Another quick question... (high pass filter)

[ollie]

Hi,

I'm wanting to create a 'throwaway' effect that creates a lo-fi, 'old radio' kind of sound. I scoured YouTube for an example of what I'm after to post but can'tfind anything, I hope my description is enough.

To do this I thought I could use a passive high pass filter with a high threshold (around 13-15kHz). As far as I know this should only take 2 components: a voltage divider with a capacitor on top and a resistor at the bottom with the output from between. All I get is a highly attenuated signal out though. When I increase the size ofthe resistor the signal is attenuated less.

Am I missing something really obvious? Or was this never going to work from the start?

Also, as this is a passive filter it shouldn't need powering at all (if I can get it working) should it?

Thanks,

Ollie

[jacobyjd]

It would be cool to see something passive work, but I don't know how feasable it would be.

However, for what you're looking for, I've had good results with the Lofo Mofo.

http://www.geocities.com/tpe123/folkurban/fuzz/lofomofo.gif

[bipedal]

Ollie,

Something passive *could* work, depending upon how closely a simple filter mimics what you mean by lo-fi old radio sound (I imagine a touch of nasally, grainy grit when I "hear" this sort of effect in my head), but any passive filter will result in a signal reduction, meaning you'd probably need a simple gain stage for it to be useable...

Changing the resistor's size simply changed your filter's cutoff point i.e. the frequency at which the filtered part of the signal starts being sent to ground rather than to 'out'.  Here's a nice tool for determining that cutoff: http://www.muzique.com/schem/filter.htm

I suspect that sound you're referring to would be accomplished by a combo of low- and high-pass filters, basically resulting in a rather narrow bandwidth of unfiltered signal (just a guess, I don't have any expertise with vintage radio equipment).

Any chance you can borrow an EQ pedal?  Play with the filters at the low and high end of the spectrum to see if you can approximate the sound you're aiming for...  If yes, you'll have a good guide for how to build your custom "lo fi" filtering effect.

[CynicalMan]

Put an old radio transmitter and an old radio receiver in the same box! 

[ollie]

Lofo Mofo looks good, might have to go with something like that

Bipedal; that's the sound, yeah. I tried adding a simple tranny amp on the end but it didn't work, i thought it might not - been a year since I did my electronics a levels, some of the stuff has obviously gone awol lol

[MohiZ]

high pass filter with a high threshold (around 13-15kHz)

I suspect you mean LOW pass filter, because a high pass filter with that roll-off frequency would attenuate every frequency BELOW that. Low pass filter has a resistor on top and cap to ground.

13-15 kHz is a really high frequency, though, at least if you're building a guitar effect. I usually put low pass filters with a roll off at 5 kHz at the end of normal high gain distortion boxes.

[anchovie]

I suspect you mean LOW pass filter, because a high pass filter with that roll-off frequency would attenuate every frequency BELOW that.

I think he does mean high-pass, seeing as he wants to emulate an old radio the approach would be to cut out lots of lows and make it sound like it's coming through a tinny little speaker.

The key is in the name "roll-off frequency" - the range below that point isn't completely slammed down to nothing, it is indeed "rolled off" from that frequency point down to 0Hz, as a curve.

[MohiZ]

I think he does mean high-pass, seeing as he wants to emulate an old radio the approach would be to cut out lots of lows and make it sound like it's coming through a tinny little speaker.

The key is in the name "roll-off frequency" - the range below that point isn't completely slammed down to nothing, it is indeed "rolled off" from that frequency point down to 0Hz, as a curve.

Ah, I get it now. But why not try something like a 5kHz roll-off. Having the roll-off at 13 kHz would attenuate pretty much every guitar frequency at least somewhat. At 6.5kHz you would already be getting a 9 db attenuation, and that's the highest end of the guitar range.

[ollie]

Yeah, might try that when I get home. Managed to order a bag of wrong pots (d'oh) but had some weird and wonderful ones in there including a 75ohm pot which I thought might be useful here