Madrigal Imaging N380S ユーザーズマニュアル

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The Nº380S Preamplifier incorporates a special surround sound processor mode
which makes it uniquely capable of integrating the highest performance audio
with surround sound—that is, dual-purpose music and movie systems. In order to
better understand the value of this design, it is essential to understand a bit about
the nature of a surround sound processor.

One obvious and fundamental difference between stereo listening and multi-
channel listening is the number of channels that must be controlled. When listen-
ing to two-channel stereo, one must have a two channel volume control so both
speakers increase or decrease in volume in concert with one another. Similarly,
when listening to six- or eight-channel surround sound recordings (whether mov-
ies or music), one must have a corresponding six- or eight-channel volume con-
trol.

This simple fact has made combining a high quality two-channel preamp with an
outboard surround sound processor extremely difficult over the years. In fact, un-
til the Mark Levinson Nº38 was introduced in 1993, it was virtually impossible to
combine the two and have the system work reliably well.

Dolby Pro Logic decoders incorporate a form of Dolby noise reduction similar to
the Dolby B one finds in cassette decks. This form of noise reduction is level-
sensitive. That is, Dolby noise reduction intentionally treats strong signals differ-
ently than weak signals. In order to operate correctly, the signal strength of the
source must be “calibrated” to the expectations of the Dolby noise reduction cir-
cuitry. (It is for this reason that one finds “Record Calibration” features on better-
quality cassette decks.) It is therefore inappropriate to feed a surround
sound decoder with the variable output of a preamplifier. 

Were you to do

so, every change of the volume control on the preamplifier would cause the
Dolby circuitry to mistrack. In extreme cases, severe distortion can result as the
Dolby circuitry overloads.

The next logical alternative might be to use the Pro-Logic decoder ahead of the
preamplifier, sending its Left and Right outputs through the preamplifier as a se-
lectable Source. Sending the Right and Left Outputs from a surround
sound decoder to a pair of inputs on a conventional preamplifier is also
inappropriate

, since any change of the preamp’s volume control would then

throw the carefully calibrated output levels of the decoder out of adjustment,
changing the volume of the Left and Right speakers while leaving the Center and
Surround speakers unaffected.

One could attempt to restore the proper balance by marking a “calibrated” point
on the preamplifier’s volume control and then using only the Pro-Logic decoder
to adjust the volume of the system, but this method is both crude and imprecise,
yielding inconsistent performance at best.