There are quite a few applicationsof an audio power amplifierin which power and hi-ficharacteristics are of secondaryimportance. If, for instance, anactive loudspeaker for a portableradio receiver is needed, compactdimensions and low currentdrain are far more importantconsiderations.
The Medium Power HEXFET amplifierpublished in this magazine in December1993 has one small drawback: it delivers‘only’ 60 W into 8 � (or 120 W into4 ). Otherwise, it is a first class amplifierthat provides excellent music reproduction,which is evidenced not so much by measurementas by audition. To some listeners,it has a quality not unlike that of a valveamplifier. Because of its popularity and themany requests for a version with higheroutput power, it has been upgraded to providearound 90 W into 8 � (about 160 Winto 4 ).By a stroke of good fortune, a pair ofIGBTs (Insulated Gate Bipolar Transistors –see our June 1995 issue) proved ideal replacementsfor the HEXFETs used in theoriginal design. Apart from the figures forpower output, the technical specification remainsvirtually the same (see box).
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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.
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This realization prompted the presentcompact power amplifier whose quality and layout match those of thepreamplifier. It offers medium power output, is not overly complex andhas a very high slew rate.
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In this design of a simple loudspeakerenclosure an attempt hasbeen made to achieve a reasonablygood quality with a minimum ofmaterial. In spite of inexpensive drivers,this aim was met.The cross-over filter has a 6-dBroll-off, which means only one componentper driver: L1 for the wooferand C2 for the tweeter. There is alsoan impedance-correction network,R1-C1, for the woofer, which ‘flattens’the rising impedance of this driver.
This circuit automatically disconnectsa mid-range loudspeakeror a tweeter from theoutput of an amplifier when toomuch power is applied. A relaywith a normally closed (n/c)contact is used as the switchingdevice.
The potentiometer introduced by Panasonica little while ago is of a qualityexceeded only by the likes of the Penny &Giles potentiometer (which cost in excess of£ 100). The Panasonic devices have multilayertracks made from conductive plasticsand carbon, which are linked to the terminalsby silver electrodes. The five-foldwiper is also made of silver and guaranteeshigh accuracy (tracking within 0.8 dB) andsmooth operation. In other words, this is anattractive, reasonably priced, high-qualityvolume control.
To convert a standardstereo installation intoa surround soundsetup, at least threeadditional speakersare needed, of whichthe centre one mustbe magneticallyshielded. The miniaturespeakers describedin this articlehave been designedsolely for this purposeby the German loudspeakerspecialistVisaton.
Frequently,a directvoltage isrequired ina circuitthat mustbe free ofhum and noise,but not regulated.For instance,the supplyto an audiooutput stage mustbe able to vary with themains voltage and theload. Another exampleis when in the workshopa variable-ratiotransformer (variac) isto produce a well-filtereddirect voltage withgood loading capacityfor general purposes.
power supply and construction
Following the detailed description of the preamplifierin last month’s instalment, this secondand final part of the article deals with the powersupply and the construction of the completeunit. More so than usual, the power supplyforms an important part of the overall design. Itconsists of 12 NiMH batteries, size AA(RG/HP7) with a nominal capacity of 1.2 Ah,and a moderately fast charger that takes aboutthree hours to fully charge a set of flat batteries(note that it is not a good idea to dischargethese batteries completely – the nominal voltagelevel is maintained at 1.2 V during 80% ofthe discharge cycle).
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