On 4/14/2014 4:46 PM, Harry_Yingst wrote:
Phone lines are Typically 600 ohms
Actually, the Zo of phone lines at radio frequencies is about 80 ohms, and varies widely with frequency in the audio spectrum. That characteristic is determined by the diameter, spacing, and insulation of the twisted pairs used to carry the signal. They have not been 600 ohms for at least 75 years. More than 40 years ago, it was standard practice to equalize broadcast lines for extended frequency response over relatively short paths, and transformers were used to step that impedance up to the 600 ohms used by older broadcast equipment. But the broadcast and pro audio world abandoned 600 ohms as a standard about 50 years ago.
Modern audio circuits (since we converted from hollow state to solid state) use low impedance sources to drive high impedance imputs, there is no impedance "matching" except to the extent that the load impedance must be high enough that it does not draw any more current than the output stage is designed to provide.
73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
