audio electrical info

[Bill Layer]

Hi Adam,

 >But how does impedence fit?

There are two impedances to be concerned with: Source and Characteristic.
Impedance has absolutely nothing to do with the signal level, but it does
create some practical considerations. Source impedance can be thought of
simply as an impedance in series with the output of your signal source. The
lower the Impedance (Z) the more current can be drawn from the source
without affecting it.  Characteristic Z is an abstract quality of a
transmission line. When a line is said to have a characteristic Z of 50
Ohms, what is meant is that a 50-ohm termination (resistor, perhaps) will
perfectly terminate that line. Perfect in these terms means no signal
refelectins on the line, i.e. all power is transferred to the load
(termination).

>i.e. what is the difference between a line level signal, a mic signal, and
>one that goes to your speaker?{be specific if possible - what is the
>nominal voltage PP(i am assuming the means peak to peak) for each.}
>
A good line level sinal can be typically 1-10 VP-P (.35-3.5VRMS), 1Vrms is
a 'standard', with a source impedance of 10K Ohms or less. (huge
generalization) 

A mic level signal is variable depending on the type of mic element, but is
typically 10 to 100 times smaller than the line level. 150-600 Ohms source
impedance is typical

Assuming a 8ohm loudspeaker, 2.83VRMS (8VP-P)would constitute 1W of audio
power into that load. Depending on whether the amplifier is tube or SS, the
source impedance may be anywhere from 8-10 ohms (Tube OTL amps) to a
fraction of an ohm (big SS amps)

>furthermore, how do i safely attenuate a say a line signal down to 360mv PP,
>then after running it through a chip get it back up to line level (with
>minimal noise)

Use a simple resistive divider. If the line level is 3.5Vrms, use a 10 to 1
divider (1K in series with a 10K resistor, output between the 1K and
ground). (Of course, adding the divider raises your source impedance,
cutting into the signal's ability to drive the next stage... Back to that
already!). Note that real Z (DC-resistive) and imaginary Z (AC-reactive)
impedances do NOT sum directly, but have a 'root of the sum of the squares'
relationship. This is also termed "adding at right angles". Summing
resistive and reactive values produce the complex value known as impedance.


  
Bill Layer, member

The International Brotherhood of Jazz Dancers, Pastry Chefs and Nuclear
Technicians