[TheLaw]

Hello all,   I know this looks long, but I've tried to make it easy to read and descriptive. Please give it a chance.   I thought it would make a decent project for those of us with family members or friends with bad hearing...or perhaps yourself. So I'm trying to design a cheap "hearing assistant" from the ground up. I have a few questions. --- Things you should know: --I plan on using two electret condensor microphones. (Standard dime sized ones). These will be unidirectional. 680Ohm impedance I believe. --I'm getting quite good at using Polymorph, (kneadable plastic), and as funny as this sounds I may design my own headphones from this plastic and place the headphone element inside. There are several decent headphone elements on Mouser and Digikey. I'll embed the mics on each side, just as if they were real ears. I do not plan on putting the electronics in the headphone for I think it would weigh too much. The idea is to have an altoids tin or equivalently sized metal enclosure that would house all of the electronics and would have the volume control and 2-band equalizer I was hoping for. This would either go in your pocket or get clipped to your clothing. If you already think I'm crazy, you're probably right, but bear with me. --- Here's what I'm thinking for design: -Mic Preamp- 2x INA217 instrument preamp (BurrBrown/TI) -Equalizer- 2x Passive Baxandall -Headphone Amp- OPA134 or equivalent [cmoy-type] (BurrBrown/TI) -Batteries- Li-On or NiMH cell(s) ~ 5-8V (If NiMH, LM393 comparator used for charging, If Li-Ion...IDK) -Power Supply- NCP3063 step up converter to ~12-14V --> TLE2426 rail splitter to +/- 6-7VDC Does this sound feasible? I probably would have to do a prototype on a much larger board than an altoids tin, but perhaps after doing some good layout, etching, or getting pcbs made, it can be made into a small device. Also hoping that a good ground plane will help. Perhaps I will manhattan style prototype it. --- Questions. --For the INA217, the diagram looks like it wants a balanced input, however, I'm not using a balanced setup, just 2 simple electret mics. If I'm unbalanced, on which pins do I input? Anything else changed? www.ti.com/lit/ds/symlink/ina217.pdf --I would like to have a simple 2 band passive Baxandall equalizer so that the user can reduce bass or treble depending on the environment or anything else. After what stage should this equalizer go? After the mic preamp or after the headphone preamp...I doubt it would go on the low voltage mic signal...don't want to induce any more noise. --How should I run the various connections to/from the headphone (which also houses the mics)? My idea was to use a mini-XLR4 type connector and then Mogami quad conductor wire + shield. The shield being ground and it would be attached to the XLR shell. So one wire for each mic and each headphone element and shared ground. Is that a bad idea? --Finally, does anyone have any good ideas for batteries? And charging. Li-ion is preferred but I'm not too sure about charging them..yet. --- Sorry this is REALLY long, but I'm pretty excited for this and I think it might be an interesting project for me (you?) to try since it's practical. Thanks a bunch.

~ Tim

 

 

[Avro_Arrow]

That INA217 looks like it's more geared towards a conventional dynamic microphone.
Why not just use a LME49990 in a more conventional setup?
 

 

[Avro_Arrow]

I found this active tone control
 

 
From here:
http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html

 

[Nixon]

If you want a nicer looking enclosure than polymorph, you could look into 3d printing. 3D printers are getting cheaper and cheaper these days, can pick up one perfect for this project for £360 http://www.sumpod.com/

 

[Avro_Arrow]

Correct me if I am wrong, but it looks like you will need
about 35db of gain overall from microphone to headphone.
 
If I have it right, that means the microphone puts out around 0.0177 volts AC.
A gain of about 20 in the preamp stage and 5 in the headphone amp should be
enough gain. The active tone control (optional) has unity gain.
I would put the tone control after the preamp and before the volume control.

 

[TheLaw]

Okay thank you both.
 
I've been thinking. If I do not have a cable run, does the microphone still require a preamp stage? The LME49990 is super low noise but I doubt it could actually drive a 150ohm load, so essentially I use the LME49990 to cleanly boost the voltage a bit, and then run it into something like an OPA134 stage to drive the headphones? Makes sense if I understand this right, but..
 
Is the signal from an electret mic still do low to ignore the preamp stage and run it directly into an OPA?
 
The question I guess is, can I take the mic signal and run it through one voltage gain op-amp stage and drive the headphones with it? So essentially a cmoy...
 
Avro, thanks for the active schematic. Would an active equalizer have any benefit over a passive one?
 
3D printing would be the dream, but damn, I don't have any money for that. For now, it's just a thought in my mind: prototype.
 

 

[Avro_Arrow]

With a passive tone control, you can only reduce the signal level.
To get boost with a passive tone control, you reduce the overall
signal level to give to illusion of boost.
With the active tone control, no overall reduction of signal is required.
 
You can't take the signal from the microphone and use it directly
with a standard CMoy type amp. The signal requires some pre-amplification.

 

[TheLaw]

Oh right. How dumb of me.
 
 
This is essentially what an electret condenser mic looks like. It's already preamplified, though I'm not sure exacty what voltage it would output. Do you still reckon it's too low if all I want to do is drive a pair of headphones?
 

 
Thanks.

 

[Avro_Arrow]

The datasheet I read said the output of the microphone was -35 db with 0 db=1 volt.
That would be about 0.0177 volts.

 

[Avro_Arrow]

A handy conversion site...
http://www.sengpielaudio.com/calculator-gainloss.htm

 

[TheLaw]

Thanks!
 
If a sensitivity of a mic is negative and that calculator says that it is voltage loss? What does that mean...? Is that not the output voltage?
 
What is a typical line level voltage?

 

[Avro_Arrow]

Google is your friend...
 
http://en.wikipedia.org/wiki/Line_level
 
 
Quote:

Thanks!
 
What is a typical line level voltage?

 

 

[TheLaw]

Right. I was getting a little to comfortable just asking questions. So according to this...Preamping is necessary, as you tried to tell me, but I was too ignorant of course.

 

[Avro_Arrow]

So then, as you can see, a preamp with a gain of 20 or maybe even a little higher, will
get you up around "line level".
 
So lets see if we have some building blocks.
 
Electret condenser microphone, something like this:
http://search.digikey.com/ca/en/products/WM-61A/P9925-ND/252843
 
Preamp with LME49990
Standard non inverting amp with gain of 20
 
Optional tone control based on schematic above.
LME49720 may be a good candidate here.
 
CMoy of your choice, many to choose from.
 
8 cell NiHM AAA battery pack. Use center tap for GND.
NiHM much easier to use than Li-ION.
 
How is that for a start?

 

[Avro_Arrow]

If you are going to use LME49990 you might want to bump the battery pack up to 10 cells.
The LME49990 like a little more voltage.