hearing aids revisited

[MrDEB]

Ah yes R8 and R4 not requited. Basically the 10kpots and the 10k resistors are in parallel thus only 5k or so.
Started designing a pcboard in between correcting some Kritter Ridder boards I had made (missed adding Vcc connection to the pic).
Trying to procure or build a mic array( two in front and two in back) for this hearing aid but so far no real luvck. Ear buds (16 or 32ohm are planned as well.

 

[audioguru]

An array of microphones or speakers causes directionality and lobing.
An ordinary cheap electret mic is omni-directional (it picks up sounds all around it).

High quality (fairly expensive) electret mics are made with a rear port so they are directional (they pickup sounds from the front and reject sounds from the sides and back). I have one made by Audio Technica.

Will your hearing aid be stereo? Surround sound?

 

[MrDEB]

I hope to build similar but these have 12 mics/bandpss filters.
**broken link removed**
https://news.stanford.edu/news/2001/june6/widrow-66.html

https://www.electro-tech-online.com/custompdfs/2012/03/paper.pdf
looking at pots liner or audio??

 

[audioguru]

An array of mics is very directional and picks up sounds very well directly ahead. Sounds or noises from the sides or behind are cancelled. But lobes in the sensitivity are produced at certain frequencies and certain angles.

Bandpass filters cannot cancel the lobes. A complicated adaptive digital echo canceller circuit might be used.

 

[MrDEB]

one half of planned device

Before I commit to having boards MADE COMMERCIALLY I am building one half of the planned device.
We will call it (proto type test"
the schematic has 200k pots for adjusting the frequency (ALL I HAVE ON HAND ARE Bourns 3366 pots)
The final design will use 100k and 500k(LOWS) all SMD all the components will be SMD to get both halves on a 50mm x 50mm board.
Doing a DIY etch of this board (1/2 proto type) to see how well this creature will perform.
Audioguru helped a lot in the design as well as TINA circuit simulation.
If this works as planned I am ordering 10 boards which are all SMD parts. Can't beat the price of $1 per board. If anyone is interested in a board along with a BOM let me know.
Will report back on the proto type results.
NOTE in this schematic I am only using 1/2 of the TDA2822 amp. Should be loud enought?

 

[audioguru]

Your new schematic is so fuzzy that we cannot see any details.
It has a few problems:

 

[MrDEB]

I made the mistake of copy n paste from my orginal schematic w/ bioth halves connected (has a bus connection)
Will clear up the pic and repost.

 

[MrDEB]

discoverd issues with my schematic

When I changed the net name it changed several net names and I missed some wrong connections on my final design.
Need to correct but here is the 1/2 schematic.
Suggestions on disabling 1/2 of the TDA2822?
Thinking ground the inputs
hopefully this schematic is readable. I didn't resize this time.

 

[audioguru]

The parts and text on your new schematics are tiny because the parts are too far apart.

 

[MrDEB]

will redo

 

[MrDEB]

Hopefully readable

I grounded the + input 2 on the TDA2822 (I only want to use 1/2 the amp to see how much amplification I get with 4.5v Vcc.
The full version will utilize both channels (Left and Right)and all smd.
would like to know how Burt used a PDF to post a schematic?
All resistors are metal film and caps are film types except the electrolytic (Audio Grade)The 1/2 circuit is utilizing through hole components.
On the full schematic I am looking at using Tant caps as anything bigger than say 1uf are not readily available in SMD

 

[audioguru]

My goodness! Will it ever be correct?

 

[MrDEB]

Revisions corrected. Hopefully ALL

I see what your referring to on the volume control (me is not to's bright----lol
Made mistake of copying n pasting components from one schematic to another (easier than trying to locate same part from scratch. Miss some.
Thanks for being so helpful.
I missed the bias on the A inputs but the capacitor C7 was off to the left hiding among the connections.

 

[audioguru]

Now you have the +inA signal of the preamp shorted to ground through C1. Connect a 47k or 100k resistor in series between +inA and C1. +inB does not need the resistor because B opamp is inverting and +inB is not its signal input.

It looks like R3 and R4 do not do anything. I think they biased pin 3 of the preamp and powered the mic before. Remove them.

Your new volume control is correct.

 

[MrDEB]

R13 &R4 = bias

The connection of the input bias got lost?
Thinking that R3 can be removed but the condenser mic calls for a 2.2k to Vcc to power the mic.
mic output impedance is 2.2k.
Would the bias on the +inA be correct if I changed R4 to 2.2k, remove R13.
current draw on the input bias would be 9ma using 2.2k resistors. 4.5 / 5000 = 9ma
The draw with 4k resistors = 5ma (4.5 / 8k = 5ma

 

[audioguru]

On your original circuit, R13 was 4k and R4 was 6k so that with a 3.3V supply they provided 0.5mA to the mic and half of 3.3V to bias the opamp.
Now you use R3 to power the mic and you use R17 to bias pin 3 of the opamp so get rid of R13 and R4.

 

[MrDEB]

So R3 by itself will bias the op amp input?
The original circuit was from TINA showing mic input with 4k and 6k voltage divider (couldn't find file?)
Will eliminate R13 and R4

 

[audioguru]

So R3 by itself will bias the op amp input?

No.
R3 powers the electret mic.
R17 with R1, R2 and C1 bias the opamp pin 3 input at half the supply voltage.

 

[MrDEB]

Ah yea R17!!

I forgot R17 waiting in the wings.
I need to make these revisions on the full circuit schematic.
Hopefully this is complete then I can etch a board for testing.
My plan is two omni mics in front and two in the rear. A DPDT switch will shut off the rear mics (my present $$$hearing aids have this feature and when in a noisy environment it allows me to have a conservation with person in front of me)
https://www.mouser.com/Search/Produ...rtualkey66500000virtualkey665-TOS1844RC33103R

https://www.mouser.com/Search/Produ...rtualkey66500000virtualkey665-TOS1844RC33103R

both are noise canceling mics

 

[audioguru]

I think I am wasting my time with this project that is full of errors.
You did not follow my note in my attachment in post #52 about the low value of C13 cutting most high, medium and low audio frequencies but it passes ultrasonic bat frequencies well.

You have selected two mics that might be electret but the "datasheet" does not say. They are "omni", not noise-cancelling. They pick up sounds and noises all around. A noise cancelling mic is larger and has a rear port. Far away sounds are cancelled but very close sounds are louder at the front than at the rear port so there is less cancellation. A noise cancelling mic is useless for a hearing aid.

The datasheets for the mics are completely useless. Where is the frequency response curve? Where is the directivity curve?