Possible impedance matching issue with subwoofer?

Grannyring, all of the understandings you expressed your OP are correct. Whether or not significant deep bass rolloff or other frequency response anomalies will result from the non-ideal impedance match depends on whether or not the impedances that are involved vary significantly as a function of frequency.

It is very common for a tube preamp to have a coupling capacitor in series with its output. I don't know whether or not the Dude has one, or uses some other design approach, but preamps that do use an output coupling capacitor will usually have an output impedance that rises significantly at deep bass frequencies, relative to its value at higher frequencies (which is already very high in the case of the Dude). That low frequency impedance rise will cause a deep bass rolloff in conjunction with the sub's low input impedance, due to what is known as the voltage divider effect.

Also, if the preamp does not drive the two sets of outputs from individual buffer stages, and instead simply wires together the two output jacks for each channel inside the rear panel, then the low input impedance of the sub can also adversely affect the signals going to the main speakers. It is very common for a preamp that provides two sets of output jacks to not buffer them independently.

Also, if the preamp does not individually buffer the two sets of outputs the capacitance of the RCA cable to the sub can affect the signals to the main amp and speakers similarly to how the capacitance of the preamp-to-main amp interconnects can. If the total of the lengths of the two sets of interconnects is long, and their capacitance per unit length is not low, given the high output impedance of the Dude the upper treble can be rolled off, at least slightly, as a result of the interaction between that output impedance and the TOTAL of the capacitances of the two sets of cables (if the preamp does not individually buffer the two sets of outputs).

Given that your sub has speaker-level inputs, it would make sense to try hooking them up to your power amp outputs, and comparing sonics with the present arrangement. The wires from amp to sub do NOT have to be heavy gauge high quality speaker cables, because they will be conducting an extremely small amount of current (as a result of the high 20K input impedance).

Regards,
-- Al

"My preamp has an output impedance of 5k ohms, making for a possible poor match with the sub (20K input Imp.) if I use the 2nd outputs on my pre into the sub's RCA inputs."

You can add a SS Buffer ... 1 to 1 gain amp ... to 2nd output on preamp to lower the preamp's output impedance to better match your sub's 20K input

The Buffer should not add any gain but lower the impedance of the preamp from 5k to less than 600 ohms

With a 5K output I would guess the pre is Tube .. how is match to main amp ??

"I suppose I could hook the sub up by going from my amp to the high level inputs on my sub?"

I wouldn't daisy chain if I didn't have to ... keep the main amp and speakers seperate

"I could simply run a second set of speaker cables out from my amp to my 2 subs this way right?"

Then the main amp would have to drive both the main speakers and two subs ... may be to much of a load to ask your amp to drive ...

Al, understand all your comments and thanks. Very interesting stuff here. Quite a lot to know and be aware when adding subs to some systems. Yes, my pre has 3.3uf coupling caps. No, my second output is not buffered separately. I use low capacitance Blue Jean cables that are 10 foot long on the subs.

Davehrab & Al, going from my Altas amp to the sub's high level (binding posts)does not mean the Atlas amp is driving the two subs? Right? The plate amp in the sub still powers the subwoofer and the Atlas amp is simply sending out the signal for the plate amp to amplify. This is the way I have always understood it? Perhaps I am wrong?

This Burson buffer may be just the ticket. Thanks for the idea. On paper it sounds good, anything I'm missing here? Seems it allows me to get the best sound out of my subs as well as my main speakers? It costs $320 however and I wonder if just hooking up my subs from my amp's output is the best and no cost answer? As long as the amp is not driving/powering the subs, but simply sending the signal over for plate amp for amplification.

Going from my Atlas amp to the sub's high level (binding posts) does not mean the Atlas amp is driving the two subs? Right? The plate amp in the sub still powers the subwoofer and the Atlas amp is simply sending out the signal for the plate amp to amplify. This is the way I have always understood it?

Yes, again you are completely correct. The amp would see a load impedance equal to the impedance of the main speakers (8 or 4 ohms or whatever) in parallel with the input impedance of the high-level input of the sub, which for all practical purposes will be essentially identical to the impedance of the main speakers. For instance, if the high-level input impedance of the sub is 20,000 ohms, that in parallel with 8 ohms is:

(8 x 20000)/(8 + 20000) = 7.997 ohms.

My pre has 3.3uf coupling caps. No, my second output is not buffered separately. I use low capacitance Blue Jean cables that are 10 foot long on the subs.

Assuming that the length of the cable running to the main amp is also around 10 feet or less, and given the very low capacitance of the Blue Jeans cables (12 pf/ft if you are using the LC-1 cable), you should be ok in the present configuration with respect to the possible effects on the upper treble that I mentioned.

With respect to the possibility of deep bass rolloff, 3.3 uf represents a capacitive reactance (the capacitive form of impedance) of about 2413 ohms at 20 Hz. In terms of the magnitude of the overall output impedance of the preamp, that combines with the resistive component of the preamp's output impedance (which to a reasonable approximation is probably about equal to the 5K specified nominal output impedance) as the square root of the sum of the squares of the two numbers. So the 5000 ohm value at higher frequencies would increase to 5552 ohms at 20 Hz. That is not much of a difference in the context of a 20K load, but some amount of phase shift will also occur as a result of the capacitance (I won't take the time to calculate how much, but I believe it will be small), and the slight impedance rise together with the phase shift might, I suppose, be enough to be very slightly perceptible.

Also, keep in mind that if you connect the subs to the amplifier outputs instead of the preamp outputs the signals seen by the subs will reflect the sonic effects of the amplifier, which of course are not reflected in the signals seen by the sub in the present configuration. Very conceivably that could be the most significant factor in the sonic differences between the two configurations.

In any event, I would definitely give the speaker-level hookup a try, before investing in a buffer (which was an excellent suggestion, but as my math indicates might not be necessary in this case even with the preamp-to-sub configuration).

Regards,
-- Al