9
FEEDBACK
Your Phi is equipped with variable negative feedback, controlled by a toggle switch on the rear panel.
The switch has two positions: zero feedback, and 12 dB of overall negative feedback.
With many loudspeakers, the best overall sound will be attained with zero feedback.
Some speakers have significant variations of impedance with frequency; such speakers may sound better
when feedback is selected.
Think of feedback as being like a spice, and “season to taste.” Audition your system both ways, and select
the setting that provides the best overall sound quality.
When feedback is used, the amplifier will evidence less gain, less noise, lower measured distortion, and a
lower source impedance.
Keep in mind that the human ear/brain auditory system works very differently than test gear, each tending to
measure different things. As a result, there is not a uniform correlation between measurements and sound
quality.
TRIODE & ULTRA-LINEAR MODES
Triode amplification is the oldest form of amplification known, while partial triode ("ultra-linear")
operation of pentodes and beam power tubes dates back to 1937. Both modes of operation are
available with your VAC Phi amplifier via a single switch on the top plate.
Although the mode may be changed with the amplifier in operation, we strongly suggest that you
turn only operate the switch when the amplifier is off and the tubes have cooled down
[VACUUM
TUBES BECOME
HOT
ENOUGH TO CAUSE SERIOUS BURNS. NEVER TOUCH A TUBE WHEN THE UNIT
IS ON. IT MAY TAKE SEVERAL MINUTES FOR THE TUBES TO COOL DOWN AFTER THE UNIT IS
SWITCHED OFF.]
Turn the switch to the position indicated, then power the unit up and listen.
Less power is available in the full triode mode, but distortion before clipping may be reduced, as
is output source impedance. Experience reveals that the triode mode will sound somewhat lean
on some systems and better defined on others. In many ways this is an issue of system matching,
and universal recommendations do not exist. Listen to both, and select the mode most pleasing in
your system.
Note that the bias current will typically read slightly lower in the triode mode than in the ultra-linear
mode. Simply follow the BIAS procedure if necessary to restore the correct reading for each tube.
