Surround sound processors should not come after the preamp
Surround sound processors should not come before the preamp
Using Surround Sound
Processors
The Nº38 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 Dolby Pro-Logic Surround™ decoder.
The Dolby Stereo® system encodes four discrete channels into a two- channel matrix during the production of the movie soundtrack. This two-channel signal is compatible with normal stereo (and even monophonic) playback. With the proper decoding during playback, however, it is possible to recover the original four channels from the two which are present on the laserdisc or on the hi-fi videotape. These channels are Left, Center, and Right in the front, and a single Surround channel for the sides and rear of the audience. In order to recover all four channels, it is necessary to have a stereo source (laserdisc, Hi-Fi videotape and stereo TV being the most common) and an appropriate decoder.
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 differently 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 circuitry. (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 selectable 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.
