©2019 by Leonardo Laguna Ruiz - Vult DSP.

  • Leonardo

Caudal 2 FAIL!

Some weeks ago I created a post about my work on a new hardware module for Caudal. As you may have seen in the title of this post, the result was unsuccessful. I want to write the causes that way I can avoid the same problems in the future.

Caudal is a challenging module to make in hardware because it features 12 analog outputs. Adding those many outputs would make it very expensive. Think about the cost of the ES-8, which features 8 outputs, and add 50% increase to add 4 extra outputs.

Vult Caudal for VCV Rack + JW Thing Thing

For every output it's necessary a DAC channel plus at least one OPAMP to adapt the signal. If we start adding up, we need a powerful DAC and a bunch of components. A circuit like that requires a lot of space which would make the module big or needing multiple PCBs.

I decided to design it the other way around, I would find the components that I could use and then try to fit Caudal into that. My targets were:

  • Minimise the HP

  • Minimise the cost

  • Maximise the functionality

I decided to use a very specialised IC that provides 8 DAC channels and can handle high voltage (DAC7718). The problem is that the chip is expensive: ~18 euro each. The advantage of using this one is that can reduce the number of OPAMPs needed and reduce the area as well.

In order to test the IC I made a small breakout board and send it for manufacturing to a European PCB manufacturer. They have a good cost, but the fast shipping with DHL is quite expensive, in order to save some money I selected regular mail shipping. This was in November last year. Since I didn't have a tracking number, almost two months later I was contacted with the news that the package was returned to the sender. The manufacturer asked me for an updated address and re-shipped it. Three weeks after, I found that he sent the incorrect boards. At this point I have waited almost three months to be able of making a single tests with the IC. When making software I have to wait 10 seconds to compile to make a test.

Since I'm working on the other modules, I decided to prepare a bunch of prototypes plus boards for Vorg and Freak and place an order somewhere else with paying a faster and more expensive delivery method. Instead of making the breakout board I decided to make the full thing (because I thought I was very good at making circuits 😎).

I had a good record of making successful boards. The times I had to make new revisions were because of small mistakes that did not affected the actual circuit, but would not look nice in a final product. With all that confidence I designed and routed (in about 8 hours) the board. I designed it in a rush. The board is small, it has many components and all the controls are in the bottom layer.

Caudal 2 PCB

Ten days after, I received all the new boards and the first thing I did was to assemble Caudal 2. I started testing all the inputs, LEDs, buttons and knobs. Everything worked correctly.

Then I had to implement the communication protocol for the DAC. I started reading carefully the data sheet and BOOM! I found the first error. Then I found more, but nothing that a few carefully soldered wires, a few traces cut, removing the IC and cutting some pins could not fix.

Patched Caudal 2

It didn't fix it.

Maybe the IC is dead, I had to remove it twice to fix PCB problems under the IC. Maybe it's a problem in the communication protocol I have not idea what's broken right now. To be sure, I need to manufacture a new breakout board and get a new IC. One problem right now, is that the IC is not very common and I have to wait until May/June so it's re-stocked in Mouser or any other store.

Here are the lessons I learned (written as recommendations):

  • When manufacturing prototype PCBs, pay the fast shipping. Life is short and the sooner you know it you messed up, the better.

  • Before making an ambitious board, test that all blocks work separately. Do not skip if you haven't used an IC or a component before.

  • Read the full data sheet over and over. Often manufacturers write critical information in a single line of some strange section.

  • Pick popular components they are cheaper and you'll find more documentation in the internet.

  • Do not design or make boards. It's an expensive and time consuming process 😅