From: jas@proteon.com (John A. Shriver)
Newsgroups: rec.audio.high-end
Subject: FAQ's?: balanced, ferrofluid, tube manuals
Date: Tue, 22 Dec 92 16:42:46 EST
Organization: University of Wisconsin - Milwaukee
NNTP-Posting-Host: 129.89.7.4
Cc: sumant@ntd.comsat.com, njenrigh@mtu.edu
Originator: tjk@csd4.csd.uwm.edu

Balanced inputs and outputs use a pair of signal leads, that carry
the signal relative to each other, with no reference to ground.  When
the "signal" goes positive, the + wire of the pair goes positive with
respect to the - wire.  (Vice versa for "negative" signal.)

One advantage is that the shield on the cable is now only an
electrostatic shield, and is not carrying any signal current.  This
solves problems with the noise that the shield is supposed to protect
you from getting modulated into the signal by wiggling the ground.

Another advantage is that any ground potential differences between the
two components connected in a balanced manner should not have any
audible effect.  There are always ground potential differences, due to
difference power circuits, power supply leakage currents, etc.  

The degree to which a balanced input ignores anything but relative
changes between the + and - wires is known as Common Mode Rejection
Ratio.  Ideally, if both + and - go up by 20 volts, this should not be
interpreted as a "signal" on input.

Balanced inputs should result in lower noise and hum, and less
problems from cross modulation between this noise (which may well be
inaudible) and the true signal in the active circuitry.

All recording studios use balanced connections, even for digital
signals (this is AES/EBU). 


Ferrofluid is a viscous liquid which contains magnetic particles in
suspension.  Placed in the gap between the magnet and pole piece in a
loudspeaker driver, it fills the space around the voice coil.  It
offers benefits of better cooling for the voice coil, and can provide
mechancial damping for the driver "cone".


The most readily available tube manual is the General Electric one.
(Title is "Essential Characteristics.")  I don't know if it's actually
still in print, but there is plenty of inventory.  It can be bought
for US$7.95 from Antique Electronic Supply of Tempe, Arizona.

The GE manual does not have a lot of depth on each tube.  However, it
has the most important characteristics, pinouts, and covers more
different tubes than any other manual I know of.  An excellent deal.

There are also quite a few data sheets on key audio tubes in the VTL
Handbook, from Vacuum Tube Logic.  (Also available from AES.)

The other popular tube manuals are the RCA ones.  All are out of
print, and are rapidly becoming collectors items.  They may sometimes
be found at Ham Radio Flea Markets and Antique Radio Shows.  The first
was RC-1 sometime in the thirties, the last was RC-30 in the
seventies.  They also published the HB-3 loose-leaf notebook series,
but it these are much rarer, and rarely are found in versions that
reach up to the more recent tubes.  (People let their subscriptions
lapse.  My most recent one only reaches to 1957.)  The HB-3 is quite
detailed.

Other vendors had their own manuals.  Often the best data for a tube
is in the manual of the company that originally designed that tube.
This is especially true of power tubes.  (This is a virtue of the
reprints in the VTL book.)

For instance, I'm still trying to find an Amperex manual to get all
the data on the strange Amperex tubes that Tektronix used in their
last vacuum-tube oscilliscopes in the sixties.  They are not covered
in any depth in my GE manual.  (There is a lot to be learned from Tek
scope designs.)