Noise generator

The noise generator of my board tends to be contaminated by some 100Hz hum. As a matter of fact this hum is hardly earable when listening to the raw white noise signal but becomes more perceptible as soon as the noise signal is injected in a VCF. When checking the spectrum of the noise signal with a software spectrum analyzer, one can see that 100Hz, 200Hz, 300Hz, 400Hz and 500Hz overtones are superimposed to the rather flat spectrum of white noise. These overtones correspond to residual ripples from the +12V rail. Because the noise generator uses a very high gain stage (Q2) these residual ripples are strongly amplified. In order to cancel this overtones, it is necessary to insert a filter in the power supply line of Q2. This is done by inserting between the 12V power rail and R4, a 330 or  470 ohm resistor, a 22 µF cap and a 11V zener diode as shown in the modified schematic diagram. The voltage drop introduced by this mod is quite small and the overtone rejection is optimal.

For the so called pink noise (as a matter of fact pink noise correspond to a 3dB/octave low-pass slope and actually the filter is a 6dB/octave low-pass that gives rise to "red" noise !) be sure to use a 2.2nF value for C5 as stated in the B.O.M. and not to use the 22nF appearing on the schemo ! Also I found useful to decrease the value of R6 to 15k instead of 68k. This provides a greater voltage swing more comparable to that of the white noise. These pictures show the mods of the PCB.


original schematic

modified schematic

The following images show the spectra of the white noise generator of the original design and of the modified design respectively. To view these spectra in detail click on the following reduced views. Theses spectra were obtained using the audio input of an Apple Wallstreet Powerbook G3 and the MacCRO shareware providing for oscilloscope and spectrum analyzer functions. The frequency and intensity axes are in logarithmic scale. The frequency range is 10 Hz to 22 kHz.

Spectrum of the white noise signal with the original circuit

Spectrum after the circuit modification.

The following images show the modifications made to the PCB tracks and the component layout. Click on the images to get an enlarged view. You can notice on the layout that Q1 is plugged rather than soldered. This makes it handy to test various transistor in order to select the nosiest ! Once found, just cut off the collector pin, otherwise it may act as a good antenna with which you would pick the strongest local radio station!

click to enlarge

click to enlarge