Microphone Feedback to correct Woofer

[tschrama]

Hi guys,

Last weekend I got fascinated by MotionalFeedBack Woofers. I would love to try something similar, but I want to keep is very simple..

I'll atach my idea...What do you guys think of it.. any advise?


Many greetings,
Thijs

[tschrama]

A little update on the schematic, this obe should give better response, allthough the difference is minimal...



Will it work, or blow up in my face?

[hacknet]

i don`t know if i`m right but i don`t think many budget mics will do sub 80hz well. am i right?

[tschrama]

Hi hacknet


Good point. I was thinking about a popular electret from Panasonic. The WM-61 has a very flat response down to 20Hz according to the datasheet. I have recorded sub 10Hz pressure waves from closing doors with it..

But I'm not sure about this...

[li_gangyi]

I don't see the microphone in ur schematics....

[tschrama]

Hi li_gangyi,

R8/R9 represent the microphone.. This is offcourse a simplification... all kinds of delays, transfer-functions properties are not represented..

[johnnyx]

I've used cheap electret mic inserts to measure frequency response. They respond much too well to air currents, draughts and the like, so they have a good response down to fractions of a Hz. Even cheap ones. It's at high frequencies that cheap ones suffer, and maybe at high spls.

Mount the mic in front of the speaker, not inside the box. Change in pressure as cone moves in is not equal to change in pressure as cone moves out, so you will get 2nd harmonic distortion if you mount it inside the box.

The main problem I see is that you will get a power oscillator if you don't take care to minimise phase shifts. It's best to use a 'scope to monitor it. Use a pot at the output of the preamp, turn it up gradually, if it starts to oscillate turn it back down quickly. Reverse the polarity and try again. If it still oscillates, compensation might help.

Try it, I think some others on the forum have done it before.

[Sjef]

I have tried circuits like this some 15 years ago. I also tried to do it with cheap electret mic's but they have a minor problem, sound pressure. The sound pressure at this close to a speaker becomes very high and these little mic's are not able to handle it. in other words, you are getting more distortion from this circuit than without.

It's better to use piezo electric elements directly glueed on to the woofer cone. This can give very good results.

One more thing. You will need a crossover for the speaker and the same crossover for the microphone signal. Otherwise the feedback from the microphone tries to correct the signal for a flat response, in other words, it tries to correct the crossover for the woofer back to lineair.

[tschrama]

Hi Sjef,

Thanks for your comments. I was thinking about using a WM-61 with the 'Linkwitz' modification. This should minimize the distortion from the mic and make it capeble of high SPLs.. I have no idea how effective the mod is... It's a good point to keep in mind.

I don't understand you're remarks about the cross-over, could you explain?

[tschrama]

Just a little update. I've now included the electret microphone, with Linktwitz modification and bootstrapped load.. The JFET and 100pF cap represent the microphone element..