Sub connection to CARY SLP-05 preamp

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

So say if an amp with 100k input impedance, is it always better to produce the bass response than one with only 25k? Or it is just a matter of enough, excess is useless?

Or its better to ask in a broad manner, how is an amp with 100k better than one with only 25k, of course there are other factors for overall quality, but when speaking of input impedance level how it affects people's choice? Because at least you wont need to bother too much of insufficiency, other than that, whatelse?

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

Impedance is usually well wihtin range with parallel downstream loads. However, adding to the complexity, 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. So you either use 2 RCA outputs or get a high grade XLR splitter to split your XLR output (only possible if you subs have XLR inputs of course). Purist audio design makes a beautiful XLR splitter, which is an adaptor with no cable.