Tuesday, 16 December 2014

ARP 2500 - SAMPLE & HOLD / RANDOM VOLTAGE (1036 module)

In electronics, a sample and hold circuit is a device that captures the voltage of a continuously varying signal. It holds onto its last sampled value until the input is sampled again
This saved voltage can then be used to control VCOs, filters, VCAs, etc.

The ARP 2500 sample + hold module is no different.

It's official name is the 1036 - S/H - Random Voltage.

A typical sample and hold circuit stores electric charge in a capacitor and contains at least one fast FET (field effect transistor) switch and at least one operational amplifier.

It looks like the 1036 uses this design.
The schematic & BOM can be found here: GuitarFool
The OpAmp seems to be a LM301
The switch - a 2n5172 ???

As the name implies it also has random triggers and more.
The 1036 is in fact a dual sample and hold.. Each S&H circuit is identical & independent.
The 1036 also contains a voltage controlled pulse generator & a white noise source.

 Each upper section of the S&H 1036 has 3 inputs and 2 outputs.

(1) External Signal input.(this is where the voltage to be sampled goes).
(2) Clock frequency modulation (VC for the pulse generator).
(3) Sample Command Input.(The pulse goes here).
(1) Clock out
(2) Out (Sample out).

The VC pulse generator can be used to re-trigger the S-H circuit there by "re-sampling the input voltage & creating a new output voltage".

When a pulse is applied to the "Sample Command Input" (SCI), the output CV is immediately (10ms) given the same value as the input CV. This Output CV will remain unchanged until a new pulse is applied to the SCI.

There are 3 ways to generate these Sample Command Pulses:
(1) Pushing the Red Sample Button.
(2) Applying an pulse from an external source (eg a keyboard trigger, sequencer or an oscillator) to the SCI
(3) Use the Internal Clock Pulse Generator.

The frequency of the clock pulse generators are determined by the two silver knobs & the slide switches.
The pic is pretty self explanatory.

The clock frequency can also be modulated by applying a voltage to the "Clock Frequency Mod" inputs.

Here is a pic of the upper inputs/outs:

The lower in/out section of this module is more flexible than the upper since we have some attenuation pots.

These CLOCK PULSE outputs can be used to provide COMMAND PULSES. for the SAMPLE inputs.
CLOCK PULSES are +10V. Their duration lasts as long as the sample period.

Between the two red sample buttons are 3 toggle switches.
The toggle on the left connects Clock (a) with Sample & hold (a).
The toggle on the right connects Clock (b) with Sample & Hold (b).
The middle toggle connects Clock (a) with sample & hold (b).
So Clock (a) can be used to control both S&H circuits, and Clock (b) is restricted to its namesake S&H.

It can be a bit confusing, but any voltage which you wish to sample must be applied to the Ext Sig input (Not the Sample input).

For each Dual S&H module there are two Ext Sig Inputs at the top & two at the bottom on the module.
The bottom ones are really cool as the signal has attenuation potentiometers .. The top inputs are un-attenuated.

In addition to sampling external signals the 1036 can produce random signals

The control for this are these two silver knobs called the Int Random Sig. (Internal Random Signal Generators).
Advancing these knobs will introduce noise into the inputs of the sample & hold circuit.
If we sample only the random signal then you will hear a series of stepped random voltages.
The fun begins when you start to sample external voltages.

You can mix the external signal (EXT SIG) with the internal random signal (INT RANDOM SIG) before they get to the sampler.

One last point to discuss is the Trig/Gate switch.

When a pulse enters the sample input two different results can be selected depending of whether the switch is in the Trig or Gate position.

So what's the difference between a trigger and a gate?
Trigger =   short duration
Gate =      longer duration 

In the trig position, the +ve edge of the pulse will trigger the sampler for about 10ms
In the GATE position the output of the sample and hold remains 'Open' for the whole time that the command pulse is positive.

The choice that you have between using a trigger or a gate in the ARP 2500 is absent in the later ARP 2600. The model 2600 uses the 4015 sub-module and works only with sharply rising waveforms or triggers.

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