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Advice designing a bandpass filter

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Mastodon

New Member
Hi guys,
so I am trying to build a sound wave synthesizer with my genuinoUNO. It is already giving me the sounds i want and now I want to go a step further and give it some analog frequency filtering!!
(I will share my code once everything is working and tested)

Since the human hearing range ist somewhat from 20 Hz to 20 kHz I want to be able to filter frequencies in this range. So far i have included a low pass filter with a cutoff of 25kHz but to get the most of the synthesizer I want the filter to be adjustible.

The only adjustible components I have are potentiometers so everything should be controlled by changing some resistance.

I tried using 10k poties the capacitance is calculated to be
CR HP filter with R [100-10100] ohm C should be 0.63nF
for a range from 20kHz to 200Hz
and a
RC LP filter with R[100-10100] ohm C should be 0.08nF
for a range from 25Hz to 2.5kHz

But I am not sure if this is correct and I dont want to cause any of my parts to get hot or burst in flames (Yes this is an irrational fear but i am fairly new to electronics). Also I read about having a low Quality when using filters without amplification. Maybe an op amp could do some magic here. But how?

Is there a better design for a bandpass filter, possibly including some gain or even resonance frequency?
Things to be controlled: LP cutoff frequency, HP cutoff frequency, gain and resonance frequency. Is that possible?
What are typical frequency filters for sound applications, what kind of filters are usually used? How do I build one?

Sadly google wants to sell me scientific papers or send me to Amazon instead of telling me how to build what i want... Can any of you help?

I do not really want to buy expensive stuff so here is what i have available, but of course if there are any cheap essential components i will get them..

- A bunch of capacitors from 10pF to 10uF
- A bunch of Resistors from 10 to 1M Ohm
- Potentiometers from 1 to 10 k Ohm
- OP amp LM324N
- OP amp LM358P
- Transistor IRFZ44N


I am grateful for any Help ;)
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Try a "Twin T" type.

A guitar wah pedal schematic is a good starting point; some inductorless types use a twin T filter, as in this schematic - a bandpass tuned by a single variable resistance:
https://img.scoop.it/8hV7xC91D_C_wCNsxFIdhzl72eJkfbmt4t8yenImKBVvK0kTmF0xjctABnaLJIm9

The principle is an inverting amp with two feedback paths.
One is the two resistors plus capacitor "T", top left in that diagram.
The other is two capacitors plus one resistor T at the bottom.

It's only a single movable band but quite simple and a fairly dramatic effect.

Note that generally you will not get a really sharp cutoff with a single stage of normal RC filtering, more a "tone control" type effect.
This is an example of a common tone control setup:
http://www.circuitstoday.com/wp-content/uploads/2010/04/baxendall-tone-control-circuit.png

Edit - if you want finer control, consider a "graphic equaliser" circuit where you can shape the overall sound spectrum, eg.
https://www.homemade-circuits.com/10-band-graphic-equalizer-circuit-for/
 
Last edited:

Mastodon

New Member
Try a "Twin T" type.

A guitar wah pedal schematic is a good starting point; some inductorless types use a twin T filter, as in this schematic - a bandpass tuned by a single variable resistance:
https://img.scoop.it/8hV7xC91D_C_wCNsxFIdhzl72eJkfbmt4t8yenImKBVvK0kTmF0xjctABnaLJIm9

The principle is an inverting amp with two feedback paths.
One is the two resistors plus capacitor "T", top left in that diagram.
The other is two capacitors plus one resistor T at the bottom.

It's only a single movable band but quite simple and a fairly dramatic effect.

Note that generally you will not get a really sharp cutoff with a single stage of normal RC filtering, more a "tone control" type effect.
This is an example of a common tone control setup:
http://www.circuitstoday.com/wp-content/uploads/2010/04/baxendall-tone-control-circuit.png

Edit - if you want finer control, consider a "graphic equaliser" circuit where you can shape the overall sound spectrum, eg.
https://www.homemade-circuits.com/10-band-graphic-equalizer-circuit-for/
Thank you so much, Ill have to read into it but this already helps me a great deal!
 

schmitt trigger

Well-Known Member
You can also employ an universal switched capacitor filter, like the MF10, which are clock tuneable.

The clock can be built around a 555, which would allow frequency to change with a pot.

EDIT; regardless of the method you choose, 20 to 20,000 Hz is a very wide adjustment range to do it reliably with a single pot. You may want to also adjust the timing capacitor to divide the span in shorter ranges. I.e. 20-200, 200-2000, 2000-20000 Hz.
 

dr pepper

Well-Known Member
Most Helpful Member
I'm not really sure what you are filtering, if you want to use a filter to add to or create a sound effect try googling parametric equalizer, this is a equalizer that instead of having a pot to control the level at a certain freq it has a pot to control the frequency itself, so you can create a peak or dip at one freq, might be possible to voltage control too so you could control it from the 'duino.
 
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