http://www.groupdiy.com/index.php?topic=37821.msg466618#msg466618
Although non-FDA amps are shown, an FDA was really there before and can be dropped in.... This circuit is bizarre and can be improved upon/simplified depending on the application.... The better the match of the components the better CMRR performance but caveat price... I was originally stuck with only using 10k precision resistors as an exercise... This circuit should be buffered on both L & R main channels (pre vampiring of the alt jumper cables from board to board) to prevent any insertion loss from the real world build out resistances on practical real-world output stages upstream... The circuit was designed to be generic enough to be stamped/repeated and be channel agnostic such that the main channel is the channel of interest, e.g., L or R and the alt channel was siphoned off of the other board to be the opposite channel of interest in the matrix math; one big cross over set of cables to get the "alt" signals jumped over from each separate M/S board)
There is another M/S design around here that uses the THAT chips precision resistors (0.005% match IIRC), that is also pretty good, perhaps better in some aspects than the circuit I show, maybe worse in others - but is also very clever regardless of my brain droppings...
Also, included is a PDF of the basic gain versus attenuation characteristics for FDAs in a proposed stepped gain/attenuator scheme.... And the same singleton resistor values that bridge between the 10k's on the differential lines give the same figure for attenuation as they do for gain...
the caveat is the T- (or is I-??? H-???) networks in the feedback section for gain... This can hinder performance a little bit with noise / distortion et cetera (e.g., TINA-TI also shows a small uptick in these parameters when used in this configuration)... it is sometimes better just to change the resistances for the FB resistors and skip the whole T-network idea, but this is not practical in this example to maintain CMR etc....
But if the same ladder of resistors for different step attenuation/gain values (that _could_ be switch between gain versus attenuation positions with only 1 relay --- ah ha) never gets switched into the gain position, then no issue with noise/distortion perhaps....
For example, an 82k resistor will attenuate by 1dB when placed in the attenuation position, and will give 1dB of gain when placed in the gain position... Same for the resistor value that makes 36dB of gain or attenuation.... (try 160R69)... The lower in resistance value the more precise the resistor has to be, even over temperature range...
As far as the Mac... yes, one can purchase Parallels or VMware for a virtual machine that runs Windows in a window on the Mac....
