50W Electronic Amplifier Constructional Project

Introduction To 50W Electronic Amplifier

This electronic amplifier project is based on an IC amplifier module from ST Microelectronics, the TDA7294. It is intended for use as a high quality audio class AB amplifier in hi-fi applications. It has very low noise and distortion, wide bandwidth and good output current capability, enabling it to supply high power into both 4 ohm and 8 ohm loads. It has both short circuit and thermal protection.

With the addition of a handful of parts and a suitable power supply, this module will deliver over 50W RMS into 4 or 8 ohms with < 0.1% Total Harmonic Distortion (THD) and < 0.1% Intermodulation Distortion (IMD). It is also suitable as a replacement power amp stage, or upgrade for many existing amplifiers of between 30W-50W, provided they have a suitable dual supply, and most do.

The Specifications of the electronic amplifier project are:

D.C. Input : 35V
Output power : > 50W RMS, 4-8 ohm load.
Gain : 24 dB (30dB modification)
Input sensitivity : 1.3V for 50W, 8 ohm
Signal-to-Noise ratio : > 95 dB, (>105 dBA)
Frequency response : approx. 20Hz - 200kHz, �3 dB
Slew rate : > 10V/uS
THD : < 0.01%, 1W-40W, 1kHz
IMD : < 0.01%, 1W

The maximum supply voltage of the IC is +/- 40V. However the maximum dissipation of the IC can be exceeded even at a lower voltage. Therefore the supply voltage used need not be more than +/- 35V. This can be constructed using a 50V center tapped transformer, a diode bridge rated at 5A (min.) and a pair of electrolytic capacitors, as shown below. A lower secondary voltage transformer could also be used but the reduced DC voltage will result in less power output into 8 ohms. You can still obtain 50W into 4 ohms with only 24V supply rails.

A 36V C.T. transformer will give you approx +/- 25V rails. The mains transformer used should be rated at a minimum of 80VA. If you want to run two modules in a stereo amplifier you can use a common power supply. In this case the transformer should be rated at 150VA or greater.

Electronic Amplifier Circuit Diagram Description

Most of the circuitry is contained within the IC module. The input signal is applied to pin 3 via capacitor C1 and low-pass filter R1/C2. The filter improves the pulse response and helps stop RF signals. The lower -3dB point is determined by R2/C1 and R4/C3. This is approximately 20Hz for the values used. The upper -3dB point is over 200kHz. C7/C8 and C9/C10 provide extra power supply filtering or decoupling.

R3/R4 are the feedback resistors. The gain is 1+R3/R4 which is approx 16 times, or 24dB. If you wish to increase the input sensitivity you may change the resistors to suit. Changing R3 to 22k would increase the gain to 30dB and lower the input required for 50W into 8 ohm, to 0.6V, without affecting performance too much. If you reduce the value of R4 you will also need to increase C3 to maintain bass response, as this sets the feedback low frequency roll off.

Pin 10 is a mute input and pin 9 provides a standby mode. Muting should always take place before standby mode is selected. Connecting these pins permanently to the supply rail ensures that the amplifier comes on immediately on power up. Any switch-on clicks may be eliminated by increasing the time constants of R5/C4 and R6/C5 if necessary.

Ensure that a heavy duty heatsink rated at least 1.4 degree C/W or better is used.

Electronic Amplifier Parts List

The parts list of this project is as shown below.

You may download the full data sheet for the Electronic Amplifier TDA7294 here from ST website.