The quest for the ideal "stereo image" has occupied audio designers for years and it has taken many turns in both recording and equipment. One of the latest efforts to recreate that elusive quality of "three-dimensional" sound is embodied in a new preamplifier, the Carver model C-4000. It contains what the manufacturer calls "sonic holography."
Some may take exception to the term, but the C-4000 does provide enhancement of normal stereo program material. The efforts vary from the sense of a more convincing proscenium effect to an occasionally rather startling imaging or "materialization" of sounds from parts of the room that your original conception tells you can't be possible since there are no speakers there.
The trick involves some rather arcane circuitry which, to put it simply, acts on phase and amplitude variations in normal stereo program material so as to modify the channel information that otherwise would be wafted from a pair of stereo speakers. At its best, this provides an enhanced spacing beyond the speakers of the stereo material; it can be likened to the kind of presentation you might get from a binaural recording heard over headphones.
The technique seems most effective on recordings in which the studio mixdown (from multiple microphones) has not resulted in too many "built in" phase derangements. This is impossible to predict for any recording, but there are controls on the Carver unit that compensate to an extent. Another desideratum is the use of direct-radiating speakers which probably will "image" better when they are placed a little closer to each other than you may have done originally. An acoustically balanced room also helps, and your own listening position in that room can become critical, although with the new version (the M-4000B) the optimum listening spot has been widened to accommodate three listeners instead of the solo listener "width" of the first version.
Aside from the "hologram" feature, the Carver preamp has several other notable features. One is a built-in time-delay system which may be used with or without the hologram option. This section has its own 20-watt-per-channel power amplifier for driving rear speakers directly. Alternately, or in addition to that, there are other outputs that permit sending the time-delay signal to a separate power amp. You actually can set up two time-delay systems with this unit.
Also included is a very sophisticated noise-reduction circuit that really lops off unwanted signals below the normal frequency range but without degrading the music within that range. Another option supplied with the preamp is its "peak unlimiter" which provides about 6 dB of dynamic range recovery. The phono input section has selectable loading to "fine tune" the response to suit different phono cartridges.
The full use of all the options takes a lot more study of the owner's manual and a far more "activist" approach to listening than would a conventional preamp. The initial set-up of the first version took some six hours, but this time has been reduced to, at most, one hour with the aid of a special test record made by Carver.
Using the Carver M-4000B becomes some kind of sonic adventure that involves one's stereo equipment and recordings in a new and often gratifying way. Some recordings -- given the Carver treatment -- may reveal sonic and musical treasures you were not aware of before. This comes, of course, at a fairly high price: The M-4000 costs $900. For those who do not care to invest in the full preamp, there will be an add-on accessory offering the hologram feature at under $300.