ABSTRACT
In 1996, the author published a design for what is commonly known as the precision preamplifier in Electronics World; one of its main features was a treble/bass tone control with the frequencies variable over a 10:1 range. The noise can be reduced significantly by adding a little more complexity in the form of two unity-gain buffers to drive a balanced input amplifier built from much lower value resistors. The variable-gain balanced input stage for the Elektor preamp was the state of the art, when it was designed in 2011. As is now well-known, the great merit of the Baxandall active volume-control is that its control law depends only on the setting of a linear pot and not on the value of the pot track or the ratio of that track to fixed resistors. The distortion of the tone-control was measured for various other permutations of maximum boost/cut and maximum/minimum frequency.
