A premium preamp tone circuit

Whether the sound reproduction of the equipment can reach the ideal level is directly proportional to the grade of the equipment. Adding a tone circuit with frequency band attenuation or compensation in the pre-amplification of the equipment is an effective measure to improve the reproduction of the equipment.

Many years ago, I bought a high-quality pre-amplifier tone board kit and was very satisfied with it. Later, due to the upgrade of the whole machine, the tone board and the whole machine were integrated and reproduced. It has been used to good effect so far. The circuit principle and parameters are introduced as follows.

This circuit can boost or attenuate the frequency points of 25Hz, 200Hz, 1kHz, 10kHz, and 16kHz by ±18dB. The output stage adopts cross-coupled complementary emitter followers, which have good symmetry, wide frequency response, low noise, and low distortion.

The author made the following improvements during the remake process:

1. Remove the original integrated power supply part and process it separately, and connect multiple specifications of power supply filter capacitors in parallel.

2. The first-stage output interconnect capacitor is changed to a 1uF tantalum capacitor, the high-frequency capacitor of the tone part is changed to silver mica of the same specification, and the mid- and low-frequency capacitors are changed to Weimar capacitors of the same specification.

3. Replace the op amp 5532 with AD827, the high frequency is significantly improved, the mid-frequency is brighter, the low-frequency extension is stronger, and the control is better.

Suggestion: 1. Add 100uF+0.1uF in parallel before the ±power supply of the two op amps for decoupling, which is helpful in suppressing background noise.

2. Adding a small signal amplification stage before the line input will achieve better results.

Note: The power of the 220Ω resistor in the figure is not less than 1/2W, BC547 and 557 can be replaced by A733C945.