About Microphony
Every amplifying device, be it tube or transistor, produces some output with mechanical stimulation
(microphony). In fact, even cables possess microphony. Acceptability is not a question of whether
this characteristic is present but rather the degree to which it is present.
In the VAC preamplifier, every tube (except possibly V5) will emit a sound through the speakers
if it is directly struck. This is a normal situation. The exact amount of sound depends on the tube
type, its location in the circuit, and slight random variations in tube manufacture and condition.
The amount of sound emitted when the tube is struck is irrelevant. The tube and circuit card
assembly are shock mounted from the main chassis, and as such are isolated from direct contact.
When the chassis itself is struck sharply there may be either no sound or a slight sound emitted
from the speaker, but it should die away rapidly. Again, this is a normal result, and not
representative of the type of mechanical or acoustic stimulation encountered when the unit is
properly installed and playing music. However, if the sound rings on strongly when the chassis is
struck, the offending tube should be replaced.
It is best not to subject the tubes to tapping or the chassis to repeated mechanical jars, as this may
damage the precision of the tubes and cause them to become more microphonic.
About Noise
Every amplifying device, be it tube or transistor, produces some amount of noise. If fact, even a
resistor produces noise in the presence of current flow. Acceptability is not a question of whether
this characteristic is present but rather the degree to which it is present.
In the phono stage, the most likely source of noise is tube V1. This is because any noise produced
by V1 is amplified by all of the following tubes. With normal low noise tubes, the subjective level
of electronic noise will be approximately 1/3 of the groove noise, referenced to a .3 mV output MC
cartridge and a good direct to disc recording played at loud levels. Electronic noise will be
insignificant with normal recordings at normal levels.
In the line stage active modes, the most likely source of noise is V4. The audibility of noise
produced in the line stage itself is not affected by the volume control setting, and is determined by
the sensitivity of the power amplifier and the efficiency of the speakers. With a speaker of
extremely high efficiency (for example, a Klipschorn) the preamplifier should be set in the passive-
buffered mode to eliminate the noise of amplifying tube V4. Noise should be insignificant in either
mode with speakers in the typical efficiency range of 80 to 93 dB/1 watt.
To confirm which tube is noisy, you may swap the tubes between the channels. For example,
exchange V4L and V4R. If the noise changes channels the V4 tube in the noisy channel should be
replaced. Be sure to mute the preamplifier, turn it off, and allow the tubes to cool before making
swaps.