Bill Hudson wrote:
/The slope of the De/Pre-Emphasis isn't what is important here, as that won't
change in a 2 component circuit, what is important is where the circuit
starts (or finishes) working; which will affect the equalization of the
transferred audio. . .
/
You're going to have to help me with this one Kevin. The "slope of the
De/Pre-Emphasis" is "the equalization of the transferred audio."
I missed something.
I think that everyone knows Bill is correct in theory, but the practical
application of the circuit is where it's gunna bite Bill in the @ss.
You trimmed one very important passage of my message, Bill:
That being said, it does matter what the source impedance is. It
won't matter as far as the 6 dB per octave slope is concerned, but it
will matter where the knee of the circuit will occur, and how much
audio is available after it.
_The "knee" of the circuit._ If you consider that any 2 component
de-pre emphasis circuit has a starting/ending point, (some call this the
corner) then the placement of the components *will* matter. Why,
because we have practical source and output impedances we have to
consider. The knee (corner frequency) will be controlled by the
impedance of the audio source *if* you don't have a buffer op amp ahead
of the RC. If the driver has significant output impedance, that'll add
to the series resistance value and move the corner. Lets take a "for
instance"..
In the Repeater Builder AP-50
<http://www.repeater-builder.com/products/ap-50.html>, we have
components that provide Pre or De Emphasis
<http://www.repeater-builder.com/products/AP-50_Docs.pdf> depending on
what audio is available to you. Lets say for instance you have
de-emphasized audio that you want to process, and you need to
pre-emphasize the audio first. You choose a jumper setting to apply
pre-emphasis. In our circuit, we have chosen the values of C9 and R9 as
.0022 uF and 4K7 respectively. If the AP-50 is driven from a source
that is low impedance, say 100 ohms or less, the knee would be out to
about 15 kHz. However, if the AP-50 is sourced by a driver of 100K, the
knee would be down to about 725 cycles. You wouldn't want your
pre-emphasis to end at 725 cycles.
This is where complex equalization problems originate from, and why many
of us choose to bypass most/all the pre-de emphasis circuitry where
possible.
Kevin Custer
