a low cost hearing aid

Thread Starter

luc

Joined Mar 4, 2011
153
The LM386 power amplifier is cheap and is used in kids toys. It has hiss which is not good for a hearing aid. Its minimum supply is 4.0V or 5.0V. What will you use to power your hearing aid? Three button or AAA alkaline cells are 4.5V when brand new then drop to 3.0V. An LM386 might not work when the voltage is less than 4.0v or 5.0V. Find a newer amp that will work with a very low supply voltage.
i am using a 9V battery with 5V voltage regulator as my supply. So i have to use other type of amp instead of lm386? May i know lm 831 or mc33078 suitable to be a power amp?


Thanks!
 

Audioguru

Joined Dec 20, 2007
11,248
An ordinary 5V regulator has a 7V minimum input voltage. When a 9V battery quickly drops to 6V then the regulator will not regulate anything and the 5V might become 4V.
You could use a low-dropout 5V regulator with a 9V battery.

The LM831 had 16 tiny surface mount pins, was a stereo or a bridged power amp and has not been made for many years. Digikey and Newark do not have any.
The MC33078 is an ordinary dual opamp that needs a minimum supply of plus and minus 5V. It cannot drive an earphone.
 

Thread Starter

luc

Joined Mar 4, 2011
153
An ordinary 5V regulator has a 7V minimum input voltage. When a 9V battery quickly drops to 6V then the regulator will not regulate anything and the 5V might become 4V.
You could use a low-dropout 5V regulator with a 9V battery.
can i use LM2931 as my low-dropout 5v regulator? the connection is similar with 7805?

The LM831 had 16 tiny surface mount pins, was a stereo or a bridged power amp and has not been made for many years. Digikey and Newark do not have any.
The MC33078 is an ordinary dual opamp that needs a minimum supply of plus and minus 5V. It cannot drive an earphone.
how about LM675? can give me some suggestion about type of amp that normally use to power amplifier ?
 

Audioguru

Joined Dec 20, 2007
11,248
can i use LM2931 as my low-dropout 5v regulator? the connection is similar with 7805?
I use the LM2931 5V low dropout voltage regulator for all my 9V battery powered projects. Look at its datasheet to see how it is connected and to see all the different packages available. It must have a 100uF output capacitor. Its dropout voltage is typically 0.3V when the load is 100mA.

how about LM675? can give me some suggestion about type of amp that normally use to power amplifier ?
Please stop selecting power amplifiers at random without looking to see if they are available and to see the minimum supply voltage.

The LM675 is a power opamp, not an audio power amp. But if you add many external parts to bias it and apply feedback to it then it can be a 20W audio power amplifier. It is available. But its minimum supply is 16V and its max supply current when it is doing nothing is 50mA.

I have never built a hearing aid. There are many low power audio power amplifiers available but most are extremely small. Look at National Semi and Texas Instruments.

You never said the impedance of your earphone and if two earphones will be used in parallel.
 

Thread Starter

luc

Joined Mar 4, 2011
153
Please stop selecting power amplifiers at random without looking to see if they are available and to see the minimum supply voltage.

The LM675 is a power opamp, not an audio power amp. But if you add many external parts to bias it and apply feedback to it then it can be a 20W audio power amplifier. It is available. But its minimum supply is 16V and its max supply current when it is doing nothing is 50mA.

I have never built a hearing aid. There are many low power audio power amplifiers available but most are extremely small. Look at National Semi and Texas Instruments.

You never said the impedance of your earphone and if two earphones will be used in parallel.
i plan to use an earphone with impedance of 32ohm with frequency response is 20-20khz,and the sensitivity is 110dB/mW.

i search from texas about audio op-amp, there have many category which are headphone amplifier; audio operational amplifier,portable amplifier and so on, but i did't see power amplifier category, so i look into these few type of category and i found that some op-amp have power amplifier, but i notice that all of it don't have pdip pin which hard to implement into my project..
 

Audioguru

Joined Dec 20, 2007
11,248
The TDA2822M little power amplifier IC will be fine. It has two power amplifiers that can be separate or can be bridged for more output power. It can drive loads from 4 ohms to more than 32 ohms. PCB designs for both ways are shown in its datasheet.
 

Thread Starter

luc

Joined Mar 4, 2011
153
The TDA2822M little power amplifier IC will be fine. It has two power amplifiers that can be separate or can be bridged for more output power. It can drive loads from 4 ohms to more than 32 ohms. PCB designs for both ways are shown in its datasheet.

may i know the input of power-amp need a bias voltage?

Thanks!
 

Audioguru

Joined Dec 20, 2007
11,248
may i know the input of power-amp need a bias voltage?
You forgot to look at its datasheet. It shows many circuits with the input grounded through a piece of wire, a resistor and a volume control.

So its audio inputs are biased at 0V like many audio amplifier ICs.
It also has built-in negative feedback which makes its voltage gain about 90.
 

Thread Starter

luc

Joined Mar 4, 2011
153
You forgot to look at its datasheet. It shows many circuits with the input grounded through a piece of wire, a resistor and a volume control.

So its audio inputs are biased at 0V like many audio amplifier ICs.
It also has built-in negative feedback which makes its voltage gain about 90.
Is that mean i no need another bias voltage to bias the signal above 0V?
I am a bit confuse, i have only 1 output from 3-band tone control, and then it will connect it with a volume control. I am using a stereo earphone ,but i only have 1 output from volume control, which mean that i have to use bridge circuit instead of stereo circuit in data sheet?

Thanks!
 

Audioguru

Joined Dec 20, 2007
11,248
Your tone controls circuit has an output coupling capacitor to the volume control which is grounded. The TDA2822M has its input grounded by the volume control and it is designed to work properly with its input grounded so another coupling capacitor is not needed from the slider of the volume control to the input of the TDA2822M power amplifier.
 

Thread Starter

luc

Joined Mar 4, 2011
153
Your tone controls circuit has an output coupling capacitor to the volume control which is grounded. The TDA2822M has its input grounded by the volume control and it is designed to work properly with its input grounded so another coupling capacitor is not needed from the slider of the volume control to the input of the TDA2822M power amplifier.
For the input of tda2282m, there have 2 input for stereo circuit and 1 input for bridge circuit. If i am using stereo circuit, so i have to connect both input to slider of the volume control? Or i can only use the bridge circuit?

For the output of tda2282m, there have 2 coupling capacitor for each output and due to coupling capacitor is no needed from the slider of the volume control to the input, the total CR network in circuit still remain to 5, so the CR network value in the circuit is not need to recalculate?Do i need to change both output of CR value in the datasheet to match my cutoff frequency which is 10Hz for each?

Thanks!
 

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Audioguru

Joined Dec 20, 2007
11,248
I don't know how much sound is too loud for a deaf person. If it is loud enough for the deaf person to hear then it might be loud enough to destroy what remains of the hearing.

When the amplifier is bridged then its output power is 4 times higher than when it is a stereo amplifier. So use the stereo amplifier circuit but short the input of the unused amplifier to ground.

The 10k input resistor is not needed. The volume control will be there.
 

Thread Starter

luc

Joined Mar 4, 2011
153
I don't know how much sound is too loud for a deaf person. If it is loud enough for the deaf person to hear then it might be loud enough to destroy what remains of the hearing.

When the amplifier is bridged then its output power is 4 times higher than when it is a stereo amplifier. So use the stereo amplifier circuit but short the input of the unused amplifier to ground.

The 10k input resistor is not needed. The volume control will be there.
May i know, now my total of CR network become 4,so the total cutoff frequency become 40Hz, should i modify the CR value or just remain, and change it to 40Hz?

I have draw the schematic for my power-amp, can you help me check it whether correct or not?

I plan to design a Battery Life Indicator in my hearing aids to indicate when battery low,but i not sure whether the circuit work or not,so i have attach the circuit diagram below.



Thanks!
 

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Thread Starter

luc

Joined Mar 4, 2011
153
I think you made many mistakes with the power amplifier.

There are also mistakes with the battery voltage indicator because the red and green LEDs will never be turned on at the same time.

Do you mean that the CR network that connect at output is the 470uF with the load instead of 470uF with the 4.7 ohm?Because previously for the output coupling capacitor, i set the cutoff to 10Hz, so my C is 470uF and R is 33 ohm instead of 4.7 ohm. So now the R is refer to load resistance ? Do i need to sum the value of load resistance with the resistance in earphone(ect: 33ohm + RL then calculate the C)?.

For my load resistor, it should have a value lower than 220 ohm? etc: i am using a 32 ohm resistance of earphone, so my load resistor will be greather than 32 ohm but can not be too large?May i know what is the limiter use for?

Thanks!
 

Audioguru

Joined Dec 20, 2007
11,248
The load on the power amplifier is the earphones (two 32 ohms earphones in parallel are 16 ohms in total) plus the limiter resistor value if it is used.
The 4.7 ohm resistor and 0.1uF capacitor are a load only at high frequencies to prevent the IC from oscillating because the inductance of a speaker causes its impedance to be high at high frequencies. Do not change the value of the 4.7 ohm resistor.

The output coupling capacitor value is 1000uF for a cutoff frequency of 10Hz when the load at low frequencies is 16 ohms. If you use one 32 ohm earphone in series with a limiter resistor of 33 ohms then a coupling capacitor of about 220uF gives a cutoff frequency of 11.2Hz.

The limiter resistor stops the earphone from blowing up the deaf person's ear.
The amplifier has an output power of 120mW into a 32 ohm load.
Earphones produce a sound pressure of 110dB with an input of 100mW so the loudest sound will be extremely loud without a limiter resistor in series with the earphone.
You should ask an ear doctor if 90dB is too loud for a deaf person like it is for a person with normal hearing.
 

Thread Starter

luc

Joined Mar 4, 2011
153
There are also mistakes with the battery voltage indicator because the red and green LEDs will never be turned on at the same time.
i draw the schematic for battery voltage indicator and try to run it on stimulator.When my battery voltage is greather than 7.1V, my green LEDs will turned on, when my voltage below 7.1V, my green LEDs will turned of but my red LEDs does not turn on. The green LEDs should turn on when voltage is greather than 5.5V and turned off when the battery voltage is less than 5.5V, while red LEDs will turned on right? May i know was i connect wrongly?

Thanks!
 

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Audioguru

Joined Dec 20, 2007
11,248
The simulator cannot show if an LED turns on or turns off because when an LED is turned off then its voltage is "floating".
Add a resistor in parallel with each LED to draw maybe 0.2mA (then each resistor will be about 2.1V/0.2mA= 10k) so that the voltage will rise to the supply voltage when an LED is turned off.
 

Thread Starter

luc

Joined Mar 4, 2011
153
The load on the power amplifier is the earphones (two 32 ohms earphones in parallel are 16 ohms in total) plus the limiter resistor value if it is used.
The 4.7 ohm resistor and 0.1uF capacitor are a load only at high frequencies to prevent the IC from oscillating because the inductance of a speaker causes its impedance to be high at high frequencies. Do not change the value of the 4.7 ohm resistor.

The output coupling capacitor value is 1000uF for a cutoff frequency of 10Hz when the load at low frequencies is 16 ohms. If you use one 32 ohm earphone in series with a limiter resistor of 33 ohms then a coupling capacitor of about 220uF gives a cutoff frequency of 11.2Hz.

The limiter resistor stops the earphone from blowing up the deaf person's ear.
The amplifier has an output power of 120mW into a 32 ohm load.
Earphones produce a sound pressure of 110dB with an input of 100mW so the loudest sound will be extremely loud without a limiter resistor in series with the earphone.
You should ask an ear doctor if 90dB is too loud for a deaf person like it is for a person with normal hearing.
Hi, i am just testing my power amp which connecting the output of pre-amp(include mic) to a volume control ( before it have 1 CR network) and then connect it to input of power amp which have a earphone connect at the output of power-amp, and i found out that there have alot of noise ( very noisy) can be hear by earphone with the minimum volume, when adjust to maximum, it will be very loud noisy sound come out from earphone.. may i know what is the problem?

Thanks!
 
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