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....:. . :S!I Stereo compressor for CD players Ever tried to record a compact disc for use in your car's tape player? We'll bet that you struck problems. This simple compressor circuit reduces the dynamic range from CDs so that you can make good recordings. By MALCOLM YOUNG & GREG SWAIN If there has ever been an ideal music medium then the compact disc (or CD) must come pretty close. But while CDs give great music reproduction, recording the contents of a CD onto tape usually gives a disappointing result, particularly if the tape is to be used in your car. The major problem when interfacing these two pieces of equipment is that the humble cassette tape cannot handle the wide dynamic output range of the CD. As a result, high level signals will be severely clipped while low level 26 SILICON CHIP signals will be lost forever down in the noise of the recording system. If you do try to play such a recording on your car's tape player, the soft passages will be drowned out by vehicle noise while the loud passages will sound distorted because of signal overload. In short, the tape will sound dreadful. Dynamic range Some readers will not be familiar with the term "dynamic range" so let's explain what this means. In simple terms, it is the difference in level between the softest and the loudest passages on the recording. Compact disc players can handle a very wide dynamic range, up to 96dB, while tape players only have a dynamic range of about 50dB. From those figures, you can see that a tape player simply cannot handle the wide variation in signal level that a CD player can deliver. But even if it could, we don't want a wide dynamic range in cars anyway. There's no point having a tape with a wide dynamic range if the soft passages are drowned out by vehicle noise and the loud passages almost deafen the driver. To overcome the above problems, you need the CD Compressor described here. It makes soft passages louder and loud passages softer. By compressing the dynamic range, it makes music much easier to hear in noisy environments like a car. LEFT SPEAKER CD PLAYER LEFT COMPRESSOR RIGHT LEFT RIGHT SPEAKER PARTS LIST RECORD LEFT RIGHT AMPLIFIER TAPE DECK RIGHT RIGHT PLAYBACK LEFT TAPE MONITOR LOOP Fig.1: the CD Compressor is simply installed in the line between the CD player and the inputs of your stereo amplifier. Alternatively, for a direct connection, it can be installed between the CD player and the cassette deck. 3V 3V 2V 2V 1 Printed circuit board , code SC01203891, 112 x 7 4mm 4 screw mount RCA phono plugs 1 DPDT miniature toggle switch 1 SPST miniature toggle switch 1 plastic box, 160 x 95 x 55mm 1 1 2VAC plugpack 1 O PC stakes 1 aluminium ground plane , 148 x 85mm 4 5mm PC standoffs 4 1 0mm x 2mm-dia. bolts plus nuts & washers 1V Semiconductors 1V 1 1 1 1 2 - 1V - 2V -2V -3V Fig.2a: a lkHz 5V peak-topeak waveform and a 3kHz 1V peak-to-peak waveform. In use, the CD Compressor is simply installed in the line between the CD player and the inputs of your stereo amplifier (see Fig .1 ). In practice, you connect the leads from your CD player to the RCA input sockets of the CD compressor. You then take a pair of leads with RCA plugs on both ends and connect the CD Compressor outputs to the CD inputs on your amplifier. To make recordings from CDs, you use the Tape Monitor function in the usual way. Alternatively, for a direct connection, the CD Compressor can be connected between the CD player and the tape deck. When you are not recording from the CD player, you simply switch the Compressor to the bypass mode to eliminate compression. Signal degradation in the bypass mode is negligible. We measured a signalto-noise ratio in this mode of - 116dB. The CD Compressor can also be very useful when you want background music at a dinner party. In this situation, if you turn the volume right down to allow comfortable conversation, the soft passages of the music will be lost completely. By switching in the CD Compressor and keeping the volume low, you can comforta bly hear all -3V MAX DYNAMIC RANGE 6V pk-pk Fig.2b: if the two waveforms are simply fed to an amplifier, the system can be overloaded and the lkHz signal clipped. 3V --- ---- - - -- 2V LM4136 quad op amp (IC1) NE572 compander (IC2) 7812 3-terminal regulator 7912 3-terminal regulator 1 N4001 rectifier diodes (D1, D2) 4 3 .3V zener diodes (ZD1, ZD2, ZD3, ZD4) 5mm red LED Capacitors 2 4 2 4 1V 4 2 4 2 -1V -2V -3V - ------ -- - 1 OOOµF 25VW electrolytic 1 OµF 16VW electrolytic 2.2µF 25VW electrolytic 2.2µF 50VW bipolar electrolytic 1µF 50VW electrolytic 0 .1 8µF metallised polyester 0.1 µF metallised polyester 1 OOpF disc ceramic - - Fig,2C: a compressor solves the problem by attenuating the high-level signal and amplifying the low-level signal. the music while keeping it at an unobtrusive level. What it does To get a better idea of what the CD Compressor does, take a look at the diagrams shown in Fig.2. In Fig.2a , we see two separate signal waveforms: a lkHz waveform with an amplitude of 5V peak-to-peak and a 3kHz waveform with an amplitude of 1V peak to peak. Fig.2b shows what happens if these signals are simply fed to an amplifier and applied to a system with a maximum dynamic range of 6V peak-to-peak. The 3kHz signal can still be handled OK but the Resistors 4 1 OOkO 6 47k0 2 18kQ 2 3.3k0 3 1k0 peaks of the lkHz signal exceed 6V peak-to-peak and so a re clipped, thus causing distortion. Fig.2c shows what happens to the · same signals after they have been fed through a compressor. As can be seen, the original lkHz signal has been slightly attenuated to make it softer while the low-level 3kHz signal has been amplified to make it louder. In effect, the CD Compressor is basically a variable gain amplifier which is controlled by the average signal input level. At low input levels , the circuit has a high gain to boost the signal well above the noise floor. Conversely, at high inM ARCH 1989 27 47k 47k 100pF 100k +12V RIGHT INPUT RIGHT 0~ Cf100k .,. 2.2 + 25VWI .,. 1k +12V 6 16 2.2 BP NE572 IC2a 2.2 BP 3.3k 4 10 + 16VWI 1 + .,. 50VW+ 100pF 100k LEFT BYPASS OUTPUT S1b 10 .,. + 2.2 25VWI ... COMPRESS -12V 1k -12V 10 2.2 BP 11 IC2b 13 12 + 1 50VWJ ~ 2.2 BP 3.3k + 7912 7812 14 10 16VWr ~ ~K .,. .,. 2x 1N4002 01 ---0 12VAC FROM PLUG PACK CD COMPRESSOR ,~oo, GNO OUT +12V 0.1 0.1 - 12V Fig.3: the compressor circuit consists of an NE572 compandor chip (IC2) connected into the feedback path of inverting op amp stages ICla and IClc. IClh and ICld function as inverting op amp stages with gains of 2. put levels, the circuit attenuates the signal so that it doesn't exceed the maximum allowable limit to a tape recorder. Specifications The specifications panel clearly 28 SILICON CHIP shows how the CD Compressor varies its gain in response to different input signal levels. The maximum gain is about 21dB (11.5 times) for a 2.6mV input and decreases to - 7.1dB for a ZV input. This means that the circuit pro- vides about 28dB of compression over its useful range. The remaining figures for noise, distortion and frequency response . are all quite good, although not quite up to CD player standards. However, we can safely say that the CD Compressor will outperform all cassette decks and players. Circuit operation Fig.3 shows the circuit details of our CD Compressor. Note that both the left and right channels are identical, so we will look at how the left channel operates only. The design is based on a Signetics NE572 stereo compandor chip (ICZ). This chip is called a compandor because it can be used either as a compressor or an expandor. It is essentially a variable transconductance cell. What this means is that the NE572 is basically a resistance which varies in proportion to a control voltage. Let's see how this is done. To produce a very effective compressor circuit, we simply connected the NE572 (ie, the variable resistance) into the feedback path of an inverting op amp stage (IC1a). Thus, depending on the control voltage, the NE572 controls the gain of IC1a to provide signal compression. IC1a is actually part of an LM3146 quad op amp package. This device was chosen not only for its low noise performance but also because of its low cost and ready availability . The action starts on the left hand side of the circuit. Signals from the CD player are applied to the inverting input of IC1a via a 1µF capacitor and series 18k0 resistor. Together, these roll off the response below 10Hz while the 18k0 resistor sets the input impedance. The 100k0 resistor at the input ties the input to signal ground when no signal is applied. DC biasing for IC1a is provided by the two 4 7k0 resistors connected between the output and the input of the op amp (pins 3 and 1). Because the NE572 biases the non-inverting input to + 2.5V, this means that the output of IC1a also sits at + 2.5V. The 10µF capacitor at the junction RIGHT LEFT Fig.4: install the parts on the PCB as shown in this diagram. Make sure that all parts are correctly oriented and be sure to use shielded audio cable where shown. of the two 47k0 resistors prevents AC feedback via this loop by shunting any AC signals to ground. The two zener diodes, ZDl and ZD2, provide hard clipping for any signals that exceed 3.9V peak (3.3V + 0.6V). This is necessary to prevent overloading a full-wave averaging rectifier stage in the NE572 compandor. To control the gain of ICla, the output signal is fed to the rectifier (pin 3) in IC2 via a 2.2µF capacitor and series 3.3k0 resistor. The output of the rectifier then drives a voltage to current converter stage which, in turn, controls a variable resistance circuit (or variable gain cell) between pins 7 and 5. This variable resistance circuit is connected in the feedback loop of ICla and thus controls the instantaneous gain of the amplifier. Thus, the gain of ICla varies in response to the signal level on its output. A feature of the NE572 compandor IC is that the attack and release times can be varied. We simply used the values recommended in the Signetics applications literature. The lµF capacitor on pin 4 sets the attack time while the lOµF capacitor on pin 2 sets the recovery time. Following ICla, the signal feeds into ICl b which is wired as an inverting amplifier with a gain of 2. A lOOpF capacitor in parallel with the lOOkO feedback resistor rolls off the response above 16kHz. This stops high frequency noise from being fed into the recording system. Power for the circuit is derived from a 12VAC plugpack transformer. This feeds positive and negative half-wave rectifiers D1 and D2 and two lOOOµF filter capacitors. The resulting ± 17V DC rails are then fed to positive and negative 12V 3-terminal regulators. Specifications Frequency Response: 20Hz-1 6kHz (-3d8) Total Harmonic Distortion: 0 .30% at 1 OOHz; 0.25% at 1 kHz; 0.20% at 1 OkHz; 0 .12% at 20kHz Signal to Noise Ratio: -60d8 (20-20kHz); -80d8 ('A' weighted) Input Impedance: 18k0 Output Impedance: 0.30 Typical Gain: +21 .2d8 at 2 .6mV (RMS) input; +18 .5d8 at 5 .0mV; + 15.6d8 at 1 OmV; +5.1 dB at 1 OOmV; +2.6d8 at 200mV; -1.1dBat500mV; -4.1dBat1V; -7 .1d8at2V The PCB is earthed to the ground plane using a solder lug which is secured to one of the PCB mounting screws. Construction This project should be built up on the PC board pattern provided as circuit layout is critical for best performance. By using the PC pattern, you will avoid problems such as ground loops which can cause hum. Before installing any parts on the board, you should carefully inspect the copper pattern for defects. In particular, look for breaks in the pattern and shorts between tracks. It is easier to locate and repair any faults at this stage before the parts are soldered in. Begin the board assembly by installing PC pins at all external wiring points. Fig.4 shows the details. Once this has been done, the power supply components should all be installed. This involves installing D5 and D6, the two lOOOµF capacitors, MARCH 1989 29 This photo shows how the PCB is mounted on the lid of the plastic case, along with the aluminium ground plane. Don't forget to connect the earth track on the PCB to the ground plane via a solder lug as shown in Fig.4. Solder the leads to the tags of the RCA sockets quickly, otherwise they wdl melt the plastic case. Once the wiring has been completed, the leads should be bound using plastic cable ties to prevent fraying. the two 3-termmal regulators and the two 0. lµF capacitors across the regulator outputs. Be careful not to swap the two 3-terminal regulators over and make sure that the diodes and electrolytic capacitors are correctly oriented. The two regulators are installed with their metal tabs closest 30 SILICON CHIP to the edge of the board. The power supply can now be checked for correct operation. To do this, you will have to temporarily connect the leads from the 12VAC plugpack transformer. Switch on and check for + 12V on the output of the 7812 regulator and - 12V on the output of the 7912 regulator. There's a good reason for checking the power supply at this stage. A faulty supply can quickly damage an IC and a replacement NE572 compandor will set you back $6.00 or more. By the way, the AC plugpack will probably be fitted with a 3.5mm plug. We decided not to use the plug as it involves buying an extra socket which is sometimes prone to shorting out. Instead, we simply cut off the plug and soldered the two leads directly to the PC pins. If everything checks out, disconnect the plugpack supply and install the rest of the parts on the board as shown in Fig.4. It's best to install the low profile components first (ie, the resistors and diodes) and then move on to the capacitors. Push each component down onto the PCB as far as it will gD before soldering the leads. The two ICs can be left till last. Take care to ensure that each IC is installed in the correct location and is correctly oriented. Pin 1 of an· IC is always adjacent to a notch (or dot) in one end of the body moulding. With the PCB assembly now completed, attention can be turned to the plastic case. You will have to drill holes in the front of the case to accept the RCA sockets, LED and toggle switch as shown in the photograph. Another hole in the rear panel [actually one end of the plastic case) carries the ON/OFF switch. Note that the specified plastic case is actually used upside down so that the lid forms the base of the CD Compressor. The front panel label has been designed so that the large block letters "CD COMPRESSOR" sit on the top surface of the case as shown in the photo. The "ON/OFF" label is affixed to the top right hand corner of the rear panel. The best way to prepare the case is to first affix the front and rear panel labels and then drill pilot holes at each of the locations indicated. Each hole can then be carefully enlarged to its correct size, preferably by using a tapered reamer to ensure a neat finish. Clean out the holes using an oversize drill before mounting the various iterns of hardware. You will have to drill an additional hole beneath the ON/OFF switch on the rear panel to allow cord entry from the plugpack transformer. The PCB assembly is mounted on the lid of the case with an aluminium ground plane (see photo) to minimise noise. You can make your own ground plane by cutting out a 148 x 85mm piece of aluminium sheet. Trim the corners of the ground plane to provide clearance for the corner holes in the lid, then mark out four mounting holes using the PCB as a template. • Fig.5: here is an actual-size reproduction of the PC artwork. CD COMPRESSOR - The ON/OFF switch is mounted on the rear panel, with the 12VAC power cable entry directly below it. The PCB and ground plane can now be mounted on the lid of the case and secured using machine screws, nuts and washers. Note that a solder lug is fitted to one of the mounting screws so that a lead can be run back to the earth pattern on the PCB. Check to ensure that there is adequate clearance between the bottom of the PCB and the ground plane (the PCB should be stood off the ground plane by about 5mm). All that remains now is to complete the internal wiring as shown in Fig.4. There are a couple of things to watch out for here, though. First, be sure to use shielded audio cable between the input RCA sockets and the PCB and between the PCB and the BYPASS switch. Second, solder the leads to the tags of the RCA sockets as quickly as possible, otherwise they will melt the plastic and loosen the fitting. The leads from the AC plugpack POWER BYPASS + ♦ COMPRESS INPUT .,. + LEFT RIGHT ~ t_;_J OUTPUT LEFT RIGHT Fig.6: the front panel artwork has been designed so that the words CD COMPRESSOR sit on the top surface of the case (see photo). enter through the hole in the rear panel, beneath the ON/OFF switch. Bring about 100mm of cord into the case and then tie a knot to prevent it from being pulled out. The two leads can then be separated and wired to the switch and PCB as shown. Testing Before switching on, go over your work carefully and check for possible wiring errors. You should also check the PCB assembly carefully, particularly the component orientations. Now plug in the plugpack transformer and switch on. There should be + 12V at pin 11 of ICl and - 12V at pin 7 with respect to ground. Check also that pin 16 of ICZ is connected to the + 12V rail. If everything checks out OK, screw down the lid and fit four rubber feet to the bottom of the case. To check the circuit operation, connect the CD Compressor between the CD player and amplifier as shown in Fig.1. Select BYPASS and check that the signal from the CD player is unaffected. Finally, switch S1 to COMPRESS and check that signal compression takes place. Notice that the CD Compressor makes the soft passages considerably louder and the loud passages a little softer. That's it! You can now enjoy your CDs via your car's tape player. !c MARCH 1989 31