11-02-15: LewmDo you know this for a fact, or are you guessing?
To repeat, many if not most modern mono cartridges are internally
constructed as stereo cartridges, except that the outputs of the two
channels are summed prior to exit. Such cartridges will likely be responding
to vertical modulation imparted by the record grooves.
Many mono carts are NOT just summed channel stereo carts. My mono
cartridge makes NO vertical sound. NADA. It is dead quiet on a needle drop
where I can get a pretty loud THUMP with a stereo cartridge. If it were
summing two channels it would still make a big sound on a needle drop.
The mono output goes out to both speakers because its single signal, picked
up by a single coil and magnet, is split to go to both channels.
In fact, there's another advantage to a real mono cartridge--lower moving
mass. I think that's how this Audio Technica can put out 1.2mV and yet be
more detailed and sensitive than my Denon DL-160, a stereo HOMC.
What I've said about this Audio Technica cartridge is also true of the low
cost mono Grados--$90 for the conical model, $150 for the elliptical. The
suspension allows horizontal and vertical movement, but that is simply for
record safety. There are no mangets or coils to pick up the vertical
movement and then killed downstream. It is a mono cartridge with an
internal splitter to send the signal to both channels. That is why it's devoid of
phasing issues and the noise level is cut at least in half. And also why a
mono switch on a preamp won't do the same thing.