Toolbox149 5-26-2017
The whole system is bi-amped using Dahlquist high/low pass which rolls of the high pass at about 80 hz at 18 db an octive....
All of my inputs (mostly my CD equipment) feed into my primary preamp, a
Bryston BP-25. From there the signal goes to the Dahlquist LP-1.
After that, the high pass signal goes through a number of pieces
including the Sansui QSD-1 quadraphonic unit. From the QSD-1 it goes to
the C-20s and from there to the ADCOM 5500s. The low pass material goes from the Dahlquist to a EQ, then to an ADCOM 555 and finally to a pair of 15" subwoofers.
The fact that the frequency content of the signals handled by the C20 is sharply rolled off below 80 Hz helps the impedance issue considerably.
The C20 schematic I referred to earlier (which is provided in the service manual that can be found at hifiengine.com) shows that the cathode follower output stage is comprised of a 12AX7 tube used in conjunction with a cathode load resistor of 47K. The resulting output impedance, aside from the impedance of the output coupling cap, will be in the vicinity of 600 ohms. The impedance of the coupling cap at 80 Hz is about 4.2K.
The 10x rule of thumb guideline that Bob referred to applied to those impedance numbers, and considering also that the 18 db/octave rolloff will still result in significant frequency content extending somewhat below 80 Hz, suggests a minimum load impedance of about 50K, which is what your Adcom 5500s have been providing. That guideline is further reinforced by the fact that in the case of a cathode follower it is generally desirable, for reasons other than impedance compatibility, to keep the external load impedance greater than the value of the cathode resistor.
50K broadens your amplifier choices considerably, of course, compared to 100K.
Good luck. Regards,
-- Al
