If it is specifically a 400CX-2, then the center position is the correct position for normal stereo play. Unlike any other piece of Fisher gear, the Stereo Dimension control on the 400 CX-2 is quite unique, and unlike any other I know of in the industry during its day. With this particular unit:
1. When the control is rotated counter clockwise from a center position, the control acts like any other Fisher Stereo Dimension control, wherein the individual channel signals are gradually merged, so that at the extreme counter clockwise position, full mono operation is achieved.
2. In the center position, normal stereo operation is achieved, with full separation achieved between the two stereo channels.
3. As the control is rotated clockwise from a center position, the circuit then begins to cross-couples itself, in such a way as to gradually add a reverse Channel A signal into the Channel B signal, and a reverse Channel B signal into the Channel A signal. The result is that at full clockwise rotation, all stereo information that is common to both stereo channels is completely canceled out in both channels, leaving each channel to only reproduce the stereo information that is unique to that channel. Based on the source material being played, this can produce some interesting results:
A. With monophonic sources, turning the control to the extreme right results in a complete cancellation of all sound, since there is no unique stereo information in the two channels to reproduce once the common information between the channels is canceled out.
B. With modern stereo recordings, turning the control clockwise acts to actually expand the stereo image, since the unique stereo information in each channel becomes emphasized by way of the common information beginning to be minimized. Extreme clockwise position is extreme in the effect, and not enjoyable in my opinion.
C. With old (i.e. early) stereo recordings, where the stereo effect was extreme to begin with in the recording (Mamas in one channel, Papas in the other, bass and drums in one channel, strings and guitars in the other, etc), turning the control clockwise from a center position has virtually no effect at all, because the unique stereo information in each channel makes up virtually the entire sound of each channel. Therefore, there is nothing common to both channels to cancel out as the control is advanced.
It is the ability to actually expand the stereo sound stage (with the right program source) that makes the Stereo Dimension control so unique on the 400CX-2 preamps. Those who further analyze the effects of this control will realize that in the expand position, the common information between the two channels is being electrically canceled out as explained above, while the unique stereo information in each channel is actually being audibly cancelled out, since in the extreme position, Channel A is reproducing its own unique stereo information, and, an anti-phase signal of Channel B's unique stereo information as well. Of course, Channel B is doing the same thing relative to Channel A's unique stereo information. Theoretically, this should result in the cancellation of all audible sound, but as we know, if you connect two speaker cabinets to the output of a mono amplifier, but out of phase with each other, there is hardly anywhere near a full cancellation of sound. It is only the low frequency signals that audibly act to cancel each other out, while mid and high frequency signals produce little cancellation. In developing this unique control, Fisher took advantage of this fact, since most stereo information only occurs in the mid to high frequency range anyway -- where little audible cancellation would occur. It is a unique control to say the least!
I hope this helps!
Dave