In article ,
Patrick Turner wrote:
On May 23, 5:38*am, John Byrns wrote:
In article
,
*Patrick Turner wrote:
Quad-II used a kind of floating paraphase. Nobody much understands
exactly how it works but fact is that the output of one EF86 is
reduced
by resistance divider and fed to the grid of another EF86 and both
thus have a stage which effectively has 6dB of local positive FB. Both
EF86 have to make up to about 40Vrms for each KT66 grid, ( depending
on the load ).
It's not really that hard to figure out how it works, a lot of the
quirkiness is
due to the acrobatics needed to apply negative feedback to the common
cathode of
the voltage amplifier and phase splitter. *This was probably a bean counter
thing to eliminate a couple of electrolytic capacitors.
The solution to the problem of low open loop gain suited bean counters
at Quad. But if you make the two EF86 behave as a real LTP with common
cathode R to -400Vdc, or a CCS, and without the mild 6dB PFB boost of
the paraphase inverter principle used, then the differential gain is
halved and if the same amount of NFB was applied globally, you'd need
2.8Vrms for clipping power instead of only 1.4Vrms.
You keep talking about "the mild 6dB PFB boost of the paraphase inverter
principle". I don't understand where you are getting this 6 dB boost? The
floating paraphase has the same gain as the concertina phase inverter, how does
the concertina equal the gain of the floating paraphse without PFB, or does the
concertina also use PFB? In any case only one out of the many many possible
variations of the floating paraphse could be said to incorporate PFB. I'm not
an expert on the LTP, and am not a big fan of it, I suspect that the truth here
is that the LTP throws away 6 dB of gain relative to the concertina and floating
paraphase.
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/