MiniMod Sample Hold & Slew (silver edition)

MiniMod Sample Hold & Slew

Sample and Hold modules date back to the dawn of modular synthesis and are a very useful way of creating staircase waveforms, random and semi-random control voltages and even treating audio signals. We have added every possible feature to this module so that it can cover a huge range of duties and it goes way beyond regular Sample Hold and Slew functions. It really has all you will ever need in a Sample, Hold and Slew module!

Sample and Hold section
It can be easily switched between Sample & Hold or Track & Hold modes, and the unique “Restrict” control affects the way the hold output changes with respect to the current input level and previously held output level. With the control fully off the unit behaves just like any other S/H, but at fully clockwise the output is restricted to move by only a small amount after each new sample. The output will still have the same range as the input but will need more hold commands to get there.

The first mode is the 'S-HOLD' mode. This mode is the traditional sample and hold (S/H). It has a signal input, a signal output and a clock input. The module's job is to memorise the input voltage and present that to the output socket on command of the rising edge of a clock signal. The clock signal is any signal input you like, but it is traditional to use a gate type pulse. If a signal is inserted into the clock input, when the signal rises over 1V or so, the command is given to hold. The hold output will then remain fixed no matter what the input signal is now doing. That is until the command to hold is given again whereby it attains a new output value.

The second mode is 'T-HOLD'. This is a track and hold and performs in the same way as the so-called 'sample and hold' module on the Korg MS-20. With a clock input above 1V then the output of the module will follow in the input signal. When the clock input falls below 1V the output will then be held and not change again until the clock input rises again above 1V. This effectively lets the input signal through unchanged when the clock is high and will hold the last voltage sensed when the clock goes low.

The restriction control affects the way the hold output changes with respect to the current input level and previously held output level. With the control fully off the unit behaves just like any other S/H, but at full, the output is restricted to move by only a small amount after each new sample. The output will still have the same range as the input but will need more hold commands to get there. Here's how it works - there are two analogue memory units connected in cascade or series. The second one is designed to hold the previously held value of the first memory unit. The output of the module is derived from the first memory unit. However, the input to the first memory unit comes from a mix of the voltage on the sample input socket and the output of the second memory unit. The degree of mixing is determined by the restriction pot. With the control fully counter clockwise, the input to the memory unit is derived solely from the input socket. With the control fully clockwise, the input to the memory unit comes mostly from the second memory unit.

This is an analogue sample and hold circuit, therefore the hold function itself relies upon storing voltage in a capacitor rather than in a digital memory chip, and as there is no such thing as a perfect capacitor, with time the stored hold voltage will very gradually droop as the capacitor loses charge through internal leakage. The droop is relatively small and for most synthesiser patches it is not a problem, and it only becomes noticable with very long hold times.

The hugely versatile sample and hold circuitry used on this module was designed by, and built under licence from Tony Allgood of Oakleysound.com. Tony has been designing great synth circuits for decades and his skills and design expertise are legendary.

Slew Generator with Gate function
The Sample Hold & Slew module also includes a very well featured Slew generator, which includes a Gated Slew function. It can be used independently or chained with the S&H module. Slew mode can be switched between Linear or Exponential Slew types. The SLEW control varies the time constant of the slew or lag generator. The slew generator is essentially a one pole low pass filter. However, rather than dealing with audio signals it is specialised in dealing with control voltages. It is designed to slow down any rapid changes in the input signal. We have also included a Gate input (SL GATE), so that the Slew effect can be switched in or out of circuit using an External CV or gate pulse Also, the Slew control logic can be reversed with a front panel switch that doubles as a manual Slew on / off switch. An indicator LED lights when Slew is active.

In addition, the Slew Generator has a polarising option - it has slew up only, slew down only or slew up and down modes, which are selected with a three-way toggle switch.

The slew input socket is normalised to the output of the sample/hold module, thus the slew generator can used to remove the sharp steps created by the sample/hold function. Using the Slew together with the S&H section (with noise as the sample source) can produce some excellent fluctuating, smooth random voltages, ideal for adding in small amounts as a modulation to "humanise" VCO pitch, PWM or wavetable selection.

Noise source with colour and clipping
There is also an inbuilt analogue Noise generator, which is normalised to the input of the Sample and Hold section, and there are both regular and clipped Noise outputs – the Clipped Noise is particularly useful with the sample and hold section, as it is weighted to produce higher random voltages. Both Noise outputs are fed through an internal “Noise Colour” control – at centre position it allows the noise through unfiltered, turning the control anti-clockwise introduces low pass filtering of the Noise, and rotating it clockwise introduces high pass filtering. The Noise source itself is based on an analogue bandgap shunt regulator configured in avalanche mode, and it generates white noise with a gradual roll off above 8kHz. It is a little “grainier” in character than the noise from the AJHSynth MiniMod Glide & Noise module (which uses a reverse biased transistor as a noise source), so is a useful compliment to this module as it does not generate exactly the same sounding Noise.

http://www.ajhsynth.com/SampleHold.html