I did notice that input impedance for balanced inputs is lower than that for unbalanced inputs in bryston amplifiers. do you know why it is that the input impedance for the balanced inputs is lower than the input impedance for the unbalanced inputs?I took a look at the schematic for one of their amplifiers, the 3BSST, shown here. (Bryston is the only current manufacturer I am aware of that has schematics for many of its products on its website). The amp has an input impedance spec of 50K for the unbalanced inputs, and 20K (described as "10K per leg") for the balanced inputs.
The input stage active device, referred to on the schematic as a DOA68, I believe is an op amp that is custom made for Bryston. In the unbalanced input configuration it is used as a non-inverting amplifier with a gain of either 1 or 2, selectable via a switch. Circuits in which op amps are used in a low-gain non-inverting configuration have extremely high input impedance, in this case such that the overall input impedance of the circuit is essentially determined by the 49.9K resistor R6.
In the balanced input configuration, the op amp is of course used as a differential amplifier. That kind of op amp configuration has a much lower input impedance than a low gain non-inverting configuration.
The overall input impedance in the balanced mode is essentially determined by a combination of the various 20K resistors you will see on the schematic. The input impedance of the Channel 1 inverting leg is equal to the parallel combination of R1 and R1A, which is 10K.
In the high gain (1V) configuration, the input impedance of the non-inverting leg is equal to the parallel combination of R2 and R2A (10K) in series with R4 (20K) resulting in an overall input impedance of 30K.
In the low gain (2V) configuration, the input impedance of the non-inverting leg is equal to the parallel combination of R2 and R2A in series with the parallel combination of R4 and R5, resulting in an overall input impedance on that leg of 20K.
So the input impedance description for the balanced inputs of "10K per leg, 20K overall," I don't think is entirely accurate, but the higher input impedance on the non-inverting leg won't have any adverse effects, as I see it. It is common for differential amplifier stages to have unequal input impedances on the two legs.
I suspect that the reason the 20K resistors were not chosen to be significantly larger, such as 50K, is that in the differential amplifier configuration various non-ideal characteristics of the op amp itself, such as perhaps what are called input offset currents, as well its own input impedance, would then have become significant.
Regards,
-- Al