On Wed, 7 Sep 2011 15:58:48 -0700 (PDT), Engineer
wrote:
So it is a horribly bodged solution to a problem that has no business
being there in the first place? The idea of implementing negative
feedback and then defeating it by killing the open loop gain it
mediates is beyond ludicrous.
d
Not a bodge... steering the Nyquist plot around the "minus 1" point
for all frequencies is established practice in feedback control
systems. It's also perfectly acceptable here. BTW, the higher break
point (of the two) for the VLF shelf is well below 100 Hz... around 15
to 20 Hz is typical to stop a 1 Hz or so VLF oscillation. I'll agree
that the removal of one of the RC couplings in the forward path would
be good, but hard to do when you have a separate driver stage after
the phase splitter. There are direct-coupled DC-amplifier designs, but
they are far more difficult to implement than a VLF shelf and, anyway,
not needed. Also, a massively larger primary inductance in the OPT
primary would be next to godliness... but most of us (in NA, at least)
cannot afford that so we use the best Hammond iron we can afford or
find a decently large OPT from a recycled high grade amplifier!
Cheers,
Roger
Don't know where you got those numbers. It does this:
http://www.soundthoughts.co.uk/look/shelf.png
d