Testing the Juno, Moog and JP6 filters

Juno
 

The Juno filter seems to work fine as well. The output is +/- 500mV from a +/-5V input through a 230k input resistor. 

The filter output is compensated on the voice card, with a gain of 5. This will however only get +/-2.5V from the input. Not sure what is going on there but it can be tested more thoroughly when testing with the voice card.

Edit: I misread the schematics, I put an external 230k resistor on the input when the Juno board already has one, meaning the input was attenuated by 50%

Resonance, VCA linear FM and cutoff CVs work fine. The 12/24dB switch has not been tested, neither has the temp sensor.

Moog

The Moog filter also works as it should. The range of the 2p and 4p trimmer CVs are sufficient, the reso balance trimmer needs around 4.6V for centering which is a bit too close for comfort.

The resonance CV at 5V may not be entirely sufficient for self oscillation but that has to be retested when the filter is connected to the voice card.

JP6

The JP6 filter works fine out of the box. I haven't checked the output levels but all filter variants look good and resonance works fine.

Testing the bus mixer and the waveshapers

Not much to say really, the bus mixer works perfectly with VCAs, switches and I2C, though I haven't checked the polarities and gain.

Oh, and for future testing: When leaving out the VCAs and connecting input to output on the FX VCAs - the input 47k resistor and the output 47k resistor will act as a resistor divider. Thus, the output is half of what would be expected, this threw me off a bit before I realised what was going on.

The waveshapers also seem to be working fine - I only tested with a non-symmetric triangle wave from my function generator, but all trim pots and CVs seem to work as they should.

These were the ones I most expected to work. Next up are the filters, I don't particularly look forward to testing those...

Errata

This is a collection of known PCB fuckups.

56 ch sample-and-hold card v1.0

- Rear rows of the output connectors are not connected to CV output so no CVs will arrive at the mainboard.

Mainboard v1.1.1

- IC2, selector switch for Waveform/PCM Wave: The control and output pins are mixed up. For testing, a workaround is a manual jumper and no DG413 switch, since PCM Wave is a future ad-on anyway. Can also be solved by creating a custom card with an SMD DG413 on it, with the pins routed correctly

- Waveshaper sub oscillator waveform selectors: Waveforms for sub-1 and sub-2 have been reversed, so when sub-1 is square, sub-2 is saw and vice versa. Can be fixed on an updated waveshaper card or the same way as the IC2, by doing a custom DG413.

VCO v2.0

- The saw output from the saw never reaches above 4V, it flatlines halfway through. This is caused by the 1k/1k resistor voltage divider in front of the comparator used to create the square wave. Also, the 1k/1k divider reduces the 0-8V saw to 0-4V, it should have been 0-5V. 

This seems to be fixed by replacing the divider with 100k/168k.

Distortion, multi FX v1.1

When doing the multi-FX board I used the wrong version of the distortion circuit, so this one has no output gain. Thus, the output is way too low.

There are no spare op amps on the board, but there IS one spare OTA. Perhaps I can reconfigure that one as gain.

Juno filter

- Not sure this is an error yet, but the output directly from the circuit seems to be +/-500mV from a +/-5V input. It should be +/-1V I think. I misread the schematics, I put an external 230k resistor on the input when the juno board already has one, meaning the input was attenuated by 50%

Testing the CV generator card

I've finally started testing the new cards. First off is the 56 channel CV generation card.

After first testing the 4ch DAC on a breadboard, driving the old 16ch sample and hold cards, and making sure the code ran perfectly on that, I connected the DAC to the CV board and the CV board to a Teensy 4.0 and...

...nothing.

No output at all on the output pins.

A quick check in Eagle revealed the first major bug - I've forgotten to connect the rear row of the connectors, only the front one is connected. Not a problem for testing but it will require a lot of soldering to bridge all the 56 outputs. 

I then switched to the rear pins and...

...again, nothing.

Damn. I do however try to run the DAC at 50MHz and 5V logic supply, from a 3v3 Teensy. It wouldn't be surprising if that lead to some issues.

 I left the card for a few days and returned yesterday. This time I disconnected the DAC and put it on a separate breadboard, running 16 wires over there. I could then connect probes to the inputs and outputs to see if the DAC itself was actually doing anything.

And what do you know, this time everything worked immediately! Very strange. Perhaps the long wires or the connected probes changed something.

Thus, today, I moved the DAC back to the CV board, but left it slightly high to be able to connect clip on probes onto the DAC board pins while it was inserted into the CV board. And this time it worked!

Finally, I disconnected the probes, one by one. When I got to the probe on the SPI clock pin, everything stopped working. I reconnected it and disconnected all the rest, and it started working again.

As a last try, I connected the probe to the other end of the SPI clock wire, close to the teensy. This time it stopped working too.

Sooo... not sure EXACTLY what is going on, but perhaps the added load/capacitance of the probe slows down or filters the clock slightly?

Anyway, once I got everything working, the output looks great!

Closeup of one of the outputs shows a fairly stable level, the noise seen is 0.005V which is most likely the minimum step on the scope ADCs.

 

Four simultaneous outputs

 

Update:

I've tested all channels and they work great. I also tried reducing the SPI speed to 25MHz to see if that changed anything with regards to needing the probes on the clock bus but it didn't help.

I did notice too that the sample and hold op amps get very hot but I experienced the same on the previous 16ch board so I'm not too worried.

Logic level converter

I have designed a small 4 channel logic level converter board based on the TI SN74LVC2T45 IC. It will translate the 3v3 Teensy level to the 5V the DAC expects. I use 5V on the DAC as that's necessary to run at 50MHz.

Most level converters are way too slow for 50MHz, but the TI chip datasheets says that 420 Mbps is possible when doing 3.3V to 5V translation. We'll see.

I also have the option of rewiring the CV board by cutting a jumper and soldering another one, to make the DAC run at 3V3. My most recent breadboard tests seemed to show that it would be possible but it DOES feel a bit risky for a final design.