[Avro_Arrow]

The next thing to think about is mounting the microphone
in a parabolic reflector...

 

[TheLaw]

Avro, thank you for the continued response.
 
Would LME49720 work alright for the mic preamp? Then use it again for the active equalizer. Then perhaps an AD823 or OPA2134 for the "cMoy" section.
 
I've never designed heaphones from scratch, so I might just experiment with that first.
 
Parabolic reflectors. I made some for my router once. Perhaps if I moulded some "dishes" into the heaphones, it would work similarly? Or were you just trying to be funny?

 
I guess one of my biggest questions is how to get the signal back and forth from the headphones (which also would have the mics) to the amplifier unit. XLR connector was my first guess. Do you think it would be wire to preamp the mics while they are still in the headphones (before they are sent down the wire to the box)? In this case, I would then need to get an XLR with more pins so that I can supply a voltage to the headphones also. So like an XLR6 connector?
 
Pin 1- Mic L +
Pin 2- Mic R +
Pin 3- Headphone L +
Pin 4- Headphone R +
Pin 5- 5V
Pin 6- -5V
Shell- Ground
 
Perhap something like this: http://www.markertek.com/Cables/Bulk-Wire-Cable/Bulk-Audio-Cable/Belden-CDT-Inc/9536-060500.xhtml?9536
 
 
Would that work? That seems mighty complicated, plus the fact that I don't know where I'm going to locate decent flexible 6 conductor wire? Or is this being over engineered? Nothing wrong with a big common ground is there?

 

[Avro_Arrow]

I would run the lines from the mics separately from the lines to the headphones.
If you pack them into the same cable you will likely have an oscillator.
 
Actually I was being serious about the reflectors...
They use just such a set up at sporting events to capture commentary
from the field. That would only be useful if the mics were not attached
to your head 

 

[Avro_Arrow]

You could probably use the LME49720 for the preamp, it's not quite
as good as the LME49990 but it works at lower voltages.

 

[TheLaw]

Righto. Do you think it would be overly ambitious to try to put the mic preamps in the headphones?

 

[TheLaw]

Also RE: parabololic reflectors, how would YOU implement them?

 

[Avro_Arrow]

For IEM...I would say it's overly ambitious.
For over the ear headphones I would think it would be quite doable.
Acoustic feedback would be a problem with co-locating the headphones
and the microphones though.

 

[Avro_Arrow]

They would be useful for listening to something that was very quite or far away.
The commercial versions ether have a pistol grip or are mounted on a tripod.
 
Some examples:
http://www.spyegg.com/parabolic-microphone-listening-device.htm
 
Quote:

Also RE: parabololic reflectors, how would YOU implement them?

 

 

[Avro_Arrow]

Fred_Fred did a project where he made an electronic stethoscope.

 

[TheLaw]

Hmm since I probably won't need a hearing aid for another 50 years, maybe I'll change the direction of this project to something like..yes..a "spy" amplifier as you have linked to.
 
It would be MUCH easier to do this via a single unit versus trying to futz around wtih having half of the unit in some diy headphones and half in a box plus wiring.
 
So lets see now...

 

[wakibaki]

A couple of things you could think about:-
 
LME49725 is very close to LME49720 performance-wise, but has a typical quiescent current of 6mA as opposed to 10mA, if you can get them. Unfortunately Farnell have discontinued them, but it's still an active device.
 
You could sign up at ti.com and ask for some samples. Free samples. TI have bought Nat Semi but both did free samples. TI might not have got their act together with the National parts yet, but no harm in trying.
 
OTOH, NE5532s are almost certainly good enough and you can get 20 for $4.50 shipped from ebay.
 
Unless you're determined to put the charger in the unit, you could use tubular Lithium cells with an external charger (no design issues), they have better energy density than NiMH and you need fewer to get a decent voltage. Ultrafire protected CR123As from dealextreme.com are pretty good.
 
A graphic equaliser or at least parametric tone controls might be more useful than simple bass and treble for somebody with hearing problems. A graphic built with preset pots and SMT components will still probably fit in an Altoids tin. It's a set and forget application.
 
w