[sdiy] Mixing PWM audio from 2 different devices.

Benjamin Tremblay btremblay at me.com
Sat Jul 22 06:03:27 CEST 2023


Okay, here’s what happened.

I upgraded the power to my quad opamp to a nice bipolar 12V supply.
I was then able to bias my opamp to ground instead of VCC/2. 

The sound was louder with less distortion. 
I was able to find a sweet spot where if my trim pots for both inputs were set correctly, the squeal was greatly reduced. 
However, whenever I played a note on the synth, the squeal rode in on the volume envelope, and faded out when I released the key. 

Looking to see if there were any more low-hanging fruits, I popped out the op amp chip, a lowly low-impedance LM324, and replaced it with a SynthCube TL074. 
Bingo. Audio is clean and quiet. Still a little background pitch. If anything too good for Muppet Babies. 

I am using the HIFI output circuit described on the Mozzi site: https://sensorium.github.io/Mozzi/learn/output/
However I had not #defined HIFI correctly so it was STANDARD_PLUS mode. HIFI isn’t working. I get a lot of distortion. 
(And yes, right there I see a notch filter which I am not using.)
I am using the reference output circuit described in the DSP-G1 notes: https://cdn-shop.adafruit.com/product-files/2400/2400.pdf

I have 1/4 TL074C with a 100K resistor for inverting feedback. 
Opamp non-inverting input is grounded. 

Mixer resistors at the inverting input are 15K each.
At the other end of each mixer resistor I have a 10K trimmer.
Across each trimer I have ~4.7nF plastic capacitors (passive lowpass filters on the input signals).

Going into each trimmer resistor I am decoupling the synth signals with 10µF electrolytic capacitors in hopes of some bass. 
(The DSP outputs 3.3v and the Arduino outputs 5V, and they pull to ground on LOW, so yeah.)

If I invest some time into more intelligent filtering I will probably go to the happy place.

This has gone from unbearable to maybe good enough. Now I have no excuse but to focus on “features” in the firmware.

Benjamin





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