I wonder if anyone else who regularly plays an electric guitar in different venues, has the problem I have ... the hearing aid induction loop is picked up by the pickups in the electric guitar and is amplified by the guitar amp. This amplified sound is then picked up by the mikes, amplified and fed into the hearing aid loop, causing a "howl-round" or feedback situation. With the hearing aid loop running right round the building it is very hard to get away from it.
A guitar pickup is a coil of wire with a permanent magnet inside it. The steel strings move in the magnetic field and generate an alternating current in the coil. The trouble is that any alternating magnetic fields will also be picked up by the coil and amplified. The usual problems are caused by hum fields around the mains transformer in the guitar amp, but a lot of halls and churches now have induction loops that provide a signal for people with hearing aids that work on the same principle.
Now the obvious solution is to use a "humbucking" pickup. This type of guitar pickup has two coils, wound in opposite directions, and wired in series, so that any background magnetism is cancelled out. One coil has a magnet running north-south, the other is south-north so the signal from the guitar strings is in-phase and adds together. It is the perfect solution, background hum is cancelled and the wanted signal is doubled.
I blogged about it a few weeks ago. It looks very similar to a Fender Precision Bass, and, since 1957, these instruments have been fitted with a pair of pickups wired in a humbucking configuration. Wikipedia page about the Fender Precision Bass
So what's going on? Why do I have such problems with my bass?
Replacement pickups are available for a few tens of pounds, but if I changed them, how would I know whether the problem was solved? I need some way of exposing the guitar to a magnetic field like the one from the hearing aid loop, in a controlled way, so that some comparisons can be made.
I did some reading about hearing aid loops, and they have to meet certain standards (BS7594 / IEC60118-4). They are designed to produce a field strength of 100mA/m. They have compression circuits to keep the field strength within that range even when the person speaking into the microphone varies their volume. So 100mA/m looks like the field-strength to aim for.
I decided to make a Helmholtz Coil. Wikipedia Page about Helmholtz Coils
This sort of apparatus looks like a pair of hoops. When fed with a current (d.c. or a.c) the coils produce a uniform field in the space between the two coils. It is also possible to do a sum, based on the number of turns of wire, the diameter of the coil and the current flowing trough it, to work out the exact strength of the field inside.
The equation above looks a bit scary, but it turns out that if we want to calculate the field in terms of mA/m we can leave out the permeability, and the 4/5ths raised to the power of 3/2 works out to be 0.71554.
True RMS Multimeter across the resistor. By ohms law, 1mA produces a voltage of 100mV across the resistor.
But I'm getting ahead of myself, I need to make a former to wind the wire on ...
Then I wound the 53 turns of wire on the former. To make this easier I found an old plastic pill-pot which was an exact fit in the centre hole of the wooden disk. I could use that as an axle to rotate the disk around. I set up the spool of wire on a big screwdriver in a vice so that it would dispense wire freely. I made a mark on the wooden disk so I could count the turns. I used 0.56mm dia. enamelled copper wire - but almost any type of wire would suffice.
The photo below shows the final set up with a guitar in the coils. the Levell oscillator is feeding an a.c. signal into the coils and the meter is used first to set the current to 1mA (100mV across the 100 ohm resistor) and then to measure the output from the guitar pickups.
As long as the frequency is below 1kHz, the meter reads very accurately. If I wanted to look at how the pickups performed at higher frequencies, I would need to use an oscilloscope or a different kind of meter.
So what do the results show?
The black-coloured guitar is a useful comparison because it has hum-bucking pickups in bridge and neck positions and a single coil pickup in the middle. I measured the output of the guitars at three different frequencies, 50Hz, 100Hz (mains hum frequencies) and at 800Hz, more representative of a hearing aid loop. On the black guitar the humbucking pickups had almost no signal at any frequency - and the background hum (with the oscillator switched off) was low too. The middle, single coil pickup picked up the Helmholtz signal quite strongly giving an output of about 10mV at 800Hz. But the output from the bass was twice as strong with over 20mV. So clearly the bass pickups are behaving like single coil pickups - not humbuckers.
E string 89.9mV
A string 57.7mV
D string 56.8mV
G string 80.8mV.
This may seem a bit over the top, but I want to be able to compare the outputs of different pickups. You will see that 20mV is about the quarter of the output of the guitar when a string is plucked. A most unacceptable level of background signal.
The reason that the bass guitar picked up more signal from the Helmholtz coils maybe because the pickups are physically bigger. The signal output is proportional to the area of magnetic flux which the coil encloses, so a larger diameter coil will have more magnetic field lines passing through it.
So the next step is to get some new pickups and see how they perform.