The phase inverter is a very fast active unity gain circuit that inverts the phase of the output signal (turns it upside down). If you connect to a stereo power amp with one channel seeing the normal phase, and the other channel seeing the inverted phase, and take the outputs (speaker connections) from the hot on one channel and the hot on the other channel (ground connections on the amplifier not used) then this essentially sums the voltage swing of the two channels. If you had a (for example) 200 watt per channel stereo amplifier, swinging plus and minus 60 volts, then summing the outputs would yield a plus and minus 120 volts (in theory) in the mono bridged mode, yielding over 800 watts. Things don't work quite this way because the power amp power supply will likely sag, but in general you can assume a three times stereo power rating in the bridged mono configuration. The connections cannot be used with some amplifiers that do not have a common ground connection (they may be making high power by already having four small amplifier channels inside already bridged for two higher power channels). In addition, in the bridged mode the outputs of the amplifier sees the speaker load at half its normal impedance (8 ohm speaker looks like 4 ohms, 4 ohm speaker looks like 2 ohms, and so on) so bridged operation is not recommended for inefficient low impedance speakers. This is also true with vacuum tube amplifiers, because as the load impedance is not matched to the speaker output terminal of the same impedance, the amplifier power is drastically cut. With an 8 ohm speaker you can connect to the 4 ohm taps and get much more useful power but with a 4 ohm speaker you would need a non-existant 2 ohm tap, and the impedance mismatch will cut the power to the speaker in half, nullifying the effect of running bridged mode in the first place.
You cannot reverse the phase of a preamp signal just by swapping the interconnect cable connections, that simply ties the signal to ground, result, no signal at all. You must have an active inverter signal to do this.
AVA phase inverters are either free standing units, or built into the preamplifier as an option. They are not built into the power amplifiers.
One musical issue with a bridge circuit is that it must have a very wide bandwidth to eliminate the phase lag through the active bridge circuits. With most inverter circuits, the phase lag results in a late inverted signal out, and when the signals are summed at the amplifier, the result is a waveform with a huge amount of crossover distortion, much like a Class B amplifier. You get a lot more power, but not clean power.
Note that the phase inverter can also be used for providing a balanced line signal as it does have a plus output, minus output, and common ground. All that is necessary is to wire the preamp or inverter box end with two RCA jacks per channel. We do not have enough demand for this to justify providing a three pin connector on our inverter or preamp chassis, everyone would have to pay extra, and few want or need this function.
Frank Van Alstiine
