be a resulting logarithmic attenuation of the bass signal.

Figure 4, Treble Control Minimum Cut

When VR2 is at the top of its resistance track as in Figure 4, high frequency signals are passed directly to the output through C3. Low and mid frequencies are passed entirely through the bass control circuit.

Figure 5, Treble Control Maximium Cut

When VR2 is at the bottom of its resistance track as in Figure 5, high frequency signals are prevented from reaching the output by C4 which is seen as a short to ground. Once again, low and mid frequencies are passed entirely through the bass control circuit.

As before, the treble control will not spend all of its time at the extrema. There is a logarithmic relationship between its attenuation of high frequency signals and the vertical position of the wiper in VR2. As the wiper is lowered, the attenuation increases with little to no effect on low and mid frequencies.

The combination of filters working in unison proves to be a very powerful tool for tone control. When working in unison, each filter diverts the unwanted signal away from the other and can reduce its value to a pleasant level for listening purposes.

The project’s level of success depended on the audible difference created by adjusting the bass and treble control knobs on the radio. This can most easily be tested by simply listening to the radio and turning the potentiometers. Testing this in the lab using a radio proved difficult, as we struggled to receive an audio signal inside of Broun Hall in previous labs. Given this limitation, the test had to be performed using the function generator and oscilloscope. Proving the bass and treble cutting audibly was done by inputting low and high frequency audio signals on an AM wave and adjusting the potentiometers to influence the output.

When the potentiometers are configured for maximum cut, the output waveform should point to a frequency response similar to the one in Figure 6.

Figure 6, Frequency Response for Bass and Treble Cut

When the potentiometers are configured for maximum cut, the output waveform should point to a frequency response similar to the one in Figure 6.

Figure 7, Frequency Response for Bass and Treble Boost

Note: The term boost is used above because although the bass and treble signals are attentuated slightly in this configuration, their amplification is higher relative to the mid-band signals.

An issue with using the FFT was that we cannot input a broad band of signals simultaneously to produce graphs as seen in Figures 6 and 7. We can overcome this by sampling signals from the upper, middle, and lower portions of the audio spectrum and recording their responsiveness to adjustments of the potentiometer.

The components to be used are all included in the kit acquired at the beginning of the course except for the potentiometers. The parts list for the Baxandall filter includes the following components: two 10 kΩ resistors, one 1 kΩ resistor, two 100 kΩ potentiometers, three 22nF capacitors, and one 220nF capacitor. The 100 kΩ potentiometers can be purchased on Amazon at $6.25 for 5. Since they are the only parts needed for the Baxandall filter that need to be purchased, the budget for the final project comes out to be $6.25.

We came into this project with a primary goal of adding bass and treble control to the radio. The idea for such an improvement came when the radio was first operational, and the radio broadcast we received couldn’t be understood due to the booming bass. From our original idea of paralleling high and low pass filters, we discovered the Baxandall Filter, which has an established reputation for controlling the level of high and low frequencies sent to the amplifier.

Overall, I learned a great deal form this project. I enjoyed utilizing this personal webpage to monitor my lab experience throughout the semester, learning about RF systems and developing the skills to design and test useful radio circuitry.