VAC PA35/35 - Manual (Page 10)

TIPS & ADVICE
A Word About Feedback
A control is provided by which controlled amounts of loop negative feedback may be introduced. The settings
are as follows:
Position Comments
A no global feedback at all
B 1 dB negative feedback
C 2 dB negative feedback
D 3 dB negative feedback
E 4 dB negative feedback
F 5 dB negative feedback
A given amount of feedback reduces the sensitivity of the amplifier by that amount. For example, when switching
from 0 dB to 2 dB of feedback, you will need to turn your preamplifier up 2 dB to achieve the same volume.
As small amounts of feedback are introduced, the sound will tend to tighten up and be more controlled. At the
same time, the frequency response of the amplifier/loudspeaker combination will change. With dynamic
speakers, 0 dB is often satisfactory, and excessive amounts will dry and brighten the sound too much. With
highly reactive loudspeakers, such as electrostatics, more feedback may actually produce a more open, airy
sound.
There is no "optimal" setting for this control in an engineering sense. While feedback improves static damping
and lowers some forms of distortion, it also weights the relative balance of distortion components to a higher
order. In other words, there's a little good and a little bad about feedback. Listen, and use the setting that
provides the best overall subjective performance of your system.
For further information please read the Tips section entitled Tuning Your System.
A Word About Grounding
The PA35/35 amplifier is designed to be used with a three prong power cable. The ground pin connects directly
to the chassis, while the audio ground couples to the chassis as determined by the GROUND switch. In the
"Normal" position, audio and radio frequencies ground information is referenced to the chassis, but DC is
blocked. In the "LF" position, the time constant of the reference network is increased, coupling still lower
frequencies. Each of these positions will sound different and may produce different noise floors. The correct
setting depends on how the rest of your system is grounded. Try each, and use the setting that produces the
best sonic performance. Retest the settings any time you change a component in your system or change any
power wiring.
Please note that some power line "conditioners" or "enhancers" do unusual things with the power line grounds,
and may thereby create noise and sound quality problems.
For further information please read the Tips section entitled Tuning Your System.