Audio filter advice sought

Thread Starter

upand_at_them

Joined May 15, 2010
621
Hi guys,

I'm going to be toying around with audio filters and wondered if there are standard capacitors that I should buy. Either type or value, or both. Right now I have a microcontroller 10-bit DAC* feeding an on-chip op-amp and going to a small speaker. I have the OPA set up to buffer the DAC, but I recognize that I should be making it as an active LPF. This isn't going to be high-end stuff, but about me learning about filters. Based on some online LPF calculators the capacitor component values are within the range of several capacitor types: polyester film, polyester Mylar, monolithic ceramic, and regular ceramic. Is there a preferred type among those?

Are there beginner-friendly books I can get? I have "The Active Filter Handbook" by Tedeschi somewhere in a box of unread books if I can find it.

Also, is there an objective means a hobbyist can compare setups? I have a Rigol DS1052E that has FFT, but other than printing out the FFT plots and comparing ones with drastic differences I wouldn't know which setup is better/best.

Any input?

Thanks,
Mike

* And I'll probably experiment with PWM too.
 

Audioguru again

Joined Oct 21, 2019
2,353
An opamp does not produce enough output current to drive a speaker. A speaker needs a power amplifier.
A filter is usually placed at the input of an amplifier. A lowpass filter can be made with a series resistor then a capacitor to ground. It is only one "order" then it is so simple that its reduction of high frequencies is gradual.

Your 10 bits of DAC needs more "orders" to sound better than a buzzer. A Sallen-Key second order or third order active lowpass filter tutorial can be found in Google.

Do you know the cutoff frequency you need?
 

Wolframore

Joined Jan 21, 2019
2,176
Sounds like a fun experiment, try a power amplifier @Audioguru again is right, you won't get enough current out of an opamp to drive that speaker. For the filter you will have to calculate the values based on your cutoff frequency. The formula is fc=1/(2 pi R C) Where fc (f of c) is the cut off frequency. Play around with it. I suggest picking capacitance value first because resistor values are easier to match. Try things and see what you find and ask yourself why does it work? Why doesn't it work like you thought.
 
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