[Amps] Direct rectification of AC mains to derive the amp VDD, supply

[Jim Brown]

On 9/17/2013 11:59 AM, Manfred Mornhinweg wrote:
> No
> problem with that. Given the choice, I also find an isolated power
> supply and grounded amplifier block to be a cleaner layout - but the
> economy and greater efficiency of a direct line supply is attractive
> too, and for this I'm willing to use a floating amplifier!

There are several fundamental issues here, and your post indicates that 
you're generally aware of most of them. Nearly all relate to safety, and 
are written into the electrical Codes (laws) that govern buildings. They 
are not "dumb" or out of touch with reality -- rather they are based on 
very solid good engineering practice. So for the benefit of others who 
might be aware of them, here they are.

#1 -- In most parts of the civilized world, in any SYSTEM, there must be 
one, and ONLY one, bond between neutral and ground (the Green wire in 
North America, Protective Earth in EU, etc.) .In North America, that 
bond must be at the point where the SYSTEM is established.  In most 
homes and small businesses, there is only one system, and it is 
established where power enters the building. This point is called "the 
service," or "the service entrance."  This point also requires a 
physical connection to the soil.

A new system is established by a transformer, either one in a UPS, or an 
isolation transformer, or a step-down or step-up transformer. Thus, the 
neutral must be bonded to the Green Wire (or Protective Earth) at the 
secondary.

#2 -- ALL grounds in a facility MUST be bonded together.

#3 -- ALL exposed metal parts that "could be energized" MUST be bonded 
to the Green Wire (or Protective Earth).

#4 -- In most of the civilized world, load current is permitted ONLY on 
the Phase (hot) and neutral conductors.  It is both illegal and a major 
violation of good engineering practice  to connect a load between a 
phase (hot) and the Green wire (or Protective earth).

#5 -- For purposes of EMC, all cable shields MUST be bonded directly to 
the shielding enclosure of equipment to which they are terminated, and 
all balanced cables must either filtered or bypassed.

#6 -- For purposes of lightning safety, cable shields MUST be bonded to 
earth.

Most safety codes make equipment with more stringent construction 
requirements exempt from the Green wire. In North America, this is 
called Class II equipment, and it has either no exposed metal parts, or 
is double insulated so that there is no possibility of electrical 
shock.  Those consumer devices with direct rectification are built with 
no exposed chassis, and no external connection to ground (or a 
transformer coupled connection).

These  fundamental requirements combine to make direct rectification of 
the mains more than a bit impractical if you care about safety and 
freedom from EMC issues.  Your post, which outlines the sort hoops one 
must jump through to get there, suggests that the cure may be worse than 
the disease.

73, Jim K9YC