18W Class B audio amplifier

Circuit diagram:

Component:

P1______________50K Logarithmic potentiometer (or 47K)

                    (Dual concentric shaft dual-coupling for stereo)

P2______________20K linear potentiometer (or 22K)

                    (Dual concentric shaft dual-coupling for stereo)

P3_____________100K Linear Potentiometer

                    (Dual concentric shaft dual-coupling 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 Capacitor

C6_____________120nF 63V Polyester Capacitor

C7______________22nF 63V Polyester Capacitor

C8,C10_________220nF 63V Polyester Capacitor

C9______________22µF 25V electrolytic capacitor

C11, C12________220µF 25V electrolytic capacitors

IC1__________NE5534 Low-noise single operational amplifier

Q1____________BD440 60V 4A PNP transistor

Q2____________BD439 60V 4A NPN transistor

J1______________RCA Audio Input Jack

Comment:

To meet the repeated requests from the communicator, a medium-power audio amplifier with tone control included in the feedback path was finally designed.

To avoid an excessive increase in the number of parts due to the addition of tone control, based on successful...

 45W Class B amplifier

The same guiding principles were used to design a simple amplifier circuit, but the excellent NE5534 integrated circuit was used instead of discrete component operational amplifiers to drive the output "damping" transistor.

A special circuit configuration allows the NE5534 to be driven to its limits, enabling the entire amplifier to deliver relatively high power output without problems. Therefore, 18W of power can be obtained with an 8-ohm load using a +/-20V supply voltage.

Despite the added complexity of tone control, the amplifier has an input sensitivity of 130mV RMS, allowing direct connection to most different audio sources without the need for a separate preamplifier.

The total harmonic distortion data is extremely low, far below that of comparable audio amplifiers using a single integrated circuit audio amplifier.

One interesting feature of this amplifier is that it requires no setup whatsoever.

Bass and high frequency response:

Power supply circuit diagram:

Power supply components:

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_____________Single-pole single-throw main power switch

T1______________230V primary, 30V center tap or 15+15V secondary.

                    Main power transformer, approximately 45 to 60 VA or 1.5 to 2 A.

PL1_____________Main power connector with cable

Comment:

The power supply section is simple. The recommended component values ​​apply to the stereo version of this design. For a mono amplifier, a lower-power mains transformer with a 30V center tap or a 15+15V secondary winding can be used, with a rated power of approximately 24 to 30 VA or 0.8-1 A.

Note:

The Q1 and Q2 power transistors should be mounted on a heatsink of at least 100x50x25mm.

+22V and -22V are the absolute maximum permissible supply voltages for the NE5534: Do not exceed these limits.

If a higher voltage is required from the power supply, consider using a regulated power supply. Please refer to [link/reference].  

  Simple, efficient, and regulated power supply  

 .

Technical data:

Output power:

 18 watts RMS into 8 ohms (1kHz sine wave)

 Input sensitivity:

 130mV RMS for full output

 Frequency response @ 1W RMS and tone control at the middle 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