active bandpass filter design

[birt]

I'm trying to design (and i'm more of a trial and error type of guy without theoretical knowledge :p ) a bandpass filter. i found various calculators on the net.
like this one: http://www.captain.at/electronics/active-filter/?freq=1850&band=3100&gain=10&cap=3&Q=0.6&R1=17113&R2=34227&R3=-59479&xcap=&xR1=&xR2=&xR3=&xQ=&xfreq=&xband=&xgain=&Rs=&series=0&Rl=&Ru=&tcap=&captol=1&tR1=&tR2=&tR3=&restol=1&minQ=&maxQ=&minfreq=&maxfreq=&minband=&maxband=&mingain=&maxgain=

what i want to do is cut everything lower than 300Hz and everything higher than 3400Hz. if i use the calculator i mentioned i get a resistor value below 0... i realised i don't have a clue...

so i might need some help with this -what i thought was an easy to solve- problem

Bert


edit: i need this to use before a very small speaker (like from a telephone) because otherwise guitar (and other instrument) sounds destroy it

[FatMike]

Hi,

try this one:

http://www.4qdtec.com/graph.html


The circuit is quite simple and works great. With those fomulae given you can easily design your own bandpass filters (choosing the desired Q and frequency values). It is quite similar to the Boss GE-7 or the Electro Harmonix graphical EQ.

You can replace the transistor with an opamp, if necessary (voltage follower stage, connect minus input and output).



FatMike

[R.G.]

So is this a lo-fi telephone style filter or an active crossover?

"Bandpass" filters with high and low frequencies separated by more than 2:1 are best implemented as a high pass and a low pass filter cascaded, not as a single bandpass. Under an octave wide you can do OK with a single bandpass filter.

I suggest you read about crossover filters. A good place with some cookbook designs to try is http://sound.westhost.com/projects-3.htm at Rod Elliot's Elliot Sound Products.

[birt]

i think i found what i need.

this might be the most stupid question but is it better to run a high pass filter into a low pass or the other way around? i think it probably doesn't matter but maybe someone here can prove me wrong.

[Rob Strand]

Theoretically it makes no difference but in practice it does.

The best ordering of filter stages is one which reduces noise and maintains full dynamic range (ie. no clipping). Optimal ordering is a non-trivial exercise.  In your case putting the LPF last means the noise from the HPF will be filtered, and reduced, by the LPF.  It's often only a small improvement.