The objective is to make a neckloop that works much better than what's available, has plenty of volume, doesn't need you to ever change its battery, and can be plugged into regular audio stuff that normally drives earbuds or can serve as a good sounding plug-in headset for standard 2.5 mm jacked cell phones.
In many cases the strength of the magnetic field at the hearing instrument’s telecoil turns out to be about 20% to 25% of what it would be If the telecoil were perfectly aligned in the center of the neckloop. So that means that you want to generate about 5x more field strength at the loop's center than you need at the telecoil.If you want 100 ma/meter at the telecoil you need 500 ma/meter in the loop.
The field strength at the neckloop's center equals the current times the number of turns divided by the loop diameter. If you want 500 ma/meter and the loop diameter is .25 meters then:
I*Turns = 500*.25 = 125 ma-turns.
If you try 8 turns then:
I = 125/8 = 15.6 ma
For example at 8 turns the loop has negligible impedance. Just driving the loop at this level with a 16 ohm source would require an open circuit rms voltage (audio) source of .253 vrms. If you put a 16 ohm resistor in series with the coil to match the source (strongly recommended), you would need a source with .5 vrms open circuit. In other words your radio would have to be able to drive a 16 ohm load at about a 5 mw level. Many sources may operate at levels far below this.
More turns give you greater output. A 40 turn loop gives you 20*log(40) or 32 db more gain than a one turn loop and 20*log(5) or 14 db more than an 8 turn loop. So for a 40 turn loop you would need a source with about .1 vrms open circuit voltage to produce an effective 100 ma/meter field at the telecoil.
Using a resistor to make the load impedance 16 ohms, the required output from the device would be (.1^2)/16 or .7 mw. This should be available from most devices that drive earphones.
Design approach and prototype details will follow.
3 comments:
I am having difficulty securing the plastic tubing holding 40 turns of wire. Any photos available showing mounting method inside box?
Do you accept other hearing related info such as EQ sources and bare 5 channel EQ pcboards? Very new to Blogging, not sure how to create new subject to post off topic about EQ items
Thanks, Dave_S
Google McMaster Carr for tubing etc.
This was meant to be a very rough prototype. The box is just a split box normally used for housing 4 AA cells and gutted out.
The holes for the wires etc. were drilled so that half the hole was on each side of the split halves. Rubber/plastic grommets were captured in the split holes.
Mounting was not worried about for this quick prototype. Securing was via hot melt, cable relief just used heat shrinkable tubing or small cable ties etc.
We have an issue that's come up here to take care of before I can get back to this. We hope to resolve it very soon.
The EQ board sounds interesting. I think you have to have a general Google account to create a new subject. I'll have to check on that and respond with a comment.
OK, figured it out. You should be getting an email shortly and be cleared for creating a new subject.
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