Possible impedance matching issue with subwoofer?

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?

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

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.

Hi All,
While it is true that many manufacturers including ARC do make a balanced designed preamp, they don't conform to the balanced spec. (I'm sure this has a name, I'm not aware of it.) But I was having a similar problem as Grannyring between my ARC REf 3 and my Xilica Crossover. I made a quick call to ARC to see is my suspicions of incompatability were correct. The tech at ARC said, there is no point in even trying to get the two to work together that the ARC preamps could not be configured to work with low impedence input and outputs because of the circuit design. His quick less than two minute explanation was sell your ARC unit and don't purchase another unless the equipment you use are compatible with the ARC design.
A subsequent conversation with the guys from Atamasphere provided me with much of the explanations I outlined above but I did leave out all the technical reasons why tube preamp designs do not conform to the balanced standard that the music industry uses today which prevents impedence matches between equipment. The audio industry has adopted the balanced connector part of the equation and the cables are built correctly by the cable guys, it is the electronics guys who are at fault.
If all audio equipment was built to the balanced specs the music industry uses, we would not have these issues of impedence mismatching. And, I believe the music industry does successfully mix tube gear with solid state with no problems.
Not to sound like I'm picking on the tube manufacturers alone, but shame on the subwoofer guys for not dealing with this problem in their designs as well. If one is going to design products with class D amplifier circuits, then care should be taken to raise the impedence so that it matches so that it conforms with an industry standard as well. One wonders how many subwoofers have received bad reviews or that have had unhappy customers because of an impedence mismatch, and not the design and construction of the woofer itself.

Wanted to update folks on my progress. Well I have two subs now hooked up to my system. I have a/b'd the sound with and without the subs. When I turn the subs off I also remove the ic's from my preamp to compare the sound.

No loss of highs. No loss of bass. No bad anything. So what I feared as an impedance mismatch did not materialize. Most interesting as on paper one would think I could have an issue with impedance between my preamp and sub plate amps.

Al, you certainly got this one correct.