The testing focused on maximum output
power capabilities and amplifier linearity. the high degree of linearity of the bridged circuit which directly translates into a cleaner sounding more transparent amplifier. Other bridged circuit topologies that use the output of one amplifier as the input to the second inverting amplifier inherently possess higher THD and noise that will degrade the solution’s sound quality.
The input impedance of the inverting amplifier is essentially resistor, Ri. The value of this resistance affects the gain setting of the amplifier as well as the low frequency rolloff in conjunction with Ci. There is a tradeoff between having a low frequency rolloff, a high input impedance and a small capacitor size and value. It is critical to have a flat band response down to 20Hz while it is equally important to have a high enough input impedance so that heavy loading does not occur from the preamp stage.
Using large valued low-cost capacitors implies the use of leaky electrolytics which affect the output offset voltage. Electrolytic capacitors are also less linear than other premium caps and should not be used in the signal path when not necessary. This tradeoff issue is the toughest
portion of the design. The amplifier gain setting is just as one would expect for an inverting op amp. Of course, the input impedance issue can be quickly resolved by using a
voltage follower as an input buffer, but it was omitted from this design to minimize cost and simplify the design. The values provided in the bridged schematic are at a good tradeoff point.
There is sufficient input impedance for practically all audio op amps, the closed-loop gain setting is 11 for each amplifier, (gain of 22 overall) while the capacitor value of 4.7μF sets the low frequency −3dB rolloff at about 7Hz.