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Patrick Turner
 
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Phil Allison wrote:

"west"

Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power
Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback?


** The former industry standard for hi-fi loudspeakers was 16 ohms - so
most designs were optimised for this load. It only became 8 ohms ( or
lower ) with the advent of output transformer-less SS amplifiers. The 16
ohms tap also gives the highest voltage level.

What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to
the
8ohm?


** You will get about 3dB more gain and 3dB more THD.


True, but if the global feedback resistor was reduced by about 30%,
the gain, ie, the sensitivity of the amp could be kept
constant, and with it the thd.

The trouble is that 30% of the secondary winding is then unused, and leakage
inductance
will have risen, so stability could be affected.

The connection of the NFB to the actual point of speaker take off is
better imho, because the speaker signal is fed back to the amp more directly,
and better HF response is obtained.
Its ok if the amp is then stabilised properly.

But many makers don't like the effort and complexity of
changing matching taps *and* NFB networks because
so many of the ignorant general public make mistakes with such things,
and the smoke gives a maker a bad reputation...

Patrick Turner.




........... Phil