ELECTRO-ACOUSTIC RESEARCH 3 Mini Eurorack Module User Manual

ELECTRO-ACOUSTIC RESEARCH 3 Mini Eurorack Module

Introduction

In 1971, Morton Subotnick played a pivotable role in defining the music composition department at the at newly-formed California Institute of the Arts in Valencia, CA. Coming from New York, he assembled a group of colleagues to join him in that endeavor: composers Barry Schrader from the University of Pittsburgh; Ingram Marshall, Charlemagne Palestine and Serge Tcherepnin from NYU; and in an advisory role: Donald Buchla.
Through this affiliation with Don Buchla, CalArts took delivery of the very first Buchla 200 Electric Music Box. At the time the school’s campus was still under contraction, so initially the system was delivered to Mort Subotnick’s home studio in Tarzana, CA where it would be utilized as the primary instrument for his seminal piece Sidewinder.
Benefiting from the expansive technology advancements of the late 60s, among the design innovations of Buchla’s new music box was the inclusion of Integrated Circuits (ICs), and an opto-isolating device called a VACTROL. Of particular relevance here were the vactrols used at the core of the Model 292 Low Pass Gate – which acted as a VCA in the 200 -an eclectic choice that most engineers wouldn’t have considered as vactrols are, by nature, slow reacting.
A vactrol-based Low Pass Gate utilizes fixed resonance unity gain Sallen and Key two pole filter typography, also known as a VCVS or Voltage Controlled Voltage Source. Variable center frequency is obtained by changing the value of two resistors simultaneously, which, as figure 1 indicates is ideally suited for a dual-element vector despite the speed penalty which would, over decades, serendipitously become its legacy: the Buch la 292’s infamous ‘ringing’.

FIGURE 1 • SALLEN AND KEY FILTER TYPOGRAPHY

In 2004, EAR (then ‘Plan B’) released the first LPG to the Eurorack market – the Model 13 Dual Timbral Gate. In 2020, while tasked to reconfigure the modular synthesizer EAR had designed a decade ago for the band Tool, a one-off 4HP wide single LPG was developed specifically for that system and is the basis of the M13 Mini. Like the original Timbral Gate, the Mini incorporates additional circuitry not found in other vactrol LPGs which significantly decreases the bleed associated with Don’s original circuit when adapted for the higher amplitudes of Eurorack signals. As a bonus, this addition also enables the user to adjust the vactrol decay time.

Summary of controls

Connecting and mounting your m 13

Before installing your module, verify that your system’s power supply (PSU) is capable of providing an additional 40 mA current required to operate the Model 13.
As with all Plan B/EAR modular products, the Model 13 Mini adheres to the mechanical and electronic conventions of Doepfer MusikElektronok as outlined on their website at www.doepfer.de/a100_man/a100m_e.htm. Power is connected via the 16-to-10 pin flat cable included with your module. While all EAR products are diode-protected to safeguard against power connection errors, EAR cannot be responsible for damage to other modules which may occur as a result. As the module will withstand reversed power applied to it, a keyed connector is not required.
You will notice that one end of the cable terminates to a 10 pin dual row header connector, while the other end is 16. The 10 pin end is intended for module termination and the 16 pin end to your power busboard.
With the system power turned off, first connect the flat cable to the module. Notice that down the length of the cable one side is colored red. That red stripe should face the RED BAND marking on the module’s 10 pin power connector.

FIGURE 2 • POWER CONNECTION VIA FLAT CABLE

Now connect the other end of the flat cable to one of the 16 pin power connectors on your bus board, making sure that the cable’s RED BAND is FACING DOWN towards the -12 VOLT

MARKING on the bus connector. Your last step before powering up is securing the module in place by installing the two mounting screws. Please note that in some cases your faceplate may become marred by the head of the screw. Plastic washers (not included) will stop this from happening.

Basic operation

The primary function of a Voltage Controlled Amplifier (VGA) in synthesizers is shaping sound. An external voltage can be applied as automation to raise or lower the amplitude of the signal being gated. LPGs serve this same function utilizing an AC coupled fixed resonance 12dB/oct lowpass filter.
Unlike the original M13 and the Buch la 292, the Model 13 Mini only has two operating modes: LOW PASS (filter only) and COMBO (low pass and VGA modes combined). Each mode has very distinct attributes which will be demonstrated in the test patch in figure 3 below:

FIGURE 3 • MODEL 13 MINI TEST PATCH

The signal path (black lines) is quite simple: A ramp wave output of a VCO routed to the INPUT of the Model 13 with it’s OUTPUT connected directly to speakers without additional processing. The COMBO MODE should not be engaged – (the LED should be off).
The control architecture (red lines) is also straight foward: an AID or AIR Envelope Generator* which responds to rising­ edge triggers configured as a 1-2 millisecond transcient by setting the ATTACK and DECAY pots fully counter clockwise. The ENVELOPE OUTPUT of the EG should be routed to both the Model 13 VC INPUT and into a VC FREQUENCY INPUT of the VCO with it’s Level Attenuator fully clockwise (set to max).

• An ADSR may also be used by setting the ATTACK, DECAY, SUSTAIN and RELEASE pots fully counterclockwise.

FIRST PATCH VARIATION • SAWTOOTH WAVE IN LOW PASS MODE
Once the patch in figure 3 is loaded on your modular, first verify that the M13’s MANUAL OFFSET POT is fully counterclockwise and the COMBO MODE pushbutton is DISENGAGED (LED is off). Connect a square wave LFO into the Envelope Generator’s GATE or TRIGGER INPUT. Once the EG begins firing it will produce a string of relatively short, slightly muted filtered beats with a distinctly percussive attack due to the frequency transient applied to the oscillator. Regardless of the short envelope you have loaded (1-2 milliseconds) the LPG’s output decay will be significantly longer – approx. 300 milliseconds. Welcome to vactrol’s slow reaction time!

SECOND PATCH VARIATION • SAWTOOTH WAVE IN COMBO MODE
Depress the COMBO MODE button so that the LED IS ON to engage the LPGs characteristic ‘ringing’ effect. You will notice the decay has increased even father, lasting anywhere from .5 from to 1 second depending on the setting of the trim pot on the back side of the module (reference the last page of this manual for the trim adjustment procedure). Along with the extended duration, COMBO MODE increases the high frequency transient response of the filter which adds a natural sounding ambience to short percussive events.

THIRD PATCH VARIATION • SINE ON SINE IN COMBO MODE
As a third variation, change the input wave shape from a Sawtooth to a Sine wave, then introduce a second Sine VCO to frequency modulate the first oscillator. Adjust the amount of FM to taste and experiment with the frequency of the modulating VCO. Lower frequencies produce deep wood tones while higher frequencies sound metallic

FORTH PATCH VARIATION • REPLACE MODEL 13 MINI LPG WITH A STANDARD VCA
To fully appreciate the Low Pass Gate, replace the Model 13 Mini with a standard VGA. Due to the short envelope required for the initial test patch, the VGA output will consist of short unpitched clicks, dramatically different to the result when the same envelope is applied to the Model 13. Again, welcome to vactrols and why you will learn to love them!
While Low Pass Gates have a magical effect on audio signals, unlike standard VCAs they cannot gate control voltages efficiently. The ringing that gives vactrol-based LPGs their unique personality translates to poteen to when gating DC or sub-audio AC signals. For that reason, the inputs have been AC decoupled to block signals of 12 hertz less.
Apart from the trim pot adjustment procedure on page 13, this concludes the user section of this manual. The remaining pages are instructions for those who have purchased the Model 13 Mini as a kit. Thank you for purchasing EAR products …. enjoy your Low Pass Gate and tell your vactrols you love them!

Model 13 mini kit: inventory

Congratulations on your purchase of an EAR Model 13 Mini Vactrol LPG Kit. You will find the following:

1. Qty 1 Main Board – gloss black soldermask
2. Qty 1 Pot Board – gloss black soldermask
3. Qty 1 FR4 Faceplate in flat black with gold flash graphics
4. Qty 11 OOK Linear Potentiometer with associated mounting hardware (nut/washers)
5. Qty 1 Red Illuminated Latching Pushbutton
6. Qty 1 Davies type 1900H Splined Control Knob
7. Qty 3 3.5MM Jacks with associated mounting hardware (round nuts)
8. Qty 3 Jack Washers
9. Qty 110 Pin to 16 Pin dual-in-line Euro rack format power cable

General assembly instructions

The EAR Model 13 kit does not include all the components needed to build this module. A Bill of Materials is included on page 9 listing all the additional parts, with Mouser part number provided where applicable. Building this kit involves soldering 0805, SOIC and SOT23 format SMT components. Prior SMT soldering experience is required. Page 9 maps all component locations, as well as which side of the board thru hole parts are installed.
If hand soldering the SMT components it is suggested that you begin with IC 1 (TL074), as it is the most challenging and will be easier to rework without the clutter of other parts crowding around it.
Assembly should be completed in the following order:

• Placement and soldering of all SMT components.
• Installation and soldering of jacks.
• Installation of the 10-pin header power connector.
• Pot Board assembly, then mounting to the Main Board.
• lnstallalion and soldering of the pushbutton
• Installation and soldering of vactrols.
• Installation of faceplate and associated hardware / faceplate alignment.
• Making endless bloops and bleeps geared to drive your cats cray.

Smt component placement reference

Thru hole component placement reference

model 13 mini kit: BOM

For SUPERCHARGED operation change the value of R3 from 4,7K ro 15K

Assembly/modification/calibration guidelines

COMBO MODE PUSHBUTTON INSTALLATION
The Combo Mode pushbutton is illuminated with an LED and is polarized with a Cathode (-) and an Anode ( +) lead. On the base of the switch you will see an embossed + and – marking indicating the direction of polarity. Install the pushbutton facing the thru hole (faceplate) side of the Main Board (opposite side as the silkscreen marking) so that the LED’s CATHODE(-) is inserted into the designated PCB hole as shown (fig 5).

SINGLE/ DUAL VACTROL INSTALLATION
The Model 13 main board is equipped to accept either a dual VTL5C3/2 vactrol or two VTL5C3 vactrols (the brands of which can be mixed). Their location, and the thru holes they are inserted into vary for these two configurations. Figure 6 shows the different silkscreen and thru locations for installing either A) two VTL5C3 vactrols, or B) a single VTL5C3/2. If you’re using a /2 dual a jumper will need to be installed into the thru holes marked INSTALL JUMPER VTL5C3/2.

FIGURE 6 • VACTROL INSTALLATION (VTL5C3 OR (VTL5C3/2)

Figure 6 above shows the same area of the board, highlighting which markings and which thru holes to focus on depending on the vactrol configuration being installed. Diode symbols are provided on the PCB indicating polarity. Vactrols made by Perkin Elmer, Vactec, Excelitas Technologies and Xvive are clearly marked indicating which leads are the LED end, as well as the polarity. In all cases, the+ LED lead is inserted into the left hand thru hole, also marked with a+. Cool Audio vactrols are unmarked, and orientation is determined by the diode end (it has one long and one short lead), with the long lead corresponding to the LED ANODE which should be inserted into the upper left thru hole. Cool Audio parts purchased with a M13 kit have been hand-marked with a L which should be oriented as shown.

POT BOARD SUBASSEMBLY

The M13 kit includes a small 14 x 16mm printed circuit board – the POT BOARD which adapts the 1 00K Level pot to the stack height of the 3.5mm jacks. Its artwork is symmetrial – you can solder the pot and header connector to either side, as long the two are on opposite sides of the board from one another. The header connector must be installed first. You’ll notice that the pins on either side of the connector are different legnths. As figure 5 shows, solder the SHORTER PIN end into this PCB.

FIGURE 7 • POT BOARD SUBASSEMBLY

Next install the 1 00k pot to the opposite side of the assembly. There are two unplated slots on the pot board. Insert the longer flat tabs at the base of the pot into these slots A) making sure the shorter standoff pins on the base of the pot straddle between the rows of header pins (ref: fig. 7) and B) that the PCB and the base of the pot are parallel to one another. Once soldered, fold the two flat tabs inward on the opposite side of the PCB.
Now install the pot board subassembly onto the thru hole (faceplate) side of the main PCB, making sure that the two board surface are paralell to one another. This is required to maintain alignment between the board components and their faceplate clearence holes.

FIGURE 8 • POT BOARD INSTALLATION

FACEPLATE ALIGNMENT

z Axis alignment: Three round spacers have been included in your kit. They must be placed on the jacks like standard hardware washers, but underneath the faceplate. They are required to maintain proper height in relation to the Offset Level pot so that the faceplate and PBCA are parallel to assure the Combo Mode pushbutton travels freely.
X and Y Axis alignment: Position the faceplate & install the round jack nuts. Before they are fully tighened, jog the faceplate’s horizontal & vertical position to align the Combo Mode pushbutton through its clearence hole.
Once you are satified with the alignment, you can apply final tighetning to the jack nuts and install and secure the pot hardware and the Davies 1900H knob. Resist the temptation to over-tighten the jack nuts. The jack body and threads are made of a relatively soft material, which will hold tension without the need for a washer or excessive tightening.

FIGURE 9 • FACEPLATE ALIGNMENT AND HEIGHT ADJUSTMENT

BEFORE THE JACK NUTS ARE FULLY TIGHTENED, JOG THE FACEPLATE INTO HORIZONTAL & VERTICAL ALIGNMENT UNTIL PUSHBUTTON TRAVELS FREELY THROUGH IT’S CLEARENCE HOLE
HARDWARE STACK: PLACE 1 MM ROUND SPACERS ON JACKS UNDER FACEPLATE FOR PROPER HEIGHT

FINAL ASSEMBLY STACK
Fig 10 gives a side view of a completed Model 13 kit. Note which components are on which side of the PCB. This shows a VTL5C3 configuration. If using a single VTL5C3/2, there would be only one vactrol. On the back of the PCBA, 1/2 inch up from the power connector on the left side is a thin 1/2″ exposed trace labelled “Ground Clip” for a scope probe ground .

Figure 10.

THE SUPERCHARGE OPTION
The Model 13 has been optimized for clean, noise free operation. However, a simple modification – the substitution of a single resistor (R3) from 4.7K to 15K – will introduce a blush of distortion. Great for harmonically rich signals. With purer tones where you don’t want clipping the input must be attenuated 2 volts Vpp (approx 3 dB) prior to gating.

FIGURE 11 • SUPERCHARGED MODE

Calibration

A unique feature exclusive to Plan B/EAR vactrol-based Low Pass Gates is the addition of compensation circuitry designed to improve the bleed associated with VTL5C3 LPGs in the amplified Eurorack signal environment (20v Vpp vs. Buchla’s 3v Vpp line level standard). Another benefit of this modification however it allows the user to adjust vactrol decay time – the ringing- while in COMBO MODE operation.
Preassembled M13s have been factory calibrated to an approx half second decay. The user is encouraged to experiment with longer or shorter decays or periodically return the module to its factory settings. On the back side of the module a single trim pot TR1 located to the right of the TL07 4 Op Amp. To make this adjustment, run a harmonically robust signal such as a Square or Sawtooth wave through the M13 with a very short envelope. Depress the COMBO MODE button so that the LED lights and using a pot tweeker tool, adjust the pot setting while the sound is active to increase or decrease the decay time to your liking. You cannot damage the circuit making this adjustment.
playlistl suggested listening .Given you’ve purchased an EAR Model 13 Mini LPG, it would be fair to assume Buchla synthesizers and the esoteric music they make resonates with you. While you’re building may we suggest the following playlist of albums utilizing that very first Buchla 200 mentioned in the introduction which Donald Buchla hand delivered to CalArts in 1971.

WHO
BARRY SCHRADER
BARRY SCHRADER
MORTON SUBOTNICK
MORTON SUBOTNICK

WHAT
LOST ATLANTIS
LOST ANALOG
VOL 1: ELECTRONIC WORKS
VOL 2: ELECTORNIC WORKS 2

WHERE
APPLE MUSIC, BANDCAMP
APPLE MUSIC, BANDCAMP
APPLE MUSIC, BANDCAMP
APPLE MUSIC, BANDCAMP

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