IP phone to analog phone or modem audio interface circuit

Posted on Feb 6, 2014

The transformer is a 600:600 ohms transformer (also named 1:1 ratio 600 ohm transformer) : more or less the same number of turns on both primary and secondary coils, and optimized to work at 600 ohms load. Selectronic sells one here. I measured the primary and secondary coils on the one from Selectronic: about 75 mH inductance and 50 ohms

IP phone to analog phone or modem audio interface circuit
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resistance. - the variable resistors need to be fine tuned for operating at best impedance match with the IP phone. The one near the IP phone speaker will probably need to be tuned at a high value (about 10K ohms); it directly controls the amplification of the signal you ear. The other needs to be very low since it is only here to attenuate the level at which your voice is sent to the IP phone, since the microphone you replaced with this module probably sends a lower signal than this module. - the two 100 ohms resistors will dissipate about 600 mW each : avoid using those standard low-cost half watt resistors (or use 150 ohms standard ones instead. ). - the two 300 nF capacitors (and their associated resistors) act as high-pass first order filters (Fc is about 300 Hz) to eliminate noise and perform echo cancellation. While I was conceiving this telephone interface circuit, I used my Sound Blaster Extigy sound card from Creative Labs as a function generator. I encountered weird effects and I realized the headphones output had a not so good transfer function. I plotted the voltage transfer function with my APPA 106 multimeter. Here it is: After weeks having bad dreams about this weird transfer function, I suddenly realized, one night, that it was obviously a Head Relative Transfer Function (HRTF) ! Here is a plot of a standard HRTF (extracted from this web site ):

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