Possible impedance matching issue with subwoofer?

Input impedance is stated to be 20K so the preamp sees 20K to the sub and whatever ?? to the amp, but with both in paralell if the amp was 20k as well then the overall input imp. would be only 10k and that could affect freq. response on both the sub and amp. Al is correct, and using the speaker level inputs will negate the paralelling of the signals and offer less problems. Especially if they are using a passive preamp. The only thing to watch out for with using speaker level inputs is if you have a fully differential amp and the sub ties the grounds together to form the mono signal to the woofer, but that is another issue since a good sub will use a floating ground of about 100 Ohms or so.

If you hook the sub up in parallel to the preamp you will go from an output to input impedance ratio of 94 to only 4. I feel that will certainly result in high frequency sparkle loss. You should look into a buffer between the preamp and sub for the line level inputs. The Musical Fidelity X10 v3 has a high input impedance around 400K that will mate well with your amp. I think the ratio would then be about 44:1 and certainly acceptable compared to 4. I am speaking from experience here, not just textbook. I noticed a definate drop in the highs when I hooked up my sub the same way without the buffer. These buffers don't cost that much. Just get one and try it both ways... you will hear a difference.

First off, I must say that Al definately knows more about electronics/stereos than me from my reading of these forums for so long. So with a bit of confusion I am asking - "Why aren't we seeing this the same way?" This is what I think:

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.

Won't this cause rolloffs of the frequency lows and highs? It did when I hooked my passive preamp to my amp and sub by splitting the signals, but when I used a buffer to the sub it sounded much better through my mains.

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.

Al, what are your thoughts here?

Thanks Al for taking the time to write such an informing response. In my case I noticed a distinct reduction of the sparkle in the highs. So I got a much better sound using the buffer, then I just concluded that it's that impedence issue that I had read about on these forums. Not knowing that it is much more complex then that, I figured the ratio thing was written in stone. So now what comes to mind is the Seierra Audio amps and preamps that have reportably excellent sound that have identical output to input impedence (75 Ohms) and use a BNC coaxial wire to connect them. This effectively is a ratio of only 1:1 yet people say it sounds great. So as you say there is more to it.

Thank you for sticking with this thread and I'm sure that others as well as myself have benefited from this discussion.