HiFi stereo preamplifier with tone control

Posted on Oct 18, 2012

The preamp featured is very straightforward to make on the PCB, and has an innovative tone defeat function. Rather than completely disable the tone controls, they are massively de-sensitised, and when `defeated` have a maximum range as shown in Fig. 3 (below). This can be increased if desired, so you can have two tone control settings, one with the normal 10dB boost and cut, and the other with a very subtle 3dB boost and cut - this will be enough (surprisingly) for very minor adjustments such as you might need for day-to-day listening.

HiFi stereo preamplifier with tone control
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As you can see, the PCB is very compact. The volume pot is actually spaced a little further apart than the others to allow a larger knob, since this is the most commonly used control in any preamp. The use of 16mm pots makes for a small and neat layout, and makes it very easy to include the preamp with a power amp, making a complete integrated amplifier system. he input stage is configured as shown with a gain of 2 times (6dB), and also acts as a buffer for the tone control circuit. The tone control is a basic Baxandall type, but the addition of R117, 118 and 119 provide flexibility and easy reconfiguration that is not available with the traditional arrangement. The tone control (and overall) performance is shown in Figure 2 (10% steps of the pots), and it can be seen that the midrange is barely affected. This is in contrast to the majority of designs, where the controls are centred on 1kHz, and there is a very audible effect in the midrange frequencies. For those who absolutely do not want to use tone controls, I suggest the DoZ preamp (Project 37) or Project 88 - both were designed with no tone controls and are more in the line of true minimalist designs. Note that although shown using TL072 opamps, OPA2134 or anything else that suits your purposes may be used instead. I do not recommend using anything less than a TL072, even for the workshop or rumpus room, as there will be excessive noise and limited frequency...

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