Audio Amplifier Projects

single-chip AF power amplifier
If you’re looking for an audioamplifier that produces a lot ofpower using an absolute minimumnumber of components,this one is for you.Burr-Brown’s OPA541 is apower opamp capable of operationfrom power supplies up to}40 V and delivering continuousoutput currents of up to5 A. Internal current limit circuitrycan be user-programmedwith a single external resistor,protecting the amplifier and theload from fault conditions. TheOPA541 is available in an 11-pin power plastic package andan industry-standard TO-3 hermeticpackage. The former isused here.

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PA300 power amplifier
Taken by themselves, the properties of thePA300 amplifier are not revolutionary. Buttaken in combination, they show somethingspecial: a robust 300 watt hi-fi poweramplifier that is not too difficult to build.

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passive VU meter

If you want to fit your loudspeaker enclosure(s) with a drive indicator, it is best touse a unit that does not need a powersupply. This article describes a passive VUunit for smaller amplifiers and loudnesses—see Fig. 1. It consists of a rectifier and six(may be fewer or more) identical stagesthat each comprises a current source, azener diode and a light emitting diode,LED. The current sources are built fromJFETs with interconnected gate-source terminals—here, Type BF256A. The saturationcurrent, IDSS, of these devices with adrain-source voltage, UDS = 15 V is about 5mA. This current is not exactly constant,but is perfectly all right for driving a lowcurrentLED and will not exceed the permissiblevalue of 7 mA. Networks R2-D2,R3-D4, and R4-D7 are protection circuits;they prevent the drain voltage of the relevantJFET rising above 30 V, which normallydestroys the transistor.

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AF input selection

Use is made of a single-pole, 12position rotary switch, S1, with whichand 12 pull-down resistors the inputsource is selected. Since only one resistoris linked to the positive supply lineat any one time, the current drawn bythe circuit is only 15 μA, which, in thecase of a battery operated preamplifier,is an important advantage.

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digital VU meter - 2
After last part detailed description of thedesign and facilities ofthe accurate, multifaceted digital vu meter,this part’s final instalment deals with the construction and operation of the instrument.

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opto-to-coaxial audio converter

Most audio equipmentproduced in thepast five to ten yearshas digital inputs andoutputs. It is a pity,though, that there aretwo standards: opticaland coaxial, and notall equipment hasboth. Fortunately, theconnection difficultiesthat may ensue areeasily resolved by theuse of the adaptordescribed in this article.This enables anyoptical output to beconverted into acoaxial one or viceversa.

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digital-audio-input selector

As the name indicates, the selector isintended to choose one of up toeight digital audio signal inputs,which it does with the aid of a multiplexer.The multiplexer, IC6 is controlledby preset up/down counter IC2. Thecounter is set with DIP switch S3(note that the MSB switch is notused in this application).The various inputs are selectedwith press-keys S1 and S2.

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COPYBIT INVERTER

The inverter can be included in theS/PDIF link between any digital a.f. signalsource (such as a DCC recorder, a CDplayer, a DSR receiver) and a (second) DCCrecorder without the need of opening ormodifying any of the equipment. Duringthe copying, any copybit is inverted and atthe same time the category code is altered.This means that the S/PDIF signal so modifiedis accepted by the recorder as if itcomes from a CD player (so that unlimitedcopying of the signal becomes possible).The inverter also offers a number of otherfacilities, such as S/PDIF detection.

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COPYBIT ELIMINATOR REVISITED

Our February 1994 issue carried anarticle describing an inexpensive andstraightforward circuit for eliminating thecopybit from a digital S/PDIF* audio signalto enable users to copy (digitally) their ownmusical work many times without degradationby the SCMS**. The present article describesan updated version of that circuit,which can be used with the latest DAT, DCCand MD players.

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clipping indicator

A little while ago areader wrote to saythat he had foundoverdrive on some ofhis compact discs.This sort of newscomes of course likea thunderbolt since itis assumed by mostpeople that compactdiscs are examples ofthe quality of today’sdigital technology.The first reaction tosuch an allegation isone of outright disbeliefor at least scepticism.Moreover, it hasbeen alleged by otherreaders that severalproducers haveadmitted (sic!) tooverdriving, that isclipping, of CDs atthe request of the relevantartists. Be thatas it may, it was reasonenough to designan indicator to bringoverdrive to light andhelp the consumer inhis/her quest not tobuy flawed CDs.

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