[sdiy] Mixing PWM audio from 2 different devices.

Richie Burnett rburnett at richieburnett.co.uk
Sat Jul 22 19:02:53 CEST 2023

LM324 <--- not even good enough for Muppet Babies.


---- Benjamin Tremblay via Synth-diy wrote ----

>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.
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