Telex Model 530 Cordless Headphone

I look after the PA for my local church. As with most such systems, it includes an inductive loop system. This is fundamentally quite simple, and consists of a single long turn of wire around the inside wall of the church driven by an amplifier. The amplifier contains an aggressive automatic level control (AGC), and attempts to maintain a constant drive level over a wide range of signal levels. A suitably configured hearing aid will receive this signal and deliver it to the listener without unwanted acoustic interference. One issue is that unless you have a suitable aid, it is not easy to tell whether the loop is working or not. Some decades ago, a "Telex Model 530 Cordless Headphone" was purchased. This is a headset with a loop receiver and amplifier built in. It is many years since this was last used and nobody seemed to know much about it. I did find this catalogue entry - from 1989!

Telex 1989 Catalogue Page

Removing the pads from the earphones reveals a battery compartment. Unfortunately, I can find no reference to what battery it requires. Several batteries and cells would physically fit in the holder. The required item would be approximately 12 mm diameter and 28 mm long. This isn't much help, as after some research I concluded that the voltage could be 1.5, 3.6, 7.5 or 12 V! Telex do still exist, but I an not confident that I would receive much help for such an old product. Dismantling revealed the following:

Disassembly

 

Please ignore the glue stick, used to prop the headset apart! The circuit is very simple.

Schematic

Ignore the "12 V" marked by the battery as this was my initial guess based on dimensions. The circuit seems to operate nicely from anything above about 2 V, but was prone to oscillation. The IC is marked "MCC-2637632", but I could not find any information on it. The single electrolytic capacitor is fine in both value and ESR, so I left it alone. The preset potentiometer sets the bias level. I was surprised by the lack of supply decoupling, especially considering the cable run from one side of the headset (with the battery) to the other (with the amplifier). Adding a 100 μF capacitor between pins 2 and 4 of the IC made it stable. In the end, I opted to add a micro USB socket on a cable and wired it across the battery holder contacts. This allows the amplifier to be powered from any convenient USB battery pack. Given the low current consumption (a few mA), it would last very many hours powered this way.

I tried this in the church. It is possible to reduce the PA loudspeaker amplifier drive signals to zero and play music through the loop amplifier only. This allows me to investigate the effectiveness of the loop in various areas without having to contend with audio through both the speakers and headset. It loop really is very effective, with only a few small dead spots.