Homemade Electronic Transmitter. (Telephone Microphone).

[ThePillenwerfer]

Some time ago I posted about an electronic transmitter I’d built using a circuit originally published in Elektor magazine and then re-published in the VMARS newsletter. See: http://www.vintage-radio.net/forum/s...d.php?t=109784

At the time I didn’t have a telephone landline; the ’phones I fitted these transmitters to were ones I was fixing up for a then friend who was an antique dealer. They passed the tests we subjected them to but when I did get a landline (which I didn’t really want but it’s cheaper to have one than just broadband) and used them in anger I found a problem: I was deafening myself.

Obviously the gain needed reducing but doing so was beyond my extremely limited knowledge of transistors. I did find that upping R3 — which connects the electrect to the base of the first transistor — from 4.7KΩ to 100kΩ helped but that seemed only slightly less crude than sticking insulation tape over the electrect.

Looking on the internet revealed very few alternatives. There were two or three circuits that did the same thing with a single transistor but I found that these designs either didn’t work at all or not well enough to actually use. After taking ideas from several I came up with a configuration that does work.

I’d always thought that the Elektor circuit was needlessly complex and mine is much simpler; it’s simple enough to make ‘dead-bug’ style so it can be squeezed into things like Trimphones or even into old Transmitter No 13s making a drop-in replacement to use in a Handset 164. It can also be made on Veroboard if you don’t have facilities for making PCBs.

I attach a ZIP containing PCB details.

A resistor in the emitter is a more reproducible way of reducing gain, a lovely simple circuit BTW.

[paulsherwin]

Lots of designers don't like collector feedback bias, but I've never had any problems with it in my crude efforts, and it keeps the circuit nice and simple.