VAC PA160 Mk I - Manual (Page 7)

7
Output Impedance Matching
We strongly suggest that you experiment with the three available impedance connections for the best sonic
match with your system. Since no loudspeaker represents an unchanging impedance at all frequencies, it is
impossible to assert with certainty which output tap is appropriate to use. In many systems an amazing
difference in sound will exist between the various impedance taps.
You should consider the output impedance markings on your VAC Power Amplifier as follows:
"8 ohms" matches loads between 4 ohms and 8 ohms
"4 ohms" matches loads between 2 ohms and 4 ohms
"2 ohms" matches loads between 1 ohm and 2 ohms
Most loudspeakers vary outside of any one of these ranges, which is why experimentation is essential. We
often find that matching a speaker's minimum impedance is more important than matching its nominal
(average) impedance.
If you bi-wire your system (run separate speaker leads from the amplifier to the high and low frequency
transducers) you may discover that two different impedance taps work best. For example, with the early
Martin Logan Sequel II we found that the bass speaker was best matched with the 4 ohm tap, while the
electrostatic panel was best controlled by the 2 ohm tap. To achieve this connection, the black leads of both
speaker cables connect to "G", the red lead for the panel connects to "2", and the red lead for the woofer
connects to "4". With later Sequels we use the 4 ohm connection for both drivers.
Sonic Troubleshooting
If the sound of your system ever seems strange, or if there seems to be a difference between the channels,
check all of the switch settings and connections. The two chassis should be set to the same tube type,
grounding, operating mode, feedback level, and output impedance.
If the settings are the same, try removing and cleaning the cable contacts - sometimes materials are found
on RCA plugs and XLR pins that aren't readily visible, but can dull the sound.
If the symptom persists, move the tubes one at a time between the left and right channels. Any sonic problem
that changes channels when the tube is moved is probably the fault of the tube itself, which is readily
replaced. Be careful not to burn yourself on hot tubes, and to readjust bias settings when you move output
tubes. Do not remove tubes while the amplifier is running, or run the amplifier without tubes.
If a static, rushing, or hissing sound is heard, it may be the fault of a tube. As above, move the tubes one
at a time between the left and right channels, starting with the front-most 6SN7 tube. Listening after each
move until you hear the noise. When the noise changes channels, the culprit will be the tube just installed
in the newly noisy channel, and should be replaced with a new or known good tube. If a tube is not
identified, reverse the interconnect cables at the back of the PA160's only (left input cable moved to right
power amplifier, etc.). If the noise changes channels, it is originating in the preamplifier or associated source
component.