sensitive low frequency filter

[MP1]

I probably should not have gone on so long winded with that last post. Let's continue the filter design theory in the theory section if you want to continue the discussion there. Lets keep this thread going for the low pass filter Rich needs to get going, ok?

MP

 

[electrodoc2]

HOOORAY!! .. after telling the missus off for throwing my velleman kit instructions. i finally found this online.

Scrap the opamp idea. what would i have to do to this circuit (PMK103) to make it indicate 100hz - 250hz ish??

Thankyou.

View attachment 35345

 

[audioguru2]

Electrodoc,
What happened to the previous low-bass circuit? Now you want 100-250Hz high-bass? You're making a colour-organ! I can see midrange coming next, then........
It would be complicated to add low-pass and high-pass filters to that kit circuit, and calculating its existing response is difficult due to its feedback at each stage.

 

[electrodoc2]

I require a led to flash to the Bass Beat of music using a microphone as a pickup source, hopefully using the PMK103 circuit outlined above.

 

[audioguru2]

Electrodoc,
A very simple 200Hz low-pass filter can be added to the Velleman kit:
1) Add a 3.3nF capacitor across RV1.
2) Add an 18nF capacitor across R13.
The LEDs will flash to the bass and beat of the music.

 

[electrodoc2]

??? Thanks audio guru. I used a 22nf + a 2.2nf for the 18nf and the 3.3nf respectively ( numbers on caps where 223 (22nf) and 222 (2.2nf). If these capacitor code translations are wrong then please mention it.

shurely these values should be ok? but it wouldnt play ball.

The 2.2nf was tried between the current limited power output of R8 and T1 collector. also tried between the wiper of RV1 and T1's collector and between the wiper upto R8'S output. But to no avail.

It would flash to the beat of the music but also flash to any other noise (ie talking in the room).

Please help.


thanks,, Rich

 

[audioguru2]

Electrodoc,
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.

 

[electrodoc2]

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..

www.electronics-lab.com/forum/index.php?board=15;action=display;threadid=793&start=0

 

[audioguru2]

Electrodoc,
Did my latest modification (100Hz cutoff) and recommendations work?
Is the music played through big speakers, or is it live?
Please describe the "beat" sound: Bass drum, bongo, wood-blocks or cymbals?

Don't connect the capacitor to the wiper.

 

[audioguru2]

Electrodoc,
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.

 

[electrodoc2]

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.

View attachment 35371

 

[audioguru2]

Electrodoc,
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.