Schematic Volt

This is a great audio amplifier circuit based on single power IC TDA7294. TDA7294 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 4Ω and 8Ω loads. It has both short circuit and thermal protection, so is quite robust.

Component part list:

R1_____________ 150R R2, R3, R5_____ 10K R4_____________ 680R R6_____________ 22K IC1____________ TDA 7294 Heatsink

C1____________ 1u5F or 1u C2____________ 2n7F C3, C6________ 22uF 63V C4, C5________ 10uF 63V C7, C9________ 2200uF 50V C8, C10_______ 100nF

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). a similar circuit was published in elektor magazine, 11/96.

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.

• 2N3055 – a complementary Silicon Epitaxial-Base planar NPN transistor mounted in Jedec TO-3 metal case for use as power transistor.
• BD243 – an epitaxial-base Silicon NPN transistor intended for wide variety of high speed switching and power amplifier applications such as series and shunt regulators, and driver and output stages of high-fidelity amplifiers.
• BC303 – a PNP silicon planar epitaxial transistor used for AF drivers & outputs, for AF medium power amplifiers, and for switching applications up to 1 A.

R1__________100K   1/4W Resistor

R2___________50K   1/2W Trimmer Cermet

R3__________330K   1/4W Resistor

R4____________1M2  1/4W Resistor

R5__________470K   1/4W Resistor

R6,R7_______500K   1/2W Trimmers Cermet

R8____________1K5  1/4W Resistor

R9-R11______470R   1/4W Resistors

C1___________47pF  63V Ceramic Capacitor

C2__________100nF  63V Polyester Capacitor

C3___________47µF  25V Electrolytic Capacitor

C4____________1µF  25V Electrolytic Capacitor

D1______BZX79C5V1 5.1V 500mW Zener Diode

D2_________1N4148  75V 150mA Diode

D3-D5________3mm.  Yellow LEDs

IC1_________LM339  Quad Voltage Comparator IC

SW1__________SPST  Slider Switch

B1_____________9V  PP3

Clip for 9V PP3 Battery

Circuit operation:

• D5 illuminates at 2W
• D4 illuminates at 12.5W
• D3 illuminates at 24.5W

Notes:

• The simplest way to connect this circuit to the amplifier output is to use a twisted pair cable terminated with two insulated crocodile clips.
• Setup is best accomplished with an oscilloscope or an audio millivoltmeter like the one described in these pages. Precision Audio Millivoltmeter
• A 1KHz sine wave generator with variable output is also required (see a suitable circuit in this website also). 1KHz Sinewave Generator
• Connect the generator to the amplifiers input and the Audio Power Indicator to the output of the amplifier, in parallel with the oscilloscope probe or the audio millivoltmeter input.
• When using high power outputs disconnect the loudspeakers to avoid Tweeters damage and connect in their place an 8 Ohm 20-30 Watt wirewound resistor.
• Remember that VRMS output is equal to output Peak-to-Peak Voltage divided by 2.828.
• RMS power output in Watts is equal to VRMS² divided by speaker impedance (usually 8 or 4 Ohm).
• Example: set the output of the 1KHz sinewave generator to read 14V on the audio millivoltmeter (24.5W @ 8 Ohm). Set R2 until D3 illuminates, and be sure that D3 turns-off when diminishing a little the generators output.
• Do the same with R7 for D4 and R6 for D5. The readings of the audio millivoltmeter must be 10V (12.5W @ 8 Ohm) and 4V (2W @ 8 Ohm) respectively.