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Re: Sound card question



On 10/02/2015 10:18 PM, rlharris@oplink.net wrote:

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




Ok, I think we are not fighting over anything. I did not assume speakers
as the destination when I suggested a Y adapter. _Then_ the OP said he wanted
speakers. I did not assume, as you apparently did, that he was thinking of
powered computer speakers. With the exception of some Logitech powered
speaker systems, practically all others are junk, with no attempt at
fidelity. So I would not immediately think of "computer speakers."
And yes, the input impedance of any of such systems is at least a
couple of K-ohms, and will not load the Y adapter. Another responder,
as you no doubt know, suggested an old boom-box with an external audio
input jack. That might be the simplest answer of all, if there's one kicking
around somewhere.

Cheers--doug
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