MFOS Micro Sample & Hold

Article by Ray Wilson · Music From Outer Space

A compact sample-and-hold module, small enough to add onto an existing synth, with a built-in clock and a glide control.

The finished Micro Sample and Hold circuit board, fully populated. A green PCB silkscreened "MFOS JUNE 2011 REV 1" and "MICRO SAMPLE & HOLD", with an LF444CN quad op-amp in the centre, ranks of colour-banded resistors, two blue electrolytic capacitors, yellow film capacitors and a transistor, plus screw terminals marked GND, +12V and -12V down the left edge. — The populated Micro Sample & Hold board.

Introduction

[RAY'S TEXT — introduction]

Circuit Description

[RAY'S TEXT — circuit description]

How the sample and hold works

[RAY'S TEXT — walkthrough of the scope traces below]

Input voltage

Oscilloscope capture: two triangle waves, one yellow and one cyan, riding on top of each other. Cursors read CurA 7.40V and CurB -7.20V for a 14.6V difference, so the incoming signal swings both above and below ground. Both channels at 5V per division, 500 microseconds per division. — The incoming triangle wave, swinging roughly ±7V around ground (14.6V peak to peak).

Level shifted voltage

Oscilloscope capture of the same pair of triangle waves after level shifting. Cursors now read CurA 9.60V and CurB -400mV, a 10.0V difference, so the waveform sits almost entirely above ground instead of straddling it. Both channels at 5V per division, 500 microseconds per division. — The same signal after level shifting — now sitting between about −0.4V and +9.6V rather than straddling ground.

Sampling the level shifted signal

Oscilloscope capture: a smooth yellow triangle wave with a cyan stepped staircase traced over it. The staircase holds each sampled voltage flat until the next sample, producing clear rectangular steps that follow the triangle's slope up and down. Both channels at 2V per division, 50 milliseconds per division. — The sampled output (cyan) holding each value flat until the next sample — the staircase that gives sample and hold its sound.

Sample clock versus output

Oscilloscope capture with the sample clock on the upper trace in yellow, a repeating train of about ten sawtooth-edged pulses, and the sample-and-hold output on the lower trace in cyan, descending in flat steps with one step per clock pulse. Both channels at 5V per division, 20 milliseconds per division. — Clock above, output below — one new held step per clock pulse.

The differentiated trigger pulse

Oscilloscope capture: the yellow trace shows a tall narrow spike that rises almost vertically and then decays away exponentially, while the cyan trace below steps from a low level up to a high level at the same instant. Both channels at 5V per division, 100 microseconds per division. — The clock edge (cyan) differentiated into a brief spike (yellow) — the short pulse that actually opens the sampling gate.

Low frequency input, high sample rate

Oscilloscope capture: a smooth yellow sine wave with a cyan sampled version drawn almost on top of it. Because the sample rate is high relative to the input, the steps are very small and the staircase tracks the sine closely. Both channels at 2V per division, 200 milliseconds per division. — Sample fast enough relative to the input and the steps shrink until the output nearly redraws the original.

U1D pins 12 and 13

Oscilloscope capture of the clock oscillator core: a yellow ramp that climbs steadily then drops sharply, and a cyan ramp moving the opposite way, the two crossing repeatedly. Cursors read CurA 1.72V and CurB -2.00V, a 3.72V difference. Both channels at 1V per division, 10 milliseconds per division. — The oscillator core at U1D pins 12 and 13 — the ramp that sets the sampling rate.

Glide

[RAY'S TEXT — glide / slew explanation]

Oscilloscope capture with glide partway up: the same yellow sine and cyan sampled output, but now the vertical edges between steps have become sloped ramps instead of instant jumps, rounding off the corners of the staircase. Both channels at 2V per division, 50 milliseconds per division. — Glide partway up — each jump becomes a ramp, softening the corners.

Oscilloscope capture with glide high: the cyan output has stopped looking stepped at all and is now a smooth wavy curve that lags behind the yellow sine wave, rounded and shifted later in time. Both channels at 2V per division, 50 milliseconds per division. — Glide high — the steps smooth away entirely into a lagging curve.

Schematic

[RAY'S TEXT — any schematic notes]

View the Micro Sample & Hold schematic as PDF »

PC Board

Parts placement (silkscreen)

Parts placement drawing for the Micro Sample and Hold board. Component outlines are labelled with their designators: U1 in the centre, resistors R1 through R18, capacitors C1 through C8, transistor Q1, diodes D1 to D3, and connection points X1 to X8. Marked "MFOS JUNE 2011 REV 1" at the top and "MICRO SAMPLE & HOLD" at the bottom, with a note that the PCB size is 1.70 inches wide by 2.40 inches high. — Parts placement — PCB is 1.70″ × 2.40″.

Top copper

Top-layer copper trace artwork for the Micro Sample and Hold PC board, shown as black pads and tracks on white. — Top copper artwork.

Bottom copper

Bottom-layer copper trace artwork for the Micro Sample and Hold PC board, shown as black pads and tracks on white. — Bottom copper artwork.

Board Assembly

[RAY'S TEXT — assembly notes]

Assembly drawing — legend view (PDF) »

Assembly drawing — value view (PDF) »

Panel

Panel drawing (PDF) »

Connecting it to the Sound Lab

[RAY'S TEXT — wiring it to the Sound Lab Mini-Synth]

Wiring diagram for connecting the Micro Sample and Hold to a Sound Lab PC board. The board layout is shown with blue callout lines running from labelled tags X1 through X8 to their connection points, and three power leads leaving the right-hand edge - green from GND, red from +12V and black from -12V - annotated "To extra power connections on the Sound Lab PC Board." — Connection points X1–X8, and the three power leads that pick up from the Sound Lab board.

Parts List

[RAY'S TEXT — parts list. Use a table with class "rc-table" to match the other MFOS pages: Qty / Description / Value / Designators]

All work © Ray Wilson / Music From Outer Space — musicfromouterspace.com. The Micro Sample & Hold, its circuit, text and images are Ray's.

Mirrored and recoded by Mess O' Pedals to preserve Ray's work and keep it accessible. Only the HTML is mine — the design, the words, and the wit are all Ray Wilson's.