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sensitive low frequency filter

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If the filtered circuit picks-up voices then obviously the 200Hz cutoff is much too high.
For 100Hz, connect a 6.8nF (682) capacitor across RV1, and a 33nF (333) capacitor across R13. It is a very simple 2nd-order filter, so don't expect a miracle. Besides, if a voice occurs in a filter's passband then it will pass it.
Of course, you can help matters if you move the microphone closer to the "beat" and farther away from the voices.
I hope that your "beat" isn't coming from a cheap little radio.
I also hope that your "beat" isn't from high-frequency bongo drums, wood-blocks or cymbals.

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thanks for that audioguru, but i do know about the frequencies of audio structures, i'm just not that good at filter theory. would you mind explaining the theory behind my filter?

I'll start a new thread in the theory forum. Here's the follow on link..


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I just realised that your kit is designed for voices, not music.
C1, C2 and C3 are much too small to pass bass frequencies. Use 0.47 microfarads for C1 and C2, and 1 microfarad for C3. If using polarized capacitors, connect the negative terminal to the transistor's base.
Then maybe my previous 200Hz filter will be OK.

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The filter is working very well now since changing C1, C2 and C3's values to those you specified ,thanx guru.

However the frequency response is very dependant on the setting of RV1 and the intensity of the audio signal, with OR without the 3.3nf cap.

The unit will be used in a very loud room and music volumes will be lower in some parts of the room and higher in others. So this problem is an issue which would need resolving.

Changing the microphone type or adding desctrete extra stages isn't a problem for me, one i was toying with is the addition of the filter circuit here.


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I am glad that the modified circuit works well.
Additional modifications:
1) It will probably be even better with the 100Hz cutoff bigger capacitors.
2) As I explained in your other post (Theory), the additional filter cap across RV1 operates poorly because the low input resistance of the T2 stage loads it down. It works much better with the volume control turned way down. If you replace T2 with a high-input-impedance darlington transistor, and change R4 to about 330K, then this filter cap will work well at any setting of the volume control.
3) Of course the filtered circuit will still respond to loud voices since the simple filter has only 2-stages, and the voices may have frequencies in, or near, its pass-band. An 8-stage, switched-capacitor filter chip could be added, but may not give much better results.
4) Changing the microphone type won't make any difference.
5) Your audio tone-control circuit is designed to gradually boost or cut frequencies below (for bass) or above (for treble) about 1KHz, so won't work for just the beat. Even if it was modified to boost below 100Hz, it won't make much difference because it has only a 1-stage filter.

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