Require An Aid For Hearing Aid

[audioguru]

The electret mic is operating at such a low current that its impedance is 10k ohms. The 100nF capacitor in parallel with it has an impedance at 2kHz of only 800 ohms so it is a "short circuit" at a mid frequency.

The 47nF capacitor to ground is also a "short circuit" at a mid frequency.

They kiss the high audio frequencies, "Goodbye".

 

[leonidus]

Where do you get the idea that a capacitor acts as a "short circuit" at mid-frequency?

From, Xc=1/(2ΠfC)!!!
Say, a cap with 10uF capacitance at 1kHz has a reactance of 15.915Ω can be assumed to be short.
Also, AG has given an expln. regarding the ckt..
How a coupling cap b/w transistor1 & pot would solve the prob. of shorting?
A resistance offered to dc current(Rdc) tends to 0 for an inductor. So, I think the impedance mentioned of spk. is its ac resistance & should be taken as a short in dc analysis. Pls. explain.

 

[colin55]

You really don't have the correct concept of a capacitor. Firstly you have to explain where the capacitor (electrolytic) is placed in a circuit, then you can talk about its impedance at a particular frequency and we call this CAPACITIVE REACTANCE.
You cannot say a "capacitor acts as a "short circuit" at mid-frequency" because you are inferring (saying) that DC will pass thorough it.
That's why you need to use the correct terminology.
A capacitor or electrolytic has a low or very low impedance (resistance) at high frequency due to the value of CAPACITIVE REACTANCE of the capacitor.
Even though the word "impedance" is generally applied to the value of an inductor, it can also refer to the state of a circuit where the resistance is HIGH or LOW but this resistance is not simply made up of a set of resistors. In this case we say the circuit has HIGH IMPEDANCE or LOW IMPEDANCE.

 

[Sceadwian]

colin, I didn't know DC classifies as 'mid frequency' If you're talking about a capacitor and someone says 'it's a short at mid frequency' that's pretty easy to understand, I've even seen it written in text books and on web sites like that. You could probably spend hours arguing about what exactly classifies as a short though =) According to my multimeter though anything under about 50 ohms rings the continuity buzzer

 

[colin55]

We are on two different trams.

 

[audioguru]

Instead of the Talking Electronics project with its very narrow audio bandwidth and high distortion, this circuit project be used: Amplified Ear - RED - Page38

 

[leonidus]

You really don't have the correct concept of a capacitor. Firstly you have to explain where the capacitor (electrolytic) is placed in a circuit, then you can talk about its impedance at a particular frequency and we call this CAPACITIVE REACTANCE.
You cannot say a "capacitor acts as a "short circuit" at mid-frequency" because you are inferring (saying) that DC will pass thorough it.
That's why you need to use the correct terminology.
A capacitor or electrolytic has a low or very low impedance (resistance) at high frequency due to the value of CAPACITIVE REACTANCE of the capacitor.
Even though the word "impedance" is generally applied to the value of an inductor, it can also refer to the state of a circuit where the resistance is HIGH or LOW but this resistance is not simply made up of a set of resistors. In this case we say the circuit has HIGH IMPEDANCE or LOW IMPEDANCE.

Hey Colin! Does capacitive reactance changes from place to place?
By the well known formulae of reactances(Xc=1/(2Πfc),Xl=2ΠfL), I don't think so. Also, I was telling about the ac part of current for which a cap. can be treated as a short in mid freq. range if Xc is low as in about 50Ω. For a dc instead it is treated as an open.

 

[colin55]

Hey Colin! Does capacitive reactance changes from place to place?
By the well known formulae of reactances(Xc=1/(2Πfc),Xl=2ΠfL), I don't think so. Also, I was telling about the ac part of current for which a cap. can be treated as a short in mid freq. range if Xc is low as in about 50Ω. For a dc instead it is treated as an open.

I don't know what you are talking about.

 

[audioguru]

Instead of cutting all the mid and high audio frequencies, remove the 100nF capacitor across the microphone and remove the 47nF capacitor at the base of the second transistor.

To avoid acoustical feedback howling use headphones that seal (over the ears or that fit tightly inside the ear canals) and move the mic away.

 

[leonidus]

audioguru
Instead of cutting all the mid and high audio frequencies, remove the 100nF capacitor across the microphone and remove the 47nF capacitor at the base of the second transistor.

To avoid acoustical feedback howling use headphones that seal (over the ears or that fit tightly inside the ear canals) and move the mic away.

AG, I checked the freq. response of the ckt. using PSPICE & I got to know that high freq. response & the max. value of gain is mainly affected due to the 47n cap. while the 100n one across the mic. hardly affects. So, I have removed it from the ckt. Whereas previously only while making the layout I had kept mic. away from the spk.

Thank U Guys For Ur Help!!!

 

[audioguru]

Your Pspice is stupid and doesn't know that the electret mic that is operating at a very low current is a high impedance. It thinks it is a very strong 30A generator.

In my simulation I have the electret mic with a 10k ohms impedance. The resistor divider that it feeds in parallel with the 100nf capacitor causes 1416Hz to be down -3dB and higher frequencies are reduced more. The 47nF capacitor also cuts high audio frequencies. The result is a very muffled sound.

EDIT:
Here is the electret mic in my simulation:

 

[leonidus]

Your Pspice is stupid and doesn't know that the electret mic that is operating at a very low current is a high impedance. It thinks it is a very strong 30A generator.

In my simulation I have the electret mic with a 10k ohms impedance. The resistor divider that it feeds in parallel with the 100nf capacitor causes 1416Hz to be down -3dB and higher frequencies are reduced more. The 47nF capacitor also cuts high audio frequencies. The result is a very muffled sound.

EDIT:
Here is the electret mic in my simulation:

Thanx for showing that.
[Acc. to PSPICE] For an i/p of 5mV, max. o/p at collector of 3rd transistor is just about 9.5mV although I removed the 2 caps. Whereas Collin told it is 1.5V?

colin55
The collector of the second transistor will swing nearly 1v and thus the output transistor will swing almost 1.5v but it will have about 100 times more “current” to drive the earpiece and achieve the greatest volume.

If the 10k of mic. not considered then upto 39.5mV. This wouldn't be enough, would it? Plssssss explain.

 

[audioguru]

The simple circuit is not biased properly and does not have any negative feedback.
The result is severe distortion.

 

[leonidus]

What should I do now???
Pls help urgently.

 

[leonidus]

I am doing the ckt. without automatic volume control:
https://www.electro-tech-online.com/attachments/amplified-ear-with-modification-png.28125/
Its freq. response simulation result is better than the previous one.

 

[leonidus]

Sorry for multiple posts.
The resistor b/w the C & B in biasing of 1st transistor is not named. I think its R2 1MΩ. Pls. tell me am I correct/not?
Also does the pot behaves as a volume control? or else what? And does it help in avoiding motor boating?

 

[bigkim100]

Remember, the average joe thinks that all a hearing aid does is amplify what it hears over a broad range of the audible frequency range. While this may have been true when they first came out... hearing aids now usually just amplify the frequencies that the individual needs help in hearing. As we age, certain frequencied of hearing are lost. Very seldom does a person lose all their hearing accross a wide range of frequencies, unless there hearing has been damaged somehow. As well, hearing aids may even mix other frequencies into the input to cancel out other unwanted noise. I remember that my dads hearing aid helped him in noisy situations, pick out specific discussions...without his aid, it all just sounded like one big wall of noise. Its very simplistic to call this amplifier a "hearing aid"....just as calling the slabs of glass on the bottom of a Coke bottle " Corrective Lenses"\
My son actually has one of these amplifiers that use the circuit the OP has provided..except its called a "Super Snooper"
Its built into a wee tiny plastic bean shaped case around 1inch, along with a pair of headphones. Its supposed to amplify conversations so that you can snoop in on them, but its soooo noisy, loud and distorted...you cant tell what anyone is saying. They are available at Dollar Stores here in Canada, and come with a extra set of 2 3AG batteries to power the unit.

 

[audioguru]

The BC547 transistor for Q1 has a wide range of current gain. Its average is about 150 at the 0.1mA used. Then the collector voltage will be about 1V and the base voltage will be about 0.58V and the base resistor will have a voltage drop of 0.42V and a current of 0.67uA. Then its value is 62k ohms.

C5 has a high value of 470uF that will prevent motorboating.

 

[leonidus]

The BC547 transistor for Q1 has a wide range of current gain. Its average is about 150 at the 0.1mA used. Then the collector voltage will be about 1V and the base voltage will be about 0.58V and the base resistor will have a voltage drop of 0.42V and a current of 0.67uA. Then its value is 62k ohms.

C5 has a high value of 470uF that will prevent motorboating.

But in ckt. expln. its given that "This operation is accomplished by Q3, modifying the bias of Q1 (hence its AC gain) by means of R2." From this the unknown resistor has a value of 2MΩ(as per given) & u r calc. as 68kΩ. Which 1 2 be taken? I am going to solder the components in 2 days Pls tell. Also in place of 2k I got a 4k7Ω pot. Will it be OK?

 

[audioguru]

But in ckt. expln. its given that "This operation is accomplished by Q3, modifying the bias of Q1 (hence its AC gain) by means of R2."

Correct. But you are making a different circuit.

From this the unknown resistor has a value of 2MΩ

No.
1M (R2) plus 100k (R7)= 1.1M, not 2M. The 1.1M connects to +1.5V. My 62k connects to only 1.0V.

Which 1 2 be taken?

It depends on the current gain of the first transistor which could be 60 to 400. I calculated for a transistor (with a current gain of 150 at 0.1mA. If your transistor has higher current gain then the collector voltage will be too low and the base resistor must have its value increased which is quick and simple.
Use a BC547B transistor if you want it to work with a 62k or 68k resistor. The BC547B has its current gain selected for a narrower range.

Also in place of 2k I got a 4k7Ω pot. Will it be OK?

Then the gain of the circuit will near half. The volume control pot is supposed to be 22k, not 2k.