A simple audio frequency white noise generator

[AB2EZ]

I posted the schematic of a simple, battery-operated, easy-to- build, low-cost, audio frequency white noise generator on my web site. I use one of these to test the frequency responses of various pieces of audio equipment, my overall audio chain, and the end-to-end frequency response of my transmitter: between the input to my mixer and the output of my off-air monitor.

http://mysite.verizon.net/sdp2/id15.html

Here is the text from my web site:


Below is the schematic of a simple, audio frequency white noise generator. This simple piece of test equipment is very useful for measuring the frequency responses of audio systems.
 
It uses a Zener diode as a white noise source. The Zener diode is biased into breakdown to produce approximately 0.1 ma of average current through the 47kohm resistor. More current is ok... if you wish to use a higher voltage to bias the Zener diode. In this case, two 9-volt batteries are used as a bias voltage source for a 12 volt Zener diode.
 
The current flowing through the 47kohm resistor is (as desired) very noisy, because the Zener diode's avalanche breakdown process greatly increases the shot noise associated with the average current.
 
The op-amp acts as an impedance converter (~ 50kohm input impedance, and ~ 10 ohm output impedance) and also provides a voltage gain of 23 dB.
 
The 100pF capacitor limits the bandwidth of the white noise to an upper frequency of around 50 kHz. (taking into account the input inpedance of the op-amp). The 0.22 uF coupling capacitor (for DC blocking) limits the lower frequency of the white noise to around 15 Hz. The 16 uF output coupling capacitor limits the lower frequency of the white noise to around 100Hz (with the 100 ohm output load shown in the schematic). A larger coupling capacitor (~80 uF) would be useful for producing a lower frequency (~20Hz) cutoff of the white noise. To drive an 8 ohm load, an even larger output coupling capacitor (~1000 uF) would be needed to produce a low frequency cutoff of the white noise of  20 Hz. .
 
The output of the op-amp can drive the line level input of a mixer. 

Best regards
Stu

[K3ZS]

Stu,

I need just the opposite.   My 1967 vintager all solid state stereo amplifier has become a white noise generator in one channel.  It puts out white noise after being on for a few minutes.   Are there any components prone to do this after aging, or maybe some bias settings in the power amp are messed up.   It's in the PA part, not any of the preamplifiers.

[wavebourn]

You may use  a small signal transistor, Base - Emitter junction breaks down on a voltage about 4V and generates nice white noise.

[W1RKW]

Stu,

I need just the opposite.   My 1967 vintager all solid state stereo amplifier has become a white noise generator in one channel.  It puts out white noise after being on for a few minutes.   Are there any components prone to do this after aging, or maybe some bias settings in the power amp are messed up.   It's in the PA part, not any of the preamplifiers.

Freeze spray components, lightly. Try not to overspray adjacent componets. You should be able to isolate the component.  Probably a semiconductor or a cap. Focus on the early stages of the PA section like the differential amp and stages up to the drivers and final.  Is there any sign of DC showing up on the output?

[K3ZS]

Thanks for the info.   I'll check the DC on output.   It is capacitor coupled, not sure if its single voltage or zero DC before th output caps.  Got to get some freeze spray.  It makes the noise after warmup, so the freeze spray will work.  Its an old Lafayette FM stereo.    Years ago I built a new phase-locked loop decoder to replace the original circuit.    Its my first stereo I ever had so I will put in some time on it.   Sort of like the first ham rig.

[Bacon, WA3WDR]

I once saw solid tantalum caps create crackly noise like a dirty pot because they had been designed in reverse-polarized.  It wasn't like white noise, though.

[W1RKW]

Thanks for the info.   I'll check the DC on output.   It is capacitor coupled, not sure if its single voltage or zero DC before th output caps.  Got to get some freeze spray.  It makes the noise after warmup, so the freeze spray will work.  Its an old Lafayette FM stereo.    Years ago I built a new phase-locked loop decoder to replace the original circuit.    Its my first stereo I ever had so I will put in some time on it.   Sort of like the first ham rig.

Bob,
I'd be inclined to think you have a noisy transistor though a cap certainly can give you what you're getting.  I know on some of those older receivers the power amp section used a varistor or a dual or triple diode pack attached to the heatsink for output transistor bias control.  The diode pack usually was a culprit too for noise generation.  The Lafayette being as old as it is more than likely has some leaky electrolytics.

In addition to freeze spray you may want to rig up a small light bulb with a shroud around it to be used as a pin-point heat source so you can slip it over a suspect component.

GL
Bob

[WBear2GCR]

Stu,

I don't know why you'd need or want a white noise generator to test the freq response of audio gear today?
How do you get from white noise to frequency response?


The way to go is to download one or more of the completely wonderful and free FFT based audio testing softwares and get your results that way using a computer's sound card.

With a modern >1gHz clock speed computer, try Speakerworkshop.

       _-_-WBear2GCR

[AB2EZ]

Bear

Good comments!

1. I use my computer (soundcard + a freeware application called Spectrum Laboratory) as an audio spectrum analyzer. I launch white noise into the input of the system under test... and I observe the system's frequency response via the shape of the spectrum of the signal produced at the system's output.

2. Yes... I do use PC-hosted software applications to produce audio test waveforms for various types of measurements; including various applications which can synthesize white noise.

However, I find this little hardware-based white noise generator convenient in situations where I don't want to coordinate between the multiple applications (test signal generators, spectrum analyzer; volume level meter) that share the two sound cards I have on my radio room computer, or when the system I'm testing in on the kitchen table.

Best regards
Stu

[wavebourn]

If it is for an audio frequency response equalization,

I don't use any noise generator. I have a Behringer 31-band graphics EQ with FBQ technology. It is the simple to use and genuine technology: each filter has own indicator (a LED on the slider), so I put my mic in front of speakers then increase gain until it starts oscillating, then I pull down the slider with the brightest LED, and so on, untill the frequency characteristic of the overall channel, including the room, is flat.