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Everything Else » Personal sound Amplifier

May 28, 2013
by dgikuljot
dgikuljot's Avatar

Hey guys, So my grandfather has recently lost the ability to hear a conversation going on at normal volume. He can however hear on the phone and through headphones.

I wanted to build him a little device where the sound can be picked up by a normal electret microphone and played through headphones. I have looked at lots of schematics but am not really sure which one to go through. Basically I just want to build a circuit that can pick up sound from a distance you would normally be able to hear from and amplify it and play through the headphones. What is the simplest, yet decent quality, and low power consumption circuit you guys can recommend?

Any suggestions or schematics would greatly be appreciated.

May 28, 2013
by BobaMosfet
BobaMosfet's Avatar

First, a word of caution. You don't want to damage his hearing further or rupture an eardrum. Because you are considering hooking an unthinking machine into a biometric capacity-- inducing sound into perishable human tissue, you need to be cognizant first of the risks. Volume and frequency.

Limit volume, and limit frequency.

You can use a piezo, a bjt as your amp, and then you'll need to use a few resistors and capacitors to make the appropriate filter(s) to clip out unwanted (60Hz) noises, as well as dangerous frequencies. Find the Q-point on the BJT, and you can use it to limit the amplitude (volume).

Happy hunting,

BM

May 29, 2013
by pcbolt
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dgikuljot -

It's interesting that your grandfather can hear a conversation on the phone because the phone circuitry limits frequencies to about 300 Hz to 4,000 Hz, well within the limits of human hearing (30 to 20,000 Hz). As people age they usually lose the high frequency which explains why normal conversations get garbled since "S" sounds and other high frequencies get lost. Oddly enough, if you talk to the back of someone's head, the cranium acts like a low pass filter and cuts out high frequencies.

I'm not sure what kind of headphones your grandpa is using, either powered and amplified or just the type you plug into an Ipod, but the common element is background noise suppression. Usually the output from any device is "line level" being defined as just under +0.5 volt to -.5 volt with low output impedance. This should be the target range for your amp.

With these ideas in mind, you should try to build a device that amps up the tiny voltage from the electret mike to the +/- .5 volt range and attenuates low frequencies (which he should hear) and pass frequencies in the 4,000 Hz to 8,000 Hz range. You should also heed Bobamosfets advice and limit higher frequencies and volume.

There are commercial product on the market that do all this, but I certainly understand the sentiment in building one for him.

May 30, 2013
by dgikuljot
dgikuljot's Avatar

Hey Guys, Thanks for the replies. I didn't know things would be this complicated. Certainly doable, just requires alot of planning. My theory is he can hear everything on phone and such because the frequency of the voice probably gets changed. Also maybe it is because the voice is being placed right next to his ear versus from a few feet away.

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