18W Class-B Audio Amplifier with Tone Controls

No need for a preamplifier - No setup required

Incorporates Baxandall Bass and Treble Controls

Circuit diagram:

18W Class B Audio Amplifier


P1______________50K  Log. Potentiometer (or 47K)
                     (twin concentric-spindle dual gang for stereo)
P2______________20K  Linear Potentiometer (or 22K)
                     (twin concentric-spindle dual gang for stereo)
P3_____________100K  Linear Potentiometer
                     (twin concentric-spindle dual gang for stereo)

R1_____________820R  1/4W Resistor
R2______________68R  1/4W Resistor
R3_______________1K8 1/4W Resistor
R4______________10K  1/4W Resistor
R5_______________1K  1/4W Resistor
R6,R7__________100R  1/4W Resistors
R8_____________330R  1/4W Resistor
R9______________47R  1/2W Resistor

C1_______________1µF  63V Polyester Capacitor
C2______________33pF  63V Polystyrene or Ceramic Capacitor
C3______________10nF  63V Polyester Capacitor
C4,C5____________1nF  63V Polyester Capacitors
C6_____________120nF  63V Polyester Capacitor
C7______________22nF  63V Polyester Capacitor
C8,C10_________220nF  63V Polyester Capacitors
C9______________22µF  25V Electrolytic Capacitor
C11,C12________220µF  25V Electrolytic Capacitors

IC1__________NE5534   Low noise Single Op-amp

Q1____________BD440   60V 4A PNP Transistor
Q2____________BD439   60V 4A NPN Transistor

J1______________RCA audio input socket


In order to satisfy repeated requests by correspondents, a medium power audio amplifier incorporating tone controls in the feedback path was finally designed.
To avoid an excessive increase in parts-count, due to the addition of the tone controls, a simple amplifier circuitry was designed on the same guidelines of the successful 45 Watt Class B Amplifier, but using the excellent NE5534 IC instead of a discrete component op-amp to drive the output "dumper" transistors.

The particular circuit configuration, allowed to push the NE5534 exceptional capacity of driving the output transistors near to its limits, enabling the whole amplifier to deliver relatively high power outputs without problems. For this reason, it was possible to obtain 18W into an 8 Ohm load using a power supply voltage of +/- 20V.
Despite the complication added by the tone controls, the amplifier has an input sensitivity of 130mV RMS, allowing to connect directly to its input the most disparate audio sources without the need for a separate preamplifier.
Total Harmonic Distortion figures are astonishingly low, much lower than comparable audio amplifiers using a single-IC audio amp.

An interesting feature of this amplifier is the absence of any kind of setup.

Bass and Treble Frequency Response:

Bass and Treble Frequency Response

Power Supply Circuit diagram:

Power Supply for the 18 Watt Class B Audio Amplifier

Power Supply Parts:

R1_______________1K8 1/4W Resistor

C1,C2_________4700µF  25V Electrolytic Capacitors

D1_____Diode bridge  100 to 400V, 1.5 to 4A
D2______________LED  Any type and color

SW1_____________SPST Mains switch

T1______________230V Primary, 30V Center-tapped or 15 + 15V Secondary,
                     about 45 to 60VA or 1.5 to 2A, Mains transformer

PL1_____________Male Mains plug with cord


The power supply is straightforward. The parts values suggested are suited to power a stereo version of this design. For a mono amplifier a less powerful mains transformer can be used, having a 30V Center-tapped or 15 + 15V secondary winding, rated at about 24 to 30VA or 0.8-1A.


Technical data:

Output power:
18 Watt RMS into 8 Ohm (1KHz sine wave)
Input sensitivity:
130mV RMS for full output
Frequency response @ 1W RMS and Tone Controls in center position:
-1.2dB from 50Hz to 15KHz; -3dB @ 20Hz; -2dB @ 20KHz
Total Harmonic Distortion @ 1KHz:
500mW 0.005% 1W 0.0035% 10W 0.0035% 18W 0.008%
Total Harmonic Distortion @10KHz:
500mW 0.007% 1W 0.004% 10W 0.01% 18W 0.024%
Unconditionally stable on capacitive loads