A stereo mixer amplifier for mains and battery operation

The DIY mixer amplifier shown here has a maximum output of approx. 2x40 watts. It is suitable for 230 volt mains operation and for connection to 12 volt vehicle electrical systems. There are four mixable stereo inputs to which e.g. keyboards, outputs of stereo guitar effects devices, MP3 players or notebooks can be connected. It is therefore well suited for field days, outdoor parties and street music performances. By connecting one or more microphone preamplifiers, the device can also be used as a karaoke amplifier. There is enough space in the device to install them if necessary.

In addition to the power supply and display unit, the amplifier is essentially divided into three functional units, namely the four-channel stereo mixer with four TDA8199, the tone adjuster with the TDA1524A integrated circuit and the stereo power amplifier with the TDA8560Q IC. By using the TDA8560Q integrated circuit designed for the car hi-fi sector, the circuit complexity for the output stage is extremely low, especially since it contains both stereo channels. The achievable output power is 25 watts into 4Ω and even 40 watts into 2Ω per channel. The IC must be mounted on a large heat sink.

It is noteworthy that this performance is achieved with the comparatively low operating voltage of only 14.4 volts. This is made possible by output stage bridge circuits. It should be noted that the loudspeakers must therefore be connected to a floating connection. Due to the low supply voltage, the amplifier can also be operated with rechargeable batteries. The high level of operational reliability is also advantageous, because the IC is equipped with a short-circuit and overtemperature protection circuit.

Clipping operation or excess temperature can be read out via the digital diagnostic output (pin 12). If pin 11 is not connected to positive, the output stage switches to standby mode. This enables an electronic switch-on delay to be implemented using a simple RC element. All you have to do is connect a resistor between pin 11 and plus and an electrolytic capacitor from pin 11 to minus. With a 10kΩ resistor and a 100µF electrolytic capacitor, the time constant is one second. With about this delay, the power amplifier then switches on smoothly and without annoying crackling.

By using the TDA1524A as the control unit for the bass, treble, volume and balance setting, the effort could also be kept small here. The advantage of this circuit is that only mono potentiometers with linear setting characteristics are required. In addition, the wiring effort to the front panel is minimal. Since the settings are made exclusively via control DC voltages, the lines do not need to be shielded. The IC also offers the possibility of a contour / loudness function (aurally correct volume), which is not used in the existing device. The setting range for the bass ranges from -19 to + 17dB. The heights can be adjusted within a range of ± 15dB.

The amplification of the tone setting assembly is 21.5dB when the volume is fully turned up (tone control in the middle position). Because this amplification is not required in the existing concept and the sound adjuster assembly provides an output level that is too high for the power amplifier, adjustable attenuators are inserted between the sound adjustment unit and the power amplifier, as can be seen in the photo. If inserted before the sound adjuster, its dynamic range would not be fully exhausted, which would ultimately lead to a poorer signal-to-noise ratio. As can be seen from the circuit, the attenuator can be adjusted so that both amplifier channels can be adjusted to exactly the same gain (balance adjuster in the middle position).

When designing the level display, the bridge circuit of the output stage had to be taken into account, because neither of the two speaker terminals is connected to ground. In the circuit used, the diode connected as a indication rectifier is connected to the loudspeaker terminals with high impedance, so that adverse effects with regard to the distortion factor are negligible. The lines from the output stage outputs to the display board are twisted. This measure prevents them from acting like antennas and thus reacting to the output stage inputs via the wiring, which could result in high-frequency oscillations.

The display instruments are illuminated with colored LEDs, which results in a greater color contrast. The lighting with incandescent lamps, which is common for such displays, was ruled out, as unnecessarily high power consumption should be avoided with a view to battery operation. The LEDs are operated in the usual way with series resistors on the internal 12-volt supply voltage.

The TDA8199 circuit used in the mixer unit was actually developed for volume adjustment in TV receivers. It is two-channel, i.e. intended for stereo, and is therefore required once per mixing channel. As a result, it is required a total of four times for the four existing mixer channels. The mixer assembly must be terminated with a very high resistance, a condition that is met by the directly following module with the TDA1524A.

A TDA8199 contains two voltage-controlled amplifiers, the amplification of which can be adjusted synchronously via a control DC voltage in the range from approx. 0 to 5 volts. As a result, only simple mono potentiometers with linear characteristics here also are required for level control of both stereo channels. Excellent synchronism is achieved. In addition, scratching noises caused by the potentiometers are suppressed because they are not in the signal path. Furthermore, the wiring is simplified considerably by the level control based on DC voltage. Therefore only the 5V line, the ground line and four single-core lines for the individual channels have to be led to the potentiometers to the front panel. No shielding is required here either.