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Add sparkle to your tapes with this Stereo Audio Expander This little project will add sparkle to your music cassettes, video or FM receiver by increasing the dynamic range of the signal. It uses only a handful of parts and runs from a 12V AC plugpack. By DARREN YATES 24 SILICON CHIP AVE you ever compared the sound from your compact disc player with your cassette deck or video? The difference is pretty much chalk and cheese! Unless you have one of the new VHS hifi video recorders, compact discs represent the closest thing to "perfect sound" by a big margin. The first thing you notice when listening to an ordinary audio or video tape is the background hiss, which is quite audible on some of the older models. And, of course, tape hiss will be quite audible if your cassette deck does not have Dolby noise reduction. What we are really talking about here is dynamic range. This is the range between the softest and loudest signals that can be reproduced by a system. In practical terms, the loudest signal is the highest level that can be reproduced without distortion while the softest signal represents the lowest level that can be discerned above the noise floor of the recording medium; ie, tape, disc, etc. Compact disc players have a dynamic range of about 90dB . This means that the ratio between the loudest and smallest signals coming out of your disc player is about 30,000:1. By contrast, the dynamic range- of an audio cassette deck with Dolby is about 50dB or about 300:1. Based on these figures, it's not hard to understand why CDs produce a far greater H OUTPUT VARIABLE TRANSCONDUCTANCE AMPLIFIEII INPUT BUFFER VARIABLE TRANSCONDUCTANCE AMPLIFIER DC BIAS' EXPANSION COMPRESSION Fig.l(a): basic scheme for an audio compressor. It consists of a variable transconductance amplifier in the negative feedback loop of an op amp. range of sound levels than audio tapes (ie, they have a greater dynamic range). Recording problems These days, most music is recorded on CDs and audio tape. But if you try to copy a CD straight onto audio tape, two things will happen. First, very quiet passages on a CD will disappear into the hiss and noise of the audio cassette; second, the loud passages will become distorted. Both of these problems occur because of the restricted dynamic range of audio cassettes. You can think of it as trying to fit a big box into a smaller one. You can't do it unless you crush it and bend it all over the place. Signal compression Fig.l(b): to achieve audio expansion, we simply swap the VTA from the feedback loop to the input circuit of the op amp stage. The larger the signal, the lower the resistance of the VTA and the higher the gain of the op amp. and a few well-known FM radio stations, you will notice that there is very little difference between the loud a.nd soft passages. It's particularly noticeable on the sound track of many video tapes , especially when there is a quiet passage, how the background hiss slowly increases as if someone is turning up the volume. You'll also notice that as soon as someone speaks, for a very short time afterwards, the sound level will be quite high and then drop to its normal level. This is an example of a compressed signal. It may squeeze more sound information onto the tape but what results is a fairly constant sound level which bears little resemblance to reality. Wouldn't it be nice if we could expand this range out again, back to its original level? This Audio Expander does just that. Essentially, it performs the reverse function of the CD Compressor project by "stretching" the compressed dynamic range out again. It can thus be used to complement the CD Compressor or to expand any music signal that has undergone compression during recording or transmission. NE572 AG C RECOVERY+ BUFFER C ATTACK+ - - t - - 1 RECTIFIER Fig.2: inside the NE572 compander IC. The incoming audio signal is fed to a gain cell (~G) & also to a rectifier stage to derive a control current. Signal-to-noise ratio (at 3Vpp & 1kHz): flat, -84.SdB; 20-20kHz, -88.SdB; A-weighted, -104d8 Fig.1 shows the basic scheme. In Fig. l(a), we achieve compression by connecting a variable transconductance amplifier (VTA) in the negative feedback loop of an op amp. The VTA acts like a voltage controlled resistor which is dependent on the incoming signal level. The larger the signal, the lower the resistance of the VTA and the lower the gain of the op amp , so that the resulting signal level remains fairly constant. In Fig. l(b), we simply swap the VTA from the feedback loop to the input circuit. It still works in the same way: an increasing voltage causes an increase in gain or a decrease in impedance in ·the VTA. The difference is that we now use this to produce an overall increase in the gain of the following op amp stage. Input impedance: 50kQ The NE572 compander IC Typical gain (at 1kHz): -17dB at 10mV input; -13.1dB at 20mV; -5dB at 50mV; 1d8 at 100mV; 6.9d8 at 200mV; 14.6dB at 500mV The Audio Expander has been designed around the NE572 stereo compander IC; the same chip used in The CD Compressor project described in the March 1989 issue of SILICON CHIP solved this problem by "shrinking the box". In other words, it compressed the 90dB dynamic range of the compact disc down to the 50dB range of audio tape by proportionally adjusting the level of the signal. ' If you listen carefully to many prerecorded video and audio cassettes Specifications Frequency response: 20Hz-80kHz (-3dB) Total harmonic distortion (THD) at 3Vpp: 0.9% at 100Hz; 0.125% at 1kHz; .08% at 10kHz; .09% at 20kHz MAY1991 25 100pF .-------------•12V 2.2 50VW - + LEFT~ INPUT .,. 18k 16 + ~ 100k 100k .,. .,. IC2a NE572N LEFT OUTPUT 4 + 10 + 50VWJ 16VWr 1 100pF 2.2 50VWI - RIGHT~ INPUT 100k 18k + 100k BYPASS 11 IC2b T" ~ S1b 10 12 t GND GND f 0--0 12VAC PLUG-PACK OUT 0.1 1k IN 470 25VW AUDIO EXPANDER Fig.3: the circuit consists of two identical channels plus a power supply. For the left channel, the incoming audio is buffered by ICla & then fed to the NE572 compander IC. The higher the input signal, the lower the impedance of the gain cell in IC2a & the higher the gain of the following op amp stage (IClb). the CD Compressor project. We published an article in the March 1989 issue which gave the details of the NE572 but we shall briefly go over it again for readers who missed it. Take a look at Fig.2 ; this shows the internal sections of the IC. The three main sections are the rectifier, a buffer stage and a variable gain cell (~G). The latter can be thought of as the voltage controlled resistor. The input signal is fed to both the gain cell (~ G) and the rectifier. Depending on the signal level, the rectifier produces a proportional DC current which becomes the control signal. This signal is then fed to the buffer stage where two external capacitors are used to independently set the attack and decay response times. The buffered signal is then used to control the gain cell and this in turn 26 SILICON CHIP .,. 14 m m OUT RIGHT OUTPUT 2 2 IN 'T EXPAND controls the gain of the following external op amp stage mentioned previously. In use, the Audio Expander is simply installed in the line between the music source (eg, a cassette player) and your stereo amplifier. Once installed, it can be left permanently in position and then switched in and out of circuit as required, using the BYPASS switch. Circuit diagram Refer now to Fig.3 which shows all the circuit details. As you can see , it consists of two identical sections, one for each channel, so we'll just explain the right channel. The input is coupled to the circuit via a 2.2µF capacitor which is grounded on both sides by lO0kQ resistors. This sets the input impedance to 50kQ + - 0.1 OUT LED1 -12V· as well as providing a DC charge path for the capacitor to prevent "pops" when you first turn it on. Cl is a 4136 quad op amp which has a very low noise level and is also quite cheap. ICla is connected as a buffer stage and provides a low-impedance source for the compander IC (IC2a) so that the rectifier section doesn't load the signal down. As mentioned earlier, the buffered audio signal is fed to both the gain cell and the rectifier inside IC2a, via pins 7 and 3, respectively. The rectifier converts the AC audio signal into a proportional DC control voltage and feeds ·it to the buffer stage inside the NE572. The two capacitors on pins 4 and 2 (lµF and lOµF) set the attack and decay time constants. After buffering, the DC signal is used to control the impedance of the gain cell between pins 7 and 5 of IC2a. The higher the signal, the lower the impedance of the gain cell. This variable impedance is con- PARTS LIST , 1 Mount the two 3-terminal regulators so that their metal tabs face each other, as shown here. The prototype board used PC stakes to terminate external wiring leads but you can solder the leads direct to the board if you wish. nected in series with the input of amplifier stage IClb, and thus controls the gain of this stage. An 18kQ resistor in the negative feedback loop sets the maximum gain ofIClb, while the parallel l00pF ceramic capacitor rolls of the response above 80kHz. Thus, as the audio input signal increases, the gain ofIClb also increases and so we get signal expansion. DC bias for the non-inverting input of IClb is supplied by pin 6 of IC2a and is about +2.5V. This is filtered by a lkQ resistor and 2.ZµF electrolytic capacitor. Because of the bias arrangement used, the output from IClb (pin 4) is at a DC level of +2.5V. It is therefore RIGHT OUTPUT fed to the output socket via a 2.ZµF DC blocking capacitor and referenced to ground via a lO0kQ resistor. Switch Sla is included in the output path so that the expander can be switched in or out of operation. Power supply Power for the circuit is derived from a 12VAC plugpack supply. This feeds positive and negative half-wave rectifier diodes Dl and DZ, and the resulting DC is filtered by two 470µF capacitors. The resulting ±17V DC rails are then fed to positive and negative 3-terminal regulators to derive ±12V supply rails. Power on/ off indication is provided ·~ 1 plastic case, 150 x 90 x 50 1 PC board, code SC01105911 , 117 x 76mm 1 front panel label, 87 x 57mm 1 rear panel label, 10 x 20mm 1 SPST toggle switch 1 DPDT toggle switch 12 PC stakes 1 12VAC plugpack 1 5mm LED bezel 4 5mm spacers 4 panel mount RCA sockets 1 3.5mm panel mount phono socket Semiconductors 1 4136 quad low noise QP amp (IC1) 1 NE572 dual compander (IC2) 1 7812 + 12V regulator 1 7912 -12V regulator 2 1N4004 diodes (D1 ,D2) 1 5mm red LED (LED 1) Capacitors 2 4 70µF 16VW PC electrolytics 2 10µF 16VW PC electrolytics 10 2.2µF 50VW PC electrolytics 2· 1µF 50VW PC electrolytics 2 0.1 µF 63VW 5mm-pitch metallised polyester or monolithic 2 100pF ceramics Resistors (0.25W, 5%) 6 100kQ 2 3.3kQ 2 18kQ 3 1kQ Miscellaneous Hookup wire, tinned copper wire, screws, nuts, washers, 600mm of shielded audio cable. 2 2uF 7812 ••• OGI LEFT OUTPUT RIGHT INPUT LEFT INPUT. Fig.4: check the orientation of each part carefully as it is installed on the PC board (see Fig.3 for pinout details). Note the use of shielded cable for the connections between the PC board, S1 & the RCA input sockets. MAY1991 27 monolithic types can be used here. This done, solder in the two 100pF ceramic capacitors and the electrolytics. Again, use the overlay diagram to make sure you install each one in the correct place and that the electrolytics are installed with the correct polarity. Construction of the PC board can now be completed by soldering in the two 3-terminal regulators and the ICs. The circuit diagram (Fig.3) shows the pinouts for the 3-terminal regulators. When they are installed on the board, their metal tabs should face each other as shown in the photographs. Check the PC assembly carefully before moving on to the next stage. Final assembly The PC board is mounted on the lid of the case on 5mm brass spacers and secured with machine screws & .nuts. Keep all wiring leads as short as possible & take care to ensure that no shorts are created when the lid is screwed down. The unit is powered from a 12V AC plugpack supply. by LED 1 which is wired in series with a lkQ current limiting resistor across the -12V rail. Construction All the components , except for the front and rear panel sockets and switches, are mounted on a PC board coded SC0l 105911 and measuring 117 x 76mm. We strongly recommend that you use this board, as the circuit layout is critical for good performance. By using the board, you will avoid problems such as ground loops whioh can cause hum. Before mounting any of the parts, check that there are no shorts or breaks in any of the PC board tracks. If you find any problems, use a sharp knife or a spot of solder to repair the fault as appropriate. Once the board is correct, refer to the overlay diagram shown in Fig.4. Begin the assembly by installing PC stakes at the external wiring points, then solder in the wire links, diodes and resistors. Check the polarity of the diodes against the overlay diagram to make sure you get them the right way around. If you're unsure of the resistor colour codes, check them on your digital multimeter or refer to the accompanying table. Next, install the two 5mm-pitch 0. lµF capacitors. Either polyester or At this stage, you can attach the Dynamark labels to the front and rear panels of the specified plastic case. Note that the case is actually used upside down, so that the lid forms the base of the Audio Expander. The front panel label has been designed so that the large block letters "AUDIO EXPANDER" sit on the top surface of the case as shown in one of the photos. The "ON/OFF" label is affixed to the top right hand corner of the rear panel. Once the labels have been attached, you can use them as templates to drill holes for the switches, LED bezel and the four RCA sockets. The holes for the LED bezel and the bypass switch are 7mm diameter, while 9mm holes are used for the RCA sockets. In addition, you will have to drill a 7mm hole directly under the ON/OFF switch to accept the power socket. You'll probably find it easier to drill out small holes to begin with, and then slowly ream them out until you get a snug fit. The PC board is mounted on the lid of the case on 5mm spacers and secured using 3mm screws and nuts. These holes can be marked out by using the PC board as a template. Af- RESISTOR COLOUR CODES 0 0 0 0 0 28 No. Value 4-Band Code 5-Band Code 6 100kQ 18kQ 3.3kQ 1kQ brown black yellow gold brown grey orange gold orange orange red gold brown black red gold brown black black orange brown brown grey black red brown orange orange black brown brown brown black black brown brown 2 2 3 SILICON CHIP AUDIO EXPANDER ~ N POWER BYPASS + + INPUT EXPAND ~ ~ OUTPUT + + + + LEFT RIGHT LEFT RIGHT SPOT A BARGAIN Fig.5: here are the full-size artworks for the front & rear panels. THEN CONTACT US FOR MORE INFORMATION UHF REMOTE CONTROL: The one and only - $49.90 for the Rx+ Tx, extra Tx $17.00 ter drilling, mount the board in position, then complete the wiring as shown in Fig.4. Since the power requirements for the Audio Expander are quite small, light-duty hookup wire is sufficient for the power supply leads, for the LED wiring and for the wiring between the RCA sockets and S 1. Check the LED polarity carefully - its anode lead is the longer of the two and goes to the lkQ resistor. The earth lugs attached to the RCA sockets are all connected together by soldering them to a length of tinned copper wire. Once this has been installed, the rest of the wiring can be run using shielded cable, as shown in Fig.4. Try to keep all the leads as short as possible. The best way to do this is to install the wiring with the lid of the case lying flat on the benchtop and the case standing upright immediately behind it. Before applying power, go over your work carefully and check for wiring errors. If everything is OK, apply power and use your multimeter to check that the regulator outputs are at ±12V. If these voltages are incorrect, switch off immediately and check the power supply wiring. Finally, you can check the circuit operation by installing the Audio Expander between your cassette player and stereo amplifier. First, select BYPASS and check that the signal from the cassette player is unaffected. If that's OK, select EXPAND --you should immediately hear clean, crisp sound with some added dynamic range! SC 1 cl SMALL D.C. MOTORS: 3-12V, 2 for $5.00 LASER: Small gunsight/pointer IR NIGHT VIEWER KIT: Large prefocussed tube plus electronics kit, a few only at $200.00 CRYSTAL LOCKED ULTRASONIC: Has relay output, assembled PCB, open transducers and circutt - Top unit! $24.90 NICAD BATTERY PACKS: 7.2V/500mAH (6 pack of penlights). easy to seperate $11 .00 LASER MOTOR DEFLECTION KIT: 2 motors, 2 small mirrors, 2 wirewound pots (Speed control), plus simple cct/diagram $16.00 , .... DUAL ELEMENT PYROELECTRIC SENSOR: Not only 1/2 price of inferior single elements units, but comes with fresnel lens and cct. $9.90ea, 1O+ $9.00ea.. We have seen elsewhere for $20.00ea. MINI NIGHT VIEWER KIT: Small electronics kit, plus small prefocussed fibreoptically coupled tube wich responds to visible and IR light $359.00 BIGGER LASER: 5SmW Large laser HEAD plus potted 12V professional power supply: 1/2 price at $500.00 DYNAMIC MICROPHONE INSERTS: High quality, unidirectional $10.00 LASER TUBE: 350mm long, 2-3mW wtth clips, ballast resistor and spec's $150.00 1 [Z] T"' T"' g:i U') OATLEY ELECTRONICS 0 T"' T"' 0 0 0 INFRA-RED LASER DIODE POINTER • KIT: $99.00 STEPPER MOTORS: Driver cct. included $12.00, driver IC available. MASTHEAD AMPLIFIER: Proven performer, complete with casing $31 .00 (No plugpack). ~tl 1I HALL EFFECT IC'S: Info supplied, 10 for $20.00 6.5AH GEL BATTERY: Brand NEW, fresh stock, $29.99 Inc. P+P (Aust. only) Testing 0 VISIBLE LASER DIODE POINTER - KIT: $220, with the casing etc. $239.00 en ~ Fig.6: check your PC board against this full-size pattern before installing any of the parts. In particular, look for shorted tracks due to incomplete etching. PO BOX 89, OATLEY, NSW 2223 Telephone: (02) 579 4985 Fax No: (02) 570 7910 Certified p&p: $5 inAust. NZ (Ainnail):$10 Fax orders are accepted with credit card payments. Melbourne Distributor • Electronics World (03) 723 3860 or (03) 723 3094 MAY 1991 29