"François Yves Le Gal" wrote:
On Mon, 23 Jul 2007 17:05:50 GMT, Patrick Turner
wrote:
I just downloaded a copy of the SP8 schematic but its such a poor scan
I cannot read the R numbers.
Have you tried the ARCDB site? They've got quite readable schematics of most
versions of the SP-8 as well as of many vintage ARC products.
http://www.arcdb.ws/SP8/SP8.html
Thanks Francois,
The schematic here is exactly the same one as at
http://www.analogstereo.com/audio_re...ce_manuals.htm
But your source is much clearer, because ARC scanned it at higher dpi.
I was right. Alter gain by varying R27 between points A&B.
Do NOT change the value by adding or subtracting another R to what is
shown as R83, 33k,
which is a dc carrying load R for V6.
Minimum gain is when R27 is omitted entirely.
Benn should try omitting R27 immediately, and if ordinary levels are
enjoyed with gain pot set to the 12 o'clock position then all is well
and nothing
more needs to be done.
However, a good tech would perform a stability check.
Its unlikely that loooong cables with high capacitance effects are being
used,
and unlikely HF oscillations will occur with the increase in NFB by
removing
R27.
The reason is that the removal of R27 also increases the local current
NFB
of V4, thus reducing the open loop gain of this triode and thus the open
loop gain of the
two triodes, V4, V5, so although the global NFB is increased, open loop
gain is decreased,
and so the amount of reduction of open loop is kept more constant this
way.
Patrick Turner.