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Selective Inverter

NOTE: R15 should be 22k not 22 ohms as shown on the PCB

The Synthasystem designs and schematics are all
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z 1975, 2010, 2011 Nyle Steiner
They may not be used for profit, sale, or reproduced in any way without the express consent of Nyle Steiner

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Function

Nyle had a client who wanted to be able to invert a phrase without having to play it inverted. So, Nyle came up with this little module.

Honestly, I haven't had much time to play with this one. That said, here is my best description. This module is designed to be used with Control Voltages. The intent is to be able to set an arbitrary voltage as a "zero" point so that when you play above this point by for example, a step through the module, it outputs a voltage a step lower than the "zero". If you play down a step, it outputs a voltage up a step.

The Synthacon (not Synthasystem) owner's manual is the only place I've found information on this module.

The Selective Inverter is a programmable contrary motion generator. It will generate the inverse of any phrase played by the performer. The invert reference point is instantly programmable by foot switch. Input can be from any voltage control source, such as a keyboard, sequencer, pre-amplified acoustic pickup, etc.

I found some more info thanks to an owner of a Synthasystem.

The Selective Inverter is a six input voltage summer with two outputs (A&E) that track each other in unison except when a trigger is applied. This causes output B to invert and move in exact contrary motion to output A. Ie, when output A is moving negative, output B is moving positive, when output A is moving positive, output B is moving negative.

The point where A and B meet is the point where A and B are at the instant the trigger is applied. When the trigger is released, output B moves instantly to wherever output A happens to be.

For example, this feature facilitates the playing of contrary motion from any devised note when using a keyboard. When using a foot pedal to apply the trigger, one can be playing several oscillators in unison and stop on any chosen note such as C and push the pedal. While the pedal is held, a scale of D, E, and F can be played. All oscillators driver by output A will play D, E, and F while all all oscillators driver by output B will play Bb, Ab, and G. If the pedal is now released all oscillators driven by output B will go from G to F. If the pedal is pushed again, F now becomes the point of unison and a scale played G, A, B, will cause all oscillators driver by output A to play G, A, B and all oscillators driven by output B to play D#, C#, B. Whenever the pedal is released, all oscillators will play in unison and track each other.

The Selective Inverter has six inputs, three of them feed through an input pot, and have a maximum voltage gain of approximately 2. They are marked VR at the input jack. The other three are fed straight in at a gain of 1. The first four of the inputs on the left side are non-inverting while the last two on the right are inverting, ie, with no trigger applied, a positive voltage fed into a non-inverting input will drive both outputs positive. A positive voltage fed into an inverting input will drive both outputs negative. Regardless of what inputs are used, output B always does the opposite of output A during the time a trigger is applied to the inverted trigger input or the pedal is pressed or the invert button is pressed.

The invert point can be set at ground and made non-selective by moving the invert point switched to the "fixed" position.

Since this module is a voltage summer, it can be fed into the calibrated input of each VCO. Several sources such as sequencers, keyboards, sample and hold, envelope generators, etc. can be added together to modulate several VCOs simultaneously and maintain perfect tracking from all sources.

this module can also be used as a six input audio mixer using output A as the signal output.

A voltage pot with an on-off switch is also on the front panel. When this switch is on, the outputs can be swept over a wide range by turning this pot.



Use

Use this module to invert an incoming control voltage using an arbitrary voltage as the zero point. This module is designed to work with DC control voltages although you can feed any signal into it you want, but the output is pretty weird and nothing you would expect.

Inputs/Outputs

This module has two fundamental inputs and two outputs. When the "Inverter" is not active:
When the "Inverter" is active:
That is, Out A and Out B are the same until the inverter is activated. The non-inverted inputs and the inverted inputs are summed and pass through to both outputs. When the Selective Inverter is activated, the "inverted" or mirrored version is sent to Out B.

(Don't confuse the inverted inputs with the Selective Inverter function. All the inverted input does is invert the signal coming into the SI and then sums it with the non-inverted signal. This sum is what drives the sample and hold circuit and is also the signal which is inverted or mirrored about this "zero" point.)

To operate the Selective Inverter, input a gate, push and hold the "Inv" button, or press the connected foot switch. This causes the SI to capture and hold the currently summed CV input as long as the switch, gate, or pedal (hereafter referred to as "gate") is active.

In the "Select" mode and an active gate:

The sum of the inverted and non-inverted inputs and voltage (if on) are sampled when the gate becomes active. This sample is the "zero" about which the incoming CVs are inverted or mirrored.

Again, the sum of all the input signals is inverted about this "zero". The signal sum is routed to Out A and the inverted signal is routed to Out B.

For example if the sum of the input signals is 6.75 volts, and the zero is 5.0 volts, Out 1 will be 6.75 volts and Out 2 will be 5.0 - (6.75 - 5.0) = 5.0 - 1.75 = 3.25 volts.

The sample is controlled by a foot pedal, trigger signal, or an "Inv" pushbutton on the panel.

There is a jack for a pedal switch, this should be a normally open SPST type switch. To activate the selective invert, the tip will be grounded to provide an "S" trigger/gate.

In "Fixed" mode when there is an active gate:

The summed input signal is inverted or mirrored about ground. All else is the same as for "Select" mode.

Knobs

This module has four potentiometers:
Switches

There are three switches:
General Connections

Hooking it up is pretty simple. Connect a CV (or more) to an input. Connect Out 1 to a VCO and Out 2 to another VCO. Play to the key where you want to invert. Play the key, press and hold the panel button, foot pedal, or feed in a gate.

You can also feed the output into any other source which looks for a control voltage for an input, like two VCAs, VCFs, Phaser, etc to get an increasing effect from one, diminishing effect from the other for stereo effects.

Component Selection

Resistors

This module was originally built with carbon core, 5% resistors with one or two 1% metal film resistors. So, you have a wide range of options here. I recommend using 1% tolerance, metal film resistors everywhere.

This project uses three sets of resistors which need to be matched to 0.1% or better if you have the patience. You can purchase them to this tolerance, or hand match them with an ohm meter. The actual value isn't as important as matching them to 0.1%. They can be any value between 36k and 47k, so long as they match in their respective groups.

Capacitors

There are probably a billion different ceramic capacitors at a place like Mouser. Pick a capacitor that can fit the hole easily, typically 0.1 inch on centers.

Pick good quality electrolytics where designated.

C1 needs to be a temperature stable, low leakage Polystyrene or similar. It is holding a voltage and the droop will be in part determined by how good this capacitor is. The problem is I couldn't find a big enough Polystyrene capacitor. A good second choice is polypropylene or polycarbonate.

See here for a good discussion on capacitor types by Harry Bissell. Thanks Harry.

ICs

LM741s. There are "better" op ams out there, but the qualities of these will have an effect on the sound. If you want to use better ones, go ahead. There are many to choose from. If you want the original, use the 741s.

This module also uses a CA3140 in the "hold" section. These are available at DigiKey. Be careful handling these as they are sensitive to static damage.

Diodes

Nothing special, 1N4148s are fine.

Pots

Your choice for your panel. If you use the panel I laid out, the holes and spacing will work for the Alpha 12 and 16mm pots. You can probably use nicer BTI, Bourns, etc. 9mm pots with "pot chiclets"

Trimmers

Use good trimmers, please. A good Bourns multi-turn trimmer like Bourns 3296Y series will fit the pad layout and work well.

Jacks

For the panel I laid out, a good 3.5mm or 1/8 inch jack will work. I use the Switchcraft 42A Tini-Jax true 1/8 inch jack. These are switched jacks and they work with 1/8 inch plugs and 3.5 mm plugs.

Switches

Use a good quality SPST and SPDT switch.

Build Notes

Soldering

I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.

Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.


Power Supply Regulation/Filtering

Some additional comments here. These modules are tested to run on +/-12 VDC. The original power supply in the Synthasystem was +12/-10 VDC due to how Nyle designed the -10 volt section of the power supply, not for any magical requirement to have -10 volts.

The power/regulation section has 2 voltage regulators on it which can be set to +12/-10 (or +/-12 volts) depending on your needs. If you are coming from +/-15 volts, you need both regulators and you may as well set one to -10 volts.

If you are coming from +/-12 volts, technically you don't need the regulators, but if you want, install the negative one and set it to -10 volts. The LEDs are not strictly needed. They are there to establish a base current draw so the regulators will work.

Important... if you don't install the regulators, you have to install a jumper between pins 2 and 3 as shown on the Power/Regulation PCB or you won't get power.

Component Placement

FET, C1, and R13

Like in the sample and hold, this module has a "hold" circuit with high impedance nodes. To help prevent leakage, I didn't use a ground plane and used a spread mounting for the FET and tried to keep the high impedance pads close together and away from other runs. There isn't much you have to worry about since the PCB layout constrains how these components are installed. Just a FYI thing.

Mounting

This PCB has four holes to allow flexible mounting configurations.

Calibration

Panel Notes

There is nothing too special. I suggest using connectors on the PCB and jacks on the flying wires. The spacing and holes are setup for Alpha 16 mm or 12 mm pots. The jack holes are 0.25 inch in diameter.

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