good band pass filter schematic?

Started by caress, January 31, 2007, 03:13:04 PM

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caress

i'm interested in a decent active band pass filter schematic centered around 2khz with maximum cutoff and relatively high Q.  more or less an am radio-style sound...i have the active filter cookbook and started an idea, but i'm still working on wrapping my head around the math.  right now i'm using a LPF into a HPF, but am unsure about getting a higher Q...
if anyone has a simple schematic that could help me out or just any ideas on getting the math right, it would be greatly appreciated.  thanks

Mike Burgundy

I used to have a spreadsheet around that I built for just such a purpose. You fill in the component values for a run-of-the-mill gyrator parametric EQ circuit and it spews out the Fc, F-6, Q and range of frequency. Can't find it right now but if I run across it I'll drop you a line. I'll have a look at my backups tomorrow or the day after.
This won't give you a true bandpass, rather a really big boost (or cut) at the chosen frequency, but I like the sound and versatility of this myself.

John Lyons

I don't have any solid information for you but I wonder if you make a multi pole low pass filter into a multi pole high pass if it would raise the peak?
I know that multiple poles increase the roll off DB for LP and HP filters. I have not tried it but what about the LOFO MOFO?
Even for a lo fi sound you want to have some low mids and highs. If you just have the mid bands it gets real ugly quick. (in a bad way)

Take a look at the EQs on Geofex and ESP (elliot sound pages)  you would have to compound those filters but it would give you a starting place. 
John

Basic Audio Pedals
www.basicaudio.net/

caress

i don't want any lows, low mids or highs.  just the area around 1-2khz +/- with a boosted Q around the center point.  if i could cut the other frequencies completely or close to it, that would be perfect.  i've used many different plug-in filters to get this sound easily, but i was interested in making one for someone else.

ugly is only one point of view... ;)

Meanderthal

 Try a wah circuit for resonant bandpass filtering.
Or the lo-fo mofo maybe for that tiny speaker phone/pocket radio sound.
I am not responsible for your imagination.

John Lyons

Quote from: caress on January 31, 2007, 06:59:08 PM
i don't want any lows, low mids or highs.  just the area around 1-2khz +/- with a boosted Q around the center point.  if i could cut the other frequencies completely or close to it, that would be perfect.  i've used many different plug-in filters to get this sound easily, but i was interested in making one for someone else.

ugly is only one point of view... ;)
Have you played any of these plugin simulations through a guitar amp (assuming it's a guitar circuit) 1-2K is a very nasty frequency. Instant headache material. I'm not questioning your style of music or taste. I'm just telling you from experience that the frequencies by them selves is a  nail in the forhead type sound. Although this may sound even better to you....  at any rate, try iy out...

John

Basic Audio Pedals
www.basicaudio.net/

R.G.

Pretty simple, really. State variable filter. Three or four opamps, but Q's in the hundreds available.

For bandpass filters, bandwidth in octaves is the inverse of Q. A 1 octave wide filter is a Q=1 filter. A half-octave wide filter has  q=2, and so on.

For realizable filters, the ultimate slope of a single section is -6db/octave, so you'll need multiple sections if you want to really eliminate the sidebands.

I'm thinking that the plugins you're using are DSP based plugins, and you may have problems creating that same filter in the analog world. You may need a second or third order highpass just below the band you want, an equivalent order low pass just above what you want, and a state variable working in the overlap.  That should get you closer to what you describe.

If, however, what you want is AM-radio style sound, try the Lo-Fi at GEO first.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

caress

john - i tested out on my fender twin and a peak around 1.4-1.6khz sounded good.  higher than 2khz and around 600-800hz were both painful...

r.g. - the filter doesn't have to sound exactly the same as the plugins, just approximated...i was originally thinking two dual opamps for a 3rd or 4th order LPF and HPF (just below and above the band...exactly) with an output amplifier tagged on.  i guess i wasn't too sure if i actually needed a state variable filter as well...hmmm...i might try the lo-fi after all to get my mind working.  are the two transistors any noisy bjt's?  what are the opamps used?  would duals be ok;  one for the buffers, on for the filters?

DDD

Try the input filter from the B@ss Metalzone (approx 900-1000 Hz). Or couple such filter in series. Very simple and 10 million times tested circuit ;-)
Too old to rock'n'roll, too young to die