Sample And Hold With VC-Clock (+/-12V to +/-15V)
Article by Ray Wilson
This is an intermediate to advanced project and I do not recommend it as a first project if you are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building, troubleshooting and electronics experience is assumed. Additionally, electronic equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully.

Features

Introduction

Sample and hold circuits are used to create repeating or non-repeating cycles of control voltage by sampling a signal fed into the input at a rate set by the sample clock. At each sample the input voltage is stored on a capacitor and held there until the next sample. The held voltage is fed to the output which is used to control another module. It is interesting to sample various waveforms and hear the remarkable patterns that emerge as you vary the sample frequency or the frequency of the sampled signal. When noise is sampled then random non-repeating patterns are output.

Sample And Hold With VC-Clock MP3 Samples

Paolo of Italy's S&H saying Ay yay yay yay...
Paolo's S&H saying Doh-wit Doh-wit.
Paolo's S&H modulating a Ramp oscillator.
Paolo's S&H modulating a Sine oscillator.
Paolo's S&H modulating a Triangle oscillator.
Paolo's S&H modulating a Sine oscillator (w/Portamento).
Paolo's S&H modulating a Square Wave oscillator.
Paolo's S&H demo of rate range.


Sample And Hold With VC-Clock Schematic Page 1 PDF

Op amps U1-A, U1-B and associated components comprise a linear voltage to exponential current converter. It is used to control the rate of the sample clock. External control voltage may be applied to the CV1 and CV2 inputs to modulate the sample rate. The panel layout shown only uses CV1 but you can, of course, use them both if you desire.

U1-C is an integrator and the current flowing out of its inverting input into the collector of Q2 causes its output to ramp up. When U1-C's output goes above the threshold (plus hysteresis) of comparator U1-D, U1-D's output goes from approximately -V to +V and turns on NFET Q4 for 100 uS causing it to neutralize the charge on C13 (integrator cap). This causes the output of U1-C to go to ground. R18 is used to increase the time that the NFET needs to discharge C13. This is so the output of the U1-D comparator stays high for approximately 100 uS. That is the time it takes the cap to discharge to below the threshold (plus hysteresis) of U1-D at which time U1-D's output goes low. U1-D's output is the sample pulse.

U1-D's output is stretched by the circuit of U2-B and associated components to provide trigger pulses. Cap C8 charges via diode D2 and R13 during the 100 uS sample pulse time. When the sample pulse ends the cap discharges through R19. The voltage on the cap stays above the threshold of comparator U2-B and associated components for approximately 2 mS causing U2-B's output to go from -V to +V for about 2 mS. The high excursion of U2-B's output becomes the trigger output after it is rectified and dropped on R20. Thus the trigger pulses go from ground to about +V.

U2-B's output is further stretched to drive the LED circuit of Q3 and associated components. Without stretching the pulse the LED would not stay on long enough to be visible. U2-A and associated components comprise a sync input that can cause the sample clock to pulse and reset. To use this as an external clock turn the panel sample rate very low and then the samples will occur at the rate of the synchronizing clock source (this can be any square wave input). You can get interesting effects by varying the panel sample rate while applying a synchronizing clock.

The 100 uS -V to +V sample pulse is applied to the gate and drain of NFET Q5 which turns it on and passes the pulse through it's drain and to the gate of Q6. This arrangement isolates the gate of Q6 quite well so that R40 turns it completely off until the sample pulse. I have seen very low droop between samples 20 seconds apart.

The voltage at the source of Q6 is presented to the polystyrene (or polycarbonate - low leakage is the key) storage cap C17 during the sample pulse. When the sample pulse is low the voltage is held on C17 until the next sample.

R29 is used to attenuate the sampled input signal. The output voltage is essentially the same as the sampled input. I suggest input voltage in the range of -5 to +5 volts (experiment and of course you can attenuate with R29).

R21 and C14 are used to provide glide between samples. SVO1 is affected by the setting of R21. SVO2 is the straight sampled voltage and does not have a glide circuit.

Approx. Current Consumption
+12V 16mA
-12V 13mA


Sample And Hold With VC-Clock PCB Parts Layout (Parts Side Shown) PDF


Sample And Hold With VC-Clock PCB Part Values Layout (Parts Side Shown) Larger GIF

I find this view useful when I'm populating the board. I don't have to go back and forth from the designator to the value. It speeds up construction. Click the "Larger GIF" link and print the image as landscape.


Sample And Hold With VC-Clock PCB Bottom Copper (Parts Side Shown)


Sample And Hold With VC-Clock PCB Top Copper(Parts Side Shown)


Sample And Hold With VC-Clock PCB Top Silk Screen


Sample And Hold With VC-Clock PCB Populated


Sample And Hold With VC-Clock on the Breadboard

This is what I mean when I say breadboard first. I use solderless breadboards. When one gets ratty and the contacts don't grip anymore I throw it away and get a new one to avoid problems.


Sample And Hold With VC-Clock Front Panel and Wiring PDF


Sample And Hold With VC-Clock Project Parts List

Qty. Description Value Designators
1   LF444 Quad Op Amp   LF444   U3  
1   TL074 Quad Op Amp   TL074   U1  
1   TL082 Dual Op Amp   TL082   U2  
5   1N914 Sw. Diode(s)   VALUE   D4, D5, D2, D3, D1  
3   2N3904(s)   2N3904   Q2, Q1, Q3  
3   2N5457 N Channel FET   2N5457   Q4, Q6, Q5  
1   LED   LED   LED1  
2   Potentiometer(s)   100K   R29, R2  
1   Potentiometer   1M   R21  
10   Resistor 1/4 Watt 5%(s)   100K   R9, R6, R16, R28, R23, R24, R30, R8, R13, R10  
2   Resistor 1/4 Watt 5%(s)   10K   R4, R38  
4   Resistor 1/4 Watt 5%(s)   1K   R36, R37, R34, R17  
2   Resistor 1/4 Watt 5%(s)   1M   R33, R19  
1   Resistor 1/4 Watt 5%   20 ohm   R27  
1   Resistor 1/4 Watt 5%   200K   R40  
9   Resistor 1/4 Watt 5%(s)   20K   R15, R31, R32, R20, R25, R14, R35, R41, R1  
1   Resistor 1/4 Watt 5%   2K   R12  
1   Resistor 1/4 Watt 5%   39K   R39  
3   Resistor 1/4 Watt 5%(s)   4.7K   R22, R5, R7  
1   Resistor 1/4 Watt 5%   4.7M   R11  
2   Resistor 1/4 Watt 5%(s)   47K   R3, R26  
1   Resistor 1/4 Watt 5%   6.2K   R18  
1   Ceramic Capacitor   .001uF   C8  
1   Ceramic Capacitor   .0047uF   C15  
1   Polystyrene Capacitor (low leakage)   .01uF   C17  
2   Ceramic Capacitor(s)   .022uF   C13, C2  
7   Ceramic Capacitor(s)   .1uF   C5, C6, C10, C11, C14, C7, C12  
1   Ceramic Capacitor   100pF   C1  
3   Ceramic Capacitor(s)   10pF   C16, C18, C3  
2   Electrolytic Capacitor(s)   10uF   C4, C9  

Miscellaneous