Tools to capture electromagnetic fields and alternatives to telephone pickups?

[Lando]

Hello everyone.

First of all, if this is not the correct forum for this post, staff, please feel free to move it.

I am a part of a small community that is tangential to field recording, in that we attempt to archive recordings of speakers in public places, with the best quality and least noise, background or otherwise, as possible. The main "tool of the trade" besides using a normal mic close up to the speaker is using small telephone pickups (https://www.newark.com/productimages/standard/en_US/39C0931-40.jpg), which work quite well by capturing the electromagnetic field around the speaker to capture the audio. However, I have noticed that a specific older model of telephone pickup works better than others that I have tried, with a stronger input and better overall frequency range. At this point, I have ordered and tried a total of five different pickups, with no new results to show of any models that can produce better recordings than the main one already known to the community.

A few people in this same community have recently brought back up the idea of possibly winding our own pickup coils or otherwise trying to contract some to be manufactured. However, I know basically nothing about these, besides that they have a coil of wire in them, and I don't know what would produce the best/loudest/most detailed recordings. Through some research and talking to others, I have seen inductors that can be purchased and wired to 3.5 mm cables, and that guitar pickups are another possible option; however, I have no idea what I am looking for specification-wise.

If anyone is able to provide info regarding other options that can capture recordings from speakers' electromagnetic fields, or the technical details regarding this broken pickup () or other specifications, I would greatly appreciate it. Thanks!

 

[Voyager10]

Hello and welcome! Sorry you've not had any responses yet, it's a rather unusual problem.

Just out of interest, have you tried electric guitar pickups? There's a whole world of different types available for experimentation.

 

[Lando]

Hello and welcome! Sorry you've not had any responses yet, it's a rather unusual problem.

Just out of interest, have you tried electric guitar pickups? There's a whole world of different types available for experimentation.

I have not, but they are definitely on my radar. Just not sure where to start with them as it's a nontraditional use for one - my main goals are getting the strongest signal and best frequency range possible. Additionally, I don't know if there are any unique concerns in using a guitar pickup for this instead of a telephone pickup or inductor, or the quality of the resulting sound for that matter.

 

[Tubetec]

I have one of those wireless phone bug pickups since my teens I got in Radio Shack or Tandy electronics ,
As wayward youths we used them to record the incoming side of prank phone calls impersonating students in the classes parents calling other parents to complain or other simillar false pretences,
Years later we were still rolling around on the floor with laughter listening to the 'Tape' .

Its not really a high quality sound the magnetic pickup ,
but it does have the benefit of immunity to external sources of acoustic noise ,

One thing that might improve the performance of the telephone bug pickup is a balanced connection , typically they come wired to single ended 3,5 mini jack .
Copper foil screening could help keep out RF mush also ,

 

[Tubetec]

You have a break in the coil or more likely where the cable joins ,
you may be able to unwind a few turns off the coil and solder it up ,
Use the multimeter to see if you can find coil continuity ,

You can rewind the coil easily by chucking up the bobbin in a cordless drill,
finding the wire diameter with and without insulation is the next step .

 

[ruffrecords]

Whatever transducer you choose it would almost certainly benefit from a local buffer/amplifier stage. This could be made phantom powered which would allow you to use long cable lengths without risk of interference or degradation of the signal.

Cheers

Ian

 

[cyrano]

I think investing in a few rolls of enameled copper wire would be wise.

When you shop for magnetic pickups, there's usually no info at all. So rolling your own is simple. It's just one coil. The more windings, the better sensitivity. Within reason, of course.

I also have one of these Radio Shack (Tandy over here) phone pickups. I had long forgotten I still had that, but clearing out the last boxes from the move, it re-appeared.

It's a nifty idea picking up audio from speakers with a coil.

 

[abbey road d enfer]

If anyone is able to provide info regarding other options that can capture recordings from speakers' electromagnetic fields, or the technical details regarding this broken pickup (

I would suggest you start by measuring teh wire gauge with a micrometer or caliper.
That will allow determining roughly the number of turns.
If unwinding partially does not allow you to find where the winding is cut, you may want to rewind it completely. It takes some patience but it's not difficult.
Alternatively you may find a winder, for whom it would be an easy task.

 

[Tubetec]

Have a look at Kingkorgs low impedence pickup thread ,
he uses standard off the shelf 'lucky bag' audio transformer , dismantles the core and places a magnetic slug into the coil .

Adding a buffer seems like a good plan , but simply rewiring the coil with two core and screen connection might effectively reduce RF pickup .
From memory the coil has a Resistance of a few hundred ohms ,
its destined to be plugged into the unbalanced mic input on a casstette player ,
thats usually around 100kohms .

 

[ct.waveform]

I would think your problem is not so much the coil or pickup you use as the load you put the signal into. This is essentially an old-fashioned problem; the one of how to get the full audio band out of a high value inductor - as found in a guitar pickup or a phono cartridge. Choice of load resistance and capacitance is critical. In your favour is the fact that this response can be made astonishingly flat; for example a Stratocaster in standard setting is almost ruler flat. Otoh, quite a few MM phono cartridges truly struggle to reach 15kHz. Against is the fact that you will ideally have to model the circuit or do the maths of it. Not a difficult model but it is the very minimum you should do.

As far as core materials are concerned there's not much wrong with cast iron as in the photo. It's nearly identical to low carbon steel. If you are making just an inductor you'll get some improvement in distortion and losses with a laminated core but initial distortion can be quite high on all these. It doesn't appear saturation is a problem here, so a rod ferrite core would improve things on all fronts; especially low level distortion. (You can't use toroids or other closed inductors in this application, even if it could be arranged topologically.) The pertinent elements here are that, although ferrite saturates earlier, iron and especially iron dust/powder cores have much higher distortion at low levels than ferrites do and all of them have too high a permeability to be used without an air gap.

When I'm looking for stray magnetic fields I tend to use a 0.47 or 0.5mH air cored inductor. I have some on ferrite cores too, if space is limited. I was recommended this value by a friend at Naim Audio some decades ago and have stuck with it ever since. While this works into almost every load without causing problems, the problem with such a small value of inductance is signal level. If you need much more signal then you are definitely breaking out modeling software of some sort. If you are not a Spice user then you could probably adapt NuHertz's Filter-Free software which does 2nd Order filters inside its freeware part. I don't normally recommend NuHertz because of the technical misunderstandings I've seen, but they aren't so gross as to affect something as simple as this. What makes this software good for your application is that it allows you to vary the output resistance so, level aside, you don't even have to worry about the wire gauge.

Just punch in the bandwidth you want (and, say, a 2nd order Butterworth low pass) and see what it gives you when it has something close to the values you've been using. You may find that the other 4 devices are better than you think when given the correct load for your task.

 

[Doug_]

I take it that you are recording the signal from the hearing loop set to couple to hearing aids?
If this is the case, from my investigation it seems this is now old technology and modern hearing aids are not commonly fitted with a coil for this, the connection is now via Bluetooth but I have yet to discover how this works in public spaces. Please advise if this is not the case, I would like to understand current practice.

I guess with the advent of the cell phone and reduction of magnetic style handsets will reduce the prevalence of such loops.
I do have and old pickup similar if not the same as the one previously illustrated and this works extremely well with my H5 Zoom recorder with a simple 2.5mm to 1/4” adapter. No amplification required. As others have said this is a simple coil, probably with a small iron or ferrite core to concentrate the magnetic field.

I live in Sydney, Australia and few public places are fitted with hearing loops - a small number of churches and some railway stations but that was all I could find. I don’t wear hearing aids so I can’t say my survey was extensive.

The biggest problem in recording this way are stray magnetic fields in the audio spectrum. Think electrical cabling and switching power supplies!

Ask any gigging electric guitar player about dodging noise on stage, The hum-bucker guitar pickup was a solution to the ‘extranious’ noise. Guitar pickups have a magnet as the core and pick up primarily the change in flux from the steel string vibrating in its field.

So my take on grabbing and recording these broadcast signals now is to figure how to connect to public Bluetooth sources. I expect these would be perfectly clean in comparison.

 

[abbey road d enfer]

My understanding is the OP does not try to record signals from induction loops, which, as you mentioned are less and less used.
I think he likes to put his sensor in the vicinity of loudspeakers, where it picks up the leakage magnetic field coming from the voice coils.
Maybe I'm completely wrong...

 

[Lando]

My understanding is the OP does not try to record signals from induction loops, which, as you mentioned are less and less used.
I think he likes to put his sensor in the vicinity of loudspeakers, where it picks up the leakage magnetic field coming from the voice coils.
Maybe I'm completely wrong...

This is correct.

 

[Voyager10]

Do you have any form of 'reference' setup, like some commonly available loudspeaker, and arrangements for holding the coil in a fixed position?

If you can make frequency response and sensitivity measurements in a consistent way it's generally easier to see the effect of small changes.

 

[abbey road d enfer]

I believe the main subject here is how to optimize both signal level and frequency response.
Level is directly related to the number of turns of the coil and how much of the stray field is picked up by it.
There's a limit to increasing the number of turns, because it also increases the inductance, which in turn limits the frequency response.
Guitar pick ups have an inductance of a 3-10 Henrys, which results in an impedance of 200 - 600 kiloohms at 10 kHz, which imposes a load impedance of more than 500k - 1.5Megohm. Connecting to a standard unbalanced mic input of about 5-10 kohm, results in an attenuation of 25-40dB at 10 kHz.
In order to provide a better response, the inductance should be around 100mH.
For balanced inputs with an impedance of about 2kohms, the inductance should be about 20mH.
The presence of an iron core considerably increases the level, by concentrating the magnetic flux lines, but also increases the inductance compared to an air-core inductor.

 

[Tubetec]

A copper foil screen around the coil might be worth a try ,

These devices were made to attach to the telephone ear piece in very close proximity to the voice coil , Im not sure how well they would work on a PA cab , the other issue there is you might have a metal front grill on the cabinet and no access to the magnet from behind ,
the sound might also have passed through a crossover of some sort ,