Stereo Pre-amplifier & tone control kit

This is a pre-amplifier and tone control unit which will make a useful addition to any of our power amplifier kits. It is extremely easy to construct due to the use of a special IC, the Philips TDA 1524A.

This contains all the circuitry required to provide up to 20 dB of voltage gain, +/-15 dB of bass and treble control, channel balance, and a switchable loudness contour. Because it uses voltage control, linear single gang pots can be used , which lowers the costs and reduces tracking rrors. It also allows for control by switches or microprocessor if desired.

Simple construction, few external components, low noise and distortion, and has a switchable loudness.

This circuit will drive most power amps.

Power Supply: 15V DC supply or 12V DC battery. 50 mA min.

Maximum output: > 3 V RMS

Maximum input: > 300 mV, G = 20 dB 3V RMS, G = 0dB

Input Z > 10 k ohm

Output Z < 300 ohm

Frequency response: < 10 Hz to > 50 kHz!

+ / - 1 dB THD at 1kHz : < 0.1 % at 1V output

Signal to Noise ratio: > 80 dB, G = 0 dB

Bass boost/cut ~ 15 dB at 50 Hz

Treble boost/cut ~ 15 dB at 15 kHz

All signal processing is done within the TDA1524A by voltage controlled amplifiers and voltage controlled filters. The IC provides a fixed voltage (~ 3.8V DC) at pin 17, and this is used by all the variable resistors to provide an adjustable DC voltage to the appropriate control pins. Current sensing is used to provide a flat response when R5 is connected to pin 17, and a loudness contour when disconnected. 100 nF capacitors are used on each pot to decouple any AC signals from the control inputs. 10 uF capacitors are used to couple both input and output signals whilst blocking DC. R1 and R2 are to ensure stability with capacitive loads. R3 and R4 are to make sure there is no DC spikes at the output sockets if the load is switched. C3 and C4 control the loudness contour. C5 and C6 are to ensure HF stability. C15-17 provide power supply filtering. D1 provides protection in case of incorrect supply polarity. The LED is a power on indicator and may be omitted if not required, or preferably mounted on the enclosure. If you are not using a switch pot, you can connect an external switch across the P1 switch pins, or connect a wire link there and switch the power supply. The power supply is critical to the noise performance of the pre-amp. An on board regulator is provided to reduce mains hum. If you wish to use it with a car or other 12V battery, then you should omit the 7812 regulator, and place a wire link between the regulator input and output pin positions on the PC board. Do not short to earth! This will be necessary because the regulator must have an input voltage at least 2-3V greater than its output, for it to maintain regulation. However a regulator will not be necessary with a battery supply. If using a plug pack, its output voltage should be 15 to 18vDC. Because most plug packs have poor regulation, one rated at 12V DC will often be around 15V when lightly loaded. The current drain of the pre-amp is less than 50mA, so many 12 V supplies may be adequate. If you are using a 15-20V supply for your power amplifier, you can use that as your pre-amp supply as well. Make sure you test the voltage first in all cases.