diy low-pass filter from toy keyboard

[redepoch7]

So I'm trying my hand at some mods to a toy keyboarding.

I am trying to create what I think is a low-pass filter. Kind of a wah sound if you will or really any kind of EQing that cold add to the sound. I'm not great with electronics and I'm using these types of little projects to learn.

I looked up basic RC and RL low-pass filters online and found a few simulators which I used to get an idea of what I wanted to do.

So...

I removed the speaker (i think it's a little 8ohm) and took those lines into the bread board. I have a pot (10k) between the leads of the speaker out lines. Branching off of that is a 0.1uF capacitor.

The reason I chose these values is because I have only 25ohm, 10kohm, and 50kohm pots. So, I plugged that into the calculator first. Then found the value of capacitor needed to start trimming out frequencies in the 1000hrtz range.

I understand that the frequencies that the human ear can hear(audio frequencies) are around 50-2000 (is that right?).

Anyway, I got it all breadboarded up and all I have to show for it is a volume pot...

Any ideas on how I could make this better? or work at all???


schematic:

-->(audio in)--------[10k pot]--------------(audio out) -->


The part below intersects right after the "10k pot" and before the "audio out"

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[.1uF]
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[ground]

 

[crutschow]

You can not use a 10kΩ pot to properly control an 8 ohm speaker. It will basically give you a very low signal until the very end of the pot. Because of the varying impedance of a speaker, any frequency control you try to do will be hard to predict.

You could try the 25Ω pot, but that would require a 40µF non-polarized cap to get the same rolloff that 10kΩ and 0.1µF gives. And it will still vary the volume along with the frequency.

A good human ear can hear frequencies from 20Hz to 20kHz.

If you want to properly adjust and control the frequency response you should add an audio amplifier to drive the speaker, so that you use higher impedance components or an active filter to adjust the frequency at the high impedance input to the amp.

If you want to see what various circuits do to the frequency response I recommend you learn to use the LTSpice simulator, which is free from Linear Technology. It allows you to learn how circuits work without actually having to built them.

 

[audioguru]

If you use a resistor feeding a capacitor to ground as a lowpass filter then amplify it, the output will simply sound muffled.
A wah, wah circuit is a bandpass filter, not a lowpass filter and its peaked frequency changes to produce the "wah" sound.
The circuit is ahead of the speaker's amplifier.

 

[redepoch7]

I made anouther post about this but I will sum it up here quickly...

I didn't know that about the speaker impedance... I'm just starting to pick up electronics.

Ok so I went anouther route...

In that same simulator applet that I used to figure out the first schematic... it has an active low pass filter.

here is the schematic:

**broken link removed**

those are 100uF electrolytic capacitors.

I am having a similar problem as before. I can get the thing to lower the volume... is that from the 10k pot??? I assume so as per your explanation above.

I'm using one side of a TL082 dual op amp... Again, just starting out with this stuff so I'm trying different things in my box-O-parts.

any thoughts???

 

[MikeMl]

I answered this in the other thread on this topic