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Build a high-quality ALC for PA systems Designed specifically for PA systems, this high quality Automatic Level Control will maintain a constant output level over a wide range of input levels. It is ideal for setting the volume level in applications which are not monitored by a sound system operator. By JOHN CLARKE Maintaining a consistent volume level from a public address (PA) system is a constant problem requiring continuous adjustments. This is because as people speak, they continually move towards and away from the microphone or even sway from side to side. While this move­ment can 42  Silicon Chip be almost imperceptible, it can have a drastic effect on the volume level. Whenever there is a different person speaking or when music is played, there is again a volume change. These variations can be adjusted by the sound system operator who rides the volume control to maintain an audible level at all times. This can be satisfactory in most instances where there is an operator but for unattended sound systems, an automatic control is a great advan­tage. Again where there are remote loudspeakers driven by a sepa­ rate amplifier, adding an automatic volume control can ensure that the sound levels are consistent regardless of the source level. Ideally, an automatic level control should have as little effect on the sound quality as possible. To this end we have used low distortion and low noise components and have provided adjust­­ments for all the main paramet­ ers. In this way, the control opera­tion can be tailored to your requirements. For example, you may wish to use the unit as a compressor, where the dynamic range of the sound is restricted. This type of response is useful in areas which have high ambient noise. In this case the attack and decay times would be adjusted close to their fast response settings. The gain limit control is adjusted to set the threshold below which the output signal will drop off in level. If set to its lowest setting, the ALC will maintain a constant output for signals down to 2.5mV. This is a very low signal level and will probably be too sensitive for most applications. Adjustment of this control is usually made during listening tests so that the normal range of input signals are effectively amplified. The Automatic Level Control is housed in a small plastic case. A potentiometer is provided for the output level adjustment, while the gain limit, attack and decay controls are trimpots which can be accessed by a screwdriver through holes in the front panel. A power switch and indicator LED are also provided. On the rear panel are two RCA sockets for the input and output signals and a DC socket for the plugpack. Block diagram The general arrangement of the ALC circuit is shown in Fig.1. The input signal is amplified by 5.5 before being fed to IC2, a voltage controlled amplifier (VCA). IC2 is an Analog Devices SSM2018 VCA which features a 117dB dynamic range, .006% THD at 1kHz and unity gain, and a 140dB gain control range. In addition, it has the option to set the output amplifier in class A or AB modes. We opted for the class A mode since this provides excellent distortion characteristics. The AB mode improves noise performance by 3dB but distortion is 10 times higher. The change from one mode to the other is easily implemented by altering a resistor value. IC2 feeds the output level control VR5 and thence the output amplifier IC1b. It also feeds a precision fullwave rectifier. This rectifier monitors both the positive and negative signal excursions which are converted to a DC level by the following filter. The resulting output is applied to the VCA control input which adjusts the gain of this device so that the signal level is constant. The response of the filter will determine how quickly or slowly the gain of the VCA is controlled. Fig.1: the signal output from the VCO drives a precision full-wave rectifier to derive a control signal. This signal is then used to control the gain of the VCO, so that its output remains constant. A gain limit adjustment by way of VR4 and buffer amplifier IC3d prevents the control input from going below a certain preset voltage. This limits the overall gain of the VCA so that automat­ic level control is initiated from a preset minimum signal. Circuit description Fig.2 shows the complete ALC circuit. Input signals to amplifier IC1a are AC-coupled to pin 5. The 100kΩ feedback resis­tor and the 22kΩ resistor to ground set the gain to 5.5. Thus, a 1V input signal produces a 5.5V signal at pin 7. IC2 is the VCA, with the gain controlled by the voltage level at pin 11. The 18kΩ input resistors to pins 4 & 6 and between pins 3 & 14 set Features • • • • • • • Low noise and distortion Constant level over a 52dB input range Adjustable control level (gain limit control) Adjustable attack and decay times Output level control 12VAC plugpack powered Compact size the VCA gain to 1 when pin 11 is at ground. The 47pF capacitors at pins 5 & 8 and 3 & 14 are for compensation Performance Frequency response ��������������������-3dB at 40Hz and 20kHz (measured below compression limit) Signal-to-noise ratio ����������������������87dB with 22Hz to 22kHz filter and 89dB A-weighted at 100mV signal limiting; 74dB with 22Hz to 22kHz filter and 76dB A-weighted at 15mV limit­ing; with respect to 1V RMS output. Harmonic distortion at slowest attack and decay settings...............<.015% at 1kHz, 10kHz and 20kHz for .......................................................18mV to 1V input levels ALC input range ����������������������������2.5mV to 1V Attack time ������������������������������������1ms to 150ms Decay time ������������������������������������20dB/second to 6dB/second Maximum input before clipping �����1.35V RMS Output level �����������������������������������0-1V RMS March 1996  43 Fig.2: the circuit is based on an Analog Devices SSM2018 VCA (IC2). Op amps IC3a & IC3b, together with diodes D1 and D2, form the precision full-wave rectifier. Its output appears at pin 1 of IC3b and is applied to pin 11 of IC2 via D3 and VR1. and rolloff at high frequencies. Op amps IC3a & IC3b, together with diodes D1, D2 and asso­ ciated resistors, form a precision full wave rectifier. When the input signal goes positive, the output of IC3a goes low and the gain set by the 20kΩ input and feedback resistors is -1. This signal is seen at the anode of D2 and is coupled to the inverting input of IC3b via the 10kΩ resistor. The gain for IC3b is set by the 10kΩ and 180kΩ resistors at -18. The overall gain for the input signal is therefore -1 x -18 = +18. Note, however, that there is an extra path for the input signal via the 20kΩ resistor at pin 2 of IC3b. This signal gives a negative signal at the output of IC3b with a gain of -9. Adding the two gains gives us +9. For negative signals, the output of 44  Silicon Chip IC3a is clamped due to the conduction of diode D1. The signal then passes via the 20kΩ resistor connecting to pin 2 of IC3b. IC3b inverts the signal and provides a gain of -9. Since the input signal is negative, the signal at pin 1 of IC3b is positive. The full-wave rectified signal is fed via D3 and VR1 to two back-to-back 100µF capacitors. Diode D3 allows the 100µF capaci­tors to be charged up via VR1 but they are only discharged using VR2 and a 470kΩ resistor. This allows separate control over attack and decay times of the voltage applied to pin 11 of IC2. Trimpot VR3 sets the ALC threshold and it is buffered by IC3c which then offsets the inverting inputs of IC3a and IC3b. VR3 is set so that the signal output at pin 14 of IC2 is at 1V under high signal conditions. Op amp IC3d and VR4 set the gain limit. Basically, VR4 is set so that the voltage at pin 11 of IC2 cannot go below the level clamped by D4 and the output of IC3d. Naturally, pin 11 of IC2 can go above this clamp voltage since D4 is then reverse biased. After it has varied the gain of the input signal, the output of IC2 is AC-coupled to the output level control, VR5. This is buffered by unity gain buffer IC1b. Its output is AC-coupled to the external amplifier via a 10µF bipolar electro­ lytic capacitor and a 100Ω resistor. Power for the circuit is derived from a 12V AC plugpack. This is fed via S1 to D5, D6 and two 470µF capacitors to provide positive and negative supply rails. These are then fed to two 3-terminal regulators (REG1 & REG2) to provide balanced ±12V supply rails. Construction The automatic level control is built PARTS LIST 1 PC board, code 01303961, 98 x 98mm 1 plastic case, 111 x 45 x 140mm, Arista UB14 1 12VAC 300mA plugpack 1 self-adhesive front panel label, 95 x 33mm 1 self-adhesive rear panel label, 95 x 33mm 1 SPDT toggle switch (S1) 1 1MΩ miniature vertical trimpot (VR2) 1 50kΩ miniature vertical trimpot (VR1) 1 22kΩ miniature vertical trimpot (VR4) 1 5kΩ multiturn top adjust trimpot, Bourns 3296 (VR3) 1 20kΩ log pot. (VR5) 1 15mm knob 2 panel mount RCA sockets 1 insulated panel mount DC socket 1 400mm length of hook-up wire 1 100mm length of shielded cable 1 60mm length of tinned copper wire 9 PC stakes Semiconductors 1 LM833 dual op amp (IC1) 1 SSM2018TN VCA (IC2) (available from HarTec Ltd) 1 LF347 quad op amp (IC3) 1 7812 +12V regulator (REG1) 1 7912 -12V regulator (REG2) 4 1N4148, 1N914 signal diodes (D1-D4) 2 1N4004 1A diodes (D5,D6) 1 3mm red LED (LED1) Fig.3: install the parts on the PC board as shown here. Make sure that all polarised parts are correctly oriented and take care to ensure that REG1 and REG2 are not transposed, as they are different devices. on a PC board coded 01303961 and measuring 98 x 98mm. It is housed in an Arista UB14 plastic case measuring 111 x 45 x 140mm. Two selfadhesive labels, each measuring 95 x 33mm, are fitted to the front and rear panels. The PC board layout and wiring diagram is shown in Fig.3. Start your assembly by checking the PC board against the published pattern. Repair any broken tracks or shorts that may be evident. This done, insert the ICs, diodes, resistors and links in the locations shown. Take care with the orientation of the ICs, noting that IC2 is oriented differently to the other two. The accompany­ing resistor colour code chart should be used in selecting each resistor value. Alternatively, use a digital multi­meter to meas­ure them. The diodes must be oriented with the polarity shown – the banded end is the cathode (K). Nine PC stakes are required for external connections to the PC board. When these are in, install the capaci- Capacitors 2 470µF 16VW PC electrolytic 2 100µF 16VW PC electrolytic 3 10µF 25VW PC electrolytic 1 10µF bipolar electrolytic 1 3.3µF bipolar electrolytic 5 0.47µF MKT polyester 1 270pF ceramic 1 68pF ceramic 2 47pF ceramic 1 10pF ceramic Resistors (0.25W 1%) 1 470kΩ 3 20kΩ 1 180kΩ 2 18kΩ 3 100kΩ 1 10kΩ 1 24kΩ 1 4.7kΩ 4 22kΩ 1 100Ω March 1996  45 Fig.4: full-size etching pattern for the PC board. 46  Silicon Chip tors taking care to orient the electrolytics with the polarity shown. The regulators are next and they are oriented with their metal tabs away from the 470µF capacitors. Insert the 7812 into the location nearest D5. Take care to insert each of the trimpots into its correct position. The LED is mounted at full lead length and bent over at right angles so that it goes into its matching front panel hole. It would be a good idea to sleeve one or both of the LED leads to prevent them from shorting together. When complete, the PC board can be secured inside the case by mounting it on the integral standoffs, using the self-tapping screws provided. Affix the adhesive labels to the front and rear panels and drill out the holes for the output level pot, the power switch and the rear panel components. Drill 3-4mm holes for the limit, attack and decay trimpots. A 3mm hole is required for the LED. Secure these components to the front and rear panels, attach the knob and slide the panels into the case. Affix the rubber feet to the base of the case. You can now do the remaining wiring inside the case. Use short lengths of shielded cable for TRIMPOT CODES Value 1MΩ 50kΩ 22kΩ 5kΩ The rear panel carries RCA sockets for the input and output connections, plus a DC power socket. Code 105 504 223 502 the input and output connec­tions as shown in Fig.3. Adjustment & voltage checks Check your work before applying power. Using a multimeter, check that pin 8 of IC1, pin 2 of IC2 and pin 4 of IC3 have +12V present. Similarly, check that pin 4 of IC1, pins 16 & 10 of IC2, and pin 11 of IC3 are at -12V. If the LED does not light, it is probably connected the wrong way around. Trimpot VR3 can only be set by applying a signal to the input. You can use a signal generator set to about 500mV and 1kHz or use a signal such as that from a CD player, tape deck or the audio output from a video player. Connect your multimeter to the output terminals and adjust it to read AC volts. Turn VR4 (the gain limit trimpot) fully anticlockwise and adjust VR3 for a 1VAC reading. Now the ALC is ready for testing on a signal. You can hook the unit up to the line output of the PA system or signal source as mentioned above and the output to the input of a power ampli­fier. Set the output level and adjust the gain limit so that with no signal there is no evidence of noise. Now you can experiment with the attack and decay controls for the type of compression or automatic level SC control required. AUTOMATIC LEVEL CONTROL OUTPUT + MIN GAIN LIMIT MAX + 12VAC IN DECAY ATTACK POWER + + + + LOW HIGH SLOW FAST SLOW FAST + OUT + + IN Fig.5: these full-size artworks can be used as drilling templates for the front and rear panels. RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  3 ❏  1 ❏  4 ❏  3 ❏  2 ❏  1 ❏  1 ❏  1 Value 470kΩ 180kΩ 100kΩ 24kΩ 22kΩ 20kΩ 18kΩ 10kΩ 4.7kΩ 100Ω 4-Band Code (1%) yellow violet yellow brown brown grey yellow brown brown black yellow brown red yellow orange brown red red orange brown red black orange brown brown grey orange brown brown black orange brown yellow violet red brown brown black brown brown 5-Band Code (1%) yellow violet black orange brown brown grey black orange brown brown black black orange brown red yellow black red brown red red black red brown red black black red brown brown grey black red brown brown black black red brown yellow violet black brown brown brown black black black brown March 1996  47