Audio Amplifier Projects

In these days ofminiaturization, fewpeople seem to feela need for largeloudspeakers in theirliving room. But,although they wantsmall boxes, they stillwant good quality. Thewall box presented inthis article meets thoserequirements. It is atwo-way system housedin a small, shallowenclosure that isinconspicuous in theliving room.

Brief parameters
Design Two-way or three-way, vented
Woofer WS13BF (Visaton)
Tweeter DTW95NG (Visaton)
Wideband driver FRWS5 (Visaton)
Frequency range 70 Hz – 20 kHz (–3 dB)
Crossover frequency About 2 kHz
Music power 60 W
Nominal impedance 4 Ω
Volume 5 litres
Dimensions 300×418×138 mm (H×W×D)
Special aspects Wideband unit provides
sideways radiation

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S/PDIF test generator
The generator is intended primarily for checking S/PDIF (Sony/Philips Digital Interface Format) receivers and any associated digital-to-analogue converters (DAC) and/or output filters. The externalclock – standard TTL level – enables 128 sample frequenciesto be generated. The clock may also be used for generating standardfrequencies with the remaining inverters serving as crystal oscillators(provided a 74HCU04 is used).

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simple copybit killer
The Serial CopyManagementSystem (SCMS)prevents digitalcopying of audiomaterial from thesecond generationonwards.Unfortunately, thisprotection alsoworks on homerecordings and soprevents homerecordists fromdigitally copyingtheir ownmusical work more thanonce without degradationby the SCMS. This problemhas been highlightedbefore in this magazine,but it has not gone away– reason enough todescribe a simple andinexpensive circuit forpermanently removing thecopy-prohibit-bit from theS/PDIF* audio signal.

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sampling rate converter
Although there arestill dyed-in-the-woolsound technicianswho swear by thegood old analoguerecording technology,most others, bitten bythe digital audiorecording bug, do notwant to revert to analoguerecording. Theyhave experienced thepleasures of loss-freeprocessing and copyingof recordings atdigital level. Thesepleasures turn to frustration,however,when they want toconvert a DAT recordingon to a CD. Thiscannot be done justlike that because thetwo have differentsampling frequencies:DAT 48 kHz, and CD44.1 kHz. To overcomethis difficulty, aconverter as describedin this articleis required.

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50 W a.f. amplifier
Although audio enthusiastscome inmany sizes andcolours, not many ofthem are prepared tospend a lot of money,time and effort tobuild an a.f. amplifierusing up to 40 transistorsto improve thedistortion by a fractionof a per cent.Therefore, the amplifierdescribed hereshould appeal tothose enthusiasts. Itis compact, presentsno problems and yethas properties thatmake it fully suitablefor all but the mostdemanding audio applications.In short, anamplifier that isgeared to the practicalaudio buff.

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30 W AF amplifier for cars
The power that can be obtained from a standard car radio amplifieroperating from a 12 V car battery is 5–6W, which (formany listeners) is not really enough for satisfactoryhi-fi reproduction. It is, of course, possible to boost the 12 Vsupply with a power inverter, but that is fairly expensive andnot always acceptable. Now, a Philips IC enables audio powerof about 30 W to be obtained from a 12 V car battery.

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1-watt BTL audio amplifier
The TDA8581(T) from PhilipsSemiconductors is a 1-wattBridge Tied Load (BTL) audiopower amplifier capable of delivering1 watt output power into an8-Ωload at THD (total harmonicdistortion) of 10% and using a 5-V power supply. The schematicshown here combines the functionaldiagram of the TDA8551with its typical application circuit.The gain of the amplifiercan be set by the digital volumecontrol input. At the highest volumesetting, the gain is 20 dB.Using the MODE pin the devicecan be switched to one of threemodes: standby (MODE levelbetween Vp and Vp–0.5 V),muted (MODE level between1 V and Vp–1.4 V) or normal(MODE level less than 0.5 V).The TDA8551 is protected by aninternal thermal shutdown protectionmechanism.The total voltage loss for bothMOS transistors in the complementaryoutput stage is less than1 V. Using a 5-V supply and an8-Ω loudspeaker, an outputpower of 1 watt can be delivered.

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supply board for output amplifiers
Apart from their electronic configuration,all output amplifiers comprisethe same elements: an amplifierboard, a mains transformer, a bridgerectifier, and electrolytic smoothingcapacitors. The board is normallyscrewed to the heat sink, while thetransformer and bridge rectifier arefixed to the bottom of the enclosure.Often, there is no such defined locationfor the electrolytic capacitors.These are mounted on a piece ofprototyping board, or to the bottomof the enclosure with suitable brackets.

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SINGLE-CHIP 50 W AF AMPLIFIER
The chip on which the amplifier isbased, a Type LM3876, is a memberof the Overture family from National Semiconductor,All members of this family arepin-compatible and mutually interchangeable.They are typified by an internal protection(called SPIKE). In practice, the differencebetween them is the power output.The series was described on the basis of theLM3886 in an earlier issue*.The PCB has been designed so that itcan accommodate the LM3876 (50 W) aswell as the LM3886 (150 W). Because ofthis, pin 5 of the IC on the board is connectedto the positive supply line. This connectionis not needed for the LM3876,since its pin 5 is not (internally) connected(NC).

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