hearing aid

Thread Starter

syshanika

Joined Apr 11, 2010
16
by the way i'm going to reduce the gain of lm 386 to about 20 using a resister between pin 1 and 8 of lm386 ..so will it work????
and it seems i have come across with new problem that i cant maintain constant volume seems i need something like automatic gain control ..is there any possibility that i can use a AGC circuit with my circuit

thnks.
 

Audioguru

Joined Dec 20, 2007
11,248
The LM386 power amp IC is supposed to use a capacitor or a capacitor in series with a resistor to increase its gain. With nothing between pin 1 and pin 8 then its gain is 20. A resistor without a series capacitor messes up its DC biasing.

An NE5532 has the same pins as an LM358 but has very low noise and a wide bandwidth.

An NE570, NE571, NE572 and NE575 used to be pretty good automatic level controllers but are not made anymore. You can rectify the signal and use a Jfet to attenuate it when sounds are too loud.
 

Thread Starter

syshanika

Joined Apr 11, 2010
16
yeah ..i changed pre amp to ne5532 ..seem much better ..and i thought of remove cap between pin no 1 nd 8 of lm386
by the way i have no idea abt control the gain using jfet would be thankful if a circuit diagram can be given
by the way i found these circuits
http://www.sound.westhost.com/project67.htm
bt i cant figure how to use these in my priject need some help to select one gain control circuit
 
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Thread Starter

syshanika

Joined Apr 11, 2010
16
at last it is done it's working quite well thnks all of u for ur help nw doing some developments is there any way of shield the equipment easily from cell phone signals it make high noise
 

retched

Joined Dec 5, 2009
5,207
Yup. Its rather easy too.


Turn the cellphones off. ;)

Very short leads, use only shielded wires, and filter out anything above the frequencies you dont care about.
 

Audioguru

Joined Dec 20, 2007
11,248
In addition to using a shielded audio cable from the microphone if the entire circuit is in a metal case connected to 0V then cell phone signals will not affect it.
If the circuit is built on a breadboard then everything will affect it.
 

Audioguru

Joined Dec 20, 2007
11,248
I think the aluminum foil will block electro-static waves but will pass electro-magnetic waves. I think a radio signal has both. Try aluminum foil to see if it is effective as a shield.
 

Audioguru

Joined Dec 20, 2007
11,248
so a parallel capacitor is a simlple low pass filter
where should i use that capacitor ?
parallel with mic or parallel with output
An RC is a lowpass filter. The resistor is in series and the capacitor shunts higher frequencies to ground.
If you connect a capacitor across the extremely low output impedance of an opamp or a power amp then the amp will probably oscillate.
 
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