On Fri, October 2, 2015 5:34 pm, Doug wrote: >> In this context, mis-use of terms "impedance" and "power" is adding >> nothing but confusion. > > I have no quarrel with the advice that follows, but I do not understand > the comment above.
Doug, As I recall, the thread began with the need to monitor the audio signal being fed into a transceiver from the headphone-level audio output of a computer. Specifically, the question was whether a Y-adapter would suffice. In this application, the Y-adapter connects in parallel the input of the transceiver and the input of the powered speaker system. We immediately get into trouble if we say "load" instead of "input", because the term "load" implies a transfer of power, and in this application there is no transfer of power from the computer output to either the transceiver or the speaker system. The most simple, natural, and common solution is to use an inexpensive (about twenty-dollars) set of powered speakers. The term "powered speaker" commonly is used for an integrated unit which consists of speakers (left and right), amplifier(two-channel), and power supply. The input (whether jack or cable with a 3.5mm stereo phone plug) of a powered speaker has an impedance in the range of 10,000 Ohms to 100,000 Ohms; this is for each channel. Such a high impedance is all but invisible to the output of the computer, because the output of the computer is designed to pump 10 to 20 milliWatts into a pair of 8 Ohm earbuds. So we take a functional system (the computer output feeding a signal into a transceiver) and connect in parallel the powered speaker, which adds to the computer output an almost-invisible load. And, like the powered speaker system, the transceiver input has a high impedance. So, essentially, the computer HEADPHONE output (which has a very low impedance) is running unloaded. The power consumed by the speaker mechanism (which consists of voice coil, magnet, and paper cone) is of no concern to us, for that power is furnished entirely by the power supply which plugs into the 120V receptacle. Inasmuch as powered computer speaker systems are a commodity item (twenty dollars), there really is no incentive to purchase components and build anything. RLH
