"Jim Lesurf" wrote in message
...
In article , Peter Chant
wrote:

QUAD used for the PSU in their 34 preamp might avoid switchon
thumps/clicks. That uses an op-amp to center the ground about
the
polarities of a single psu potential difference. Means 'both
rails come on
together' as soon as that op amp wakes up. :-)

If I follow you correctly then the ground, phono screens on input
output, are connected to 0V which is as op-amp output?

If I get a chance I'll dig out my copy of the diagram and put a scan of
the
relevant bit on the web so the details will be clear. But the trick they
employed was to use a nominally floating single pd from the PSU. Then had
an op-amp generate a mid-way voltage which is then used to define and hold
a '0V' line for the bulk of the circuitry. This then is connected to the
screens on the phono sockets, etc, so becomes 'ground'. They also chose to
make the mid-point *not* the half-way point and produced asymetric rail
voltages for other reasons. Quite an interesting design.


Using an op-amp to create a mid-point ground wasn't new in the Quad 34. I
first met it in a Weir bench PSU in the early 1970s (albeit with a discrete
component amplifier). It's a convenient way of creating a variable, balanced
dual rail supply as there is no need to try to make two regulators track.
Simply use one variable regulator to generate the rail-to-rail voltage and
rely on the amp to create the mid point.

I used an op-amp to create a mid-point ground in an active audio
balanced-unbalanced converter that I built in the late '70s and have used
the same technique in various other similar boxes since. All those had
equal voltages either side of the ground, but I noticed that a PPM card made
by Audix that I acquired from eBay recently used an op-amp to generate a
ground that wasn't mid-way between the rails.

David.