Hi Koestner,
A preamp to amp connection should have an impedance ratio of at least 40:1 from the amps input to the pre out. So if the OP's pre has an output impedance of 5K, and his amp has an input imp of 450K, then his ratio is 90:1
Now if he hooks up the sub using the 2nd line level output on his preamp and the sub has an input inp of only 20K, then the 20K and the 450K are seen to be in parallel by the preamp and thus the preamp is now seeing an input impedance of only 19K, and a ratio of 4.
Agreed 100%, assuming that only a nominal preamp output impedance (5K in this case) is known. If the worst case (maximum) preamp output impedance across the 20 Hz to 20 kHz frequency range is known, the 40:1 figure could be comfortably reduced to 10:1.
Won't this cause rolloffs of the frequency lows and highs?
Sometimes.
Achieving a high ratio of load impedance to output impedance will definitely eliminate the possibility of rolloffs or other frequency response irregularities due to impedance incompatibilities. However, that does not NECESSARILY mean that a low ratio will cause problems.
If all of the impedances that are involved do not vary significantly as a function of frequency (meaning that they are essentially resistive), and if the sum of the capacitances of the two interconnect cables is kept appropriately low in relation to the output impedance of the preamp at high frequencies, the only consequence of the low ratio would be a slight attenuation that would affect all frequencies equally, and that would be easily compensated for by turning the volume control up slightly.
Earlier in the thread we established that cable capacitance is a non-issue in Grannyring's case. The very high nominal output impedance of the preamp, and the relatively high value of its output coupling capacitor, as I see it suggest that the output impedance of the preamp is likely to vary relatively little as a function of frequency. My analysis earlier in the thread indicated that the output impedance rise at low frequencies is likely to affect the deep bass very minimally, if at all. And I see no particular reason to suspect in this case that the output impedance will rise significantly at high frequencies (which would cause an upper treble rolloff, given the 19K loading), although there are at least a few preamps for which that does occur.
The usually unspecified input capacitance of the sub and the power amp could also affect the highs, in conjunction with the preamp's output impedance, but the total cable capacitance of only 159 pf (13 feet x 12.2 pf/ft) would seem low enough to provide a reasonable margin for that.
I see this as a necessity for the OP to at least try as a ratio of 4:1 is a serious mismatch compare to his original value of 90.
So given the foregoing, I see it as something to consider, but I would keep in mind what I consider to be a significant chance that there may not be a problem at present that it would solve (although it could conceivably affect the sound produced by the sub just as a result of its own intrinsic sonic character).
I would also suggest that further consideration be given to trying the speaker-level connections. After thinking further about the issues Grannyring cited concerning uncertainty about the grounding configuration in the sub, I don't see that that has any relevance. What is needed is identification of a connection point to the circuit ground of the Atlas amp (perhaps Aesthetix can confirm whether or not a chassis screw would be such a point, or it could be checked with a multimeter). The negative high-level input terminal of the sub would be connected to that point, with the positive high-level terminal of the sub being connected to the positive output terminal of the amp. If that is done, the internal grounding configuration of the sub would be irrelevant, as I see it, aside from the usual possibility that a slight ground loop hum could conceivably occur.
Best regards,
-- Al
