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Wednesday, April 1, 2009

Portable 9V Headphone Amplifier

High Quality One-IC unit
Low current consumption

After several requests by correspondents, the decision of designing a 9V powered Headphone Amplifier was finally taken. The main requirement was to power the circuit by means of a common, PP3 (transistor radio) alkaline battery. So, implementing a low current drawing circuit was absolutely necessary, though preserving a High Quality performance.

The appearance of the 5534 low-noise op-amp at a reasonable price was much appreciated by audio designers. It is now difficult or impossible to design a discrete stage that has the performance of the 5534 without quite unacceptable complexity. 5534 op-amps are now available from several sources, in a conventional 8-pin d.i.l. format. This version is internally compensated for gains of three or more, but requires a small external capacitor (5-15pF) for unity-gain stability. The 5532 is a very convenient package of two 5534s in one 8-pin device with internal unity-gain compensation, as there are no spare pins.

The 5534/2 is a low-distortion, low-noise device, having also the ability to drive low-impedance loads to a full voltage swing while maintaining low distortion. Furthermore, it is fully output short-circuit proof.

Therefore, this circuit was implemented with a single 5532 chip forming a pair of stereo, inverting amplifiers, having an ac gain of about 3.5 and capable of delivering up to 3.6V peak-to-peak into a 32 Ohm load (corresponding to 50mW RMS) at less than 0.025% total harmonic distortion (1kHz & 10kHz).

If we consider that the mean current drawing at a power output of 15mW per channel is around 12-13mA (both channels driven), this Headphone Amplifier will become a 'must' for many DIY enthusiasts needing a High Quality, High Performance portable device.


P1______________22K Dual gang Log Potentiometer
R1,R5___________18K 1/4W Resistors
R2,R3,R4,R6_____68K 1/4W Resistors
C1,C2,C6_________4µ7 25V Electrolytic Capacitors
C3,C7___________22pF 50V Ceramic Capacitors
C4,C5,C8_______220µF 25V Electrolytic Capacitors
IC1__________NE5532 Low noise Dual Op-amp
J1____________3.5mm Stereo Jack Socket
SW1____________SPST Slide or toggle switch
B1_______________9V PP3 Alkaline battery Clip for PP3 Battery

Technical data

200mV RMS for 15.6mW RMS output
350mV RMS for 50mW RMS output

Maximum undistorted output: 3.6V Peak-to-peak

Frequency response: flat from 40Hz to 20KHz; -2.3dB @ 20Hz

Total harmonic distortion @ 1KHz: <0.025% style="font-weight: bold;">

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