VAC PA90 - Manual (Page 14)

14
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 production 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 positive leads of both speaker cables connect to "+8/+4/+2" on
the back of the VAC Power Amplifier. The negative speaker lead for the electrostatic panel
connects to "-2 ohms", while the negative speaker lead from the woofer connects to "-4 ohms".
On later production Sequels we use the 4 ohm connection to 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.