LM386 Hearing Aid Circuit needs improvement.

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gary350

Well-Known Member
The ½w circuit that I built works but it needs improvement. Electret Mic is not sensitive I need to talk 4" from the mic to get a good clear output to ear phones. Which mic will work better? I need ear buds not these ear phones. There are no specs on these ear phones my TV has trouble getting a loud signal our of these ear phones. Power supply is 12vdc 1.8a. I wonder if ear bud package has specs? I am off to Walmart in about 1 hour to look for ear buds. If I can make this circuit work better I will make another circuit board so I have an amp for each ear. Amplification is 200, maybe it needs to me higher or a better Mic? I also have a bass & Treble circuit I can add on for a small amount of tone control. I only have LM386 not the LM386 n-1.

 
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Most deaf people have "normal for their age" high frequency hearing loss. My hearing aids boost high audio frequencies twice as much as a treble tone control that is at maximum.

The sensitivity for your electret mic and preamp is normal for a microphone in your hand in front of your mouth but the 200 gain preamp is feeding another power amplifier that has more gain.

Do not use the preamp that has an output of 1/2W because any loud sound will destroy your hearing. 0.1W will be loud and can be produced by the LM386 powered from 6.5V and feeding a pair of 32 ohm earphones.

Try this gain boosting circuit with an Rf of about 47 ohms:
 

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Is this a new circuit to replace the circuit that I built?

What is IN.? Is that a mic?

rf 10 ohms = 1800 gain.

What about volume control?

Yes, I have high frequency hearing loss. Sound needs to be 60db for right ear to hear it, 100 db for left ear to hear it.
 
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My LM386 circuit is the same as your circuit except pin 1 and pin 8 have gain boost parts added. Try using 47 ohms for a gain of about 340 times.
Since you have high frequency hearing loss then the amplifier might be clipping at low frequencies.
 
Wow, ear buds make an amazing improvement I can hold the mic at arms length 26" away and talk, I hear it perfect. But I have terrible feed back. How do i stop feed back. ???


 
gary350 said:
Wow, ear buds make an amazing improvement I can hold the mic at arms length 26" away and talk, I hear it perfect. But I have terrible feed back. How do i stop feed back. ???


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The microphone is picking up the output of your headphones (ear buds).

You need to put a baffle around the microphone to make it directional (and turn down the volume to the speakers (earphones). I once used 1-1/2" pvc pipe with a bottom cap, mounted an electret inside of a 1/2" pvc pipe, wrapped it with fiberglass insulation and slid it sinside a 1-1/2" pvc pipe. Make sure the microphone is well recessed deep into the tube (4-6"). This worked pretty well for a shot-gun type microphone and minimized feedback from the stage speakers.
 
I replaced the AC/DC power supply with 3 Lithium batteries and all the feed back is GONE. Wow that was unexpected. The circuit is almost good enough to use as is for a hearing aid. A 47 ohm resistor is a very easy test to do next, before making cardboard ears.

I put the amp mic inside a 12"x12"x24" cone about 10% volume is too much. I can hear the neighbor talking 230 feet away but I can not hear birds in the trees. Wife says she can hear birds. Will a high pass filter make the circuit have more high frequency.?

 
If you cannot hear birds then you also probably cannot hear an old phone ringing or hear a fire or CO alarm (like me without my hearing aids). You need a high frequencies boost.
If the earphones are vented then the high frequencies boost will probably produce feedback.

My hearing aids have vented earmolds so that my ears do not feel plugged up and so that my own voice sounds natural. The frequency response has some notching at the feedback frequency.
The high frequencies boost is enough to sound good and causes no feedback.
Automatic Gain control and Compression are included so that low sound levels are amplified as much as is needed but loud sounds are not amplified as much to avoid overload distortion and so that the battery charge lasts longer.
 
You can get some small mic modules that have a preamp with AGC included. I don't know if they could help to get better gain and level control? Add again preset between that and the LM686.

Search "Adafruit MAX9814".

Example:
 

I would like to have some of those programmable mics. I can see my cone is totally wrong. A straight tube will work best it will receive a cone shape signal, it needs to be designed to receive a range of 3' to 5' to talk to people in that range also another tube to receive 7' to 10' range. 3 different tubes for 3 different ranges with a selector switch might work great. Cones and tubes do not amplify, they concentrate same as a magnifying glass. A tube with a trumpet shape end will be better.

 
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Each of my hearing aids has four microphones for directionality, left, right, front and back. The earphones do not have front and back but somehow I can hear front and back directions. Most of the time, the directionality keeps the mic with the loudest voices turned on and the other mics turned off. The mics are not abruptly switched on and off, instead they are faded, added or subtracted.
 
The electret Mic is noise cancelling in the rear mesh for short range reception. By sealing the rear and sealing the front except for a pin hole aperture in front, you will match impedances to transfer more acoustic power. I did this for an old webcam, and then it worked well as a living room + kitchen speakerphone mic with software half-duplex "ducking" = where extra loud near-field sounds from your voice are attenuated to your earbuds. This can work even better with dual mics to cancel near-field differential and boost common-mode far field.

If you still get feedback, we need to know the pitch and what makes it better and worse.
 
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If you have matched microphones (at least the same microphone from the same manufacturing lot), you can put two at 90° to the target direction and subtract out the average signal from the target microphone's signal. No software needed, just wire in as an op amp adder circuit.
 
That won't perform ducking, just far field cancellation
 
No one sells toy trumpet horns anymore. I can't make a trumpet shape with cardboard. I could build a, 4, 6, or 8, side funnel shapes. Cut end off of a 2 liter soft drink bottle radius is in the wrong direction. Soft drink can aluminum has memory cut pieces refuse to be bent to any shape it tries to curl up again. Empty 1 gallon milk jug ends have a handle. I need two, 4" or 6" diameter trumpet ends, ???.
 
Today I built a trumpet shape but not with a continuous changing radius. I did 25° and 55° the tube is 1" square 8" long. This cardboard trumpet is surprising how well it works. I only have amp volume at about 10%, no hum, no feed back. I can hear wife talking at the other end of the house. I turned volume down to ??? maybe 4% for room use volume only needs to be about to talk to people 10 ft away. I assume trumpet shape is picking up a much better signal for mic to receive so well. Now that I have built this trumpet I am sure I can build another square trumpet with a nice radius. If I could find a graph of a real trumpet radius I could copy it very close. This is a fun and very educational project the amp does not need high implication like I thought, the trumpet needs to be able to pick up a good amount of sound for the mic. This is what you call, research and development.

 
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