hearing aids revisited

[MrDEB]

Sorry I missed C13 but did change the others as suggested.
Will search fo different mics. I was under impression NOISE CANCELLING meant LOUD noises were shut off.
Going to recheck my entire schematic as I allowed components from my bat project to filter into this one.

 

[MrDEB]

Went over schematic Grounded the +inputs on the 4364 op amps and the +input on the 2822.
Changed the 5nf to .1uf
the mics I found are omi directional
https://www.electro-tech-online.com/custompdfs/2012/04/POS-1644R-C3310-2-R.pdf
they are smd but no frequency graph.

 

[MrDEB]

found a curve graph for similar
https://puiaudio.com/product-detail.aspx?categoryId=4&partnumber=TOM-3050L-R
this has the highest db sensitivity

 

[audioguru]

I think your opamp is too noisy to be a mic preamp and your maximum gain is way too high at 68,400 (yes, more than 68 thousand when a tone control is at max).
A sensitive mic preamp has a gain of only 200.

But how does anybody except your hearing doctor know how deaf you are? Deaf like a toad?
I have never looked at how much attenuation of sounds occur to a deaf person who can be helped with a hearing aid. 10dB? (3.16 times) 20dB? (10 times).

 

[MrDEB]

perhaps lower the pre amp gain by changing R5 which sets the gain on the pre amp?
I will look for a lower noise op amp in TINA as it has LOTS of examples.
Thinking just build as is and I can change the gain seeing how I am using through hole on this unit for testing purposes.

 

[MrDEB]

Found better op amp

low noise etc
https://www.ti.com/product/opa4134
here is the schematic we have so far. NOTE the pots are 200K as that's what I have on hand. The full circuit will use 100k and 500k as noted in the Baxandall.
I noticed the input cap WAS on the wrong side of the input bias. This TINA simulation has it on the correct side.
I changed the input signal for 2.2k internal resistance
Grounding the unused +inputs I assume is correct way to disable the unused portions of the TDA2822 and the OPA4134?

 

[audioguru]

Your new preamp circuit WILL NOT WORK!

Please learn that the (+) input of an opamp MUST have a DC bias voltage and the (-) input is biased by the negative feedback resistor.

 

[MrDEB]

BAD picture & missing V+

The connection wasn't there . I had to enlarge to see if there was a connection at R5 to C6
here is corrected enlarged pic.
The graph looks better?
I was under impression that R2 provided the bias but I failed to change the J1 to V+

 

[MrDEB]

Forgot the graph

All the pots are at 50%
Going to do a noise and transient analysis

 

[audioguru]

I think the resistor that powers the mic should be supplied through an RC filter so that noise on the power supply is not fed to the input of the preamp.
I don't know where the 5V supply will come from. If it is a battery then it will bounce up and down with the signal which produces "motorboating" when amplified.

Your circuit has two circuits producing a bias voltage but only one circuit can bias both opamps.

EDIT: I forgot to say that since C6 has the huge value of 10uF then it passes frequencies down to one cycle per 6 seconds.
If it is 10nF (0.01uF) then it passes frequencies down to 16Hz.

 

[MrDEB]

Will change the bias (only one needed)
Voltage supply will be a battery so will add an RC filter.

 

[MrDEB]

A 1M resistor in series w/ .01 cap parallel with battery?
Searched for power supply RC circuits.

 

[audioguru]

The battery needs 100uF parallel to it.
The 2.2k resistor that powers the mic should be fed through a 1k ohm resistor from the battery then the two resistors should have a 47uf or 100uF capacitor to ground where they meet.

 

[MrDEB]

revisions and additions

made some changes seeing how R15 and R2 can be eliminated with the bias from R16 & R13?

 

[audioguru]

Yes, eliminate R15, R2 and C14. Connect R17 to "bias". Change C6 to 0.01uF (10nF) and change C3 to 10uF.

 

[MrDEB]

Think I have all corrections

Used the TINA schematic to re check my connections
When we get all done I hope to send you a PCboard seeing how I am going to order 10 minimum order.

 

[audioguru]

Here we go again and again.

 

[MrDEB]

I wondered about that resistor. The bias is connected in my schematic but contemplated if the 100k was needed seeing how the op amp input is biased.

 

[MrDEB]

I wondered about that resistor. The bias is connected in my schematic but contemplated if the 100k was needed seeing how the op amp input is biased.

 

[MrDEB]

Hearing aid update
Have all parts purchased but before having PCboard made I want to double check my schematic
https://www.electro-tech-online.com/threads/posting-a-two-page-schematic.128132/
I hate to be a bother but I plan to offer a PCboard for assistance in this endeavor. AudioGuru is on my list. Contemplating http://iteadstudio.com/ for boards. Board design is pretty much done but any suggestions are welcomed.