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I'm looking for a filtering solution that can shut off the highest frequencies of the signal

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Amanda Lee
I'm looking for a filtering solution that can shut off the highest frequencies of the signal
on Mar 6, 2017 at 9:22:57 am

First time here, so here I am with an electronic-related question. Please note that altough I have a scientific background, I'm quite new to electronics.

Setup

I'm trying to build a small audio amp using a LM386 chip. (Datasheet for reference: http://www.kynix.com/uploadfiles/pdf2286/LM386N-1.pdf )I found a lot of designs on different websites, and I ended up building this :
http://thoduv.free.fr/lm386.jpg

The cap between pins 1 and 8 sets a higher gain for the amp, and if I understood correctly, the feedback betweens pins 1 and 5 helps cutting higher frequencies and reducing hiss.
The input is a simple electret mic, and all this is powered by an Arduino board.

Problem
For my application of this amp, the microphone and the speaker are very close from each other, and I need a high gain. You might see the problem coming : I have to avoid the audio feedback (larsen) between them. This is why I added a simple RC low-pass filter at the end of the audio path (dotted box on the schematic). Its cutoff frequency is roughly around 1kHz, which should be ok because feedback yields frequencies higher than 4kHz.

This seems to be reducing the feedback a bit (I can put the mic and the speaker closer), but it isn't enough : I still get feedback at the same frequencies.

I tried to add a filter right after the mic output (before the amp), but then I can't hear any output signal in the speaker : it seems to be cutting all the input signal.

So, I'm looking for a filtering solution (either using extra components, or taking advantage of the LM386) that could help completely shutting off the highest frequencies of the signal.

I hope my setup and problem is clear. If not, do not hesitate to ask for more details !
Thank you !


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Richard Crowley
Re: I'm looking for a filtering solution that can shut off the highest frequencies of the signal
on Mar 6, 2017 at 8:58:19 pm

If the feedback frequency(s) is STABLE (doesn't change in frequency) then you could use a "notch filter" to remove that feedback frequency from being amplified. This is the method used in large sound reinforcement ("PA") systems. There are sophisticated filters which automatically detect feedback frequencies and attenuate them from being amplified.

You did not reveal what this project is supposed to do, but using a microphone and speaker in close proximity practically guarantees acoustic feedback problems. Might there be some other way of preventing the feedback?

---------------------------------------------------------------------------------
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.


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Peter Groom
Re: I'm looking for a filtering solution that can shut off the highest frequencies of the signal
on Mar 7, 2017 at 1:23:01 pm

Id suggest that, rather than designing out the issue from the electronics by reducing its frequency ability, you could design out the problem by modifying its use!
1) As Richard says, having the 2 near each other is a lesson 1 no no.. But ALWAYS keep the microphone behind any speaker.
2) Consider a better suited microphone. Too general a mic will always struggle. Try a lip mic or headworn mic as these only pick up from a very limited area around the mouth.
3) Consider a feedback controller in the space as heard in rock concerts.
4) Notch filter
Peter

Post Production Dubbing Mixer


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