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 6 decibels for all output taps. For
more information consult VAC Technical Monograph 90-9.
TIPS & ADVICE
A Word About Output Stage Operating Mode (triode internal wiring option)
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.
VAC can provide directions allowing a skilled technician to convert the output stage to triode operation.
