Building the NEW VCA Tile

Notes:

  • Latest version of the PCB has (redundant) trimming components removed, and no offboard power PCB.
  • The circuit is a mostly faithful copy of the Sims 13700 VCA as described in his paper.
  • Component values changed for a 0-5V CV range.

There are four PCBs:

  1. Front panel.
  2. Interface (holds pots and jacks).
  3. Main
  4. Offboard Power. Not used on new revisions.

BOM:

Qty
Value
Device
Package
Description
Source 1
Source 2
110kTrim Pot3296WMultiturn trimmer. Optional. Not on new revisions.http://www.taydaelectronics.com/potentiometer-variable-resistors/cermet-potentiometers/3296w/10k-ohm-trimmer-potentiometer-cermet-25-turns-3296w.html
1100Resistor0805
1100kResistor0805
4100nCapacitor0603
210kPotentiometer9MM SNAP-IN9mm vertical snap-in pot, Alpha / Panasonic style with 6mm built-in knob.http://www.taydaelectronics.com/potentiometer-variable-resistors/rotary-potentiometer/linear/10k-ohm-linear-taper-potentiometer-round-knurled-plastic-shaft-pcb-9mm.htmlhttps://shop.erthenvar.com/collections/9mm-potentiometers/products/9mm-vertical-panel-trimmer
110kResistor0805
110kTrim PotTC33XSMT trimmer potentiometer part number TC33X. Not on new revisions.https://www.digikey.ca/product-detail/en/bourns-inc/TC33X-2-103E/TC33X-103ECT-ND/612911
110uCapacitor0805
1120pCapacitor0805
11MResistor0805
11kResistor0805
11kTrim PotTC33XSMT trimmer potentiometer part number TC33X. Not on new revisions.https://www.digikey.ca/product-detail/en/bourns-inc/TC33X-2-102E/TC33X-102ECT-ND/612910
220kResistor0805
12n3906PNP-TRANSISTOR_SOT23SOT23PNP TRANSISTOR
1330pCapacitor0805
233kResistor0805
1390pCapacitor0805
162kResistor0805
1LM13700MOTASO16Dual OTA with buffers.
33.5mm Audio SocketTHONKICONN3.5mm socket for Eurorack modular synths "Thonkiconn" style.http://shop.erthenvar.com/collections/accessories/products/3-5mm-inline-jackshttps://www.thonk.co.uk/shop/3-5mm-jacks/
1TL072OpampSO08Almost any dual Opamp can be used.
1Tile Power Tail"Futaba J" type power lead.http://pulplogic.com/product/tile-tail/EBay
1Pin Header (Male)1X04Header for Offboard Power PCB
1Pin Header (Female)1X04Header for Offboard Power PCB
1Pin Header (Male)1X02PCB Connector
1Pin Header (Female)1X02PCB Connector
1Pin Header (Male)1X03PCB Connector
1Pin Header (Female)1X03PCB Connector
15z1Zener DiodeSOD3235.1V Zener Diode

Schematic:

Latest:

Previous:


Alternative Build

The current PCB has three trimming resistors (labelled as R-PD, R-PU, and R-B) to fine-tune CV bleed-through. These aren’t needed and will likely be removed on future revisions. They are bypassed with the “0r” resistor.

 

 

Older PCB:

The items outlined as “OPTIONAL” in the above schematic can be left out. They allow fine-tuning to lower CV bleed-through (on right of PCB) and ensure 0V CV attenuates completely (on left side).

In practice the circuit is fine without them and they will likely be removed on future revisions.

To remove, don’t place/install the parts highlighted in yellow, and replace the indicated 100R resistor with a 0R.

If keeping them, only install one of the pots on the lower-right. A multi-turn was added for very fine control of bleed-through trimming.


Build Instructions:

Clean PCB with isopropyl alcohol to remove any manufacturing residues.


Begin by soldering the surface mount devices.

The unlabelled 0604-sized parts are the 100n 0604 bypass capacitors.

Ensure the diodes and ICs are in the proper orientation.

 


Solder components, header, and power cable to the Offboard Power PCB. The only devices that need to be mounted are the four capacitors and two diodes.

The power cable and header are mounted on the opposite side and soldered on the component side. The cable should go through the strain-reliefs (see top pictures). If not using the strain-reliefs the cable can be soldered on the opposite side.


Add the male and female headers.

It doesn’t matter which PCB get the male or female headers, just that they’re on the right side. The sides with the surface-mount components face the same direction (downwards when module is installed), and the PCB with most of the components is on the “bottom”.

(The old PCB is shown here.)

Alternatively, only male headers can  be used to connect the PCBs together permanently, but troubleshooting will be more difficult if there are problems.

 


 

Solder the pots and jacks to the PCB. It’s useful to use the front panel to ensure they’re soldered flush and aligned.

 


Testing and Configuration:

 

Check all connections for shorts, particularly solder-bridges between IC pins.

Before applying power, check for shorts on the tile power connector.

Set both trimmers to the middle position (if using them).

Apply power and check that none of the components are getting hot.

 

Connect an audio source and a CV source and check that these and the knobs work.

Trimming:

The latest version of the PCB without trimmers require no trimming.

The left trimmer adjusts the gain, and the right adjusts the CV bleed-through. Both set to the middle position is okay if you don’t have the ability to trim.

To adjust gain (left trimmer):

  • Patch audio signal into audio input.
  • Patch 5V into CV input.
  • Turn “In Level” and “CV Amount” to full.
  • Connect voltmeters or oscilloscope channels to the audio input and audio output.
  • Adjust the trimmer so that both levels are close to equal.

To adjust CV bleed-through:

  • Remove cables from input.
  • Patch a fast square-wave (>100Hz) into CV input.
  • Turn “In Level” and “CV Amount” to full.
  • Listen to the audio output. You will need to turn it up (and don’t forget to turn it down afterwards!)
  • Adjust the trimmer to reduce the tone induced by the CV. It won’t be removed completely.

Done!