HEADPHONE AMPLIFIER
HEADPHONE AMPLIFIER
On much audio equipment, the headphone output is simplyderived from the loudspeaker output via a series resistor:not a very elegant design! The present circuit describes a‘real’ headphone amplifier that can be added to mostequipment, but may also be used as a stand-alone unit.
On much audio equipment, the headphoneoutput is simply derived fromthe loudspeaker output via a series resistor:not a very elegant design! The present circuitdescribes a ‘real’ headphone amplifierthat can be added to most equipment, butmay also be used as a stand-alone unit.Although many audiophiles still believediscrete components are best, the relentlessprogress of integrated circuits can not bestopped: these devices get better and better.Even top-quality commercial equipmentis now loaded with ICs and no one candoubt their quality and reliability. In manymodern CD players, preamplifiers and digital-to-analogue converters (DACs) there ishardly a transistor to be found. Only the designof power amplifiers often still relies ondiscrete devices. The present amplifier isbased on an IC: a surface mount device(SMD) Type TDA1308T from Philips Components.The IC was developed specifically for useas a headphone driver: the enthusiasticclaims of the manufacturer as to its qualitiesappear to be rather less exaggeratedthan is often the case (see opposite page:Performance). A signal-to-noise ratio of 110dB and a distortion factor of <0.009%(with a 5 k� load) are undeniably good.
The IC can be used to good effect in CDplayers, DCC players, keyboards, laser discsystems, multimedia amplifiers, and more.It draws a quiescent current of only 3 mAand can work from supplies of 3–7 V. Thelatter makes it suitable for use in either battery-powered circuits or in standard mainsoperatedequipment. Its dynamic range isgood, its bandwidth is 5.5 MHz and its slewrate is 5 V μs–1.The (simplified) internal design of the ICis shown in F i g . 1. The differential inputstage uses MOSFETs, M1, M2, is providedwith current mirrors and is powered by acurrent source, J1. The input stage is followedby two operational amplifiers, A1 andA2, that drive output stages M3 and M6,which are also MOSFETs.
The advantage ofMOSFETs is that the necessary input biascurrent is very small: typically 10 pA;moreover, the swing of the amplifier withhigh impedance loads is nearly equal to thesupply voltage.The inverting and non-inverting inputsof the op amps have an excellent commonmode suppression that ranges from the negativesupply voltage to 1.5 V under the positivesupply voltage. The IC can be fed fromsingle as well as bipolar supplies. Theclosed-loop gain can be set with two externalresistors.The outputs are short-circuit-proof andtotally free of switching noise. The humsuppression is 90 dB.