Analog outputs controlling filters, vco's and so

#1

Hi Would it be possible to connect 2 analog filters to the 2 analog outputs on the axoloti?
I see in the PINOUT doc that PA5 and PA4 are analog outputs. Not sure but i guess this are 3.3 v outs.
What is the resolution of this DAC's?

The midibox platform has a very nice module which provide 8 cv outs at 12 bits (DAC TLV5630). stable enough to drive vco's. it can also provide bipolar voltages.
I think the DAC is connected to the SPI interface and the midibox platform also uses the STM32F4 although they use freertos

#2

The gpio DACs are 3.3V/12bit.
Connecting an external dac through SPI is possible, a search here will give you some examples.

#3

Great i will have a look

#4

Hello,

I made a very simple CV output to control an analog filter. (old Frostwave MS20 clone).
I used an NPN transistor, and it's working really well!
I suppose it's not going to work for 1/V per octave, but for controlling something like a filter it works like a charm!

In the proces of making the simple circuit I think I fried PA4... PA5. is still working, but PA4 only outputs very small values in the mV range in stead of the full 0 to 3.3V.

I am very much a beginner when it comes to making these kind of circuits. Here is my simple circuit:

a)Analog GPIO PA4 (or 5) is connected to the BASE of the transistor.

b)The collector is connected to the TIP of my jack socket --- a 10k resistor --- the positive 9V lead of the Axo external power

c)The emitter is connected to the RING of the jack socket --- the GND of the GPIO --- the GND of the Axo power.

So it seems to works like that. But if I disconnect the GND of the GPIO it also still works.

So my question: what is the right and safest way to do this? Do I need to connect to the GND on the GPIO pins or not?

I also connected my new CV-out to the Axoloti input jack, since I don't need an input for this project. I soldered it to the pins just behind the jack input (with the markings T and R) on the Axoloti.
Is this safe or is there a possibilty of frying something?

Thanks very much for community insight!!!

If anyone is interested I'll post some pictures and sound.
Having an analog filter on my Axoloti-synth is a lot of fun and sounds GREAT!!! Also frees up some resources too!