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krfjr1965 10th March 2010 02:47 AM

Headphone amp with mixed-in telephone audio
Hi everybody.

This is something I would build. I have a job sitting at a desk. From time to time at work, I need to take and make telephone calls so I need to hear my desk phone ring. I would like to build an amplifier which will allow me to listen to an audio source such as an ipod or CD player, and has a 4-wire landline telephone connected to it, where both the audio source and both sides of the telephone conversation are mixed together, with possibly a buffer amplifier of some sort. I would also like to be able to talk on the telephone through this device, so I would need to know what kind of microphone I need for this.

Would anyone here have any ideas on how I could produce something like this?
Any help would be greatly appreciated.


wwenze 10th March 2010 03:11 AM

How about a computer with a modem?

Artie 10th March 2010 09:13 AM

Check out how they do it in the Yamaha CA-400 integrated amp:

ca-400-s-corrected.jpg (jpg version - around 1.7M)
ca-400-s-corrected.png (png version - better - around 3M)

Check out on the far right. The mic preamp has a variable level that you can "mix" with the audio signal. The volume, or mix pot, also has the on/off switch built in. Shouldn't be too hard to duplicate this circuit.

(Or, you could just round up a nice old Yamaha CA-400.) :D

Edit: Ignore the few colored circles. Those were relevant to another post . . . somewhere else.

GPG 10th March 2010 03:50 PM

I had a similar requirement a few years back. I tried some op-amp circuits with a jfet to reduce the gain of the music when the announcement came on. I never did get it quite right and resorted to just mixing the two signals together.

I came across this app note from THAT Corp that even gives the function a name.
Maybe this will do what you want.



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