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As you can see, the Interactor Aura amplifier module comes in an attractively finished plastic case. It has two thumbwheel controls and two slide switches to control its operation. There are two LED indicators, one for power and one to indicate the onset of clipping. A solution waiting for a problem Jaycar Electronics are selling Aura Interactor amplifiers for a knockdown price of just $5 and the challenge is to put them to good use. We’ve drawn out the circuit diagram and made some measurements. Now what can you do with them? By LEO SIMPSON For some time now, Jaycar Electronics have been selling an attractive little module which goes by the name Aura Interactor amplifier. The Aura Interactor was (is) intended to drive a body blaster cushion from a computer games console or perhaps from the subwoofer signal in a home theatre system. As sometimes happens, the product was a monumental flop and so Jaycar is selling them off by the bucket-load at $5 each. Probably most people will just buy them and throw them into the junk 96  Silicon Chip box to be “ratted” at some stage in the future. We thought that it was a shame that such a nicely finished module should end up wasted in this way. So we set out to draw up the circuitry and see if it has other applications. Fig.3 shows the complete circuit and right from the outset, we have to admit that we don’t fully understand how it functions. Nor do we have time to fully analyse it. Hey, it could have been interesting to run it through the Electronic Workbench simulator reviewed elsewhere in this issue. What would the simulator have made of it? Again, we didn’t do it simply because we did not have the time. Let’s just run through a brief description of the circuit. The input signal from a stereo source is coupled via a 3.5mm jack socket to switch S2a and then via a 1µF capacitor (C7) to the volume control R44. From there it goes to op amps U7a & U7d which provide about 75 times gain and function as a low pass filter, rolling off signals above 2kHz. The output of U7d drives comparator U6b which squares up the signal and feeds it to the two flipflops in U3, a 4013 dual-D flipflop. Thus if the output of U6b is 120Hz, for example, the two square wave outputs from U3 will be at 60Hz and 30Hz. These two signals are used to control two inputs, pins 10 & 11, on a 4051 1-of-8 de­multiplexer, U4. As used here, the 4051 has eight inputs (pins 1, 2, 4, 5, 12, 13, 14 & 15) which can be switched through to pin 3 depending on the binary signals Inside the amplifier module. Note that it has quite a small heatsink for the class-B power stage and so it can only deliver its full power in short bursts. present at control pins 9, 10 & 11. We’ve already mentioned the signals fed to pins 10 & 11. The control signal to pin 9 comes from switch S1 and is high (+5V) in Music mode and low (-5V) in Games mode. By the way, all the ICs runs from ±5V supply rails so the CMOS chips effectively are running from a 10V supply. The signals to the eight inputs of the 4051 come from op amp U7d and via diodes D4 & D5 which considerably distort the signal and also via op amp U5b which provides a phase inversion of 180° (ie, it is a unity gain inverter). How does all this hang together? When the unit is in Music mode, the signal passes through the 4051 essentially unmodified to the following power amplifier. In Games mode though, the signal being fed to the power amplifier is practically unrecognisable, as can be seen in the lower trace of the scope waveforms Fig.1: these oscilloscope waveforms show the action of the 4051 in adding sub-harmonics. The top trace is a 120Hz sinewave while the lower trace is the mangled waveform which has a repetition rate of 30Hz. in Fig.1. The 4051 chops up the input waveform, adding bits that are out of phase and distorted, to obtain a waveform which has a substantial sub-harmonic content. As can be seen from the scope waveforms, the input waveform of 120Hz is turned into one with a repetition rate of 30Hz. This is just what would be required to rumble the Interactor cushion but it sounds pretty horrible if you feed it to a loudspeaker. The scope waveform of Fig.2 shows the frequency divider action of the flipflops. The top waveform is a 120Hz Fig.2: these waveforms show the action of the frequency divider circuit based on the 4013 dual flipflops, U3a & U3b. The top trace is a 120Hz sinewave while the lower trace is a 30Hz square wave, taken from pin 12, U3b March 2000  97 sinewave while the lower trace is a 30Hz square wave. Power amplifier Now let’s have a look at the power 98  Silicon Chip amplifier and like everything else in this circuit, it is unusual. Op amp U5a provides most of the voltage gain and it drives a complementary emitter follower pair, Q3 & Q4. These drive the output stage which is another complementary pair, Q1 & Q2, driven in common emitter mode. Neither of these transistor pairs has any quiescent current so the amplifier Fig.3: the circuit incorporates a frequency divider stage which provides control signals to a 4051 switching circuit and this adds sub-harmonics to the signal waveform when the unit is in Games mode. runs in pure class-B. Naturally there is some crossover distortion but the very substantial feedback applied back to the op amp’s input keeps the crossover distortion to fairly modest levels. Yes you can hear it and it means that it is not a hifi amplifier by any means but it is quite satisfactory for voice and other non-critical applications. The amplifier has two feedback net- works; one for AC signals, via C8, R36 and C101 and one for DC, via R12, C9 and R37. Why have they done it this way? It beats us. Much of the circuit seems unnecessarily complicated. March 2000  99 This view of the Interactor Aura amplifier module shows the two slide switches and the audio input socket (Source). Some readers have tried to modify the amplifier to provide some quiescent current but this is not really practical without re-designing the output stages. In any case, the amplifier can’t handle much quiescent current because it has a very small heatsink considering its nominal maximum output rating of about 20W into a 4Ω load. In fact, this level of output power would only be possible in very short bursts because the heatsink is just not capable of dissipating any appreciable power. Threshold muting One way in which the module does reduce the effective power output or duty cycle is with the threshold muting circuit, involving comparators U6a & U6d. U6a has its non-inverting input connected to the wiper of potentiometer R45. This is labelled as a “filter” control on the Interactor case but it has nothing to do with signal filtering. Instead, it acts as a signal threshold control for U6a; signals below the preset threshold do not pass through. Those that do pass through U6a are rectified by diodes D2 & D3 and the resultant DC voltage is fed to comparator U6d which is used to switch the Inhibit (INH) input, pin 6, of the 4051 demultiplexer, U4. Thus in the Games mode only high level signals from computer games, such as explosions, gunfire and so on, are fed through to be mangled by the signal chopping circuit and then to the power amplifier. Silicon Chip Binders   Heavy board covers with 2-tone green vinyl covering   Each binder holds up to 14 issues  SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p each (Australia only; not available elsewhere). Buy five and get them postage free. Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 100  Silicon Chip REAL VALUE AT $12.95 PLUS P & P The remaining comparator in the LM339 package is U6c. This is used as a clipping indicator and it monitors the input signal to the power amplifier. The power supply is quite well designed, considering that this is a consumer product which would be normally subject to cost restraints. The separate transformer module provides 23V centre-tapped to the bridge rectifier and this produces about ±17V from the 6800µF 25VW filter capacitors. These unregulated supplies are fed directly to the output stages of the power amplifier. Low power 3-terminal regulators (U1 & U2) are used to provide ±5V to the op amps and CMOS chips, as noted above. Gift voucher That about wraps up the circuit description and we hope that readers can take the module and find interesting applications for it. In fact, Jaycar Electronics are offering an additional incentive for experimenters to put their thinking caps on. They are offering a $200 gift voucher, redeemable at any Jaycar Electronics store, for the best circuit modification or application for the Aura Interactor module. Send your idea together with a good description to Aura Competition, Jaycar Electronics, PO Box 185, Concord, NSW 2137. The offer closes SC on 31st May, 2000.