Baxandall Circuit Design

This Baxandall circuit with negative feedback is a very simplified design idea utilising a rule-of-thumb approach. An advantage of this configuration is that it saves the constructor from having to perform a plethora of mathematics usually necessary for these types of circuits. It also enables them to easily choose readily available and cheap components, which in today's economic climate is extremely important. Whilst I have published many other Baxandall circuits with calculators and formulas, I felt this was a very simple and elegant approach that is lesser known on the Internet. I hope you find this rule-of-thumb circuit useful, and I shall be bringing more simpler and elegant approaches for the hobbyist in the future.

In this design, all the capacitors are 0.068 µF, and the resistors are in proportion to the value of R. Therefore, if R were to be 5 kΩ, then the bass and treble potentiometers would be 50 kΩ. In this circuit, the maximum boost and cut achievable is ±20 dB. With the response centred around 1 kHz, the corner frequencies are very practical because it will boost or cut bass frequencies below 500 Hz, and treble above 2000 Hz. Hence this is what engineers usually deem as a standard Baxandall for general purpose audio applications, and this type of network usually connects within the negative feedback loop of almost any general-purpose operational amplifier package.

Graph

In the graph, the red lines represent ideal response, and the green curves represent actual response. Given that zeros are at 500 Hz and 2000 Hz, and that first order filters have a gradient of 20 dB per decade; we can easily calculate where the poles would be at full amplification (20 dB). In this circuit, they are at 50 Hz and 20 kHz.

Bias Current–Compensating Resistor

The resistor between the non-inverting input and ground (2×R) is a current-compensating resistor and it is usually equal to the parallel sum of the feedback resistance and input resistance. Engineers that neglect to include this resistor are treating the op-amp as an ideal device which in practice it is not. The ideal op-amp has no input current at its terminals, but in fact, in the real world, the operational amplifier exhibits small input bias currents. In addition, imbalances in its internal transistors also produce a small offset voltage between its input pins. These imbalances usually result in producing an error voltage at the output pin. By using a current-compensating resistor, we can minimize the output error voltage. You will find the mathematics and theory behind this in almost any good op-amp textbook.

Always use a bias current-compensating resistor in series with the non-inverting input when using op-amps with bi-polar input transistors, except for when the package has FET input transistors. When the op-amp package has FET input transistors, the non-inverting input usually connects directly to ground and we do not use a current-compensating resistor.

Today, there are huge numbers of op-amp packages, with differing designs, specifications, and many manufacturers produce devices that have internal compensation. In this case, an external bias-current compensating resistor will not be required. Therefore, one has to be mindful of this when selecting an op-amp device for any particular circuit.