I’m definitely very new to DIY modular, or DIY electronics of any kind for that matter, so please bear with me; for a reference point as to where I’m at, the only module I’ve successfully managed to build so far is a super-simple passive attenuator.
Anyway, the setup I’ve been using to try to test this passive RC filter is this:
My oscillator is a really simple synth from a kit that I think was meant for kids, it has a potentiometer for pitch and two others that change the character a bit (doesn’t have a filter). It wasn’t meant to be used for actual synthesis or anything but I added an output jack so that I could hopefully use it for that purpose. It’s powered by a 9V battery, and creates a pretty harmonically rich waveform (square or the like, from hearing it)
I sent the output from that into the input of the RC filter (I didn’t have a proper 10K pot so I used a 10K trimpot instead as a temporary measure, but that shouldn’t make a difference)
I sent the output from the RC filter into the 1/8" jack in my attenuator
I sent the 1/4" jack out from my attenuator into my line6 guitar amp. I made sure it was a clean tone with the tone knob all the way open, no distortion, no FX etc.
The sound from the oscillator comes in fine through the filter, I can hear the changes as I’m making them, and it’s not too noisy; it’s just not filtering anything.
I think the solder joints should be fine since the sound is still coming through, and I tested the connections going in and out of the trimpot with a multimeter and everything seemed fine on that end, so I really don’t know what I could have done wrong in this case. My only guess might be that the oscillator is too quiet, but I don’t think that would affect the ability of the filter to roll off high frequencies.
I know that this must be a really stupid mistake, so I’m sorry if I wasted your time! I’ve just tried everything I can and none of the possible issues I’ve come up with make sense.
what is the value of the capacitor you used?
it could be that its too small, making it filter only really high frequencies
I would also double check the solder joint for the capacitor (or if the cap itself is not busted) because if its not connected, it will only act as a very weak volume knob
I used a 0.1 uF/100nF ceramic capacitor, which is what was recommended in the video; in terms of it being broken, I used a brand new one and I think ceramic capacitors are pretty solid or so I’ve heard? Thanks for the recommendation about the solder joints and for helping out a newbie like myself haha, I’ll test them right away!
Edit: Tested the bonds from the cap to the pot and everything seems all good there!
It looks like you got it hooked up all right from here. If sounds comin out, i dont think it would be a bad solder anywhere but between the cap and the pot. If you think that looks alright, then im stumped.
1: What do you mean by that?
2: What kind of a signal are you using? If you use a square wave you have a lot of harmonics and can more clearly hear the effect of a filter. When using a sine you will only hear the effect of a filter if its center frequency comes near the sine’s frequency.
The filter you have now should have an audible effect on frequencies above a few hundred Hz.
1: By “not filtering anything” I mean that as I turn the knob of the pot, the sound remains unaltered (at least to my ears) and doesn’t lose any of its higher frequencies.
I learned synthesis on software synths so I guess I mean that the low pass filter isn’t really exhibiting the sound characteristics that I’ve learned to recognize that a soft synth does when I turn the LPF cutoff knob down in the filter section. I would be somewhat satisfied if it managed to turn the amplitude down without necessarily sounding like a textbook TB-303 filter, but right now it’s not discernibly affecting any parameter of the sound.
2: Right now I’m using a really rudimentary sound source that doesn’t allow me to choose the type of wave, and I don’t have an oscilloscope to prove this, but by the harmonic content alone I would say that it sounds like a pretty rich waveform (probably a square or pulse wave). When I’ve been testing the filter I’ve also been sweeping from the minimum and maximum values of the filter, so I should have been able to hear a difference either way (I think?)
Do you have a multimeter? Check the resistance from the input jack tip to the output jack tip while turning the pot; it should go from something very small to 10k. If the multimeter has the capability, check the capacitance of the capacitor, which of course should be 100 nF (or 0.1 µF). While you’re at it, if you haven’t, check resistance from input sleeve to output sleeve; it should be zero or nearly.
If all three test pass then this has to be acting as a filter at audio frequencies. Since you say it isn’t, it seems to me one of these tests has to fail. If not, can you post a video demonstration?
one thing that might be the case is that the leads on the potentiometer are not tip-wiper-tip as a regular pot, but wiper-tip-tip (just like some transistors have their legs in the BCE order instead of CBE), if its this case, you have soldered the tips of the pot and left the wiper floating, thus having a fixed resistance at all times (which would explain not hearing any change when turning the knob).
without resorting to any measurements, you could try to short the floating lead of your potentiometer with the middle one, as a quick last resort thing. If the problem still persists, then @analogoutput 's tests are the way to go
