Hearing aid design

Hi
This is my first post in this forum , and i am in need of help, very much to this post.hope you guys wont disappoint me.

First of all i am planning to implement a circuit,a so called energy saver hearing aid(that uses battery for amplifying stages in case when sound is detected), which has schematics as given below.I am aware that this needs lots of improvement in design , the main problem that i face is that what sort of earphone should i use,as sound is amplified to a very good extend ,what should be the voltage ratings? do i need any extra driver circuitry,as i dont have any preknowldge about earphone driving,and above all many more tips and improvements in design to realize this practically.
Thanks in advance

I am very worried about T3, T4 and T5 which have no feedback. Expect uncontrolled gain and lots of distortion - if it works at all!

I would think the 2 sections of LM324 will give enough gain to drive an earpiece on their own. Some kind of automatic gain control would probably be a good addition.

The output level from the electret mic is killed by the low 2.2k resistance of R1 (it should be 10k) and the low input impedance of T1.

The LM324 opamp is one of the noisiest ever made (hiss) and is not suitable for an audio preamp.

The max supply current for the 555 is about 10mA plus 1.4mA for T1 and will kill the 9V battery in about 32 hours.

The 4 transistors audio amp will have high distortion and uses a high supply current, especially the class-A output that puts a high DC current in the earphone.

Why not make or use a low power audio amplifier IC instead?
Then it will have very low hiss and distortion and its battery will last a long time without switching off parts of the circuit.

TR3 should probably be biased similar to TR2 because otherwise, the operating point can be anywhere, and possibly clip the signal.

Similarly, TR4 and TR5 are assured to be operating in their linear range and you have no control over the idle current in that circuit. A second 100k resistor from the base of TR4 to ground would set the base voltage, and a resistor between the emitter of TR4 to ground and another resistor from the emitter of TR4 to the collector of TR5 will allow you to make a closed loop inverting amplifier, thereby fixing the gain and DC operating point.

But JDT has a great point -you can get a lot of good gain with fewer parts with opamps, even though I have used the LM324 for audio, I do not recommend it where high fidelity or noise performance is important. There are so many better opamps available now. Using an opamp instead of discreet transistors, you can leave most of the worry about the operating points of the individual transistors to the IC designer and concentrate on your innovation.

JDT has another good point -a volume control is really important because it will probably need to be adjusted for different situations.

I doubt you will need another driver stage. Modern in-the-ear earphones have efficiencies in the range of 90 to 100 db SPL per milliwatts (the highest quality ones are not often only 10% to 20% as efficient for some reason). So, five or ten milliwatts would be enough in all cases except for those who are stone deaf. It might be a good idea to AC couple the earphone; you want to avoid the case in which the DC through the earphone brings the earphone so close to "bottoming out" that it distorts the audio.

By the way, you might be better off using an LMC555 to minimize power consumption, and as a bonus, you can get a higher output voltage to power your audio stage.

Hey, Audioguru, do they make very low current electret microphone elements for hearing aid applications? Running hundreds of microamps through the FET in the microphone sounds "expensive" in terms of battery life.

Very thanks JDT,but we are limited toa situation which demands only a single supply,LM324 seems to use dual supply, can you tell me a good alternative for LM324 which uses a single supply?

and can u please give a simple AGC model that i can implement to improve my current design?

Very thanks to both DickCappels & Audioguru,
So the point both of you are making is that i should use an IC instead of discrete transistors, can you suggest me few IC which would take a single supply and deliver superior voltage and/or power amplifications?

and regarding 555,Dickchappels,is LMC555 the best alternative here,what is the min voltage with which it works..

Thanks again

ANY opamp can work from a single supply if its input is biased at about half the supply voltage.
With a 6V to 9V supply, a Cmos 555 has an output high current of only about 2mA to 5mA when it has a 2V loss. Its output low current is 35mA to 50mA. Look at the very detailed datasheet for the ICM7555.

An OPA134 is a single low noise low distortion opamp that works when its single supply is as low as 5V. Its supply current is a few mA and it can directly drive a 32 ohm earphone through a capacitor so the earphone does not have DC.
So your hearing aid will be just an OPA134 opamp, a few resistors and a few capacitors.

Oh, you said "we", is this a school project?

Audioguru said:

ANY opamp can work from a single supply if its input is biased at about half the supply voltage.
With a 6V to 9V supply, a Cmos 555 has an output high current of only about 2mA to 5mA when it has a 2V loss. Its output low current is 35mA to 50mA. Look at the very detailed datasheet for the ICM7555.

An OPA034 is a single low noise low distortion opamp that works when its single supply is as low as 5V. Its supply current is a few mA and it can directly drive a 32 ohm earphone through a capacitor so the earphone does not have DC.
So your hearing aid will be just an OPA034 opamp, a few resistors and a few capacitors.

Oh, you said "we", is this a school project?

Click to expand...

Yes ,this is indeed our school project & first attempt ,
I am sorry sir that i couldnt collect any information about OPA034 , it would be fine if you could list the details(or any datasheet) about this IC.

I tried simulating the amplifier. Most transistors are saturated and the output is awful.

Sorry, I made as typo error with the part number of the opamp.
Here is the datasheet from www.datasheetarchive.com for the OPA134 opamp. The OPA2134 is a dual opamp and the OPA4134 is a quad opamp:

Thanks Audioguru
What should be the possible amplifier design? I mean, what should be the gain to be designed for?

The gain for a hearing aid should be calculated for enough loudness which is determined by how deaf a person is.
A person who is very deaf and cannot hear most things needs a hesaring aid with a lot of gain and a very high output level. The high loudness might damage the hearing even more. Acoustical feedback howling might occur.
The ratio of the value of 2 resistors determine the gain of an opamp.

Most deaf people cannot hear high audio frequencies so their hearing aid must have treble boost equalization.

Basically a hearing aid increases the volume of sound frequencies where the impaired hearing does not have enough sensitivity. It can filter noise frequencies that are higher and lower than the vowel sounds in speech. It can compress sound levels so that loud sounds are not amplified as much as lower level sounds.

Look in Google for more functions of hearing aids.