This tone control circuit is easy to incorporate in a stereo amplifier, disco unit or whatever, as it has a high input impedance (over 100kR), a nominal voltage gain of unity and a low output impedance. The usual bass and treble controls are included in the unit, with about 12 dB of boost and cut being available – at 100Hz and 10 kHz. The noise and distortion produced by the circuit are both extremely low due to the large amount of negative feedback used and the unit can handle output signal levels of-several volts rms without clipping.
Circuit in lines (Click to enlarge)
Q1 is used in a straightforward emitter follower buffer stage that gives the unit a high input impedance. C2 couples the output of Q1 into the tone control circuitry. This is an active circuit which provides frequency-selective negative feedback over an amplifier.
The amplifier uses Q2 as a conventional common emitter stage direct-coupled to emitter follower output transistor Q3. The latter gives the unit a low output impedance.
The tone control networks are slightly simpler than the usual Baxandall configuration, but give a perfectly acceptable level of performance. RV1 controls the bass while RV2 is the treble control. Feedback is at a maximum with the sliders of the potentiometers to the right and at a minimum with the sliders set fully to the left. Of course the gain of the circuit is inversely proportional to the level of feedback.
Maximum feedback therefore corresponds to maximum cut and not to full boost. The current consumption of the circuit is a little under 1mA per supply volt.
EmoticonEmoticon