VAC Vintage High Power - Manual (Page 10)

TIPS & ADVICE
A Word About 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 early Martin Logan Sequel II we find
that the bass speaker is best matched with the 4 ohm tap, while the electrostatic panel is 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.
Contrary to popular misconception, no power is lost due to unused output taps. Also, the amount of negative loop
feedback in the amplifier does not change, being fixed at approximately 8 decibels for all output taps. For more
information consult VAC Technical Monograph 90-9.
TIPS & ADVICE
A Word About Output Stage Operating Mode (optional triode switch)
Triode amplification is the oldest form known, while partial triode ("ultra-linear") operation of pentodes and beam
power tubes dates to 1937. From the engineering perspective, the triode mode differs from the ultra-linear mode in the
following ways:
1) Somewhat more distortion at low power levels
2) Somewhat less distortion at moderate power levels
3) Reduced maximum power output
4) Output impedance matching is somewhat less critical
5) Increased damping factor.
The ultra-linear mode applies negative voltage feedback from the primary of the output transformer to the screens of
the output tubes. The ultra-linear circuit displays different characteristics (efficiency, distortion components and levels,
etc.) depending upon how much of the primary winding is common to both the screen and plate of the output tubes.
The VAC implementation of the ultra-linear circuit has been selected to achieve performance quality as close to pure
triode as possible while still increasing power output and efficiency.
Subjectively, the triode mode often produces a more natural sound provided that sufficient power is available for your
application. Listen to both, and select the mode most pleasing in your system.