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You can use this Pink Noise Source as an aid to cali­ brating the Sound Level Meter described last month. It can also be used as a general purpose signal for setting the balance between loudspeakers in a multi­channel (2, 4 or more channels) system and for PA adjustments. By JOHN CLARKE BUILD THIS While noise is usually considered a nuisance, it can be useful in some cases. In audio applications it provides us with a signal which covers the entire audible spectrum. This means that there is every conceivable frequency from 20Hz up to 20kHz, all in the one signal. Armed with this type of signal we can obtain frequency response measurements and a wideband sound level output for loudspeakers. Also it provides a standard sound for subjective listening tests. With an analyser and equaliser we can also adjust the frequency levels from a loudspeaker in a particular room so that it provides a flat response across the audible spectrum. All of these measurements assume that the noise source has a flat frequency response or an equal energy per octave. This is called “pink” noise. The energy from 20Hz to 40Hz must be the same as that from 10kHz to 20kHz even though there is Pink Noise Source For sound level meter calibration & signal balancing Silicon Chip’s Electronics TestBench  91 AUDIO PRECISION SCNOISE AMPL(dBr) vs BPBR(Hz) 20.000 29 AUG 96 14:15:39 • • • • 15.000 10.000 Main Features Pink noise signal output Battery operated 0dB and -60dB levels Power-on LED 5.0000 0.0 -5.000 -10.00 -15.00 -20.00 20 100 1k 10k 20k Fig.1: the spectrum (signal output versus frequency) of the Pink Noise Source. Since the noise source is random, a second response test would no doubt reveal a slightly different result, with perhaps dips in response where slight peaks are shown and vice versa. only a 20Hz difference in frequency for the lowest octave and a 10kHz range for the upper octave. Fig.1 shows the spectrum (ie, signal output versus frequency) of the Pink Noise Source featured in this article. By contrast, the noise from electronic circuits is “white”. It has a 3dB rise in output per octave of frequency since it has equal energy per constant bandwidth. So the octave band from 20Hz to 10.02kHz will have the same energy level as the octave between 10kHz and 20kHz. Rose-coloured filter To convert white noise to pink noise we need a filter which has a 3dB/octave or 10dB/decade rolloff. This is a little tricky since a normal single pole low pass filter will roll off at 6dB/octave (or 20dB per decade). A “pink” filter is achieved by rolling the signal off in four discrete steps, Fig.2: the pink noise circuit uses a transistor noise source, two op amps for amplification and some passive filtering. 92 Silicon Chip’s Electronics TestBench introducing fur­ ther filtering as the frequency rises. Fig.2 shows the pink noise circuit. It uses a transistor noise source, two op amps for amplification and some passive filtering. An NPN transistor, Q1, is connected for reverse breakdown between the emitter and base, with current limiting provided by the 180kΩ resistor from base to ground. This provides a good white noise source but it only produces a low signal level. Op amp IC1a amplifies this noise by a factor of 101. IC1a is AC-coupled and biased to the 4.5V half supply rail to provide a symmetrical swing at its output, pin 1. The 0.27µF input ca­pacitor and bias resistor roll off the response below 0.6Hz. Similarly, the 2.2kΩ resistor and 100µF capacitor in the feedback path at pin 2 roll off response below 0.7Hz. High frequency rolloff above 153kHz is provided by the 4.7pF capacitor across the 220kΩ resistor. Following pin 1 of IC1a is a passive RC filter to roll off the frequency response at 3dB per octave. This filter 220k Fig.3 (left): the component layout and wiring details. Note that the two switches are mounted on PC stakes and be sure to mount all polarised components with the correct orientation. Capacitor Codes ❏ ❏ ❏ ❏ ❏ ❏ Fig.4: check your etched PC board against this full-size artwork before installing any of the parts. Performance Output levels ..................................60mV RMS at 0dB; 60µV at -60dB Maximum output load .....................1kΩ (for <1dB error in 60dB attenuator) Frequency spectrum ......................<0.25dB 20Hz to 20kHz (see Fig.1) Power supply ..................................7.6 to 9V at 7mA Value 0.27µF .047µF .033µF 10pF 4.7pF IEC 270n 47n 33n 10p 4p7 EIA 274 473 333 10 4.7 is accurate to ±0.25dB from 10Hz to 40kHz, assuming the use of close tolerance capacitors. The spectrum response shown in Fig.1 is that of the prototype using normal 10% tolerance capacitors. Note that the signal levels shown in Fig.1 are the actual levels at the instant the measurement was taken. Since the noise source is random, a second response test would no doubt reveal a slight­ly different result, with perhaps dips in response where slight peaks are shown and vice versa. The pink noise output is AC-coupled into op amp IC1b which has a gain of 46. This has a low and high frequency response rolloff similar to IC1a. IC1b’s output is AC-coupled to switch S2. Note that a non-polarised Resistor Colour Codes ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 2 2 1 2 2 1 1 3 1 1 1 Value 1MΩ 220kΩ 180kΩ 100kΩ 10kΩ 6.8kΩ 3kΩ 2.2kΩ 1kΩ 300Ω 100Ω 4-Band Code (1%) brown black green brown red red yellow brown brown grey yellow brown brown black yellow brown brown black orange brown blue grey red brown orange black red brown red red red brown brown black red brown orange black brown brown brown black brown brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown grey black orange brown brown black black orange brown brown black black red brown blue grey black brown brown orange black black brown brown red red black brown brown brown black black brown brown orange black black black brown brown black black black brown Silicon Chip’s Electronics TestBench  93 NOISE OUT 0dB + -60dB OFF + + ON PINK NOISE SOURCE Fig.5: this is an actual size artwork for the front panel. The construction is easy since all parts except for the RCA output socket are mounted on the PC board. (NP) capacitor is specified. This is because the noise source is designed to connect to the Sound Level Meter which would reverse polarise a normal electrolytic type. Switch S2 selects the full output (0dB) or a divide by 1000 using the 100kΩ and 100Ω resistors for a -60dB output. The 4.5V half supply is derived from a 10kΩ resistive divider which 94 is decoupled using a 100µF capacitor. The power LED is driven via a 2.2kΩ resistor while the whole supply is decou­pled using a 100µF capacitor. Construction The Pink Noise Source is housed in a plastic case measuring 130 x 67 x 41mm. The circuitry fits onto a PC board coded 04312962 and measuring Silicon Chip’s Electronics TestBench 104 x 60mm. The wiring details are shown in Fig.3. Begin construction by checking the PC board for defects. This done, install the resistors and install PC stakes at the switch positions. The PC stakes are required to allow the switches to be mounted above the PC board. The capacitors can be mounted next, while ensuring correct orientation of the electrolytics. The 10µF NP capacitor can be mounted either way around. LED1 is mounted with its leads at full length, so that it can protrude through the front panel lid. Splay the leads slightly to give the LED some vertical adjust­ment, without one lead shorting to the other. Next, insert transistor Q1 and IC1. Attach the battery holder using small self-tapping screws from the underside of the PC board. The toggle switches can be soldered in place on top of the PC stakes. Attach the Dynamark adhesive label on the lid of the case and drill out the holes for the switches, LED bezel and PARTS LIST 1 plastic case, 130 x 67 x 41mm 1 PC board, code 04312962, 104 x 60mm 1 self-adhesive label, 61 x 123mm 2 SPDT toggle switches (S1,S2) 1 panel mount RCA socket 1 9V battery holder 1 9V battery 1 3mm LED bezel 8 PC stakes 3 small self-tappers for the battery holder Semiconductors 1 TL072 dual op amp (IC1) 1 BC548 PNP transistor (Q1) 1 3mm red LED (LED1) Capacitors 4 100µF 16VW PC electrolytic 1 10µF NP PC electrolytic 1 1µF 16VW PC electrolytic 3 0.27µF MKT polyester 2 .047µF MKT polyester 1 .033µF MKT polyester 1 10pF ceramic 1 4.7pF ceramic Resistors (0.25W 1%) 2 1MΩ 1 3kΩ 2 220kΩ 3 2.2kΩ 1 180kΩ 1 1kΩ 2 100kΩ 1 300Ω 2 10kΩ 1 100Ω 1 6.8kΩ corner mounting locations. Also drill a hole in the end of the case for the RCA socket. Attach the socket and clip the PC board in place against the integral side pillars of the box. Wire up the RCA socket as shown in Fig.3. Finally, insert the battery and attach the lid with the LED bezel in place. Take care to ensure that the LED protrudes through the bezel before tightening the case screws. Testing You can test the unit by connecting the output to an amplifier and speaker. Apply power and listen to the noise which should occur after several seconds. Alternatively, look at the signal on an oscillo­ scope. A multimeter should give an AC reading of around 60mV on the 0dB range and 0V on the SC -60dB position of S2. Silicon Chip’s Electronics TestBench  95