These informations are all over the place, so let’s summarise them here.
“Mandatory Fix” (even if your module seems to work):
- replace R42 (220R) with a resistor between 1K and 4K7. With 220R the LM4040 is way out of its maximum rating and causes wobbling of the pitch, or it could just be destroyed.
Easy (no excuses to not do them if you build a new 1222):
- replace the L7805 with a L7808 because the Arduino is powered thru Vin not +5V.
- add a 10M between pins 4&5 of the 3340 to reduce the ringing on the square out. It may affect the PWM pots range… look here
- Use a low temperature coefficient capacitor for C6. The BOM specifies ceramic, but that can lead to serious temperature sensitivity. Instead use a polyester film cap, or even better, polypropylene (with a temperature coefficient of about -200 ppm/K).
adding 1K output resistors (not absolutely necessary but “good practice”).
fixing the discrepancies between output levels and DC offsets of the different waveforms.
adding a regulator for the negative supply of the 3340 ?
C5 is on the wrong side of R6.
fixes to reduce PWM detuning
An improved jack PCB could add the 1K output Rs, make all waveforms output levels 10Vpp, and provide the negative supply for the 3340.
GND and the eight jack signals are already on the existing connector.
We’ll need to find an easy place to pick up +/-12V to power the OpAmps and the negative -5V supply.
The Eurorack power connector is just in the right place Let’s pick +/-12V right on the leads of the ferrites/10Rs/schottkys/whatever you mount there.
The negative supply can connect to a pin soldered in place of one of the legs of R6 (which will be left out).
It should be possible to bodge C5 on the other side of the PCB. I didn’t try this so it may interfere with the components already on that side.
That’s a bad one to fix
There is this. Not easy to bodge into an existing PCB.
The AS3340-HYB is a small DIP-sized PCB which is supposed to be a drop-in replacement for the AS3340 with much reduced PWM de-tuning effect.
Please comment if you have tried it.
There is also a discussion about adding a trimpot but I can’t find it right away.
Ideas/suggestions/whatever to improve this post are welcome.
Especially someone who want to implement the improved jack PCB idea