tomcombriat
commented
6 months ago Content proposition:
(This comes from what I use for synthesizers powered by Mozzi)
More Advanced Filter with Volume control
Simple low-pass filtering is usually enough when outputting straight from chips without an DAC (Digital to Analog Converter), especially as they usually have a limited number of bit resolution (8 or the like).
However, more advanced outputting schemes, for instance with an audio DAC (usually 16bits), can greatly benefit more advanced filtering and power stabilization. This schematic is an example of such filtering. It implements a second order low-pass filter (which will attenuate more quickly frequencies beyond its cutoff than a first order) with stabilized audio-type operational amplifiers (TL074).
Quick functional description
Stabilization and virtual mass
The top left part of the circuit provides a clean voltage source from the 5V available on most boards. This is critically needed especially if you are powering the board via USB which is extremely noisy. This filtered 5V is used to power the Op-Amps. While we are at it, we create a virtual mass, at 2.5V. This is needed because the TL074 is not a rail-to-rail amplifier: it cannot provide voltages close to its powering values (+5V and GND here), especially towards GND. As signals created both by a DAC or PWM always have GND as the minimal value, part of the signal would be clamped by the amplifier, resulting in saturation. In order to resolve that, we are going to biased the signal to 2.5V which means that our waveform will be centered on 2.5V instead of having 0V as a lower value.
Bias and gain
The first stage after the AUDIO IN is taking care of biasing the signal at 2.5V. The first two capacitors are first removing the DC component of the signal which is then centered on 0V, and the OP-AMP biased it to 2.5V. This stage also comports a gain setting with the trimmer of 10k. The value of the trimmer should be adjusted at the end so that no saturation happens when sound is outputted by Mozzi.
Filtering
The next two OP-AMP are performing a second order low-pass-filter on the signal, removing high frequencies (DAC stepping, EM noise). It cutoff frequency can be changed by the value of one resistor between 10kHz (22kOhm) and 16kHz (10kOhm). 10kHz is usually the good choice.
Volume
The last stage implements a simple volume control with 10k potentiometer (linear). The 1k resistor attached to it gives the potentiometer a response which is closer to a logarithmic potentiometer which may be harder to find. Of course a logarithmic potentiometer (without the additional resistor) does perfectly the job. This is followed by a simple buffer to bring down the output impedance. The output capacitors are there to remove the bias and give the next stage a signal centered on GND, which preferred in audio.
Note on the DC capacitors
DC capacitors should normally be of non-polarized type. However, it can be hard to find non-polarized capacitors with values in the range of the microFarad (or expensive). The circuit here uses the trick that a couple of polarized capacitors, with reversed polarities behaves like a non-polarized capacitor. Of course, these can be replaced with a single, non-polarized one.
tfry-git
sensorium
So the other day we mentioned updating the docs. Here's a PR with a beginning regarding the "learn" section.
I set this up in my own fork, so we can have a "live demo" at https://tfry-git.github.io/Mozzi/ . Obviously the first commit, with change of title and color theme in _config.yml is just so it's easier to tell the two sites apart, and not meant to end up in the true website.
I'm not sure, whether you can technically commit to this PR, directly, otherwise I'll try to integrate comments as usual.
Misc TODO: