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Personal Noise for Tinnitus This Personal Noise Source produces pink or white noise and can be used to drive headphones or an external speaker to block out unwanted noise. It has been specifically designed for Tinnitus sufferers but can be used by anyone who wants to mask unwanted sound. W low level broad band noise. TRT does frogs in a fishpond, right outside his hy would you want a Pernot cure Tinnitus but it does make bedroom window, who play “who sonal Noise Source? Isn’t it manageable for people who are can make the loudest mating calls” there already enough noise severely affected. all night (they’re silent during the day, in this world? of course!). Now frogs are a protected Well, believe us, if you suffer from Want more information on Tinnispecies and our bloke doesn’t want the Tinnitus, this Personal Noise Genertus? There is not much more to tell wrath of the Government Inspector of ator is virtually the only treatment although there are lots of websites Frogs coming down on him. Switch on available. It is cheap and you can do devoted to it. Just call up your favourthe PNS and – ahh, bliss: sleep at last! it yourself. ite search engine (Yahoo, Google etc), type in Tinnitus and you will find lots When we said the PNS blocks noise, If you have never suffered Tinnitus, of references. that’s not strictly true. It doesn’t really congratulations; you are fortunate. block it out: it more “masks” it by You don’t know what it’s like. Tinnitus OK, that’s enough about Tinnitus increasing the ambient level so that is the perception of sound when no for the moment. Even if you don’t the unwanted noise is much less obexternal sound is present. Commonly suffer from this affliction, there are trusive. The “noise” from the PNS is referred to as “ringing in the ears”, times when a low-level noise source something you can live with – in fact, Tinnitus may sound like humming, can be really helpful in blocking out it is often quite soothing. It has been clicking, buzzing, ringing, hissing, extraneous noise. likened to what you hear from roaring, whistling or crickets. One of For example, if you are trying to a soft waterfall or a stream the staff members of SILICON cascading down rocks. CHIP occasionally experiences it and he reports that The Personal Noise Source it sounds like a tone at about is built into a small plastic or loudspeaker case and as mentioned e on ph ad he 400Hz. Fortunately, in his r fo e bl ita ❍ Su case it rarely lasts for more above, can be connected to noise output than a few minutes. headphones or to a small ❍ Pink or white ed at er loudspeaker. It can be powTinnitus may be intermitop ck pa ❍ Battery or plug ered from a DC plugpack or tent or constant and may vary e control 9V battery. It includes a volin loudness depending on ❍ Inbuilt volum ume control and can prostress, medications and the vide pink or white noise. surrounding environment. What’s the difference? Most people who experisleep and a nearby neighbour is havence Tinnitus are not really “White” and “pink” noise ing a boisterous pool party: switch on bothered by it. But some people find this Personal Noise Source (PNS) and that it seriously disturbs their sleep White noise has equal energy per you can blank it all out. Or maybe you while others find it really debilitating. constant bandwidth. So the 1kHz are trying to study and someone else band from 1kHz to 2kHz will have the For those people who are seriously in the family persists in listening to a same energy level as the 1kHz band affected, Tinnitus Retraining TherMarilyn Manson CD; again, switch on from 10kHz to 11kHz. In practice, this apy (TRT) can provide an effective your PNS and blot it out of existence. means that white noise has a 3dB rise treatment. Developed by Dr Jawel Another of our staff members has in amplitude for every octave. Jastreboff, TRT involves the use of atures Personal Noise Source Fe 62  Silicon Chip www.siliconchip.com.au se Source SUfferers By JOHN CLARKE Pink noise has a flat frequency response or equal energy for each octave; the energy from 20Hz to 40Hz is the same as the energy from 10kHz to 20kHz. In effect, this means that pink noise sounds more subdued and less harsh than white noise. Putting it another way, pink noise has more bass and less treble than white noise. Pink noise is also used for measuring loudspeaker systems so even if you don’t need this PNS for blanking out unwanted noise it could be handy if you are involved in developing loudspeakers. Circuit description Fig.1 shows the circuit diagram. It comprises a white noise source (Q1), www.siliconchip.com.au amplifier (IC1a), pink (low pass) filter and further amplification (IC1b), followed by the volume control (VR1) and power amplifier (IC2). Transistor Q1 is the noise source. Its base-emitter junction is connected the “wrong way around” so that reverse current flows. Normally this could lead to breakdown of the transistor but the 180kΩ series resistor limits the breakdown current to about 30µA so no damage occurs. Connected this way, Q1 functions like a zener diode and produces a noise signal across the 180kΩ current limiting resistor. The supply to Q1 is decoupled with a 470Ω resistor and 1000µF capacitor and a 12V zener diode regulates the voltage so that the noise level is constant regardless of changes in the supply voltage. The noise signal is coupled to pin 3 of op amp IC1a via a 0.1µF capacitor. IC1a is set to provide a gain of 11 by virtue of the 100kΩ resistor between pins 1 and 2 and by the 10kΩ resistor in series with the 1µF capacitor. The 1µF capacitor rolls off frequencies below 16Hz while the 10pF capacitor across the 100kΩ feedback resistor rolls of frequencies above 160kHz. Pink noise filter The output of IC1a drives a fairly complex RC network which functions as the pink noise filter. It attenuates the September 2001  63 white noise at a rate of 3dB per octave. This filter is accurate to ±0.25dB from 10Hz to 40kHz, assuming close tolerance capacitors. Switch S2 enables the pink noise filtering to be disabled to let the white noise through without attenuation. Depending on how S2 is set, the pink or white noise is AC-coupled to pin 5 of op amp IC1b via a 0.1µF capacitor. When S2 is closed, the 220kΩ and 10kΩ feedback resistors for IC1b set the gain at 23. Low frequency rolloff is set at 16Hz with the 1µF capacitor. The 4.7pF capacitor across the 220kΩ resistor gives high frequency rolloff above 153kHz. But not only does switch S2 determine whether or not the pink noise filter is enabled, it also changes the gain of the following op amp stage involving IC1b. When S2 is open, the negative side of the 1µF capacitor associated with a 10kΩ resistor is disconnected from ground and is effectively connected to the pin 1 output of IC1a via the pink noise filter components. This means that the output signal from IC1a is fed to both the inverting and non-inverting inputs of IC1b. The gain for the non-inverting input is 23, as noted previously, while the gain for the inverting input is -22. Adding these two gains together gives a gain of 1. This means that the gain for the white noise signal is unity while the gain for pink noise is 23. This higher gain for pink noise compensates for the signal loss in the pink noise filter. Fig.2 shows the pink and white noise frequency response for the circuit. The output of IC1b is AC-coupled via a 10µF capacitor to the volume control potentiometer VR1 and then AC-coupled again to anLM386 power amplifier, IC2. Its gain is set to 200 by the 22µF capacitor between pins 1 and 8. The amplifier drives the external speaker or headphone load via a 470µF capacitor and a 4.7Ω resistor. There is also a Zobel network, comprising a .047µF capacitor and a 10Ω resistor, which is included to ensure high frequency stability. The speaker output is connected Fig.1 (left): the reverse-biased baseemitter junction of Q1 generates the noise in this circuit. It is amplified by IC1a and IC1b and the RC network following IC1a is the pink noise filter. 64  Silicon Chip www.siliconchip.com.au AUDIO PRECISION AMPLNOIS BANDPASS(dBr) vs BPBR(Hz) 20.000 06 MAY 100 04:02:31 15.000 10.000 5.0000 Fig.2: the pink noise output has a flat frequency response while the white noise shows a rising response with frequency. 0.0 -5.000 -10.00 -15.00 -20.00 T TTT 20 TT 100 1k via a 6.5mm stereo jack so that it can drive stereo headphones (with both channels commoned to provide mono mode) or a mono amplifier. The 4.7Ω resistor is series with the jack socket is included to prevent damage to the LM386 which could otherwise occur if a mono jack is inserted into the stereo output socket. Power for the circuit is derived from a DC plugpack or 9V battery. Reverse polarity protection is provided using diode D1 which prevents reverse current into IC1 and IC2. However, the supply for Q1 is taken before the diode to allow the maximum voltage from the battery. This is important since Q1 breaks down at around 7V or so. Once the battery drops below 7V, Q1 will be no longer produce any noise and the battery will need to be replaced. Reverse polarity protection is not strictly necessary for Q1 since it would be biased in the forward direction and the zener diode, ZD1 will conduct in the reverse direction and prevent the 10k 20k voltage exceeding 0.6V. The half-supply voltage for the op amps in IC1 is set using two series connected 10kΩ resistors across the Q1 supply and is decoupled with a 100µF capacitor. The power LED is driven via a 2.2kΩ resistor while the whole supply is decoupled using a 470µF capacitor. The DC socket connects the negative terminal of the 9V battery to ground of the circuit via an internal switch contact. The contact is opened if a DC plug is inserted, thus isolating the battery from the 12V plugpack supply. Construction All the parts of the Personal Noise Source are assembled onto a PC board measuring 60 x 70mm and coded 01109011. This is housed in a plastic case measuring 130 x 68 x 41mm. Fig.3 shows the PC board overlay and all the ex- ternal wiring. Begin construction by checking the PC board for shorts between tracks or any breaks in the copper tracks. The corners of the PC board should be cut out so as to clear the pillars within the box. You can begin assembly by inserting the PC stakes followed by the links and resistors. The resistor colour codes are shown in Table 1. It is a good idea to use a digital multimeter to check each resistor value as you install it. Next, insert and solder in the diode and zener diode, making sure that they are oriented correctly. Then insert and solder IC1 and IC2. Table 2 shows the codes you may need when installing the capacitors. The electrolytic types must be oriented correctly with the positive side placed as shown on the overlay diagram. Note that one of the 1µF electrolytics is positioned on its side as shown in the photograph. This is to allow the 9V battery to fit over this area of the PC board. Transistor Q1 is inserted next, along with the DC socket, the 6.35mm jack socket and pot VR1. The pot can be mounted onto the PC stakes on the PC board if it is a long shaft type. Scrape the coating off the pot body where it will be soldered to the two ground PC stakes. If you are using a pot with a short fluted shaft, mount it directly on the box lid and make the connections to the PC board with hookup wire. LED 1 needs to be mounted with its top 29mm above the PC board. Drill holes in the end of the case for the DC power socket and 6.35mm jack Fig.3: a stereo output jack is used to allow connection of stereo headphones. If you are using a speaker, you will need to connect it with a stereo jack plug. Do not use a mono jack otherwise it will short the output. www.siliconchip.com.au September 2001  65 Parts list – Personal Noise Source 1 PC board coded 01109011, 60 x 70mm 1 plastic box, 130 x 68 x 41mm 1 front panel label, 125 x 63mm 2 SPST mini rocker switches (S1,S2; Altronics S-3202) 1 PC-mount DC panel socket with 2.5mm pin 1 10kΩ log pot (VR1) 1 PC-mount 6.35mm stereo jack socket 1 stereo 6.35mm jack plug 1 9V battery clip holder (Altronics S-5050) 1 9V battery snap 1 knob to suit VR1 11 PC stakes 1 M3 x 6mm screw and nut 1 50mm length of 0.8mm tinned copper wire 1 100mm length of light gauge figure-8 wire Semiconductors 1 TL072 dual op amp (IC1) 1 LM386N-1 amplifier (IC2) 1 BC548 NPN transistor (Q1) 1 12V 1W zener diode (ZD1) 1 1N4004 1A diode (D1) 1 5mm red LED (LED1) Capacitors 1 1000µF 16VW PC electrolytic 2 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 3 1µF 16VW PC electrolytic 1 0.27µF MKT polyester 4 0.1µF MKT polyester 3 .047µF MKT polyester 1 .033µF MKT polyester 1 10pF ceramic 1 4.7pF ceramic Resistors (0.25W, 1%) 2 1MΩ 1 220kΩ 1 180kΩ 1 100kΩ 4 10kΩ 1 6.8kΩ 1 3kΩ 1 2.2kΩ 1 1kΩ 1 470Ω 1 300Ω 1 10Ω 1 4.7Ω Table 2: CAPACITOR CODES       Value 0.27uF    0.1uF    10pF    4.7pF IEC code 270n 100n 10p 4p7 66  Silicon Chip EIA code 274 104 10 4.7 This “opened-out” photo of the project shows the PC board and its connections to the switches on the front panel. The pot, DC and output sockets are all PC board mounting. Inset is the 9V battery holder – note the nut soldered in place. Table 1: RESISTOR COLOUR CODES    No.  2  1  1  1  4  1  1  1  1  1  1  1  1 Value 1MΩ 220kΩ 180kΩ 100kΩ 10kΩ 6.8kΩ 3kΩ 2.2kΩ 1kΩ 470Ω 300Ω 10Ω 4.7Ω 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 yellow violet brown brown orange black brown brown brown black black brown yellow violet gold 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 yellow violet black black brown orange black black black brown brown black black gold brown yellow violet black silver brown www.siliconchip.com.au socket and on the side for the battery clip screw. One of the integral PC board slots will need to be removed to allow the battery clip to sit flush with the inside of the box. Use the front panel artwork as a guide to drilling the holes for the switches, LED and pot shaft. The cutouts for the switches are drilled and then filed to shape. Attach the front panel label and cut out the holes in this with a sharp utility knife. The PC board is inserted into the case with the ZD1 end going in first. The jack socket is then slid along to protrude through the end hole and is secured with its nut. Washers will be required on the jack socket inside the case. Attach the switches to the case lid and wire them as per the wiring diagram of Fig.3. Solder the battery clip leads in place and attach the battery clip holder. We found that it is easier to first solder the nut to the inside of this clip before attempting to mount it with a screw. Here’s how the whole lot goes together. The PC board snaps into place on the side guides – there are no screws to hold it in. Testing You can apply power to the circuit using a 12V DC plugpack, power supply set at 12V or with a 9V battery. LED 1 should light when powered. Check that the voltage at pins 1, 3, 5 & 7 of IC1 is at half-supply. The base- emitter voltage for Q1 should be around 7V. If it is equal to the supply voltage, check that the transistor is soldered in correctly. Note that some transistors break down above 7V and you may need to select a BC548 which has the lowest voltage across it if the PNS is to be battery-powered. The voltage at pins 3 & 5 of IC2 should be at nominal half-supply. Connect a set of headphones or loudspeaker via the jack socket and check that there is noise available both for the white and pink settings of S2. Current consumption at normal listening levels is around 25mA when driving a 4Ω speaker. This drops below 15mA with higher impedance headphones attached to the socket. This is OK for battery use but if you want to use it for long periods with an external speaker, a DC plugpack is the only practical approach. Remember that the loudspeaker must be connected via a stereo jack. If you use a mono jack, the output will SC be shorted. www.siliconchip.com.au Fig.4: actual size artwork for the PC board. Fig.5: actual size artwork for the front panel. September 2001  67