|Update nov. 25th, 2016
Sample & Hold
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Separate supply rails are used for the two sub-modules. This was made necessary to solve an issue on my first prototype, the clock and trigger stages of the S&H were interfering with the noise amplifier stage of the noise generator!
Noise submodule : Q2 is connected as a reverse polarised diode in order to generate as much noise as possible. This noisy signal is first amplified by Q3 and routed both to U3b which amplifies the white noise signal and to Q4. A network or resistor-capacitors (R2-R24,C9-C11) shapes the spectrum. The result is a low-pass filtering with a 3dB/octave slope giving rise to the so-called pink noise. Then the pink noise is amplified by U3d which is followed by two low-pass filters to provide slow varying random voltages. The first filtering stage is second order low-pass with a cut-off frequency close to 5Hz the second stage provides a 6dB/octave lowpass with a cut-off frequency of 4.7Hz with some amplification in order to level up the random voltage.
Sample and Hold : U1 is an OPA in a classical astable multivibrator configuration, with the values of R3, R4, R5, C3 and P1 the slowest rate is 0.1Hz and fastest 10Hz. The positive part of the pulse is selected by D1 and routed to a switch that is used to toggle the S&H trigger between the internal clock and an external clock. Q1 drives a LED which flashes as a function of the clock rate. C4 differentiates the clock signal in order to convert the rising edge of the clock into a brief positive pulse (10ms). U2d is connected as a comparator. D2 selects the negative part of the pulse to drive the gate of the Q2 FET.
Printed circuit boards and component layoutNOTE : the PCB and a component kit for this module are now made available at Soundtronics.co.uk
Note : in the sample and hold submodule, there is a jumper to be installed in order to choose between the linear or the log behaviour. Don't forget it.
Components and building details
|Sample and Hold submodule
|Noise generator sub-module
Setting and trimming
The trimming is quite simple, it consists of taking a batch of BC547 transistors and selecting among these which one gives the highest white noise level while being symetrically balanced around 0V level, with the flatest spectrum. In order to do so, it is a good idea to use a transistor socket for Q2, this way one can swap easily the transistors to find the best one. It is a good practice to wait about 1min before checking the noise level and quality of white spectrum in order that Q2 has reached a steady temperature.
Below are the spectra I measured on the two boards I built. Some resistor values may also be adjusted to obtain the desired output levels.
Measured audio spectrum - white noise output (0dB position set arbitrarily)
Measured audio spectrum - pink noise output (0dB position set arbitrarily)
|Name : Czaba
Modular project :
Location : Basel, Switzerland
Pseudo : Baronrouge
Modular project: JHC live lab
Location Toulon, France
Web site : http://myspace.com/patjhc
Name : Federic Monti
Pseudo : Zarko
Location Gardanne, France
Web site :