10W Audio Amplifier with Bass-boost

High Quality, very simple design
No preamplifier required

This design is based on the 18 Watt Audio Amplifier, and was developed mainly to satisfy the requests of correspondents unable to locate the TLE2141C chip. It uses the widespread NE5532 Dual IC but, obviously, its power output will be comprised in the 9.5 - 11.5W range, as the supply rails cannot exceed ±18V.

As amplifiers of this kind are frequently used to drive small loudspeaker cabinets, the bass frequency range is rather sacrificed. Therefore a bass-boost control was inserted in the feedback loop of the amplifier, in order to overcome this problem without quality losses. The bass lift curve can reach a maximum of +16.4dB @ 50Hz. In any case, even when the bass control is rotated fully counterclockwise, the amplifier frequency response shows a gentle raising curve: +0.8dB @ 400Hz, +4.7dB @ 100Hz and +6dB @ 50Hz (referred to 1KHz).


Parts:

• P1_________________22K Log.Potentiometer (Dual-gang for stereo)
• P2________________100K Log.Potentiometer (Dual-gang for stereo)
• R1________________820R 1/4W Resistor
• R2,R4,R8____________4K7 1/4W Resistors
• R3________________500R 1/2W Trimmer Cermet
• R5_________________82K 1/4W Resistor
• R6,R7______________47K 1/4W Resistors
• R9_________________10R 1/2W Resistor
• R10__________________R22 4W Resistor (wirewound)
• C1,C8_____________470nF 63V Polyester Capacitor
• C2,C5_____________100µF 25V Electrolytic Capacitors
• C3,C4_____________470µF 25V Electrolytic Capacitors
• C6_________________47pF 63V Ceramic or Polystyrene Capacitor
• C7_________________10nF 63V Polyester Capacitor
• C9________________100nF 63V Polyester Capacitor
• D1______________1N4148 75V 150mA Diode
• IC1_____________NE5532 Low noise Dual Op-amp
• Q1_______________BC547B 45V 100mA NPN Transistor
• Q2_______________BC557B 45V 100mA PNP Transistor
• Q3_______________TIP42A 60V 6A PNP Transistor
• Q4_______________TIP41A 60V 6A NPN Transistor
• J1__________________RCA audio input socket

Power supply parts:

• R11_________________1K5 1/4W Resistor
• C10,C11__________4700µF 25V Electrolytic Capacitors
• D2________________100V 4A Diode bridge
• D3________________5mm. Red LED
• T1________________220V Primary, 12 + 12V Secondary 24-30VA Mains transformer
• PL1_______________Male Mains plug
• SW1_______________SPST Mains switch

Notes:

• Can be directly connected to CD players, tuners and tape recorders.
• Schematic shows left channel only, but C3, C4, IC1 and the power supply are common to both channels.
• Numbers in parentheses show IC1 right channel pin connections.
• A log type for P2 will ensure a more linear regulation of bass-boost.
• Do not exceed 18 + 18V supply.
• Q3 and Q4 must be mounted on heatsink.
• D1 must be in thermal contact with Q1.
• Quiescent current (best measured with an Avo-meter in series with Q3 Emitter) is not critical.
• Set the volume control to the minimum and R3 to its minimum resistance.
• Power-on the circuit and adjust R3 to read a current drawing of about 20 to 25mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of J1, P1, C2, C3 &C4. Connect C9 to the output ground.
• Then connect separately the input and output grounds to the power supply ground.

Technical data:

Output power:

• 10 Watt RMS into 8 Ohm (1KHz sinewave)

Sensitivity:

• 115 to 180mV input for 10W output (depending on P2 control position)

Frequency response:

• See Comments above

Total harmonic distortion @ 1KHz:

• 0.1W 0.009% 1W 0.004% 10W 0.005%

Total harmonic distortion @ 100Hz:

• 0.1W 0.009% 1W 0.007% 10W 0.012%

Total harmonic distortion @ 10KHz:

• 0.1W 0.056% 1W 0.01% 10W 0.018%

Total harmonic distortion @ 100Hz and full boost:

• 1W 0.015% 10W 0.03%

Max. bass-boost referred to 1KHz:

• 400Hz = +5dB; 200Hz = +7.3dB; 100Hz = +12dB; 50Hz = +16.4dB; 30Hz = +13.3dB
• Unconditionally stable on capacitive loads