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Hearing aids, Inductive loops question

  • Thread starter martin griffith
  • Start date
M

martin griffith

Jan 1, 1970
0
My partially deaf neighbour cant afford to install an inductive loop
in her living room so she can hear the TV through the T position on
her hearing aids.

I've offered to see if I can make one, in exchange for home cooked
food. This seems like a fair exchange

A bit of googling suggests that a 25way cable, wired as a coil, fitted
around the ceiling edge will suffice.( approx 3m by 4m)

Any idea how much drive is required for the loop. I was thinking of a
741 and a T0220 npn/pnp output pair, run off +- 15V as a starting
point.

Any comments, better configuration info, bandwidth limits. etc. would
be appreciated


martin
 
N

Nico Coesel

Jan 1, 1970
0
martin griffith said:
My partially deaf neighbour cant afford to install an inductive loop
in her living room so she can hear the TV through the T position on
her hearing aids.

I've offered to see if I can make one, in exchange for home cooked
food. This seems like a fair exchange

A bit of googling suggests that a 25way cable, wired as a coil, fitted
around the ceiling edge will suffice.( approx 3m by 4m)

Any idea how much drive is required for the loop. I was thinking of a
741 and a T0220 npn/pnp output pair, run off +- 15V as a starting
point.

Any comments, better configuration info, bandwidth limits. etc. would
be appreciated

I've been involved in the hearing loop business several years ago and
designed an amplifier for that purpose to be used for big
installations (churches, conference rooms, etc) several years ago. I
know a few basics on this subject and also know there is a lot of
nonsense floating around (even Phillips got it all wrong in the past).

You can use a standard stereo amplifier to drive a loop. It is best to
use thin wire, like ordinary telephone cable. If you use a stereo
amplifier, you can make 2 parallel loops to get twice the field
strength. Never use multiple turns since this will cause serious
deterioration of the high frequencies. Keep in mind that the amplifier
will run hot since nearly all power will be dissipated in the
amplifier. Look for an amplifier which has a thermal shutdown.

The ceiling may not be the best position for the loop.
 
M

Mark

Jan 1, 1970
0
martin said:
My partially deaf neighbour cant afford to install an inductive loop
in her living room so she can hear the TV through the T position on
her hearing aids.

I've offered to see if I can make one, in exchange for home cooked
food. This seems like a fair exchange

A bit of googling suggests that a 25way cable, wired as a coil, fitted
around the ceiling edge will suffice.( approx 3m by 4m)

Any idea how much drive is required for the loop. I was thinking of a
741 and a T0220 npn/pnp output pair, run off +- 15V as a starting
point.

Any comments, better configuration info, bandwidth limits. etc. would
be appreciated


martin

the folks over at rec.audio.pro may be able to help you..

My GUESS is it would take at least a several Watt ampllifier...


Mark
 
M

martin griffith

Jan 1, 1970
0
the folks over at rec.audio.pro may be able to help you..

My GUESS is it would take at least a several Watt ampllifier...


Mark
well, RAP does have a couple of experts, but there is generally more
chaff than wheat there, I'll go with what Nico suggested.

I'll probably scale it down to cover the sofa area to start off with


martin
 
N

Nico Coesel

Jan 1, 1970
0
Chris Jones said:
Depending on the amplifier, it may be desirable to put a power resistor in
series with the loop to bring the load of the amplifier up to 4 Ohms or 8
Ohms, whatever the amplifier is designed to drive. This will probably mean
you'd have to turn up the gain of the amplifier a bit, but should prevent
the amplifier from being damaged. Also if there is more resistance in the
circuit, then it could be worth trying multiple turns again because the
time constant will be L/R, so it you increase R, then you may be able to
increase L also without undue loss of audio frequency response. This could
help in getting a loud enough signal.

Definitely no. Using thin wire causes the loop to have some resistance
so the amplifier is not completely shorted. An extra resistor can be
added, but not more than 1 Ohm. The typical inductance of a induction
loop is somewhere between 100uH to 200uH (depending on the amount of
steel object in the room and walls). This means that the worst case
impedance around 5kHz is 6.3 Ohms. If you insert an 8 Ohms series
resistor, you'll lose more than 6dB in field strength. If you create
multiple turns, the impedance of the loop goes up with the square of
the number of turns, hence you'll lose the high frequencies very
rapidly.

When driving induction loops, the amplifier is used as a current
source. Most amplifiers will drive a 1 Ohm load perfectly, but they
won't reach their rated power output.
 
C

Chris Jones

Jan 1, 1970
0
Nico said:
I've been involved in the hearing loop business several years ago and
designed an amplifier for that purpose to be used for big
installations (churches, conference rooms, etc) several years ago. I
know a few basics on this subject and also know there is a lot of
nonsense floating around (even Phillips got it all wrong in the past).

You can use a standard stereo amplifier to drive a loop. It is best to
use thin wire, like ordinary telephone cable. If you use a stereo
amplifier, you can make 2 parallel loops to get twice the field
strength. Never use multiple turns since this will cause serious
deterioration of the high frequencies. Keep in mind that the amplifier
will run hot since nearly all power will be dissipated in the
amplifier. Look for an amplifier which has a thermal shutdown.

The ceiling may not be the best position for the loop.
Depending on the amplifier, it may be desirable to put a power resistor in
series with the loop to bring the load of the amplifier up to 4 Ohms or 8
Ohms, whatever the amplifier is designed to drive. This will probably mean
you'd have to turn up the gain of the amplifier a bit, but should prevent
the amplifier from being damaged. Also if there is more resistance in the
circuit, then it could be worth trying multiple turns again because the
time constant will be L/R, so it you increase R, then you may be able to
increase L also without undue loss of audio frequency response. This could
help in getting a loud enough signal.

Chris
 
R

Reg Edwards

Jan 1, 1970
0
To avoid the possibility of amplitude distortion it is best to load an
amplifier with a resistance of about the same value as what the
amplifier is designed for.

So a resistance of roughly 8 ohms can be connected in series with the
low impedance loop.

The resistance has a beneficial effect - it levels the frequency
response. It allows more turns to be used in the loop without loss at
the higher audio frequencies.

The loop thinks it is being fed from a constant current source with an
internal resistance of 8 ohms. This is considerably higher than the
impedance of the loop itself even when it has several turns. It is
this constant current characteristic which maintains the frequency
response.

The amplifier is also happy because it thinks it is driving an 8 ohm
loudspeaker.

The smaller the room area to be covered, the greater the number of
turns allowed. The number of turns can be increased until the loop
impedance is several ohms at the higher audio frequencies, say 4 ohms
at 6 kHz if the amplifier has been designed for an 8 ohm loudspeaker.
But it is very non-critical.

The loop impedance is that of its inductance which can be calculated
prior to installation. or measured afterwards.

Reasonable hi-fidelity can be expected.

But performance ultimately depends on the sensitivity of the pick-up
receiver and on the level of noise, interference and 50-60 Hz mains
hum and its harmonics.
 
R

Reg Edwards

Jan 1, 1970
0
To avoid the possibility of amplitude distortion it is best to load an
amplifier with a resistance of about the same value as what the
amplifier is designed for.

So a resistance of roughly 8 ohms can be connected in series with the
low impedance loop.

The resistance has a beneficial effect - it levels the frequency
response. It allows more turns to be used in the loop without loss at
the higher audio frequencies.

The loop thinks it is being fed from a constant current source with an
internal resistance of 8 ohms. This is considerably higher than the
impedance of the loop itself even when it has several turns. It is
this constant current characteristic which maintains the frequency
response.

The amplifier is also happy because it thinks it is driving an 8 ohm
loudspeaker.

The smaller the room area to be covered, the greater the number of
turns allowed. The number of turns can be increased until the loop
impedance is several ohms at the higher audio frequencies, say 4 ohms
at 6 kHz if the amplifier has been designed for an 8 ohm loudspeaker.
But it is very non-critical.

The loop impedance is that of its inductance which can be calculated
prior to installation. or measured afterwards.

Reasonable hi-fidelity can be expected.

But performance ultimately depends on the sensitivity of the pick-up
receiver and on the level of noise, interference and 50-60 Hz mains
hum and its harmonics.
==========================================

I have made a simple calculation.

For a room 5 metres (about 16 feet) square, and an amplifier with a
resistor of 8 ohms in series with the loop, two turns of 14 AWG wire
will do very nicely.

The inductive reactance of two turns on a 5 metre square loop at 6 kHz
is 4.8 ohms.

The amplifier will think it is driving a loudspeaker of impedance 9.3
ohms and will be quite happy.

The power output required from the amplifier will depend on the
sensitivity of the pick-up receiver and the background noise level.

With a smaller room, 3 turns could be used without undue loss in the
high frequency audio response. This would reduce the power required
from the amplifier.

With a large room, 20 metres (60 feet) square, only one turn could be
used for high audio quality and a high-power amplifier with an 8 ohm
series resistor would be needed.
 
N

Nico Coesel

Jan 1, 1970
0
Reg Edwards said:
To avoid the possibility of amplitude distortion it is best to load an
amplifier with a resistance of about the same value as what the
amplifier is designed for.

The load resistance of an amplifier is dictated by its power supply
voltage and cooling capacity. Driving low impedance loads is usually
no problem as long as you don't crank the volume up all the way.
So a resistance of roughly 8 ohms can be connected in series with the
low impedance loop.

The resistance has a beneficial effect - it levels the frequency
response. It allows more turns to be used in the loop without loss at
the higher audio frequencies.

This is true for very small loops (around a sofa or coffe table) where
the impedance of the loop is very small. It won't work for a loop in a
modest living room.
 
K

Kevin White

Jan 1, 1970
0
Nico Coesel wrote:
....
You can use a standard stereo amplifier to drive a loop. It is best to
use thin wire, like ordinary telephone cable. If you use a stereo
amplifier, you can make 2 parallel loops to get twice the field
strength. Never use multiple turns since this will cause serious
deterioration of the high frequencies. Keep in mind that the amplifier
will run hot since nearly all power will be dissipated in the
amplifier. Look for an amplifier which has a thermal shutdown.
....

Do they put high-frequency de-emphasis in the hearing aid? If you
maintain a constant flux density as the frequency rises you will get a
rising amplitude response at the receiver.

A simpistic model of the transmit and receive windings would be a
transformer - if you drive a transformer with constant voltage as the
frequency rises you will get a constant voltage out.

kevin
 
N

Nico Coesel

Jan 1, 1970
0
Kevin White said:
Nico Coesel wrote:
...

...

Do they put high-frequency de-emphasis in the hearing aid? If you
maintain a constant flux density as the frequency rises you will get a
rising amplitude response at the receiver.

I don't know exactly, but I suppose they do. I do know the current
versus frequency response is supposed to be flat in order to keep the
magnetic field within limits.
 
J

John B

Jan 1, 1970
0
martin griffith scrobe on the papyrus:
loop >> in her living room so she can hear the TV through the T
position on >> her hearing aids.
of a >> 741 and a T0220 npn/pnp output pair, run off +- 15V as a
starting >> point.
well, RAP does have a couple of experts, but there is generally more
chaff than wheat there, I'll go with what Nico suggested.

I'll probably scale it down to cover the sofa area to start off with


martin

It depends on how much spare time you've got, but you might do better
to buy the amplifier and cable here:

<http://www.applesoundshop.co.uk/acatalog/AVX_Loop_Amplifiers.html>

They even sell tape to fit under carpets.
 
C

Chris Jones

Jan 1, 1970
0
Nico said:
Definitely no. Using thin wire causes the loop to have some resistance
so the amplifier is not completely shorted. An extra resistor can be
added, but not more than 1 Ohm. The typical inductance of a induction
loop is somewhere between 100uH to 200uH (depending on the amount of
steel object in the room and walls). This means that the worst case
impedance around 5kHz is 6.3 Ohms. If you insert an 8 Ohms series
resistor, you'll lose more than 6dB in field strength. If you create
multiple turns, the impedance of the loop goes up with the square of
the number of turns, hence you'll lose the high frequencies very
rapidly.

When driving induction loops, the amplifier is used as a current
source. Most amplifiers will drive a 1 Ohm load perfectly, but they
won't reach their rated power output.

The only caution I would add, (apart from not turning up the volume control
too high and exceeding the current capability of the output transistors) is
that if the amplifier is direct-coupled (no output ac-coupling capacitor)
then a nearly purely inductive load with very low DC resistance might cause
significant direct current to flow due to any DC offset in the amplifier.
If, as you suggest, you use thin wire and the wire is long enough (which is
equivalent to adding series resistance), then it may well not be a problem.

Also, when I mentioned using multiple turns, I meant maybe two or three
instead of one. For some very small rooms this may well be beneficial.

Chris
 
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