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By JOHN CLARKE Hearing Loop Signal Conditioner Want to drive a hearing loop using a conventional voltage (audio) amplifier? This Hearing Loop Signal Conditioner includes signal compression and has a treble boost control to compensate for high-frequency roll-off due to loop inductance. It uses low-cost parts and is easy to build. I F YOU ARE INSTALLING a hearing loop, you are going to need an amplifier to drive it. Commercial amplifiers specifically designed for the task are available but if you want to use a standard audio (voltage) amplifier, some form of signal conditioning is required. For loops that are smaller than 5 x 5m, signal compression is usually all that’s required. This ensures that the loop signal is adequately maintained for a wide range of input signal levels. In addition, the frequency response should roll off above about 5kHz but this is normally taken care of by the inductance of the loop. Larger loops, however, will have greater inductance and so will roll off the response earlier. This means that the input signal must be treble-boosted before it is fed into the amplifier, to compensate for 64  Silicon Chip the subsequent inductive losses in the loop. Signal conditioner The Hearing Loop Signal Conditioner described here is designed to provide both compression and treble boost. The latter can be set by the user, so that you can tailor the signal to suit your particular loop installation. In addition, the user can vary the signal level that’s fed to the amplifier. Fig.1 shows the block diagram of the unit. As can be seen, the input stage can accept either mono or stereo line inputs and these are fed in either via RCA sockets or via a 6.35mm jack socket. Alternatively, it can accept a mono balanced input or an unbalanced input via an XLR connector. From there, the signal is fed via level control VR1 to a low-pass filter stage. This filter rolls-off the response above 6kHz and has a Q of 0.9. The response of this stage is flat to about 5kHz and is designed to provide optimum results when the signal is subsequently fed to the treble boost stage that follows the compressor. The compressor stage provides a nominal 2:1 compression, so that highlevel signals are reduced by a factor of 2. By contrast, low-level signals are boosted by a factor of two. As a result, the compressor ensures a more or less constant signal level at its output, regardless of input signal variations, thereby preventing overload in the power amplifier. This signal compression in turn ensures a relatively constant field strength level in the hearing loop and this can greatly improve the audibility of speech signals. Link LK4 enables siliconchip.com.au MONO INPUT LINE INPUT L BYPASS R LK4 BALANCED INPUT OUTPUT 1 3 LEVEL CONTROL 2 LOW PASS FILTER COMPRESSOR TREBLE BOOST INPUT Fig.1: block diagram of the Hearing Loop Signal Conditioner. The incoming audio signal is first fed to a low-pass filter stage via a level control and then to a compressor stage. The output of the compressor then drives a treble boost circuit to compensate for high-frequency roll-off in the hearing loop. the compressor stage to be bypassed if compression is not required. The treble boost stage is the next in line. As previously stated, this provides boost at the higher frequencies to compensate for treble losses due to loop inductance. However, this boost stage is not like a normal treble tone control where the amount of signal boost is constant for all frequencies above the turnover frequency. Instead, it acts more like a single band boost stage in a multi-band equaliser (ie, the signal rolls off sharply at frequencies higher than the boost frequency). The idea here is to ensure that the power amplifier is not fed boosted high frequencies above about 10kHz, as this could cause instability. If instability did occur, the loop would radiate RF signals that could interfere with other equipment. The output from the treble boost circuit is unbalanced and is fed to an RCA socket and a 6.35mm jack socket which are wired in parallel. If necessary, a 6.35mm jack-to-XLR lead can be made up to connect to an XLR input on an amplifier. Although not shown on Fig.1, there are several power supply options. The unit can be powered from either DC or AC and the supply can come either from a plugpack or from the supply rails of the power amplifier. Table 4 shows the various supply rail options. Circuit details Take a look now at Fig.2 for the full circuit details. An incoming stereo signal is applied either via the two RCA inputs or the 6.35mm stereo jack socket and is mixed using two 2.2kΩ resistors to form a mono signal (ie, when link LK1 is installed). The resulting mono signal is then applied to the non-inverting input (pin 3) of siliconchip.com.au op amp IC1a via a 10µF non-polarised capacitor. By contrast, an unbalanced mono signal is fed in either via the left channel RCA socket or via the tip connection of the jack socket. However, if the jack socket is used, link LK1 must be removed to prevent the input signal from being divided by two by the right channel 2.2kΩ mixing resistor. Once again, the mono signal is applied to pin 3 of IC1a. Balanced input signals are fed in via pins 2 & 3 of the XLR connector. Pin 1 is the ground connection, pin 2 is for the non-inverted signal and pin 3 is for the inverted signal. The out-of-phase balanced signals are then fed to the non-inverting inputs (pins 3 & 5) of op amps IC1a and IC1b respectively. IC1a & IC1b together form a balanced amplifier stage. Their non-inverting inputs are tied to ground using 100kΩ resistors, to prevent them from “floating” when there is no input connection. The associated 100pF capacitors (one across the two inputs and the others between the inputs and ground) are included to filter RF (radio frequency) signals. In addition, the 100kΩ resistors to ground set the bias for IC1a and IC1b. These resistors connect either to the signal ground or to a half-supply ground, depending on the power supply configuration. IC1a & IC1b operate as non-inverting amplifiers with a gain of 3. This gain is set by the 10kΩ feedback resistors and the 10kΩ resistor between their two inverting inputs. A 100pF capacitor across each 10kΩ resistor rolls off the high-frequency response above 160kHz. The outputs from IC1a & IC1b appear at pins 1 & 7 respectively and Main Features • • • Balanced or unbalanced input Stereo mixing XLR, 6.35mm jack or phono (RCA)    inputs • Phono (RCA) or 6.35mm jack    socket unbalanced output • • • • • • Low-pass and high-pass filters Level and tone boost adjustments Signal compressor Optional compressor bypass Power switch and indicator LED Several power supply options Specifications Signal-to-noise ratio with respect to 1V in and 1V out: (1) Compressor out: 90dB (20Hz to 20kHz filter); 99dB “A” weighted. (2) Compressor in: 75dB (20Hz to 20kHz filter); 78.5dB “A” weighted. Frequency response: -3dB at 43Hz and 6.6kHz, -10dB at 10kHz (no treble boost). Treble boost: up to +16dB at 5kHz with C1 = 5.6nF. Response complements loop treble attenuation. Signal compression: typically 2:1 to -20dB input (with respect to 1V) – see graph. January 2011  65 66  Silicon Chip siliconchip.com.au 1 LK1 3 R T POSITION 3 POSITION 2 POSITION 1 K A D2 D1 100k 100k A K V– Vcc/2 R2* R1* 10k 5 6 2 1000 µF 25V A K A K 220pF 6.2k V– ZD2 15V 1W ZD1 15V 1W V– 7 1 Rb 1M TP1 4 IC1b 10k 100pF 100pF 10k IC1a 68k 1000 µF 25V THD TRIM VR2 20k 100nF 100pF 100pF 8 100nF HEARING LOOP SIGNAL CONDITIONER S1b LK3 LK2 10 µF NP 100pF POWER S1a 2x 2.2k 100k 100k 3 BALANCED AMP K A 1 µF 4 OUT λ LED1 10k 10k COMPRESSOR 4.7k 15 INV 12 (–) GAIN 13 V– 4 IC2a 10k 47k NP 6 5 V– IC3b 8 NP 10 µF LK4 100k 10Ω 7 A K 8 Vcc/2 VR3 50k 7 5.6k 8 C1* 220k 7 V– 2 3 IC5a 1.8k NP 10 µF A K ZD1, ZD2 150pF TONE BOOST 560pF V– 27k 4 IC5b TREBLE BOOST 47Ω 6 5 100nF 3 2 5.6k 1 100k 150Ω 6kHz LP FILTER 12k 10nF IC1, IC2, IC3, IC5: TL072 BUFFER IC2b D1, D2: 1N4004 V– 6 5 10k 51k 150Ω +15V 100k 10k COMP BYPASS 10 µF LEVEL 56nF +15V 47k 2.2 µF NP 100 µF 10 VR1 10k LOG 10 µF NP 1 10 µF NP IC4 RECT 4.7 µF SA571 14 Crect IN 3 2 9 THD TRIM 16 11 10k 10k 10k 10 µF 35V 1 K A LED * SEE TEXT 6.35mm JACK SOCKET OUTPUT RCA OUTPUT +15V V– 4 IC3a 1nF +15V Fig.2: the complete circuit of the Hearing Loop Signal Conditioner. IC1a, IC1b & IC2a form a balanced-to-unbalanced amplifier stage and this drives buffer stage IC2b via level control VR1. IC2b then drives IC3a which functions as a 6kHz low-pass filter. The signal is then fed to compressor stage IC4, while IC5b & IC5a provide treble boost to compensate for loop losses. Note the different signal ground & earth symbols used in the diagram. 2010 SC – 0V + CON1 CON2 DC SOCKET 6.35mm STEREO JACK INPUT L R RCA LINE INPUTS 2 BALANCED INPUT 10 µF NP 100pF +15V Compressor Response (with respect to 1V) 10 INPUT R C Fig.3 (left): this diagram shows the basic configuration of the compressor stage inside IC4. The gain element is placed in the feedback network of the op amp. OUTPUT Vref are fed to pins 2 & 3 of differential amplifier stage IC2a. For signals from IC1a, IC2a functions as an inverting amplifier with a gain of -1. By contrast, signals from IC1b are first divided by two (using two 10kΩ resistors) before being fed to IC2a which now functions as a non-inverting amplifier with a gain of 2. This means that the overall gain from pin 7 IC1b to pin 1 of IC2a is +1. As a result, the signals at the output of IC2a are now in phase and so they are added or summed to give IC2a an overall gain of 2 (ie, for balanced input signals). The resulting unbalanced signal is AC-coupled to level control VR1. By contrast, if the input signal is unbalanced, it is simply fed via IC1a and IC2a. In that case, IC2a has an overall gain of -1. VR1 is included to allow adjustment of the compressor input level (more on this shortly). However, the signal from VR1 is not fed directly to the compressor stage (IC4). Instead, it’s first fed via a 56nF capacitor and 10Ω resistor to unity gain buffer stage IC2b which has its input is biased at signal ground via a 100kΩ resistor. The 56nF coupling capacitor rolls off the frequency response below 28Hz. IC2b provides a low-impedance drive for the following low-pass filter which comprises IC3a and its associated resistors and capacitors. This filter stage is a multiple-feedback 2-pole design that rolls off the response at 6kHz. This ensures a flat response up to 5kHz which is the recommended minimum high-frequency response for a hearing loop. Compressor stage IC3a’s output appears at pin 1 and is fed to pin 11 of IC4, an SA571 compansiliconchip.com.au 0 -10 Compressor Output (dB) G -20 Rb Out -30 Rb In -40 der IC. The word “com­ pander” is a contraction of the words “compres-50 sor” and “expander” and means that the device can be used as either a signal -60 compressor or a signal expander. In this case, the SA­571 is used in its compressor -70 mode. 10 0 -10 -20 -30 -40 -50 -60 -70 The device itself conCompressor Input (dB) tains two full-wave averaging rectifiers, two gain Fig.4: this graph shows the compressor’s output as a elements and a dual op function of its input signal. It provides a nominal 2:1 amp for stereo use. Only compression but has a non-linear response with Rb one channel is used here, in (see text). however. When the device is used as a comFig.4 plots the compressor’s output pressor, the gain element is placed in response as a function of its input sigthe feedback network of the op amp, nal level. Basically, the compressor is ie, between its inverting input and set up so that it provides a nominal 2:1 output. Fig.3 shows the general ar- compression. In this circuit, however, rangement. As can be seen, the input is as the signal reduces, the gain becomes fed in via resistor “R” to the inverting non linear and is also reduced. This is input, while the non-inverting input due to the addition of resistor Rb (see is biased at a voltage above ground (ie, Fig.2). Without this resistor, the comto Vref) to allow the output to swing pressor would operate with a nominal symmetrically. 2:1 compression for signals down to In operation, the full-wave averag- -80dB (ie, below the 0dB reference). ing filter monitors the op amp’s output and rectifies the signal. This rectified Compressor circuit signal is averaged to provide a DC The SA571 (IC4) requires only a voltage that controls the gain element. few extra parts to produce a working If the signal level is low, then the DC compressor stage. As shown, the signal control voltage is low and the gain ele- from pin 1 of IC3a is AC-coupled to ment’s resistance is high. As a result, IC4’s pin 11 input, while the output the compressor provides a high signal (pin 10) is AC-coupled to the gain cell gain from input to output. (pin 14) and the rectifier (pin 15). In Conversely, if the signal level is addition, two 47kΩ resistors are used high, the control voltage is also high provide a DC feedback path from the and this reduces the gain element’s output to the inverting input (pin 12) resistance to lower the gain. As a of the internal op amp. result, low-level signals are boosted The smoothing (averaging) filter while high-level signals are reduced. capacitor for the rectifier is at pin 16 January 2011  67 Parts List For Signal Conditioner 1 PC board, code 01101111, 118 x 102mm 1 plastic instrument case, 140 x 110 x 35mm 1 front panel label, 133 x 28mm 1 rear panel label, 133 x 28mm 3 PC-mount single RCA sockets 2 6.35mm stereo PC-mount jack sockets 1 3-pin XLR panel socket (optional) 1 PC-mount DC socket 1 DPDT PC-mount toggle switch 1 10kΩ log 16mm potentiometer (VR1) 1 20kΩ horizontal trimpot (VR2) 1 50kΩ linear 16mm potentiometer (VR3) 4 DIP8 IC sockets (optional) 1 DIP16 IC socket (optional) 1 3mm green LED (LED1) 1 3-way screw terminal block (5.04mm pin spacing) 1 11-way pin header strip with 2.54mm spacing (to be cut into 4-way, 3-way & 2 x 2-way headers) 5 pin header jumper shunts 1 260mm length of 0.7mm tinned copper wire 4 No.4 self-tapping screws 5 PC stakes Semiconductors 4 TL072 dual op amps (IC1-IC3, IC5) (1µF), while Rb has a value of 1MΩ and is connected between pin 16 and the +15V supply rail. As stated, this ensures non-linear compression at low signal levels. Basically, it prevents the compressor from providing gain at these levels as this would only increase the noise. Trimpot VR2 is there to provide the distortion trim adjustment by setting the voltage applied to pin 9. Normally, this trimpot is set to its mid point. However, if a distortion analyser is available, VR2 can be set for minimum total harmonic distortion (THD). The compressor stage output at pin 10 is AC-coupled via a 10µF capacitor and 10kΩ resistor to the treble boost output stage which is based on op amps IC5b & IC5a. Note, however, that the compressor can be bypassed by installing link LK4 in the BYPASS 68  Silicon Chip 1 SA571N Compandor IC (IC4) (available from Futurelec) 2 15V 1W zener diodes (ZD1,ZD2) 2 1N4004 diodes (D1,D2) Capacitors 2 1000µF 25V PC electrolytic 1 100µF 16V PC electrolytic 6 10µF NP PC electrolytic 2 10µF 35V PC electrolytic 1 4.7µF NP PC electrolytic 1 2.2µF NP PC electrolytic 1 1µF 16V PC electrolytic 3 100nF MKT polyester 1 56nF MKT polyester 1 10nF MKT polyester 1 1nF MKT polyester 1 560pF ceramic 1 220pF ceramic 1 150pF ceramic 6 100pF ceramic C1 1.2nF - 5.6nF (see Table 3) Resistors (0.25W, 1%) 1 1MΩ 1 6.2kΩ 1 220kΩ 2 5.6kΩ 7 100kΩ 1 4.7kΩ 1 68kΩ 2 2.2kΩ 1 51kΩ 1 1.8kΩ 2 47kΩ 2 150Ω 1 27kΩ 1 47Ω 1 12kΩ 1 10Ω 11 10kΩ R1, R2 (see Table 4) position. In that case, IC3a’s output is fed directly to IC5b via a 10kΩ resistor. Treble boost As stated, the treble boost circuit works like an equaliser. This operates over a narrow frequency band and the centre frequency is set by changing a capacitor to suit the hearing loop. The equaliser is tuned to a particular centre frequency and the conventional way of doing this is to use an LC (inductor-capacitor) network. The basic scheme for a single-band equaliser is shown in Fig.5. Op amp IC5b is connected as a non-inverting amplifier. Its feedback network includes potentiometer VR3 which has its wiper connected to ground via an LC network. This LC network sets the centre-frequency of the band. It works like this: when VR3 is wound fully to the left, the LC circuit has no effect on the frequency response. In other words, an input signal passes through the circuit unchanged except for gain (ie, it has a flat frequency response). This is the “flat” setting for the equaliser. Conversely, when VR3 is rotated fully right to its “boost” setting, the LC network is connected directly to the inverting (-) input of IC5b, shunting the negative feedback to ground. At the resonant frequency, the impedance of the LC network is at a minimum. As a result, the feedback will be reduced and the gain will be at a maximum. Intermediate settings of VR3 vary the gain at the resonant frequency accordingly. The centre (resonant) frequency is obtained from the formula: f = 1/2π√(LC). No inductor Although we could use an inductor in the resonant circuit, our final circuit (Fig.2) uses a “gyrator” instead. A “gyrator” is a pseudo inductor and is based on an op amp and a low-value capacitor. Fig.6 shows the arrangement. In an inductor, the current lags the voltage waveform by 90°. However, the reverse is true for a capacitor – in this case, the voltage lags the current by 90°. Therefore, in order to simulate an inductor, this voltage lag with respect to current must be reversed. The circuit of Fig.6 works as follows. When an AC signal (Vin) is applied to the input, current (In) will flow through capacitor C and resistor R. This produces a varying voltage at IC5a’s non-inverting (+) input. IC5a is connected as a voltage follower. As a result, this op amp will reproduce its input voltage across resistor Rout at its output. This in turn causes a current (Iout) to flow in Rout and this is subtracted from the input current. The resulting total current lags the input voltage by 90°. As a result, as far as the signal source is concerned, the circuit behaves as an inductor. The value of this “simulated inductance” is given by the equation: L = R x Rout x C. By substituting the gyrator for the inductor in the circuit of Fig.5, we have the basis for a complete single-band equaliser. The value of C1 will depend on the size of the hearing loop. Basically, this capacitor is chosen so that the siliconchip.com.au IN 10k Rout Vin IC5b OUT C 150pF 51k VR3 50k FLAT Iin 27k IC5a Iout Vout R 220k BOOST C1 60mH Fig.5: a conventional single-band equaliser uses an LC network to set the centre frequency of the band. equaliser provides the correct boost curve to compensate for treble losses due to loop inductance. Smaller loops require a higher centre frequency and a shallower boost slope up to 5kHz for the equaliser. Any boost above about 6kHz is restricted due to roll-off from the 6kHz low-pass (LP) filter (IC3a). In addition, IC5b’s 27kΩ feedback resistor and its parallel 560pF capacitor provide extra roll-off above 10kHz. The output from the treble boost circuit appears at pin 7 of IC5b and is AC-coupled to the output sockets via a 10µF capacitor and a 150Ω isolating resistor. The latter prevents IC5a from oscillating with leads that present a capacitive load. The output can be taken either from the RCA socket or from a 6.35mm jack socket. Power supply Power for the circuit can come from either a 12-60V DC source, a ±12-60V DC source or an 11-43VAC source. The current requirements are quite modest at just 30mA. The simplest supply arrangement is to use a ±12-60V DC source (this type of supply can often be found in existing amplifier equipment). The positive rail is simply connected to the “+” supply input, the negative rail to the “-” input and the ground to 0V. Diodes D1 & D2 provide reverse polarity protection, while two 1000µF capacitors filter the supply rails. Zener diodes ZD1 & ZD2 protect the op amps by conducting if the input voltage rails exceed ±15V. Resistors R1 and R2 in series with each supply line limit the current through ZD1 and ZD2 when they conduct. Their values depend on the supply rail voltages and are chosen from Table 4. Note that, with this supply arrangesiliconchip.com.au Fig.6: the basic scheme for a gyrator circuit. This acts as a pseudo inductor and takes the place of the inductor shown in Fig.5 for the treble boost circuit. ment, the two different grounds on the circuit are tied together by placing link LK2 in position 2 (see Table 4). This biases the op amp inputs at 0V so that the signal swings symmetrically above and below ground. to derive the negative rail. As before, the two grounds are connected by installing LK2 in position 2, while R1 and R2 are chosen from Table 4 according to the supply voltage. Using an AC supply The circuit is a little more complicated for a single-rail 12-60V DC supply. That’s because the signal can no longer swing below the 0V rail, since there’s no negative supply rail. As a result, the op amps must be biased to the mid-supply voltage, so that the signal can swing symmetrically about this voltage. This mid-supply voltage is produced using a voltage divider consisting of two 10kΩ resistors between the positive supply rail and ground. A 100µF capacitor filters this half-supply rail and this is fed to IC3b. IC3b is wired as a unity gain buffer stage. Its output at pin 7 provides the An 11-43VAC supply can also be used to derive positive and negative supply rails. In this case, the “+” and “-” inputs are connected together using link LK3 (following S1a & S1b) and the supply is connected between either of these two inputs and the 0V terminal (ie, between either “+” and 0V or between “-” and 0V of CON1). With this supply configuration, diodes D1 & D2 function as half-wave rectifiers, with filtering provided by two 1000µF capacitors. Diode D1 conducts on the positive half-cycles to derive the positive rail, while D2 conducts on the negative half-cycles 12-60V DC supply Choosing An Amplifier To Drive The Loop Commercially available hearing loop amplifiers use current drive for the loop. An advantage of these amplifiers is that they do not require any treble boost to compensate for losses due to loop inductance. Note, however, that the Hearing Loop Signal Conditioner can still be used with current-drive amplifiers to provide signal compression and level control. In this role, the treble boost control should be set to flat. The Hearing Loop Signal Conditioner can also be used with voltage amplifiers, in which case all its features, including treble boost, can be used. The voltage amplifier chosen must be capable of driving a 4Ω load and it must also be unconditionally stable. This latter requirement is important because we don’t want the amplifier to oscillate at a very high frequency and cause RF (radio frequency) signals to be radiated from the hearing loop. Many commercially made amplifiers should be suitable, as should most of the audio amplifier designs described in SILICON CHIP. Table 5 shows some of the recent SILICON CHIP amplifiers and the recommended loop size that could be used with each. The amplifier power requirement for the loop size takes into account the fact that the loop will be about 1.7m away from the listening position. January 2011  69 PC BOARD EARTH STAKE LEVEL WIRE EARTHING THE CASES OF VR1 & VR3 TREBLE BOOST LED1 half-supply rail (Vcc/2) via a 150Ω decoupling resistor. This is then used to bias the remaining op amps. For this DC supply option, two links are required for LK2 – one in position 1 and the other in position 3. The position 1 link connects the Vcc/2 rail to the signal ground, while the position 3 link connects the negative supply pins of the op amps (pin 4 in each case) to ground. Regardless of the power supply type used, LED1 lights when power is applied via switch S1. This LED is powered from the +15V supply rail via a 4.7kΩ current-limiting resistor. Note that the +15V supply rail will be at about +12V if a 12V DC supply is used. The AC-coupling capacitors at the inputs and outputs of the various op amps remove any DC component from the signal. These capacitors are necessary when the op amp outputs are biased at half supply. For the other supply options, the capacitors prevent DC coupling to the input stages of IC1a and IC1b and prevent DC flow in the level control. S1 47 LK4 2 1 10 F NP 3 2.2k LK1 4.7 F NP 220pF LK3 6.2k – JACK IN RCA OUT JACK OUT + 100k RCA IN R D2 CON1 VR2 2.2k RCA IN L 4004 D1 R1 ZD2 15V 10 F 47k ZD1 10 F NP 100k 10 F NP 150 100pF 100pF 100pF 10 F NP 2.2 F NP 47k 10 F NP 100k IC4 SA571 10k 10k 100k 1000 F COMP TP1 100nF 4004 V– +15V 110110 1 1R2 10k 68k 1M 1 F 1000 F 560pF 15V 10k IC5 TL072 27k BYPASS 100k 12k 5.6k 5.6k 100pF 150 10k IC3 TL072 10 100k 10k 10k 150pF 1.8k POS 3 POS 2 POS 1 100 F 100nF 10nF IC1 TL072 10k 10k IC2 TL072 10k 10k 100nF 100k 51k 4.7k LK2 1nF 10 F 100pF 100pF 220k 56nF 10 F NP VR3 50k C1* P MA P O OL VR1 10k LOG Construction CON2 3 OPTIONAL XLR SOCKET FOR BALANCED INPUT (REAR VIEW) 1 2 SC Refer now to Fig.7 for the assembly details. It’s easy to build, with all parts mounted on a PC board coded 01101111 and measuring 118 x 102mm. This board is housed in a plastic instrument case measuring 140 x 110 x 35mm. Begin by checking that the PC board fits correctly inside the case and that Fig.7: follow this parts layout diagram to build the PC board. Resistors R1 & R2 and capacitor C1 are chosen from Tables 3 & 4. Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o o No.   1   1 7   1   1   2   1   1   11   1   2   1   2   1   2   1   1 70  Silicon Chip Value 1MΩ 220kΩ 100kΩ 68kΩ 51kΩ 47kΩ 27kΩ 12kΩ 10kΩ 6.2kΩ 5.6kΩ 4.7kΩ 2.2kΩ 1.8kΩ 150Ω 47Ω 10Ω 4-Band Code (1%) brown black green brown red red yellow brown brown black yellow brown blue grey orange brown green brown orange brown yellow violet orange brown red violet orange brown brown red orange brown brown black orange brown blue red red brown green blue red brown yellow violet red brown red red red brown brown grey red brown brown green brown brown yellow violet black brown brown black black brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown black black orange brown blue grey black red brown green brown black red brown yellow violet black red brown red violet black red brown brown red black red brown brown black black red brown blue red black brown brown green blue black brown brown yellow violet black brown brown red red black brown brown brown grey black brown brown brown green black black brown yellow violet black gold brown brown black black gold brown siliconchip.com.au Table 2: Capacitor Codes Value 100nF 56nF 10nF 1nF 560pF 220pF 150pF 100pF µF Value 0.1µF .056µF .01µF .001µF   NA   NA   NA   NA IEC Code EIA Code 100n 104   56n 563   10n 103    1n 102 560p 561 220p 221 150p 151 100p 101 Table 3: C1 vs Loop Size Loop Size C1 20m square loop 5.6nF (5n6 or 562) 15m square loop 4.7nF (4n7 or 472) 12m square loop 3.9nF (3n9 or 392) 10m square loop 3.3nF (3n3 or 332) 7m square loop 2.2nF (2n2 or 222) 5m square loop 1.8nF (1n8 or 182) 3m square loop 1.2nF (1n2 or 122) its four corner mounting holes line up with the integral mounting bushes. These mounting holes should be 3mm in diameter. If not, drill them out to size. The next step is to check the board for any defects, such as breaks in the copper tracks and shorted tracks and pads. That done, start the assembly by installing the six wire links and the resistors. Don’t forget the link between resistors R1 & R2 but leave R1 and R2 out for the time being. Table 1 shows the resistor colour codes but you should also use a DMM to check each resistor as it is installed. Follow these parts with diodes D1 & D2 and zener diodes ZD1 & ZD2. Check that these parts are correctly orientated before soldering their leads, then install three PC stakes to terminate the XLR socket wiring. An additional PC stake is then installed immediately to the left of potentiometer VR1 (this connects to the ground track and is used to terminate a length of tinned copper wire that connects to the bodies of the two pots). The 2-way, 3-way and 4-way pin headers (for LK1, LK2, LK3 & LK4) are next, followed by the IC sockets. Be sure to install the sockets with their notched ends orientated as shown on Fig.7. siliconchip.com.au This view shows the completed PC board. Omit the two RCA input sockets and the adjacent 6.35mm jack socket if you intend using an XLR input connector. Alternatively, the five ICs can be soldered directly to the PC board. Now for the capacitors. The MKT types can go in first, followed by the electrolytics. The electros marked “NP” are non-polarised and can go in either way around but the rest must be correctly orientated. Capacitor C1 is selected from Table 3 to suit the size of the hearing loop. Trimpot VR2 can now be installed, followed by the various connectors. However, if you are using an XLR connector for the input, then the left and right RCA sockets and the adjacent 6.35mm jack socket (input) should be omitted. This is necessary to allow space for the XLR connector on the rear panel. If you are not using the XLR connector, then install the RCA sockets and the 6.35mm jack socket as shown on Fig.7. Make sure that all the connectors are correctly seated on the PC board before soldering their leads. Switch S1 can also be installed at this stage, along with 3-way terminal block CON1. In addition, install power socket CON2 if you intend using either a single rail DC supply or an AC supply (eg, a DC or AC plugpack). Alternatively, if you intend using a dual-rail supply (ie, with “+” and “-” rails), then you should omit CON2. A grommet is then later installed on the rear panel at CON2’s location and the supply leads run through this to CON1. Installing the pots & LED1 The two potentiometers (VR1 & VR3) are mounted directly on the PC board. Before mounting them, trim their shafts to 10mm (as measured from the screw thread bush) to suit the knobs. The pots are then pushed all the way down onto the board (VR1 is the 10kΩ log pot) and their terminals soldered. Once they are in position, earth the two pot bodies by running a length of tinned copper wire between them and soldering one end to the PC stake immediately to the left of VR1. Note that it will be necessary to scape away January 2011  71 Table 4: Choosing R1 & R2 & Setting The Supply Links Input Voltage R1 R2 Links Power Input ±60VDC 1.2kΩ 5W 1.2kΩ 5W LK2 position 2, LK3 out +, 0, - ±55VDC 1kΩ 5W 1kΩ 5W LK2 position 2, LK3 out +, 0, - ±50VDC 820Ω 5W 820Ω 5W LK2 position 2, LK3 out +, 0, - ±45VDC 680Ω 5W 680Ω 5W LK2 position 2, LK3 out +, 0, - ±40VDC 560Ω 5W 560Ω 5W LK2 position 2, LK3 out +, 0, - ±35VDC 470Ω 5W 470Ω 5W LK2 position 2, LK3 out +, 0, - ±30VDC 390Ω 5W 390Ω 5W LK2 position 2, LK3 out +, 0, - ±25VDC 270Ω 5W 270Ω 5W LK2 position 2, LK3 out +, 0, - ±20VDC 120Ω 1W 120Ω 1W LK2 position 2, LK3 out +, 0, - ±15VDC 10Ω 1/2W 10Ω 1/2W LK2 position 2, LK3 out +, 0, - ±12VDC 10Ω 1/2W 10Ω 1/2W LK2 position 2, LK3 out +, 0, - 43VAC 1.2kΩ 5W 1.2kΩ 5W LK2 position 2, LK3 in +, 0 40VAC 1kΩ 5W 1kΩ 5W LK2 position 2, LK3 in +, 0 35VAC 820Ω 5W 820Ω 5W LK2 position 2, LK3 in +, 0 30VAC 680Ω 5W 680Ω 5W LK2 position 2, LK3 in +, 0 28VAC 560Ω 5W 560Ω 5W LK2 position 2, LK3 in +, 0 25VAC 470Ω 5W 470Ω 5W LK2 position 2, LK3 in +, 0 20VAC 390Ω 5W 390Ω 5W LK2 position 2, LK3 in +, 0 18VAC 270Ω 5W 270Ω 5W LK2 position 2, LK3 in +, 0 15VAC 120Ω 1W 120Ω 1W LK2 position 2, LK3 in +, 0 11VAC 10Ω 1/2W 10Ω 1/2W LK2 position 2, LK3 in +, 0 + 60VDC 1.2kΩ 5W NA LK2 positions 1&3, LK3 out +, 0 + 55VDC 1kΩ 5W NA LK2 positions 1&3, LK3 out +, 0 + 50VDC 820Ω 5W NA LK2 positions 1&3, LK3 out +, 0 + 45VDC 680Ω 5W NA LK2 positions 1&3, LK3 out +, 0 + 40VDC 560Ω 5W NA LK2 positions 1&3, LK3 out +, 0 + 35VDC 470Ω 5W NA LK2 positions 1&3, LK3 out +, 0 +30VDC 390Ω 5W NA LK2 positions 1&3, LK3 out +, 0 +25VDC 270Ω 5W NA LK2 positions 1&3, LK3 out +, 0 +20VDC 120Ω 1W NA LK2 positions 1&3, LK3 out +, 0 +15VDC 10Ω 1/2W NA LK2 positions 1&3, LK3 out +, 0 +12VDC 10Ω 1/2W NA LK2 positions 1&3, LK3 out +, 0 some of the coating from the pot bodies to get the solder to “take”. LED1 is installed by first bending its leads down through 90° exactly 8mm from its base. Make sure it is correctly orientated before you do this (see Fig.7). The LED is then installed so that it sits 6mm above the board, so that it will later protrude through its hole in the front panel. The best way to do this is to cut a 6mm-wide cardboard spacer and push the LED’s leads down onto this. Make sure that the LED goes in with its cathode towards switch S1. Resistors R1 & R2 can now be installed but first, you have to choose the power supply to be used with the device. Table 4 shows the resistor values for the various supply voltages. The links at LK2 and LK3 must also be selected according to the power supply. For a dual-rail (plus and minus supply), a jumper shunt is placed in position 2 for LK2, while LK3 is omitted. The supply leads are connected to the plus, 0V and minus supply inputs of CON1. For an AC supply, a jumper shunt is placed in position 2 for LK2, while LK3 is fitted with a jumper shunt. The supply is connected to the plus and 0V inputs of CON1 or can be connected via power connector CON2. Finally, for a single-rail DC supply, jumper shunts are placed in positions 1 & 3 of LK2, while LK3 is omitted. The supply can be fed in either via CON2 or the leads can be connected to the plus and 0V inputs of CON1. Final assembly The assembled PC board can now be installed in the plastic case. Fig.10 shows the front and rear panel artworks and these can be used as drilling templates. They can either be copied or downloaded in PDF for- Table 5: Choosing An Amplifier Module To Drive A 4-Ohm Hearing Loop Power into 4Ω Recommended Loop Size Name Issue Kit Supplier 20W 3-8m square Compact High Performance 12V Stereo Amplifier May 2010 Jaycar KC5495, Altronics K5136 30W 2.5-11m square Schoolies Amplifier December 2004 Altronics K5116 55W 2-16m square 50W Audio Amplifier Module March 1994 Jaycar KC5150, Altronics K5114 70W 2-18m square SC480 Amplifier Module January 2003 Altronics K5120 200W 1.5-33m square Ultra-LD MK.2 August 2008 Jaycar KC5470, Altronics K5151 350W Less than 42m square Studio 350 Power Amplifier January 2004 This table lists several SILICON CHIP amplifier modules that are suitable for driving a 4Ω hearing loop. The recommended amplifier will provide the correct field strength 1.7m above or below the loop. 72  Silicon Chip siliconchip.com.au The final assembly involves attaching the front and rear panels to the PC board, then sliding it into position inside the case and installing four self-tapping screws into integral spacers. mat from the SILICON CHIP website and printed out. It’s best to drill the holes using a small pilot drill and then carefully enlarge them to size using a tapered reamer. Note that if you are using an XLR connector for the input, don’t drill the holes for the left and right RCA sockets or the adjacent 6.35mm jack socket. Instead, you will have to mark out and drill a hole to accept the XLR socket. The front and rear panel labels will be supplied if you purchase a kit. If not, download them from the SILICON CHIP website as described above. The file can then be printed out onto stickybacked photo paper or onto plastic film (be sure to use the correct material for your printer). When using clear plastic film (overhead projector film), print the label as a mirror image so that the ink will be behind the film when it is affixed to the front panel. Wait until the ink has thoroughly dried before cutting the label to size. It siliconchip.com.au The rear panel provides access to the various input and output sockets, as well as to the power socket. Omit the power socket and fit a rubber grommet if you intend using a dual-rail supply (eg, derived from an amplifier). can then be affixed to the panel using an even smear of neutral cure silicone sealant. If you are affixing to a black coloured panel, use coloured silicone such as grey or white so the label has contrast. For panels that are off-white or are made of aluminium, the silicone can be clear. Once the labels are in position, leave them overnight for the silicone to cure. The holes can then be cut out using a sharp hobby knife. January 2011  73 Level TO AMPLIFIER INPUT TO AMPLIFIER Fig.9: this diagram shows how to make a 2-turn hearing loop using figure-8 cable. Use heatshrink to insulate the link between the two loops. The remaining two terminals connect to the speaker output terminals of the amplifier. Once the panels are complete, fit them to the PC board by sliding them into position, then slide the entire assembly into the base of the case. The PC board is then secured to the base using four M3 x 6mm screws that go into integral mounting bushes. The assembly can then be completed by fitting the nuts to the pots, switch S1 and the 6.35mm jack sockets before fitting the two knobs. Testing To test the unit, first apply power and check that the power LED lights. If it does, the next step is to check the power supply voltages on the board (these will vary according to the supply used). For a single-rail DC supply, the voltage between pins 8 & 4 of IC1 should be at about 15V, although this will be 74  Silicon Chip Out R L NOTE:REFER TO THE ARTICLE ON PAGE 22 OF THE SEPTEMBER 2010 ISSUE FOR INFORMATION ON DESIGNING & INSTALLING HEARING LOOPS SILICON CHIP (HEARING AID LOOP) In Treble Boost FIGURE-8 CABLE Hearing Loop Signal Conditioner Fig.8: if the amplifier used to drive the loop lacks a volume control, you can add one yourself as shown here. Be sure to use shielded audio cable for the wiring connections. Power In 10k LOG Power FROM HEARING AID AMPLIFIER SIGNAL PRECONDITIONER Fig.10: these full-size artworks can be used as drilling templates for the front and rear panels. lower if the DC supply is below 15V. The same goes for IC2, IC3 & IC5. If this is correct, check the output voltages on pins 1 & 7 of IC1, IC2, IC3 & IC5. These should all be at about half supply, or about 7.5V for a 15V (or greater) DC power supply. Now check the voltage on pin 13 of IC4. It should be at +15V but will be less than this if a lower supply voltage is used. If you are using a dual-rail supply, the voltages should be measured with respect to the 0V rail. In this case, pin 8 of IC1, IC2, IC3 & IC5 should be at +15V, while pin 4 of each of these ICs should be at -15V. Once again, these voltages will be correspondingly lower if lower supply voltages are used. stereo signal is applied to the left and right RCA sockets or to the stereo 6.35mm jack socket. Conversely, leave LK1 out for a mono signal. Note that a mono signal should be applied either to the left RCA input or to the tip connection of the 6.35mm jack input socket. For a balanced XLR connection, use the separate input connections at pins 1 (ground), 2 & 3. In this case, link LK1 is not required and is left out (as are the RCA sockets and the 6.35mm stereo jack input socket). Finally, link LK4 is fitted in the COMP position when signal compression is required and in the BYPASS position if compression is not required. Setting LK1 & LK4 The Hearing Loop Signal Conditioner is designed to accept line level Jumper link LK1 is required if a Signal levels siliconchip.com.au Loop Frequency Response (4Ω , 2 Turns) 0 -1 Helping to put you in Control Control Equipment -2 3 x 3m Temperature Sensor A DS18S20 1-Wire temperature sensor is fitted into a waterproof stainless steel probe. Accurate to ±0.5 °C over the range of -10 °C to +85 °C. Length 3.4 metres EDS-001 $49.50+GST -3 -4 -5 5 x 5m Level (dB) -6 -7 -8 Function Generator Kit Based around the XR-2206 function generator IC, it can produce sine, triangle, and 5V square waves with frequencies ranging from 15Hz to over 500kHz. SFK-001 $39.00+GST 10 x 10m -9 -10 -11 15 x 15m -12 -13 20 x 20m -14 -15 0.25 0.5 1 2 3 4 5 6 7 8 9 10 Frequency (kHz) Fig.11: these curves plot the high-frequency roll-offs for several loop sizes ranging from 3 x 3m to 20 x 20m. The larger the loop size, the greater the inductance and the greater the high-frequency roll-off. signals (ie, 774mV), while level control VR1 should be adjusted to provide satisfactory compressor operation. In practice, VR1 should be set so that there is an average of 1.8V between TP1 and 0V for a typical signal at the input (note: a “typical signal” is the program material that will normally be fed into the unit). If TP1 is less than 1.8V with VR1 set to maximum, then the gain of the IC1a & IC1b amplifier stage will need to be increased. This involves reducing the 10kΩ resistor between pins 2 & pin 6 of IC1. Final testing Once the signal levels are correct, the unit can be tested by connecting it to an amplifier and feeding in a signal to drive the loop. If the amplifier doesn’t have a volume control, Fig.8 shows how one can be added. The amplifier’s output connects to the 4Ω hearing loop and siliconchip.com.au the volume control is used to set the overall level. Fig.9 shows the way a figure-8 hearing loop is wired to the amplifier. The wire loops are effectively connected in series. Be sure to use heatshrink to insulate the link between the two loops. The output from the pre-conditioner can be taken either from the RCA socket or from the 6.35mm jack socket. A suitable lead will be required to make the interconnection to the amplifier. If the amplifier requires an XLR input, then a 6.35mm jack plug to XLR line plug lead can be made up. Pin 2 of the XLR connector is used to terminate the signal lead connection from the jack plug tip, while pins 1 & 3 are connected to ground via the jack plug’s sleeve terminal. Finally, VR3 (treble boost) can be adjusted. The Hearing Loop Tester described last month is used to check the loop frequency response. Adjust SC VR3 for a flat response to 5kHz. Triple Axis Accelerometer. MMA7341L XYZaxis accelerometer, a great low-g sensor with analog voltage outputs and adjustable sensitivity (±3 g or ±11 g), and a 0g-detect signal when the board is in free-fall. POL-1252 $17.50+GST 1 axis AC Servo Kit Consists of a 400W Brushless AC Servo motor with 1000 line encoder, AC Servo Drive and 60V 8 A power supply. Great for CNC applications. CNC-145 $624 +GST 8 Relay Card on DIN Rail Mount. We have reduced our prices for these incredibly versatile cards. Available in both 12VDC and 24VDC RLD-128 $109.95+GST Anemometer Alarm Card. Converts a Davis Instruments Anemometer wind speed and direction into 4-20mA / 0-5V signals. Can program 2 alarm relays to operate outside specified wind speeds or direction. Also Modbus connection. KTA-250 $159.00+GST Ph: 03 9782 5882 Our Catalog is Coming! www.oceancontrols.com.au January 2011  75