The input impedance of the DD-12's line-level rca inputs doesn't appear to be specified anywhere. The SLP-05's output impedance rises substantially at deep bass frequencies, to 3400 ohms at 20Hz, as indicated in Stereophile's review. The concern that raises is that if the sub's input impedance is not high (e.g. 30K or more), the sub's deep bass response will be perceptibly rolled off. The best approach, which would eliminate that concern, would be to connect your power amplifier outputs to the sub's speaker-level inputs. You don't need heavy gauge high quality speaker cables to do that, since what you would be driving is the input to the sub's amplifier, which undoubtedly draws a negligible amount of current compared to a speaker. Regards, -- Al |
25K is sufficiently higher than 3.4K to be good enough. The worst case rolloff, which would occur at 20Hz, would be less than 1db, probably significantly less than 1db. The reason I mentioned the issue is that a lot of subs have line-level input impedances in the range of 5K to 20K or so, which would be low enough to cause significant rolloff in the bottom octave if driven by the SLP-05. do I need to biwire it back to from sub to poweramp according to some threads? You may be thinking of the configuration in which the preamp is connected only to the sub, and the sub's high pass outputs are connected to the inputs of the main power amp. That would prevent deep bass frequencies from reaching the main power amp and the main speakers, which is sometimes done, although it puts the sub's circuitry into the main signal path. Given the high quality of your system, as well as the uncertainty about the sub's input impedance, that approach is most likely not preferable. If you connect the main power amp's outputs to the sub's speaker-level inputs, as I suggested, you would continue using the existing cables and connections between the power amp and the main speakers, but you would add a second set of wires between the power amp and the sub's speaker-level inputs. Those wires could be narrow gauge inexpensive wire. Regards, -- Al |
The 30K figure I originally cited was a conservative rough approximation, cited as an example (the term "e.g." which I used means "for example").
Prior to submitting my subsequent response, in which I indicated that 25K would also be ok, I did some calculations which confirmed that.
The same principles apply whether the preamp is driving a sub or a power amp, except that in general it will be a more critical issue when a sub is involved, since main speakers will typically be greatly rolled off anyway at frequencies approaching 20Hz, where the preamp output impedance is highest.
The 10x rule incorporates conservative margins, IMO, provided that it is applied at the worst case frequency, where the preamp's output impedance is highest. Falling a little short of satisfying that rule at 20Hz, as a result of a rise in preamp output impedance caused by a coupling capacitor (as in this case) can be shown by analysis to be insignificant.
Regards, -- Al |
Once the 10x rule is satisfied, or comes close enough to being satisfied (as in this case), then further increases in the load impedance seen by the preamp will provide no further benefit. The choice of an amp should be made based on other factors, obviously including listening comparisons if at all possible.
Another point to keep in mind, though, is that if a preamp has to simultaneously drive both a power amp and the line-level inputs of a sub, and if the preamp drives both of them from the same output stage, that output stage will see a load impedance that is much lower than either of the two individual input impedances. That PROBABLY wouldn't apply in the case of the SLP-05, which has both an xlr output and an rca output, but it would apply in the case of a preamp that provides two pairs of rca output jacks that are simply wired together inside the rear panel, as opposed to being individually buffered. That is a common situation. It would also apply in the situation where a y-adapter is used.
In those situations the combined input impedance is equal to the product (multiplication) of the two individual input impedances, divided by their sum. For example, if the power amp's input impedance is 50K, and the sub's input impedance is 25K, and both were driven by the same output stage in the preamp, the preamp would see a combined load of (25 x 50)/(25 + 50) = 16.7K.
Regards, -- Al |
09-12-11: Edorr ... I am using the fully balanced modwright 36.5 to drive my subs and mains, and the manufacturer explicitly advises against using RCA and XLR at the same time, because it will create an uneven load on the output transformer. I presume the Cary would have the same issue. My posts about the SLP-05 were based on the assumption that its outputs are capacitor coupled, as is the case with most tube preamps, and not transformer coupled, as is the case with your Modwright. I was also making the assumption that its rca and xlr outputs are driven by independent output stages, which is why I used the word "probably," in capital letters, when I referred to the impedance reduction that would result from driving two line level loads in parallel as "probably" not being applicable to the SLP-05. If the Modwright drives rca and xlr outputs from a common output transformer, presumably with a grounded center tap on the secondary winding, and one end of that winding going to the rca center pin, that is a completely different story, and you were correctly advised. Regards, -- Al |
Marty & Edorr, good points about RTA and equalization, with which I have no particular familiarity. I would expect, though (correct me if I'm wrong), that driving the sub and the power amp with parallel line-level signals from the preamp would be no different with respect to those functions than driving the sub from the power amp outputs, since in both configurations the sub has no control over the signals that ultimately get to the main speakers.
And the alternative of driving the power amp's inputs with the high pass outputs of the sub's crossover necessitates a tradeoff against the possible adverse sonic effects of putting a lot of sub circuitry into the main signal path. As well as the need to make sure that there is a reasonable impedance match between preamp and sub, which without further information on the sub we can't say with certainty.
Regards, -- Al |
I could be wrong here but I believe the Xover works on the line level input. This would make this a complete non-starter, since you need to apply a low pass filter. I would feel pretty certain that the sub low pass filters its speaker-level inputs no differently than it does with its line-level inputs, aside from providing different gains for the two paths. Regards, -- Al |
Marty, take a look at the manual for the Velodyne DD subs. Its descriptions of the RTA/EQ processes are similar to what you have described, but I see absolutely nothing in the manual to suggest that those processes would not occur in exactly the same manner if the sub's inputs are at speaker level, driven from the outputs of the main power amp, compared to the configuration in which the sub's line-level inputs are driven from the outputs of the preamplifier. For the design to be otherwise would make no sense, IMO. At a conceptual level I envision the design as simply dividing down the speaker-level input signals to line-level amplitudes, and then putting them through exactly the same circuitry that is used to process the line-level inputs. The only difference between the two configurations, I believe, is that if the speaker-level inputs are used, driven by the main power amp, obviously the sub's high pass outputs cannot be connected into the main power amp's inputs. But we are all agreed that using the sub's high pass outputs is undesirable in most cases anyway. Best regards, -- Al |
Richard, as I indicated above my assumption has been that there is no output transformer in the SLP-05, that it is capacitively coupled instead (as is the case with most tube preamps). You might want to ask Cary to confirm that, and also to confirm that the rca and xlr outputs are independently buffered.
As far as the impedance issue is concerned, the only way to answer that with certainty is to determine what the input impedance of the particular sub is. That figure appears to be unavailable on the web, so an email to Velodyne would seem to be in order, if you want to pursue the line-level approach rather than the speaker-level approach.
Regards, -- Al |
15K is too low to be ideal, given the impedance rise of the SLP-05's output to 3.4K at 20Hz. That is less than a 5x ratio. On the other hand, depending on the placement of the sub and speakers, the settings on the sub that you end up with, room acoustics, and certainly the source material, the effects of that will not be noticeable on a lot of material, and may not be bothersome on any material. Assuming, again, that the SLP-05's output is capacitively coupled, and that its rca and xlr outputs are independently buffered.
Also, the 15K value pretty much rules out the possibility of using a y-adapter on either of the SLP-05's outputs, because the combined parallel impedance of the sub and your main amp is only (15 x 25)/(15 + 25) = 9.4K
But I'd start with the power amp to speaker-level input approach first, which (assuming it works as we suspect) avoids all of those issues, and has the additional benefit of providing the same signal to both the sub and the main speakers. REL, in the literature on their subwoofers, makes a point of emphasizing the benefits that can provide in terms of overall coherency, since the sonic effects of the main power amplifier will be reflected in the signals to both the sub and the main speakers.
Good luck!
-- Al |
The Cary person provided excellent information. However, as I see it the information he provided makes the case for connecting via speaker-level much more compelling.
Since the positive polarity xlr signal and the rca signal are common, if you were to connect the xlr output to the main power amp and the rca output to the sub, that signal would have a load of 9.4K (see the calculation in my previous post), which would violate the 10x rule by a wide margin, and would also violate the 10K criterion Mr. Wemmer specified.
There would be other downsides to doing that, as well. At low frequencies the balanced inputs to your main amp would no longer be balanced, in terms of the amplitudes of the two signal polarities. There would also be a significant impedance imbalance on the two polarities, which would substantially degrade the noise rejection capability of the balanced interface to your main amp.
The good results Amfibius has obtained with that configuration are, I suspect, attributable to having considerably higher input impedances on both his amp and his sub. I don't know what the input impedance of his JL Audio sub is, but I see that the balanced input of his CAD-211AE amp is specified as 91K.
Best regards, -- Al |