Thank you, John (Roxy54).
Toolbox, first, I’m not sure I understand the comment just above about biamping. If it’s a quadraphonic system you’ve got four speakers, and your two Adcom amplifiers provide a total of four channels of amplification, rather than the eight channels that would be required to biamp the four speakers. Can you clarify how the Dahlquist crossover fits into the system?
Second, I found a schematic for the C20 at HifiEngine.com (one has to register there to see it), and it shows that a cathode follower stage drives the output via a 0.47 uf coupling capacitor. The impedance of a 0.47 uf capacitor at 20 Hz is about 16,900 ohms, which is extremely high. And the output impedance of the cathode follower would increase the overall output impedance further. So Kenny’s (Kdude66’s) recollection of 23K, although perhaps a bit on the high side, is in the right ballpark for the deep bass part of the spectrum. The impedance will be much lower at higher frequencies, since the capacitor’s impedance will decrease as frequency increases, but it is that **variation** in output impedance (from relatively low values at mid and high frequencies to very high values at deep bass frequencies) that will cause the deep bass roll-off Bob referred to if the load impedance is not high enough.
Which together with the fact that the C20 was designed and introduced when amplifiers were tube-based and generally had higher input impedances than most modern solid state amps, and when most users probably did not have speakers providing a great deal of deep bass extension, points to the conclusion that the 50K minimum load recommendation is aggressive if anything, by today’s standards, and an even higher impedance would be preferable. I would suggest looking for an amp having an input impedance in the 75K to 100K area. Unless, that is, you would consider modifying the C20s by installing coupling caps having much higher values. Although doing so might have sonic tradeoffs of its own, especially if you want to minimize the cost of the new capacitors.
Some of the older McCormack DNA series of amplifiers come to mind as providing 100K input impedances, and are well regarded for their sonics, and my impression is that they tend to be available used for prices in the same general area as the 2250. Some of those models may be a bit marginal for your application in terms of power capability, but if you could find a
DNA-225 for example it would be capable of more than 400 watts into the 4 ohm impedance of your Maggies.
Best of luck. Regards,
-- Al
