Hello, i want to design an low pass and high pass passive RC filter of 24db/octave, i use LT spice software to modelize the frequency of the filter i want to make but the filter slope at the corner frequency (cutoff frequency) is always very high. I believe that's because my filter is overdamped but what are the formulas that include R and C values that can make help me calculate Q's value. Thanks for your further reply.

 

A completely passive RC filter is always overdamped, and has a low Q.
They are normally not used for high roll-off purposes, since their rolloff corner is so soft.

Only active (Sallen-Key or Multiple-Feedback) RC filters can have a high Q with a sharp rolloff.

 

Thanks for your usefull reply. Do you think that, if, by example, I want an usable frequency range of 30-100 hertz, since passive rc filter is overdamped, is it a good idea to use a cutoff frequency of 300hertz for the first filtering stage, one of 600 for the second, one of 900 for the third and one of 1200hertz for the fourth? This compromise allows me to benefit of lower attenuation in the wanted frequency range, thus better linearity, all of this with greater attenuation in higher frequency. I want to use passive filter because my application is an multi-amplificaton system with passive filters in preamplification dedicated for each way (3 in total). I think that allow better sound quality because the purity of the signal is less degraded than with active rc filters. Did my idea is good or not? Also, for a low pass rc passive filter, did the value of resistor is a big concern, as i want the lowest possible value for the resistor to minimise power losses and signal reduction? Thanks

 

is it a good idea to use a cutoff frequency of 300hertz for the first filtering stage, one of 600 for the second, one of 900 for the third and one of 1200hertz for the fourth?

Don't know.
The easiest way to determine what the filter output will look like is to do LTspice simulations of the filter.

I think that allow better sound quality because the purity of the signal is less degraded than with active rc filters.

If you use a high quality audio op amp (such as the NE5532) for an active filter, any signal degradation will be insignificant and undetectable to the ear (even a Golden one).

 

Ok, i will listen your advises. And can i use an low capacitor value for an high pass rc filter (like 1nf and 24kohms) or this is bad and i must use 10 nf and 2400ohm resistor? Thanks

 

can i use an low capacitor value for an high pass rc filter (like 1nf and 24kohms) or this is bad and i must use 10 nf and 2400ohm resistor?

I think you mean 10nF and 2400 ohms.
Either should be fine.

 

A good tool for filter design -

http://www.ti.com/tool/FILTERPRO&DCMP=HPA_AMP_GENERAL&HQS=NOTAPPLICABLE+OT+FILTERPRO

It shows as "NOT RECOMMENDED FOR NEW DESIGNS", but its still a good tool.


Regards, Dana.

 

If you can be satisfied with an 18dB/octave rolloff, this tool will help you design 3-pole single op amp low-pass and single op amp high-pass active filters.

24dB/octave will require 4 op amps.

 

Jean1320...from your first post I derive that you are going to design a bandpass filter, is that correct?
As a first step - before starting any design - you must DEFINE your requirements:
* Passband with lower and upper corner frequencies
* Stopband (frequencies with corresponding minimum attenuation requirements)
* From these specifications you can derive (calculate) the necessary filter order.
(Of course filter design programs are a good help).