Personal Noise Source Features Suitable for headphone or loudspeaker Pink or white noise output Battery or plugpack operated Inbuilt volume control

Why would you want a Personal Noise Source? Isn’t there already enough noise in this world?

Well, believe us, if you suffer from Tinnitus, this Personal Noise Generator is virtually the only treatment available. It is cheap and you can do it yourself.

If you have never suffered Tinnitus, congratulations; you are fortunate. You don’t know what it’s like. Tinnitus is the perception of sound when no external sound is present. Commonly referred to as "ringing in the ears", Tinnitus may sound like humming, clicking, buzzing, ringing, hissing, roaring, whistling or crickets. One of the staff members of SILICON CHIP occasionally experiences it and he reports that it sounds like a tone at about 400Hz. Fortunately, in his case it rarely lasts for more than a few minutes.

Tinnitus may be intermittent or constant and may vary in loudness depending on stress, medications and the surrounding environment.

Most people who experience Tinnitus are not really bothered by it. But some people find that it seriously disturbs their sleep while others find it really debilitating.

For those people who are seriously affected, Tinnitus Retraining Therapy (TRT) can provide an effective treatment. Developed by Dr Jawel Jastreboff, TRT involves the use of low level broad band noise. TRT does not cure Tinnitus but it does make it manageable for people who are severely affected.

Want more information on Tinnitus? There is not much more to tell although there are lots of websites devoted to it. Just call up your favourite search engine (Yahoo, Google etc), type in Tinnitus and you will find lots of references.

OK, that’s enough about Tinnitus for the moment. Even if you don’t suffer from this affliction, there are times when a low-level noise source can be really helpful in blocking out extraneous noise.

For example, if you are trying to sleep and a nearby neighbour is having a boisterous pool party: switch on this Personal Noise Source (PNS) and you can blank it all out. Or maybe you are trying to study and someone else in the family persists in listening to a Marilyn Manson CD; again, switch on your PNS and blot it out of existence.

Another of our staff members has frogs in a fishpond, right outside his bedroom window, who play "who can make the loudest mating calls" all night (they’re silent during the day, of course!). Now frogs are a protected species and our bloke doesn’t want the wrath of the Government Inspector of Frogs coming down on him. Switch on the PNS and – ahh, bliss: sleep at last!

When we said the PNS blocks noise, that’s not strictly true. It doesn’t really block it out: it more "masks" it by increasing the ambient level so that the unwanted noise is much less obtrusive. The "noise" from the PNS is something you can live with – in fact, it is often quite soothing. It has been likened to what you hear from a soft waterfall or a stream cascading down rocks.

The Personal Noise Source is built into a small plastic case and as mentioned above, can be connected to headphones or to a small loudspeaker. It can be powered from a DC plugpack or 9V battery. It includes a volume control and can provide pink or white noise. What’s the difference?

Table 1: Resistor Colour Codes No. Value 4-Band Code (1%) 5-Band Code (1%) 2 1MΩ brown black green brown brown black black yellow brown 1 220kΩ red red yellow brown red red black orange brown 1 180kΩ brown grey yellow brown brown grey black orange brown 1 100kΩ brown black yellow brown brown black black orange brown 4 10kΩ brown black orange brown brown black black red brown 1 6.8kΩ blue grey red brown blue grey black brown brown 1 3kΩ orange black red brown orange black black brown brown 1 2.2kΩ red red red brown red red black brown brown 1 1kΩ brown black red brown brown black black brown brown 1 470Ω yellow violet brown brown yellow violet black black brown 1 300Ω orange black brown brown orange black black black brown 1 10Ω brown black black brown brown black black gold brown 1 4.7Ω yellow violet gold brown yellow violet black silver brown

"White" and "pink" noise

White noise has equal energy per constant bandwidth. So the 1kHz band from 1kHz to 2kHz will have the same energy level as the 1kHz band from 10kHz to 11kHz. In practice, this means that white noise has a 3dB rise in amplitude for every octave.

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), 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 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 220kW 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 10W resistor, which is included to ensure high frequency stability.

The speaker output is connected 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 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.

This "opened-out" photo of the project (left) 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. Below is the 9V battery holder - note the nut soldered in place.

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 external 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.

Table 2: Capacitor Codes Value IEC code EIA code 0.27uF 270n 274 0.1uF 100n 104 10pF 10p 10 4.7pF 4p7 4.7

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 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.

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 periodswith 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 be shorted.

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 180kW 1 100kΩ 4 10kΩ 1 6.8kΩ 1 3kΩ 1 2.2kΩ 1 1kΩ 1 470Ω 1 300Ω 1 10Ω 1 4.7Ω