Re: Sound card question
On 10/02/2015 02:36 PM, rlharris@oplink.net wrote:
On Fri, October 2, 2015 12:10 pm, Doug wrote:
You didn't mention that you want to add _speakers_. You might need a
small amplifier after the Y-adapter, since the speakers will be low
impedance--4- or 8 ohms. Visit ramseykits.com and find some small audio
amplifiers in kit or assembled form. They are probably overkill for your
purpose, but cheap enough. A 14W stereo kit, CK154, will run on 12 to 16
VDC at 1A. $26 for a kit, or $34 assembled. You
provide an enclosure and a wall-wart to drive it. (The "14W" rating is for
7W per
channel, and I would expect that it's a peak rating, since 16V at 1A is 16
Watts
_input_ power! I would guess that rms power is more like 2 or 3 Watts per
channel.)
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.
Speakers are rated for the _imppedance_ they present to the amplifier
which drives them. Impedance is defined as R+jX, where R is resistance
and X is reactance, which can be either negative (capacitive) or positive (inductive),
and appears on the imaginary (y) axis of a rectangular graph, thus the j term.
Power is always expressed in Watts, and is defined as current times voltage, or
in engineering terms, P = IE. I is current in Amps, E is voltage in Volts.
The voltage is either DC, or the RMS value of an AC waveform. [j = sqrt (-1)]
The "120 volts" coming out of a wall outlet is an RMS value of voltage.
(RMS value of a sinusoidal waveform equals 0.7071 times the peak value.)
So, power can mean AC audio power sent to a loudspeaker, (or AC power from
a wall outlet sent to whatever device it powers),or DC power sent to
some load, such as a flash-light bulb, the supply for an amplifier, or whatever.
Nowadays, that DC power is frequently obtained from a module that plugs into
a wall outlet, commonly called a wall-wart. DC power can also be obtained
from batteries or from the lighter socket of your automobile.
--doug
A pair of powered desktop speakers designed to plug into the output port
of a computer is a commodity item which can be purchased for about twenty
dollars at any computer store. Everything is packaged nicely and there is
a volume control, a tone control, and a power supply which plugs into a
120V wall receptacle; the power output is several Watts. The load which
these amplified speakers present to the audio source (computer,
transceiver, or whatever) is in the range of ten thousand Ohms to a
hundred thousand Ohms; in other words, the speakers present no significant
load on any audio line to which they are connected.
RLH
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