Telecoil Amplifier

[jakeottinger]

Hi this is my first post.  I've been watching the youtube channel for a while now.  I recently was tasked with finding and installing a telecoil system.  I've been doing a bit of research and I've decided to build most of the system myself using a boundary mic on the ceiling.  My question is if there was any real difference between the induction amp sold with the systems or if I can just use a regular amp and mod it to connect a long 8 ohms of wire in place of the speaker.

I am a ultra noob when it comes to electronics so please excuse me if this is self evident as to the answer.
Thanks guys/ gals.

[Richard Crowley]

Building the "transmitter" is rather trivial.  Consisting of a loop (or a few) of wire around the area, and an audio amplifier to drive it. 
Be sure to get the impedance ratio between the amplifier and the loop well-matched if you want decent performance and amplifier safety.

HOWEVER: The matter of what audio signal you are driving it with raises some questions.

First, you have not revealed what kind of situation this is? 
Is this a tiny conference room for 4 people?
Is it an executive board-room with a long table for 28 people?
Is this a night-club with occasional, casual entertainment acts?
Is this a large auditorium or theatre?

With the possible exception of the first example (a closet-size conference room), I question the validity of mounting a microphone on the ceiling and expecting decent performance.  It seems to me like the person's own hearing-aid would have a microphone closer to the source than your ceiling-mounted microphone.  Of course that depends on exactly what and where the source(s) are, and you have given us not a clue to work with.

Trying to pick up intelligible speech (especially for people who have below-average hearing) from that distance is rarely successful.

[jakeottinger]

thanks for the reply.  This is a large room with a sound system.  The majority of the signal is going to come from a mixing console.  There are people on stage with lavaliere microphones.  The main issue is when questions are asked the people in the audience do not have microphones.  I was thinking of mounting a boundary mic above the audience to be able to run that into the t-coil system as well as the output from the mixing console. 

A regular audio amplifier will work fine then?

[Richard Crowley]

A "telecoil amplifier" is just an ordinary audio amplifier.  However, remember that you need to match the output impedance of the amplifier to the impedance of your loop.  That is not a throw-away issue.  Feeding the telecoil system from the venue reinforcement system is great. But mixing in a distant mic may actually be worse than leaving it off.  Go ahead and install it, but don't be surprised if you end up not using it.

[jakeottinger]

ok great thanks.  Yes I'm very aware of matching impedance in audio gear I just wasn't sure about the inductance part of the amplifier.  If my wire is not long enough to have the necessary resistance do you see a problem with adding a resistor to the wire to get it to match? It is quite possible that the mixer output will be enough to help those with hearing aids hear the people speaking on stage more clearly and that is all we will need.  As far as the omni mic goes I can certainly see how that might not work and it may be necessary to use a parabolic mic instead and have someone operate it manually.  That is not ideal but I'm thinking out loud here LOL.  Thanks again for the help.

[mswhin63]

Often misunderstood is that Loop Amplifier are not standard amplifier, they are current or trans-conductance amplifer. The output Impedance is usually 1ohm to a maximum 2ohms. I have installed system as low as 0.5ohms but care needs to be taken.

There is a new Australian Standards AS1428.5 that is required on any new building or one built after 2010.

To be effective only a single coil is used to reduce current distortion created by additional inductance. The current output is used to determine the size of the system usually with a minimum distance in one direction. 5m wide for 150sqm area, 8m wide for 500sqm. There are many limiting factors as you are producing a magnetic field metal used in concrete re-bar will sap away some of the emitted energy. Current output ranges from 2amps for counter loops, to 60amps for up to 2000sqm. The largest single installation was at a temporary open field rock concert at 1000sqm.

In addition AGC not ALC is required as peak power can destroy the output stage either instantaneously or overheat in time.

Finally as part of the International standards the field produced 1A/m.

In short, Loop amplifiers are not your normal everyday amplifier. Especially now-a-days with smaller an smaller hearing aids with telecoils making it more importantly to have high enough power level. Hearing aids have their own standards which is why they need to match when it comes to the designs and implementation for both.

If anyone is interested, I own and run a small business that deal with hearing loss products in W Australia, for 12 years I was installing loops up till 3 years ago

So I am not talking out of my arse

I stuffed up my knee crawling on the floor installing copper foil, so I no longer do loops anymore.

[jakeottinger]

wow thanks for the great info.  So a regular audio amp won't even work? or do I need one with 2 ohm capability?

[Richard Crowley]

Certainly, ordinary audio amplifiers will work. We were doing this with ordinary amplifiers for decades before specialty products were developed.  There are several different ways of matching the amp and loop impedances.

[jakeottinger]

oh cool.  So would there be any issue with the magnetic field messing with computers or causing audio interference with other audio gear?? 

Another consideration is it possible to overload the hearing aids?  Like too much wattage blowing out people's hearing aids? or it being too loud for some and not loud enough for others?

As far as matching impedance I just have to make sure if I have an 8 ohm amp I need a long enough wire to produce 8 ohms of resistance correct? or maybe slapping a resistor in the wire if it's going to be too long?

I really appreciate the help.

[mswhin63]

For engineers. Magnetic field is relative to di/dt not dv/dt so the more turn you place in the loop coil the greater the Ldi/dt and thus the frequency response reduces. For 4 or 8 ohm amplifiers more loop turns are needed to produce the required magnetic field although the frequency response reduces due to di/dt.

So in short to produce the required magnetic field over a wide frequency response the inductance needs to be reduced as much as possible so anything more than a single loop will reduce the overall performance. Loops are restrictive enough but as most people have high frequency loss it is more important to enhance the high frequencies as much as possible.

[mswhin63]

While I was installing loops, I could only provide a system under the AS1428.5 which requires a bandwidth of 6KHz which cannot be achieved using standard amplifiers. So the specialised amplifier where created to provide good current output while maintaining frequency response. I used a audio analyser to monitor the signal strength versus frequency when testing. I was able to achieve in smaller system or nearly 10KHz while the larger system I was able to run up to 6KHz just. The trans-conductance amplifier weren't constant voltage amplifier either but was able to produce good current amplification while maintaining good frequency response.