Unworkable band pass filter due to misplacing

[elnino86]

Hello n shalom everyone,

Im designing a pre amp,bandpass filter and power amp.At first of my project,im constructing a ECM pre amp using TS974.It work well(with some help from this forum esp Audioguru).Then i combine it with power amp using TDA2822M which then fed to 32ohm earphone with 90mW power rating.They work well with total gain 10000(80dB).
Then,next thing i had do is to design a bandpass filter(200-20kHz) which require me to use 4th order sallen key filter due to its simplicity and sharpness.I test the filter.It work well as in simulation.

Then,i place it as you can see on my post diagram(between pre amp and power amp).Then,i inject 0.5mV input with 100Hz ferq.,as i measure output of the power amp,the ouput is about approx. 0.15V,which mean im still can hear the sound at 100Hz.I thought my bandpss filter doesnt work well.After several days spend on time thinking of it,i just realize that the signal was amplify at pre amp about 100 gaind,and been attenuate at bandpass filter then amplify again at powere amp which indicate that there nothing much happen on my bandpass filter.

I want reduce,in other way to silent the sound at outer freq. of my bandpass.How do i want to achieve this?My idea are to place the bandpass filter
i)after TDA2822M or
ii)before TS974.

**My first idea seem to be impossible since the TL071 will not able to drive 32ohm earphone.If i want to move on with my 2nd idea,the problem that i thought i will face is mismatch impedance.

Does anyone can help me in this prob?


-elnino wesbo-

 

[audioguru]

I didn't check your calculations for the filters so I assume the highpass is Butterworth and is down -3dB at 200Hz. Then 100Hz is down only -24dB so of course you can hear it.

If the attenuation of 100hz is -60db then it might not be heard. In one octave you need -60dB so the filter needs to be 10th order.

 

[elnino86]

I didn't check your calculations for the filters so I assume the highpass is Butterworth and is down -3dB at 200Hz. Then 100Hz is down only -24dB so of course you can hear it.

If the attenuation of 100hz is -60db then it might not be heard. In one octave you need -60dB so the filter needs to be 10th order.

This mean that i have to design 10th order Bandpass filter right?

**I need 2 quad op amp IC and 2 single op amp IC.Is there any better improvement to achieve unheard sound at 100Hz using only 4th order Bandpass filter since building 10th order filter will required large space.

 

[audioguru]

Why do you want to attenuate 100Hz?
Use a high-Q notch filter instead of a highpass filter.

 

[elnino86]

Why do you want to attenuate 100Hz?
Use a high-Q notch filter instead of a highpass filter.

Actually i want to attenuate acoustic signal below 200Hz freq and above 20kHz.The 100Hz is the signal that i used in my lab as a reference signal.

 

[audioguru]

The 4th-order Butterworth highpass filter is
-3dB at 200Hz,
-24dB at 100Hz,
-48dB at 50Hz which is low but still audible, and
-72dB at 25Hz which is not audible.

The 4th-order Butterworth lowpass filter is
-3dB at 20kHz,
-24dB at 40khz,
-48db at 80kHz and bats might still hear it, and
-72dB at 160kHz where most opamps do not work.