3 gain equalizer hearing aid simulation fact

Thread Starter

Hasan2019

Joined Sep 5, 2019
64
Dear All,

Let me introduce one of my project what I worked through 12 years back with a professor.
Still it need to be more efficient one.

Its a low cost hearing aid amplifier using LM324 as a main Op-amp.

We were trying to apply 3 frequency selecting gain circuits, 1. Bass Control 2. Mid frequency 3. Treble control.
I did apply some model from online for having suitable gain at human sensor range like 2-3KHz.
HA1.PNG

It looks clear, I have been trying

1. Treble and Bass control by frequency selecting gain, considering High, low and mid frequency gain through frequency selector.
2. Tried to boost and buck the gain of the amplifier.
3. Assume all OPAMPS are 741.
4. In mid frequency band the gain should be 17dB to -15dB. According to this design, as audio signal is fed by single input, so all frequency signal from the air will be chosen by selector, must be flat for minimum frequency gain.
5. As far as I remember, 200Hz to 8kHz, we have used signal generator.
6. My tone controls have parts to boost the frequencies and other parts to cut the frequencies. Are there any people with hearing problems that need frequencies cut because their hearing is "too sensitive" to those frequencies?


Take a look in the bode plot here,
HA2.PNG

Not sure the system works properly.
Suggest for meaningful simulation.
 

crutschow

Joined Mar 14, 2008
30,101
Typical age related hearing loss is of the high frequencies so all you should need is perhaps mid and high frequency boost adjustments.
I would think it would be rare that you would need to reduce the response at any frequency.

An LM324 is a rather noisy amp with crossover distortion so not sure how useful it will be as a hearing aid amplifier.
Also I would think you would want a micropower type amp, which the LM324 is not.
 

DickCappels

Joined Aug 21, 2008
8,812
On the other hand, if you are like me, you can no longer hear crossover distortion... You can certainly try this with an LM324 and then switch of a better quality opamp to get higher fidelity and longer battery life if you desire in the future.
 

Thread Starter

Hasan2019

Joined Sep 5, 2019
64
Typical age related hearing loss is of the high frequencies so all you should need is perhaps mid and high frequency boost adjustments.
I would think it would be rare that you would need to reduce the response at any frequency.

An LM324 is a rather noisy amp with crossover distortion so not sure how useful it will be as a hearing aid amplifier.
Also I would think you would want a micropower type amp, which the LM324 is not.
Perhaps LM324 will give the same result. I had been using breadboard, and got noise for sure. Another problem in this design is large power battery which is not realistic.
 

Thread Starter

Hasan2019

Joined Sep 5, 2019
64
On the other hand, if you are like me, you can no longer hear crossover distortion... You can certainly try this with an LM324 and then switch of a better quality opamp to get higher fidelity and longer battery life if you desire in the future.
Otherwise do you think gain curve is OK?
For high frequency gain do you have any idea that which components are dominating?
 

Thread Starter

Hasan2019

Joined Sep 5, 2019
64
I think it's too flat above 100Hz. And a gain of -228dB looks way too low for practical use.
In addition to the other mistakes already pointed out:


Do you see that U7 and U12 have both supplies fed with +9V?R12 is only 47Ω and placed across the 50kΩ potentiometer, U10 - did you mean it to be 47kΩ?You are using the pot_lin model throughout, but include the parameters Rtap=1k tap=0.5, which have no effect. Did you mean to use tapped potentiometers? (Hint: they are almost impossible to find.)R12 (47Ω)is placed across all the circuit around U7, more or less shorting it all out.The frequency selective circuit around the U10 pot is returned to the non-inverting input of U7, making it a positive feedback loop. Not that that itself stops the stage working, as it's already fatally flawed.You have set the gain of the output stage to 220x, but the 741 only has a gain-bandwidth product of 1MHz, so it will have an HF roll-off commencing about 4kHz.


Lets discuss more about this circuit again.


1. The lower cut of frequency is 48Hz


2. In the Bass control part in U6, when U9=1M is rotate clock wise, C2 gets shorted and maximum bass boost =20log (6.8+6.8/6.8)=6.02 dB.

When U9 rotates anticlockwise then , C3 get shorted and maximum Bass cut=20log (6.8/6.8+6.8)=-6.02dB.

May be this time frequency should be 250Hz.


3. For Treble control circuit, with op-amp U12, maximum boost treble, when U11 rotates clock wise, Gmax= -100k||(470k+10k)/100k||10k=20log(9.15)=19.22dB,

When U11 rotates anticlockwise then, Gmax=-100k||10k/100k||(470k+100k)=20log(0.109)=-19.25dB.

This time for treble cutt off frequency is around 1.6kHz.


4. Let me calculate the mid frequency control circuit later
 
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