I want to create a shruthi-1 with plaits oscillators.
I got the plaits oscillator code running on a teensy 4.1.
Question: Can I simply use the shruthi-1 analog filter schematics with 3.3v by simply replacing their 5v regulators or are there more modifications needed?
I’m sorry If this might be a stupid question. I absolutly don’t have any knowledge about analog signals. 
No, the Shruthi filters are designed to run on +/- 5V.
Can’t you just use 3.3V for the teensy section and +/- 5V for the rest?
yes I could. I however prefer to have the filters also running at 3.3v.
Is there an easy way to convert the shruthi filter schematics to 3.3v?
Else I guess I have to learn analog electronics
But why would you want to do that? What’s the logic behind this preference?
Also are you talking about 3.3V or +/- 3.3V?
No. To start with, the V2164 needs at least +/- 4V, and the LM13700 +/- 5V. Same for the discontinued filter ICs used in some other filter boards: +/- 4.75V for the CEM3379, +/-5V for the SSM2044.
Because I’m designing the synth from the ground up. So I have the chance to use a filter schematic that is compatible with 3.3v.
You e.g. also didn’t use 3.3v/5v schematics with ripples or blades, didn’t u?
Why would I use 3.3V or 5V for an all analog product for Eurorack?
I don’t understand your point.
With the limited experience I have on designing resource-constrained modules and audio products, and given that you have no experience with analog circuits, I suggest you to use 3.3V for the Teensy and +/-5 V (or more) for the analog section.
That’s my last piece of advice, but you’re probably smarter than me, so feel free to ignore it.
I’m sorry. I’m definitely not smarter than you. Like I said I don’t know anything about analog circuits. I simply thought it would result in cleaner circuits if I use 3.3v for both teensy circuit and filter circuit.
I thought analog circuits like filter can simply be converted between 3.3v, 5v and 12v.
Analog circuits are a big mystery to me and I miss basic knowledge about it.
I guess it would be best to use level shifters to shift 3.3v signals from teensy to 5v?
Then a couple of quick observations:
Using the same power supply for the digital and analog section of a product is not good design. Unless you use a lot of bypassing, or tolerate some amount of noise, it is best to use separate rails for the digital and analog section. I got away with it on the Shruthi, but this is certainly not “clean”.
Analog means noise. How do you fight noise? With large signal levels. How big can be your levels? As high as your supply rails. +/-12V means signals 10 times larger than with a single 3.3V rail.
Analog is easier (and less noisy, since everything is referenced to a true ground) with symmetric supplies. If you’re going to have two rails (3.3V and -3.3V), why not go for 3 and have 3.3V, -5V and +5V or even better, 3.3V, -12V and +12V?
History. You want analog? Then all the ICs (op-amps, OTAs, VCAs) were designed at a time when +/-12V or +/-15V was the norm. And it is still the norm. There is no part suitable for audio that runs on 3.3V alone (except maybe the integrated line output amp included in DAC or Codecs). Pro audio line level is +4dBu, this is 3.4Vpp. How would you reach that level with a single 3.3V supply?
Economics. Use a proven design and add $0.5 for an extra regulator and caps on your product? Or spend weeks reinventing the wheel?
Why level shifters? You just need to change the value of 3 resistors in the Shruthi filters schematics to amplify your 3.3V source.
Thanks for your help.
It seems +12 / -12 is preferred for less noise. So I will first try to use Ripples eurorack filter for my project. If I understood everything correctly I could use a DAC to control Ripples CV.
Looking at plaits and eurorack module tester power supply, I got this for 12+, 12- and 3.3v+.
And for controlling ripples filter circuit I use a DAC (circuit from tides2):
Another DAC is use for plaits oscillator output.
This isnt the right way to conver AC to DC… are you aware of the difference?
And for controlling the Filter a simple PWM + Filter is sufficient.
What do you want your input voltage to be?
I would no longer recommend using the same circuits as Ambika or the Module tester v0.1, since AC-AC adapters are increasingly difficult to find.
A simpler and modern option would be to use one of those TracoPower all-in-one DC-DC converters with 2 or 3 outputs.
Regarding your PSU schematics, you have to decide on what’s going to be the input of the 3.3V regulator: half-wave rectified AC from the wall wart, or DC from the 12V regulator. But not both!!!
You could, but PWM would work too if you want to keep things done the Shruthi way.
If you actually decide to use an expensive 4-channel DAC, why would you use another one for the oscillator output? You actually need only 4 channels (audio, cutoff CV, resonance CV, VCA CV).
Also, doesn’t the Teensy have its own DAC outputs already?
via e.g. lm158 to get the teensy 3.3v pwm to ripples 10v range?
Teensy 4.0/4.1 has no DAC anymore. But I just read that it has MQS:
The iMX6UL integrated medium-quality sound module (MQS) is used to generate 2-channel, medium-quality, PWM-like audio, via two standard digital GPIO pins.
Why 
? What about just changing the resistors setting the gain in all the input op-amps instead of making things so complicated?
I don’t know what this is… You can give it a try for the oscillators, and use PWM for the rest.
I think I finally got it. I used a circuit simulator (partsim.com) to get a better understanding of what happens with the circuits.
Ripples cv inputs go through an inverted op amp. To calculate the gain I have to adjust two resistors.
This below schematic as an example for an input should work, shouldn’t it?
On a side note: I got Teensys MQS output working.
1k and 3k are low, use values in the 10k range and then you’ll draw less current from the Teensy.
3.3V through a 1k resistor = 3.3mA drawn from the Teensy. This will quickly add up.
Also, doesn’t the Teensy have its own DAC outputs already?
Teensy 3.x series has one crispy 12 Bit DAC, 4.x series has none.
Teensy 4.0/4.1 has no DAC anymore. But I just read that it has MQS:
The iMX6UL integrated medium-quality sound module (MQS) is used to generate 2-channel, medium-quality, PWM-like audio, via two standard digital GPIO pins.
You are better off using a 32ct PT8211 DAC than the MQS output. I dont know what “medium” is to chip designers, maybe its medium in regard to playing a sample for a notification sound…
@flocked: if you like, drop me a pm for teensy specific topics
I prefer using teensy 4.x as I consider routing the filter output back to the teensy and adding some additional effects to the synth like delay or reverb. - teensy 3.x probably won’t be fast enough.
Any notes/tips for routing back the analog signal to teensy? Anything I should consider?
@fcd72 thanks for offering your help 
@pichenettes thanks that you stayed so patient with my stupid beginner questions it helped a lot.
I plan to open source this project. having a modern shruthi
Then you should probably use a codec - both your audio output and input will go through the same IC, at the same sample rate, using the same clock.
So you plan to synthesize something D/A it, feed it thru an analog Filter, A/D it, apply FX and then finally D/A it again, all with the same Teensy?
