Bridged mode operation for a stereo amplifier puts one channel 180 degress out of phase with the the other channel. The speaker connections are then made from the left HOT to right HOT speaker connections. The Left and Right GROUND speaker connections are not used.
This sums the voltage output of the two channels. Since power is a voltage squared function, doubling the voltage swing then quadruples the power output - - - up to the limits of the power supply. Of course then you need two stereo amplifiers, each in the bridge mode, to power a single stereo audio system, one amp per channel. There are several "catches" to this.
First, bridged mode operation forces the amplifier channels to each look at the speaker load as being one half of its normal rating. An 8 ohm speaker looks like a 4 ohm load, a 4 ohm speaker like a 2 ohm load and so on. Thus running bridge mode into a very inefficient and low impedance speaker probably will exceed the current and thermal limits of your amplifier and damage is then likely.
Second, the amplifier power supply will probably give up way before a four times single channel power output happens. In general you can probably depend upon three times single channel power from a stereo amp run bridged.
Third, if the bridge mode switch is built into the amplifier itself, there are two ways for this switch to effect the change to invert the phase of one channel to allow bridge mode operation. The first method is to switch in a small secondary inverter circuit ahead of one channel. This single active inverter circuit is probably a design afterthought and likely will not have the bandwidth or low distortion of the amplifier itself. The second method is to actually change the operation of one amplfier channel by changing the feedback loop to force one channel to now invert phase in relation to the other. In general, the inverted channel will not have the same bandwidth, distortion characteristics, or phase gain characteristics as the normal channel.
In either of these cases this means that the top half of the waveform and the bottom half will not be identical. The easiest problem to observe on a scope is a bit of a flat horizontal line in the wave form of a high frequency sine wave as it crosses the zero point. It will look like, and sound like, the old "notch" distortion of a Class B amplifier.
The result is that bridged mode operation will let you go much louder, but likely less cleaner than normal stereo performance where both channels are hopefully identical.
It is possible to design a phase inverter circuit that is clean enough to not get in the way of the music, but that is another story completely.
In any event, do switch your amp back to normal operation and your volume control should now work as desired.
Regards,
Frank Van Alstine
