Stereo imaging processors have captivated me, to some degree, for years. While I haven't courted obsession and sought to buy up each variant I can find, stereo imaging hardware, like drum machines, are something I have a hard time passing up. Here is the Omnisonix, Ltd. 801 Omnisonic Imager, hailing from the first stretch of the 1980's (so far as I can tell).
Overall a pretty simple device, ins and outs and an on/off switch. Let's have a closer look at that statement printed on the back..
Well. Damn. Mind you, I do like utilizing this sort of thing on stuff now and then, though to be fair, what it offers in width it subtracts in depth. It would appear I am free to use this as an effect after all: legal status of Omnisonic Imager
Let's crack it open and look inside. The wood side panels were not visibly connected with anything from the outside.
After a bit of gingerly bending and squeezing, we have this. The secret sauce is evidently contained within that red polymer block.
The secrets, they must be profound. It's funny, I have a completely passive stereo expander that is very similar in appearance (sans the supporting electronics, naturally). My guess leans on there being a phase inversion stage for each side that is "added" to the other channel, effectively subtracting the right from the left and vice versa. This could be achieved with transformers (in the case of the passive) or by op-amps (and on and on) if you have the luxury of power.
The red block can be seen through this PCB, at about the 50% - 75% (L to R) point. I count seven points in which it connects to the PCB. Three for common referenced bipolar power supply, two for right and left input and two for right and left output.
So as not to give the impression that these were all primitive gewgaws, here's a Hughes AK-100, from the minds (well, one mind, according to this article) of Hughes Aircraft, the same Hughes Aircraft that spawned the Spruce Goose. The SRS function seen on many a small boom-box type stereo is based on the Sound Retrieval System that this unit is built upon.
I'd like to take this moment to point out that simple is not always superior in the far reaching cryptic chase of ever elusive tone.
All processors take some to give some, and are usually best used sparingly (unless, well, you know...). Stereo spread manipulation (not monophonic placement in the stereo field) seems one of those uncommon tools, which seems odd to me, since it's been the better part of a century since Blumlein cut the advancing ground on stereo sound, long before we were gifted with the crunchiness of abused amplifiers.