This is a sequel to the battery mayhem post.
A couple of weeks after the battery episode, we started hearing distortion again particularly through FoH and the bass player’s monitor. The distortion didn’t manifest during the sound check but only after the whole band started playing. When we muted the bass, the problem ceased but obviously this was not a solution.
Following the battery mayhem, the active electric bass (a Fender J) and the acoustic electric guitar had been fitted with new batteries so it was unlikely to be a battery issue. We had also carefully set the gain of each instrument at the mixer to ensure there was no overload.
As we scratched our heads, we noticed that the LEDs on the hearing loop amplifier indicated it was peaking.
For those who don’t know, a hearing (or induction) loop is a special type of sound system for use by people with hearing aids. The hearing loop provides a magnetic, wireless signal that is picked up by the hearing aid when it is set to the ‘T’ (Telecoil) setting. The loop amplifier receives input from a source such as a microphone or a mixer and pushes the amplified signal out to a copper loop that encircles the area to be serviced (or alternatively to an aerial for short range applications).
Hearing loops are known to interfere with certain type of electric guitar pickups, which can operate a bit like a hearing aid on the T setting. We had heard about this on another project we are doing and, sure enough, when we turned the loop off, the distortion disappeared. We left it off for the rest of the evening – not ideal but it seemed to be the least worst option.
We didn’t tweak to this earlier for a number of reasons:
- our lack of knowledge of hearing loops
- a vague understanding (probably incorrect) that interference only occurs with single coil pickups and would not be an issue with the active bass which has dual coil pickups
- the distortion only appears when the hearing loop amplifier is pushing out a lot of signal.
We now need to work out if there is a judicious setting of levels will allow us to operate the loop so that it can be heard by people who need it, and also have electric bass in the mix at the same time. We will shortly be taking delivery of a test device so that we can hear what is going on without having to poll congregants wearing hearing aids.