R.G.
9/23/98 8:24 AM

We're being too general. 

We need to be careful with terms
here. There are four kinds of
noise to be concerned with.
There is: 

1. power supply ripple (100 or
120Hz) 
2. induced noise picked up from
outside the amp, which may be
60Hz, line synched hash
(fluorescents and SCRs) RF, or
any other stuff that radiates. 
3. noise generated inside the
circuits, primarily thermal noise
and shot noise if it's not
oscillating internally. 
4. AC hum (50 or 60Hz) 

Power supply ripple is created in
the rectifier/power supply
system, and can be contained
inside it by proper wiring and
grounding of the rectifiers and
first filter cap. No connection to
AC ground is needed, or changes
the amount of ripple. The filter
caps filter the ripple, with
perhaps a small ripple reduction
by the PT leakage inductance.
With the exception of a shield
inisde the power transformer and
freedom from AC leakage, this is
the only noise the
transformer/filter caps can do
anything about. 

Induced noise is what an AC
ground connection can reduce.
This is stuff that radiates in, RF
and the 37th through 159th
harmonics of power line hash; a
connection to AC safety ground
can reduce this by tying the entire
chassis to a low impedance point
and shunting this radiated energy
to earth ground before it gets into
the signal. 

Note that if the chassis is used as
a ground return, the stuff can
STILL get into the signal path
because of the difference in
potential from place to place on
the chassis. If you have NO
connection of the chassis to
RF/earth ground, you will have
induced noise because of the
noisy RF environment we live in.

The old 2 wire systems really
had a ground, it was just the
neutral line in the power system,
and the "line reverse" switch
contrived to tie the chassis to that
"ground" through a capacitor that
supposedly bled the radiated
(and presumably high frequency
stuff) to ground while blocking
the low frequency AC line. This
always produced some small
amount of AC leakage, so two
wire amps are always "hot" a
little. 

This radiated stuff is what you
can make quieter by "grounding"
the chassis, except for AC
leakage, which is a fault
condition anyway. Even if it
sounds like "hum", it's the
radiated noise. If you have a
series of harmonics of a note,
your ear inserts the fundamental
(see Seashore's "Psychology of
Music"), so the hash makes you
hear the hum. 

AC hum can be caused by
internal magnetic radiation from
the PT or AC filament leads, not
grounding the AC filament
supply, AC leakage from the AC
wiring or from heater electron
leakage. Of these, only the AC
filament hum and AC leakage are
grounding issues. The filament
hum you reduce by tying the
filament winding to a fixed DC
voltage relative to signal ground.

AC leakage is what can make the
amp really hum. The AC wiring
or PT are connected to the
chassis by a low enough
impedance to leak AC. This is
not only noisy, it's dangerous.
The AC safety ground can make
the amp quieter in this case by
bleeding the leaked AC to earth
ground before it gets into the
signal path. 

Note that if you use the chassis
for signal ground, you can get
hum voltages caused by
INDUCED currents flowing in
the chassis as magnetic leakage
causes the currents and a small
voltage drop. 

So, the long way round to answer
your question is "Yes, ground
connection makes them quieter,
but it's not the PT or filters that
account for it."