VAC PA160 Mk I - Manual (Page 6)

6
Feedback
The feedback controls are provided for adding controlled amounts of loop negative feedback to the amplifier.
The exact amount depends on the tube type fitted and the output operating mode selected. The exact
numbers are given in the specifications section.
The "A" setting indicates no loop feedback, while "B" through "F" correspond to increasing amounts of
negative feedback.
A given amount of feedback reduces the sensitivity of the amplifier by that amount. For example, if you add
4 dB of feedback, you will need to turn your preamplifier up by 4 dB to achieve the same volume.
As small amounts of feedback are introduced, the sound will tend to tighten up in some ways. At the same
time, the frequency response of the amplifier/loudspeaker combination will change. With dynamic speakers,
very low amounts of feedback may be satisfactory, while greater amounts may dry and brighten the sound,
add grain, or cause bass anomalies to appear. With highly reactive loudspeakers, such as electrostatics,
more feedback may 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.
Ground Configuration Switch
The way in which the components of a system relate their audio grounds to the power line ground can have
a significant impact on sound quality and noise level. The PA160 is fitted with a ground adjustment switch,
located on the top of the chassis near the output terminals. This should be adjusted for best sound, or, if
noise is a problem, for lowest noise.
The generally preferred setting for this switch is labelled "Normal" or "Lifted." This largely isolates the audio
and power grounds, thus preventing redundant signal paths between the components of your system. Under
certain unusual conditions, such as a preamplifier with a floating audio ground, a buzzing sound may be
heard in high gain configurations, such as pentode mode with zero feedback. This buzz can usually be
eliminated by setting the ground switches to the "DC" position, or by connecting the audio ground of your
preamplifier to a true ground. The latter approach may be preferable, and should be tried if you can
conveniently do so.
In very stubborn cases, try setting one PA160 to "Normal" and the other to "DC."
Some audiophiles also experiment with the "DC" ground position and the power cords on "cheater plugs."
Whatever the sonic goods or bads of this, it is usually regarded as an unsafe engineering practice and should
be avoided.
The VAC CPA1 and CLA1 preamplifiers do have floating audio grounds, so some buzz may be noticeable
in very high gain positions. If this is a problem, we recommend leaving the ground switches in the "Normal"
position, and attaching a lead from the ground post on the preamplifier power supply to the ground post on
the main preamplifier chassis.