Passive Baxandall Tone Control Circuit

This is a design circuit for tone control circuit using variant Baxandall. Variation of the famous Baxandall circuit is shown in the passive circuit in figure 1. Smoothly increasing +- 6 dB/octave slope of boost or cut is the feature of this circuit. Although the “shelves” are outside the audible range with these component values, the bass and treble filters have a shelving response. This is the figure of the circuit;

The threshold and shelving frequencies is predicted by the filter equations in figure 1a.The wiper shorts out the .033uf capacitor, when the bass control is rotated for maximum boost. A frequency dependent voltage divider that determines the shelf frequency of the boost are formed by R3 and C4. The wiper bypasses the 10K resistor. C1 and R2 form a high pass filter, when the treble control is set for maximum boost. The circuit uses commonly available parts to simplicity. Radio Shack sold even the 100k log taper pots, which are usually hard to find, as part number 271-1732 (in a stacked configuration for stereo). The pot will have about 10K on one side and 90K on the other at the midway point. The side with 10K parallels R2 for the treble control. The side with 10K parallels C4 for the bass control. Note that this circuit could be divided for an individual bass or treble control (R5 may then be omitted – it helps isolate the bass from the treble circuit when the two are put together). To avoid loading the network and affecting the response curves, the passive Baxandall must see a low impedance source and drive a high impedance load.

Volume and Balance Controls Circuit

The designed by Peter Baxandall of feedback tone control fame, amongst many other designs, there is also an active version of the 'Volume and Balance Control', which uses an op amp and a pot in the feedback loop. The log law is almost identical to that for the passive design above, but it can provide gain as well as attenuation. This circuit is using TL072 as op amp the signal input and output of the circuit. This is the figure of the circuit.


The input buffer enables the inverting stage (needed so the circuit can work) to have a very high input impedance. This would otherwise not be possible without the use of extremely high value resistors, which would increase the noise level considerably. The maximum gain as shown is 10 (20dB) and minimum gain is 0 (maximum attenuation). The input impedance is variable, and is dependent on the pot setting. At minimum gain, input impedance is the full 50k of the pot, falling to about 27k at 50% travel, and around 4k at maximum gain. These impedance figures are very similar to the simple passive version (if a 100k pot is used), and again, a low impedance drive is required or the logarithmic law will not apply properly. The actual value for VR1 does not matter, and anything from 10k to 100k will work just as well, although it will influence the input impedance. The error at 50% of pot travel is less than 5% with values from 10k to 100k.

Tone Control Circuits Using CA3140

This is a circuit for tone control in the amplifier. High slew rate, wide bandwidth, high output voltage capability and high input impedance are all characteristics required of tone control amplifiers. This circuit is built by CA3140 IC. Ca3140 is a op amp for audio signal. This is the figure of the circuit.


The Baxandall tone control circuit which is provides unity gain at mid band and uses standard linear potentiometers. The high input impedance of the CA3140 makes possible the use of low cost, low-value, small size capacitors, as well as reduced load of the driving stage. Bass treble boost and cut are 15 dB at 100 Hz and 10 KHz, respectively. Full peak-to-peak output is available up to at least 20 KHz due to the high slew rate of the CA3140. The amplifier gain is 3dB down from its “flat” position at 70 KHz.
[Project Schematic source: Intersil Corporation]