Once again I turn to my friends at All About Circuits, for help.

I have a two channel counter intercom which I would like to connect up to a standard commercial CCTV recorder.

The intercom has two channels ie one for the customer and one for the staff member. Each channel has its own electret microphone and a 8 ohm 3 watt speaker which are connected to the back of the amplifier. The intercom amplifier has no LINE OUT plug of any sort either but connections to both speaker outputs are easily done if that helps at all?

As far as I know the intercom uses one of the TDA series of amps but which one I dont know without opening it up, which can be done.

So, how would you guys suggest I go about taking the audio off both channels into the CCTV recorder. What sort of circuitry do I need and is it possible for a newbie to build? Many,many thanks-JDR04

 

It's both possible and easy. Ideally you just need impedance matching, but what you're doing isn't much different than plugging a headphone-level signal into a line-level input, which people do all the time with no special precautions. You just have to start with the output volume low, and only increase it to a level where you don't hear a lot of distortion or clipping on the line-level device. There are probably more elegant solutions, but it will work.

Oops, wait, are these both mains-powered? If so, you have to be careful they share a common ground or you could create a loop. Before you connect their grounds together, try using a 100Ω resistor between them. If it gets hot remove it ASAP. If not, measure for AC or DC voltage across it. If it's more than a few millivolts, leave it in place. If it's barely anything, you can safely remove it.

 

Thanks Wayneh for your reply. As I'm a newbie there are a few things I dont understand.
What exactly is impedence matching and how do I go about doing it?
The intercom has two channels, how do I combine both these audio channels into one output so I can connect it to the CCTV recorder?

Unfortunately I cannot reduce the volumes on the intercom amp so can I control this some other way?

Thanks a lot-JDR04