On Sun, 03 Jan 2010 15:35:54 -0600, Lostgallifreyan
wrote:

Actually, it is harder than you might imagine at first glance. Yes,
the methods are simple, but getting past preconceived notions is the
single greatest hurdle. Many engineers are ill suited to the task.


Try me. All it needs is a clear statement that I can relate to something
I've already experienced. I'm well used to being cautious about what I learn.
Unless it's as tough as atomic physics, it should be digestible, even if
slowly. And I won't be able to taste it till tomorrow... got to sleep soon.
Thanks for this, it encourages me to take the time when others do.

We shall proceed with a object lesson found in Fig. 5 of:
http://www.kongsfjord.no/dl/Amplifie...Amplifiers.pdf

Consider that this is described in the right side of Fig. 2 with all
the expectations of being "balanced." It is not. Even on viewing the
rough block diagram of Fig. 2 it is apparent in this lack of balance.
Fig. 5 and photos do nothing but support this shortfall.

At first glance, it seems as though these materials are classic text
book stuff that begs to be accepted as "balanced;" reality intrudes in
that RF (far more so than AF) finds interwinding capacitance is not
distributed equally for T2 and T3 primaries to secondaries.

The input/output windings are easily as unbalanced as one could find.
"Of Course! a protest might start. T2 is meant to support an
unbalanced input." However, a closer glance would admit that the
capacitive coupling from the top of T2 input to the top of T2 output
is NOT the same as the capacitive coupling from the bottom of T2 input
to the bottom of T2 output (irrespective of the implied ground
connecting, or not, to the middle of the T2 output winding).

The top Basic Amp is thus presented with a higher potential than the
lower Basic Amp. One might protest that this does not matter as T3
uncouples the two to present a new output through it to the 50 Ohm
output. My rejoinder would be "What practical advantage is then found
in the symmetry of upper/lower Basic Amps?" I would then point out
that the Common Modality present in the input, is amplified in the
upper half and finds its way through the system.

The solution involves complex winding taps with capacitors tied from
them to common to enforce a balance. This is not practiced
(obviously) in the object lesson presented at the page referenced
above.

A protest might erupt that this example's capacitive asymmetry only
admits of a very small imbalance. I would respond that Common
Modalities may easily support considerable currents/voltages that when
reduced through divider action are easily in excess of small signals
of interest. The Basic Amp input Z is 50 Ohms; an imbalance in the
interwinding coupling capacitance amounting to 50 KOhms will add 1000
microvolts for any 1V CM signal on the left. At 1MHz, this 50 KOhms
would be a capacitance imbalance of 3 pF. Looking at the wire
dressing of the coils in the photo would suggest 3 times this easily
(and no attention has been paid to this at all). Now extrapolate to
the 31 Meter band.... Could your AGC tolerate the local AM station's
10,000 microvolt leakage sitting on top of Radio Burundi's signal?

Grounding and shielding is not a simple topic although it is deceptive
in appearance.

73's
Richard Clark, KB7QHC