James;
Sorry for the delay in answering this inquiry. The input impedance of the 7B SST2 in balanced mode is 15K. (In unbalanced mode it is 50K). Bryston amplifiers have an input filter that slightly reduces the input impedance at 20kHz, to around 12.5K on the balanced input.
When fed from a (relatively high) preamp output impedance of 700 Ohms, that will result in a relative loss at 20kHz of 0.08 dB. If fed from a preamp output impedance of 100 Ohms, the loss at 20kHz would be less than 0.03dB. Either way, this would be considered negligible.
The losses due to cable would not materially affect these numbers. Cable impedances are low, less than 1 Ohm. Most interconnect cable will show roughly 20-25 pF of capacitance per foot. Over a 10-foot run, this will change the above figures by a small fraction of a dB, maybe 0.03-0.1 dB, depending on preamp output impedance. I would consider this still negligible.
I hope the above is helpful, but please let me know if you have other questions.
Best regards,
Chris Russell
Bryston Ltd.
