Tone control circuit based on TDA1524A

This is a low noise and distortion preamplifier and tone control circuit in a module. Using a special IC TDA1524A, this simple circuit is easy to build and can drive most of the power amplifiers.

Schematic diagram:

Preamplifier + Tone Control Component List:

Circuit Engineering:

All signal processing is done within the TDA1524A by a voltage controlled amplifier and a voltage controlled filter. The IC provides a fixed voltage (~ 3.8V DC) at pin 17, which all variable resistors use to provide an adjustable DC voltage to the corresponding control pin.

Current sensing is used to provide a flat response when R5 is connected to pin 17, and a loudness contour when it is disconnected. 100 nF capacitors are used on each potentiometer to decouple any AC signals from the control inputs. 10 uF capacitors are used to couple the input and output audio signals while blocking DC. R1 and R2 are used to ensure stability with capacitive loads. R3 and R4 ensure that there are no DC spikes at the output socket if the load is switched. C3 and C4 control the loudness contour. C5 and C6 control the treble rollover frequency. C18 and C19 are added to roll off the gain above 70 kHz. In some cases, low volume settings coupled with treble boost can cause HF instability. This should no longer be a problem.

C15, 16, 17 provide power filtering. D1 provides protection in case of wrong power polarity. The LED is a power indicator and can be omitted if not required, or preferably mounted on the case. If you do not use a switching potentiometer, you can connect an external switch between the P1 switch pins, or connect a wiring here and switch the power.

The power supply is critical to the noise? performance of the preamplifier. An onboard regulator is provided to reduce power supply hum. If you wish to use this with an automotive or other 12V battery, the 7812 regulator should be omitted and a trace placed between the regulator input and output pin locations on the PC board. Do not short to ground! This will be necessary because the regulator's input voltage must be at least 2-3V higher than its output in order to maintain regulation. However, the regulator will not be necessary when using battery power.

If using a plug set, its output voltage should be 15 to 18V DC. Because most plug sets are poorly regulated, a power supply rated for 12V DC will usually be around 15V when lightly loaded. The preamplifier draws less than 50 mA, so if you have a 12 V unregulated power supply that may be sufficient. If necessary, replace D1 with wiring, making sure to get the polarity of the power supply correct!

If you use a 15-20V supply for your power amp, you can also use it as a preamp supply. Make sure to test the voltage first in all cases.