Anushri - Assembly instructions

Anushri is made of two circuit boards. The first board (main board) is holding the power supply, the whole analog synthesis signal chain, the MIDI interface and the main processor. The second board (control board) contains all the controls (switches, pots, LEDs, and connectors). A consequence of that is that builders with a love for panel-mounted controls can build the main board and just wire all the controls to it. We don’t recommend that unless you have some insane passion to show.

Schematics and PCB

You can find the Eagle files for this board in the source code hosted on github.

Bill of materials

Bill of Materials.

Some friendly notes for the self-sourcing crowd:

Building for Eurorack

If you want to build Anushri as an Eurorack module:

The rack-mounting kit contains a spare panel that can be used to relocate the MIDI I/O and the 6.35mm audio output to the front panel.

Assembly instructions

Some equipment you’ll need:

  1. Soldering iron + solder (nothing fancy, a 15 or 20W will do).
  2. Cutters.

We assume you know soldering. If you don’t, look first at this tutorial.

**DISCLAIMER: ** We are not responsible for anything wrong (including electric shocks, malfunctions, fires, accidents involving a soldering iron) that might happen during the assembly of the kit. Everything you do while assembling the kit is at your own risks.

Resistors

The assembly starts with the main board.

All resistors have a 1% tolerance so their last ring is always brown (if it isn’t, you are looking at it backwards). It is recommended to test the value of one resistor from each strip before stuffing them in the board.

Start with the 100’s decade:

The 1k’s decade:

The 10k’s decade:

The 100k’s decade:

Diodes

Diodes are polarized, and their polarity is indicated by a ring. This should match the ring/stripe printed on the circuit board.

You can also add at this stage the quartz crystal at position Q3. It is not polarized.

Ceramic capacitors

Ceramic capacitors look like yellow or blue “drops”. They are not polarized.

IC sockets

Add the IC sockets. It is strongly recommended to align the notch of the IC socket with the notch on the symbol printed on the PCB.

Transistors, special resistors and capacitors

Add:

Electrolytic and tantalum capacitors

Add 4x 4.7uF non-polarized capacitors. Just ignore the +/- printed on the board - these parts are not polarized.

Add 5x 100uF electrolytic capacitors. These parts are polarized, the +/- is printed on the PCB. The negative lead is identified by a white stripe and a shorter lead.

Add 1x 10uF tantalum capacitor. The positive lead is identified by a stripe on the capacitor.

Power supply

Add the two voltage regulators (LM7805 and LM7905). These are two different parts, don’t mix them up even if they look similar.

Add the DC jack and power switch. You don’t have to solder the power switch if you intend to mount the unit behind a Euro panel.

Connectors

Add the main audio in/out connectors - unless you intend to mount the unit behind a panel for your Eurorack setup of course.

Add the 2x3 ISP connector (unless you don’t have an AVR programmer or are not interested in firmware development).

Solder the 4x male 2x8 connectors used to connect the two boards. Note that they are on the other side of the board.


Finally, add the MIDI connectors

That’s it! You can insert the IC. Note that the TS912 used in the first batch of kits is now replaced by the more modern TLV2372. Here’s how the finished board looks like:

Now, let’s move to the control board.

Anushri uses three different kinds of pots. Don’t mix them up!

Here is how it looks like with switches and knobs (not really necessary at this stage…) in place.

Test and calibration

VCO calibration

VCO calibration is needed only if you plan to use the CV in/out for accurate pitch control. Otherwise, you can let the software tune the unit by holding the “hold” button and pressing “run/stop”. You should hear a low-volume arpeggio (played by the unit to measure the pitch produced by the VCO for a few test voltages). When the unit is software-tuned, the VCO will react incorrectly to external VCO CV signals, and will produce VCO CV signals which are slightly off (since they have the compensation of software-tuning built into them).

The calibration procedure consists in the following steps:

  1. Power the unit on and leave it on for a few minutes.
  2. Make sure that oscillators sync is disabled. The sync switch must be to the low position.
  3. Hold the “hold” button and press “rec” to disable software correction of tuning.
  4. Set the pot labelled “2” to its minimal position.
  5. Adjust the two VCO trimmers (offset and scale) until the synth plays in tune in the C1 - C4 range. Note that tuning accuracy starts to degrade below 50 Hz due to some internal resistance on the SSM2164 inputs (which are not true virtual grounds), and above 2kHz due to integrator reset time compensation through R25 falling apart, and deviation from exponential response from the SS2164.

VCF calibration

  1. Dial a patch in which the oscillators are silent. This can be achieved by enabling sync and setting the oscillators pitch to a very low value.
  2. Disable the envelope / LFO modulation on filter cutoff.
  3. Set cutoff to a middle value.
  4. Set resonance to a maximum value.
  5. You should hear the self-oscillation tone. Adjust the V/Oct trimmer so that the intervals are respected – that is to say, when you play C3 then C4, you should hear two notes, maybe not C3s and C4s, but they must be one octave apart. If you do not have a good sense of pitch, you can try a software tuner like Tuna Pitch on OS X. If the filter is correctly tuned, you should be able to play the filter “self oscillation tone” across roughly 4 octaves with correct tuning.

Note that VCF tuning might vary a bit with temperature, so there is no need to spend too much time on this!

Mechanical assembly

The following mechanical parts are used to hold the boards and the case together:

Note that a newer revision of the case has 10 holes, not 4, on its bottom panel.

It is recommended to do the assembly from the inner to the outer - start by assembling the two boards together…

… Then screw the top and bottom case panels to them, and finish the assembly of the case with the left, right, front and back panels.

Troubleshooting

Power distribution

You can click on the image to open larger versions.

Signal path

Use one of the GND pads as a reference for voltage measurements. If you don’t have a scope, the verification of audio signals can be done by connecting the signal to a high-impedance amplifier (mixer or sound-card input, if possible switched to “high-Z” mode).

You can click on the image to open larger versions.

MCU

An oscilloscope is required for these checks.

License

This circuit and PCB layout is made available under a cc-by-sa-3.0 license.

Releases

revC

Original release.