In this project a basic preamplifier is designed and built, based mainly on the Baxandall circuits for active volume and tone controls. The Baxandall circuits exists in several variants and are very commonly used in audio circuits, for the following reasons:
• The Baxandall volume control has good linearity (in dB), which is important for an even perception of sound level variations across the range (the only exception being a rapid roll-off near the lower volumes).
• The Baxandall volume control gain depends only on the maximum gain (ratio of two resistors) and the position of the potmeter viper (linear potmeter assumed). Hence it has a large tolerance to component variations, including the potmeter.
• The Baxandall tone controls give good separation between the bass and treble ranges, even though the simpler vesions of the circuit (which is used here) have some overlap. This is illustrated in the Tech doc section.
• The Baxandall tone controls give symmetric curves for attanuation and amplification (“cut” and “boost”) in their respective frequency ranges. This is illustrated in the Tech doc section.
As mentioned, in this project the simplest version of the circuits are used, for tone controls ths so-called “one-capacitor” versions.
In addition, the preamplifier has an active input stage.
The preamplifier is intended to be used together with a power amplifier designed in another project, and with an iPhone or similar as input source. A common gain structure must be considered to match this. This results in the following requirements:
The following has been done:
Possible further progress:
Click to enlarge images below