Bass and Tone Circuit and Calculator

A bass and tone (also known as treble) filter circuit usually boosts or cuts certain frequencies of the audio spectrum. Active filters are most commonly used in the form of Baxandall-type networks that connect within the feedback loop of an amplifying block. This is one particular version of the circuit that uses least number of components. In this article we can see how to calculate the component values for any particular break frequencies.

Since potentiometers are very expensive, this calculator is particularly useful, if you wish to use any that are cheaply and readily available. You get to choose the value of R1, and you have some control over the value of R4 through the arbitrary choice of R5. The calculated value of the potentiometer must be equal to the value of R4 or greater. If you make R5 too large, then R4 also becomes large, therefore, keep R4 small. These days, 100 kΩ logarithmic (and ganged) potentiometers are difficult to find, and expensive; however 10 kΩ, and 50 kΩ, values tend to be available.

The default values are for an idealised preamplifier with an operating range of 20 Hz to 20 kHz, with ±20 dB gain. The low-frequency (FL) 3 dB corner is at 20 Hz, and the high-frequency (FH) 3 dB corner is at 20 kHz. For a 20 dB boost/cut, the gain value is usually 10.

Circuit Parameters

Once you have all the required values, you can look up the nearest standard values for the capacitors and resistors available in the E24 series range utilising the links below.

Potentiometer Types

With an active bass and tone control circuit such as this, the best type of potentiometer to use is a linear type with a centre detent. A detent consists of a special mechanism that locks the wiper at the centre position. The centre position is also where there is no boost or cut and the response is flat because the resistance on either side of the wiper is equal.

In contrast, avoid the use of logarithmic potentiometers, which are usually used with passive bass and tone circuits. With these types of potentiometers, when the wiper is dead centre, the resistance will not be equal of both sides. It is usually split by 10 % on one side and 90 % on the other. As you can imagine, with these potentiometers, the centre position of the wiper will not provide a flat response.

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