[78-L] NULLING VERTICAL COMPONENT (was: EQ - why bother? - was: stereo/mono and noise reduction

[Michael Biel]

On 11/7/2011 3:52 PM, Rodger Holtin wrote:
> The original question was how and why does A+B mono work to cancel noise,
> but lacking an answer to that, we've drifted off into finding a tuner that
> will apply A+B - which I don't really need.

Ah, I think that this was misunderstood.  Some here seem to think about
the amplifier mono/stereo switch in an integrated RECEIVER -- which has
a tuner in addition to the pre-amp and amp -- instead of a tuner which
has no amplifier but might also have a mono switch.  The tuner's mono
switch does NOT couple the left and right channel, it switches off the
left minus right sub-carrier and leaves only the station's main signal
which was the sum signal derived by the station itself.  In an
integrated receiver, the mono switch is different for the tuner rather
than the mono switch for the regular audio sources.

The earliest FM stereo tuners often had ANOTHER control -- usually on
the rear -- which would raise and lower the blend and effect of the
left-minus-right sub-carier signal on the sum signal.  This would
increase and decrease the effective separation of the stereo channels. 
Sometimes you could overdo it and eliminate the sum signal, losing the
announcer, vocalist, and anything else in the center.  THIS is what you
do when you put the stereo phono signal out-of-phase and sum it this
way.  The karaoke effect on some computer programs, and what I was asked
to do every year decades ago by some Miss Morehead State University
pageant contenders so she would have a music track to sing with. 

If you want to null out vertical noise on a lateral 78, THIS is how you
do it.  You listen to the vertical signal while changing the balance of
the two channels until you do not hear any audio creeping in.  The
sub-carrier blend control affected only the tuner section, not the
phono.  If you wanted a difference signal for the phono, you had to do
it yourself in your external wiring, and it might change the regular sum
signal when changing that wiring.  My Dynaco tube pre-amp has three
blend positions on the mono-stereo switch which was supposedly used in
the early days of stereo to reduce the hole-in-the-middle from stereo
records with no "center channel".  It just put in some leakage between
the channels to decrease the separation.  This is completely different
from the blend control for tuners that I described above.

Mike Biel  mbiel at mbiel.com