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The Loudspeaker Level Meter is built into a small plastic case and is just the shot for quickly adjusting the level of each channel in a home theatre system or public address system. Level meter for home theatre systems Setting up a home theatre system? Lucky you. Want to adjust all the speaker levels precisely? Here is the way to do it, with this handy little Loudspeaker Level Meter. It has its own inbuilt microphone and a 10-LED bargraph display to let you quickly set all channels to the same relative level. And you can use it to set up the levels in a PA system as well. By JOHN CLARKE Y OU MIGHT THINK it is a straightforward matter to set up the levels in your home theatre system but depending on your room layout and the physical positioning of the various speakers, it can be surprisingly tricky. This is especially the case when you are trying to get an overall good balance at a number of listening positions. Without the correct balance, the surround effect will not be the best it can be. Balance between the centre speaker and the left and right channels is critical since they present the front sound-scape. And as is often the case in many home theatre systems, if the centre loudspeaker is too dominant, it will detract from the imaging. With the Loudspeaker Level meter, you can set up the levels accurately and quickly. It is just a small box 22  Silicon Chip with a 10-LED bargraph display on the front. Controls include the power switch and a level adjustment. On the base is a small electret microphone for monitoring the sound level from the loudspeaker. In use, each loudspeaker is driven with a noise signal in turn and the Loudspeaker Level Meter is placed at the listening position and aimed at the speaker. The LED bargraph meter level adjustment is set so that it reads 0dB for one loudspeaker. Then the noise level of each of the other loudspeakers is adjusted at the amplifier so that they are all the same. Generally, they can be adjusted to within 1dB of each other. Relative measurements Note that the Loudspeaker Level Meter does not give an absolute sound level measurement; it is a relative measurement only, with respect to a reference level, usually 0dB, set by the level control knob. You can then measure sound levels up to 6dB higher or 13dB lower than the reference 0dB level. Most sound level meters incorporate frequency “weighting” to emulate the perceived loudness at different loudness levels. However, since this Level Meter is intended for loudness comparisons over a relatively narrow range, no frequency weighting is required. In addition to frequency response, sound level meters can respond rapidly or slowly to changes in sound levels. The Loudspeaker Level Meter LED display has a response similar to VU (Volume Unit) meters used www.siliconchip.com.au Main Features • • • • • 10 LED dot bargraph display -13dB to +6dB display range Level control Attack and decay rate follows VU standard Portable battery powered unit in recording studios to set the audio levels for recording. VU response is very similar to the perceived loudness heard by the ear for various signals that include sudden transients. Dot/bar display driver The heart of the Loudspeaker Level Meter is the readily available National Semiconductor LM3914 Dot/Bar Display Driver IC which is configured to drive 10 LEDs in dot mode. We have used the LM3914 in preference to the LM3915 which gives a logarithmic display or the LM3916 which gives a VU response, because the LM3914 is so cheap and readily available. The drawback of the LM3914 when used as a decibel display is that it has a linear rather than the preferred logarithmic display characteristic. This explains the rather unusual labelling of the 10 LEDs, which turns out to be quite useable in practice. LEDs 5 & 6 correspond to -1dB and +1dB respectively and when they are both illuminated, the level is in between, at 0dB. Fig.1 shows the internal components of the LM3914 display driver. It comprises a stack of 10 comparators, each with its non-inverting input connected to a resistor string between the RHI input (pin 6) and the RLO input (pin 4). All the inverting inputs of the comparators monitor the input signal at pin 5, via the internal buffer op amp. If the input voltage is above the threshold set on comparator 1, LED1 will light. Similarly, if the input voltage exceeds the threshold voltage for comparator 2, LED2 will light, and so on. Not shown is the internal circuitry which allows only one LED to light at a time, instead of a whole bar of LEDs which would otherwise result for a high signal level. Internal 1.25V reference The internal 1.25V reference allows www.siliconchip.com.au Fig.1: the LM3914 LED display driver IC includes 10 comparators, a 1.25V voltage reference and a signal-input buffer stage. the IC to be set up to display the range of voltages required. The resistor between the REFOUT and REFIN pins (7 & 8) sets the reference current, so with the 1.2kΩ resistor shown, the current is 1.25V/1.2kΩ or 1.04mA. This current flows through the resistors connecting the REFIN pin to ground (0V). April 2004  23 Fig.2: block diagram of the Loudspeaker Level Meter. The microphone signal is amplified by IC1, then precision rectified and filtered before being applied to the bargraph display driver (IC3). Since we are using 510Ω and 3.3kΩ resistors in series the voltage at the REFIN pin will be 1.04mA x (510Ω + 3.3kΩ) or 3.96V. The voltage at the junction of the 3.3kΩ resistor and 510Ω resistor will be 1.04mA x 3.3kΩ or 3.43V. So this gives us RHI of 3.96V and RLO of 3.43V and so the input voltage applied to pin 5 will light LEDs 1-10 when the voltage goes between 3.43V and 3.96V. This is a nominal 0.53V range. Block diagram The block diagram for the Loudspeaker Level Meter is shown in Fig.2. As shown, the microphone signal is amplified by IC1 with the gain set using VR1. Then the signal is preci- sion rectified and filtered (IC2) before being applied to the bargraph display driver (IC3). Circuit details The full circuit is shown in Fig.3. The electret microphone is powered via a 22kΩ resistor from a decoupled supply connecting to the 9V supply rail. The decoupling comprises the 10kΩ resistor and 470µF capacitor and is required to prevent the supply rail changes which occur when different LEDs light up from being injected back into this amplifier. The decoupled supply also applies a bias voltage to pin 3 of op amp IC1 via 100kΩ and 330kΩ resistors. Signal from the microphone is coupled into Parts List 1 PC board code, 01104041, 123 x 59mm 1 plastic utility case, 130 x 68 x 43mm 1 front panel label, 65 x 125mm 1 electret microphone insert 1 SPDT toggle switch (S1) 1 knob to suit 1 50kΩ 16mm log potentiometer (VR1) 1 50kΩ horizontal trimpot (VR2) 1 9V battery 1 9V U-shaped battery holder 1 9V battery clip lead 1 M3 x 6mm screw 1 M3 nut 11 PC stakes 1 50mm length of single core shielded cable Semiconductors 1 TL071, LF351 op amp (IC1) 1 TL072, LF352 dual op amp (IC2) 1 LM3914 dot/bar display driver (IC3) 24  Silicon Chip 1 16V 1W zener diode (ZD1) 2 1N914, 1N4148 diodes (D1,D2) 1 1N5819 Schottky diode (D3) 5 5mm green LEDs (LEDs1-5) 5 5mm red LEDs (LEDs 6-10) Capacitors 2 470µF 16V PC electrolytic 1 100µF 16V PC electrolytic 1 47µF 16V PC electrolytic 3 1µF 16V PC electrolytic 1 1µF NP electrolytic 1 100nF (0.1µF) MKT polyester 1 56nF (.0056µF) MKT polyester 1 100pF ceramic 1 10pF ceramic Resistors (0.25W 1%) 1 1MΩ 1 10kΩ 1 330kΩ 1 4.7kΩ 1 300kΩ 1 3.3kΩ 1 220kΩ 1 1.2kΩ 1 150kΩ 1 510Ω 2 100kΩ 1 27Ω 3 22kΩ IC1 via a 1µF capacitor. IC1’s gain is set by the ratio of the feedback resistance between the output (pin 6) and the inverting input (pin 2) to the 100Ω resistor from pin 2. The low frequency response rolls off below about 34Hz due to the time constant of the 100Ω resistor and 47µF capacitor. In practice, IC1’s gain is adjustable from 48 (when potentiometer VR1 is set to minimum) to about 548 (when VR1 is set to 50kΩ). However, if the gain is set to values above about 100, the inherent bandwidth limitation of the TL071 op amp begins to reduce the gain at higher audio frequencies. For example, at a gain of 300, the response will typically roll off above 10kHz. This limitation is not important in this application – we merely note it for readers who may want to employ this circuit in a more critical application. Precision rectifier The output from op amp IC1 is coupled via a 1µF capacitor to the full wave precision rectifier which consists of diodes D1 & D2 and op amps IC2a & IC2b. Its operation is as follows: When the input signal goes positive, pin 1 of IC2a goes low and forward biases diode D1. The resulting gain of the signal at the anode of diode D1 is set at unity by the 22kΩ resistor. This inverted signal is fed to op amp IC2b via a 150kΩ resistor. IC2b’s gain is -6.66, as set by the ratio of the 1MΩ feedback resistor and the 150kΩ input resistor. Thus, the overall gain due to this signal path is IC2a’s gain (-1) times IC2b’s gain (-6.66), or +6.66. In addition, the positive-going input signal is applied via a second path to IC2b, this time via a 300kΩ resistor. The gain of IC2b for this signal is -3.33, due to the ratio of the 1MΩ feedback resistor and the 300kΩ input resistor. Thus, the overall signal gain at the www.siliconchip.com.au Fig.3: this is the complete circuit diagram for the Loudspeaker Level Meter. IC1 is the microphone preamplifier, while IC2a and diodes D1 & D2 make up the precision rectifier. The output from the precision rectifier is filtered by IC2b and fed to the pin 5 input of the LM3914 LED display driver (IC3). output of IC2b is +6.66 - 3.33 = 3.33. When the signal goes negative, diode D2 is forward biased and so IC2a’s output is clamped at 0.6V above the pin 3 reference voltage. IC2a is therefore effectively out of circuit and IC2b then simply amplifies the signal on its own, giving a gain of -3.33. Since the input signal is negative, the output is inverted, at +3.33 times the input. Thus the precision rectifier can be seen to provide a positive output with gain of 3.33 for both positive and negative going inputs. VU response IC2b also provides low pass filtering of the rectified signal to conform roughly to VU (volume unit) standards where the output reaches the input level after 300ms and overshoots by about 1.5%. The filtering is incorpowww.siliconchip.com.au rated using the 100kΩ and 1MΩ resistors, the 56nF and 1µF capacitors and the parallel combination of the 300kΩ and 150kΩ resistors. These together provide the 2.1Hz rolloff frequency and a Q (quality factor) of 0.62. The rectified signal is then applied to the input (pin 5) of IC3, the LM3914. Trimpot VR2 is connected between the REFADJ pin (pin 8) and a 220kΩ resistor to ground and provides a DC reference voltage to pins 3 & 5 of IC2b. This is adjusted to 3.43V when there is no signal from the microphone and this will light LED1 on the display. With sufficient signal from the microphone, level control VR1 is then adjusted to light LEDs 5 & 6, indicating a level of 0dB. Varying the signal from this level will range the display from +6dB to -13dB. LED1 only shows that the signal is below -13dB. A 9V battery supplies the circuit via a 1N5819 Schottky diode (D3) to provide reverse polarity protection while minimising the voltage drop across the diode; this allows more life from the battery. The 470µF capacitor decouples the supply to the LEDs, while a 27Ω resistor and 100µF capacitor further decouple the supply for IC1, IC2 and IC3. The 16V zener diode (ZD1) allows the circuit to be powered from a 12V car battery instead of a 9V battery. The circuit could also be run from a 9V DC plugpack although this would limit its portability while doing tests. Construction All the parts for the Loudspeaker Level Meter fit on a PC board coded 01104041 and measuring 123 x 59mm. It is housed in a plastic case measuring April 2004  25 Fig.4: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly orientated. Potentiometer VR1 is secured by soldering its metal body and terminals to adjacent PC stakes (see text). 130 x 68 x 43mm. You can begin the assembly by checking the PC board for any shorted tracks or breaks in the copper pattern. Also check that the hole sizes are correct for the switch and PC stakes. You will need 2mm holes for the switch and 1mm holes for the PC stakes. The corners of the PC board need to be shaped so that the board will clear the corner pillars of the box. Start with the low profile components such as the ICs, links and the resistors. Make sure that you place the TL071 in the IC1 position and the TL072 in the IC2 position – swapping them won’t work at all! The resistors can be selected by using a multimeter to verify their values. Alternatively, use the colour code table to select the values. Trimpot VR2 and capacitors can be installed next, taking care to place the polarised electrolytics with the correct polarity. The NP (non-polarised) capacitor can be installed either way. Then install the PC stakes and the switch (S1). The shaft of the potentiometer (VR1) may need to be cut to length to suit the knob. VR1 is mounted about 3mm off the PC board and soldered to the four PC stakes which surround the pot body. Scrape the passivation coating from the pot body at the PC stake positions before soldering it in position. The three terminals are soldered to three adjacent PC stakes. Drilling the case The PC board assembly is secured to the back of the front panel by doing up the switch and pot nuts. A metal clamp is used to secure the battery. 26  Silicon Chip The lid of the box should now be drilled for the 10 5mm LEDs, the switch and pot. You can use the label artwork in this article as a drilling template. That done, place the LEDs into their holes on the PC board, ensuring the polarity is correct. Fit the lid of the box over the switch and pot and fit their nuts. That done, push each LED into its front panel hole and solder each one so it protrudes from the lid by about 1mm. The battery is fitted into a U-shaped battery clip which is secured with an M3 x 6mm screw and nut – see the photo for the positioning and orienwww.siliconchip.com.au Table 2: Capacitor Codes Value μF Code EIA Code IEC Code 100nF 0.1µF 104 100n 56nF 0.56µF 563   56n 100pF 101 100p 10pF   10   10p tation of the battery clip. A tip for mounting the clip: place the nut over the hole on the inside of the clip and then push the base of the battery into the clip to hold the nut; then the clip can be easily fastened to the inside of the box with the screw. Next, drill a hole in the base of the case for the electret microphone insert – make it a tight fit. Then wire up the microphone using a short length of shielded cable. Finally, solder the battery clip leads to the underside of the PC board at the power supply PC stake terminals. Fig.5: check your board for defects by comparing it with this full-size etching pattern before installing any of the parts. Testing Carefully check all your work, then switch on and check that the LED display works. You may need to adjust VR2 so that the lefthand LED lights with no noise applied to the microphone. If nothing happens, check the voltages. There should be about 8V between pins 4 & 7 of IC1, between pins 4 & 8 of IC2 and between pins 2 & 3 of IC3. Check that the display LEDs light up when you whistle or make a noise. Adjust VR1 and check that the sensitivity increases when it is turned clockwise. In use, you will need a noise signal Fig.6: this full-size artwork can be used as a drilling template for the front panel, if necessary. to allow setting up the speaker levels. If you are simply setting up a stereo system or measuring sound levels in a PA system, you can use a pink noise source. We published a suitable pink noise source in the January 1997 issue of SILICON CHIP. Alternatively, you can use inter-station noise from an FM tuner (ie, set it to a frequency where SC there is no signal). Table 1: Resistor Colour Codes o o o o o o o o o o o o o o No.   1   1   1   1   1   2   3   1   1   1   1   1   1 www.siliconchip.com.au Value 1MΩ 330kΩ 300kΩ 220kΩ 150kΩ 100kΩ 22kΩ 10kΩ 4.7kΩ 3.3kΩ 1.2kΩ 510Ω 27Ω 4-Band Code (1%) brown black green brown orange orange yellow brown orange black yellow brown red red yellow brown brown green yellow brown brown black yellow brown red red orange brown brown black orange brown yellow violet red brown orange orange red brown brown red red brown green brown brown brown red violet black brown 5-Band Code (1%) brown black black yellow brown orange orange black orange brown orange black black orange brown red red black orange brown brown green black orange brown brown black black orange brown red red black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown brown red black brown brown green brown black black brown red violet black gold brown April 2004  27