Induction looper

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Revision as of 22:28, 22 February 2020 by Pwuts (talk | contribs) (Class A/B)
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Project Induction Looper
IMG 5411.jpeg
Induction Loop audio on the cheap and easy
Status In progress
Contact sebastius, pwuts, noor
Last Update 2020-02-22

Plan is to make a really cheap and really portable induction loop system that we can just give to conferences for free. That will enable them to help hearing impared visitors listen in to talks via the t-coil in their hearing aids. Sidequests include using these systems to create secret audio spots for quests and other stuff.

Original inspiration for the project:

Sebastius designed a little board that allows configurable amount of loops. Prototypes have fixed settings, switches are still underway from China.

Board files:

First test was very successful!


For the driving electronics, there are basically two options:

  1. using a regular audio amplifier and matching the impedance of the adjusted loop so the amplifier doesn't shit itself
  2. building a transconductance amplifier and optimize the loop for low impedance

We could also spend €400 on a professional induction loop driver but that's not really something we talk about.

Using a regular audio amp

This is, in terms of electronics, the easiest way. It requires an amplifier (of course) and a loop with 8 ohms of impedance.

Transconductance amplifier

This is how it's done professionally. A circuit that takes voltage at the input and outputs a current proportional to the input voltage.

After some searching it seems Ali doesn't have them, so this is going to be a DIY board.

Making a simple half-wave transconductance amplifier isn't that complicated. It's the sensible (additional) requirements that have potential to make it complicated:

  • it needs to output both the positive and negative halves of the signal, not just the positive half
  • volume control would be nice
  • pls no EMPs when turning it on

But, set aside the above requirements, the minimum can be achieved with an OP-AMP, a FET or BJT and a resistor [example]. Because of the OP-AMP this is called an Operational Transconductance Amplifier (OTA).

Class A/B

Building a Class A or Class B amplifier is probably the cheapest when it comes to the BOM, but may be more complicated in terms of design and usage conditions.

The simplest would be a Class A OTA, with one MOSFET or BJT doing the hard work. To get more than just the half-wave signal, the output signal either has to be shifted so it is enterely positive, or a second, negative power supply rail is needed. The need for a symmetrical power supply would make this whole thing significantly less accessible or easy to implement. But shifting the signal upward by one amplitude means that for zero input, there will be a static output, wasting power and creating a static magnetic field.

An example of a relatively simple Class A-ish OTA (©®Pwuts™2020)

To solve the dilemma mentioned above, a Class B OTA can be built instead. It uses two transistors, allowing the direction of the current to be reversed and thus eliminating the idle current. This does need more tuning though, as the (in this case MOSFE)T will go into saturation more quickly than the one in the Class A due to the different configuration. This would cause clipping. Carefully chosen transistors or high supply voltage can be solutions.

An example of a Class B(oat) OTA

Power OP-AMP

Using a power OP-AMP like the LM675 would make this a lot easier, as there is only one building block for the amplifier (the OP-AMP itself). To build a transconductance amplifier, the feedback network has to be made so that the feedback to the V- of the OP-AMP is proportional to the current in the load (or in this case, the loop). This isn't very hard to do and can also easily be fitted with volume control, see the example below.

OP-AMPs also normally need positive and negative power supply, but their voltage supply is differential so it's also possible to supply it with e.g. +24V (Vcc) and 0V instead of +15V and -15V.

To make this work, a second supply voltage will be needed, of Vcc/2. This will be used to shift the level of the OP-AMP's input and feedback network, and can also be used to shift the output current back to zero equilibrium (the source needs to be able to handle negative current for this).

An example of a power-OP-AMP OTA with volume control and single sided power supply

to be made into wiki-compatible text:

I'll just respond to @JennyList and @NoorDraconia in one go: with some help of Benadski I changed a capacitor on the EM sniffer so it's better for low frequencies. As a result I could actually hear the transmitted sound over the EM noise coming from the wall. It's still not really an induction loop tester though, so I would advise against using it as some kind of reference. If the signal must be received in a certain space, one can make two loops at any two opposing ends (and maybe one in the middle if the other two are at the short ends) or one loop around the whole room. To get an idea of signal distribution and decay, use this: ("current loop", "loop pair stacked", "rectangular loop"). People with hearing aids can't be expected to stay stationary which must (I think) be reckoned with for signal distribution and thus placement of the loops. Apologies again for a wall of text :P