Three-Way Tone Control

Although tone control is not desirable in good-quality audio equipment, there are still instances, such as when playing well-used records, when it is. Such an add-on tone control should enable the frequency response to be altered to taste, have no detrimental effect on the audio equipment, and be fairly com- pact. The circuit proposed here meets these criteria. It is based on National Semiconduc- tors"s LM833. This dual operational amplifier has a very low noise factor (4.5 nV/Vf (Hz)), a high gain-bandwidth product (15 MHz), and a slew rate of 7W. The tone control circuit consists of three ranges, so that a presence control at around 1 kHz is possible. The opamp at the input, Ai, is connec- ted as an inverting buffer. Its non- inverting input is connected to a 10 k re- sistor to equalize the direct currents at both inputs (with respect to the bias cur- rents). This is necessary to keep the out- put of Ai near enough at 0 V because of the d.c. coupling to A2. The second opamp has ...