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Thoughts of Pass XA30.5, 60.5

After having read numerous reviews & rave about XA30.5 versus 60.5, some would prefer a 30.5 over the doubling power 60.5, some would have contraditory point of view, believing higher power always having a merits of getting fuller sound, better soundstage or remarkable improvement of overall presentation.
But i just wondering why there is not much or none would like to try bi-amping two 30.5 to feed your speakers?! That maybe the way still processing the sweetness of 30.5, but never overdriven your speaker. Provided you're not using a full-range unit speaker with one pair of terminal on each side. A friend of mine has told me that should yield a unbelievable improvement over the pair of 60.5.
Firstly, your pair of speakers would be driven with more ease since each unit of 30.5 has just need to serve half of your speaker system.
Secondly, it would be more flexible & easy to sell one out if extra power is not needed anymore for getting an easy driven speaker at a later time. Your speaker wouldn't be over driven as well as receiving a bill of less electricity payment too.
More importantly, driving ability would be better than a pair of 60.5, so you could even have more choice of less sensitivity speakers.

But the drawbacks is, your preamp should possess two pairs of output, in addition you need an extra pair of interconnects & speaker cable.

Please share your comments. Thanks!
samhvcc

Showing 2 responses by almarg

almarg's avatar
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almarg
9,670 posts
 
One thing to be careful of is the load impedance that would be seen by the preamp. The XA30.5 has an input impedance of 30K balanced and 20K unbalanced. If the preamp were driving two XA30.5 inputs, and if the preamp has two pairs of outputs that are not individually buffered (as is often the case), it would see only 15K balanced or 10K unbalanced, the same as if a y-adapter were used with a single pair of outputs. Many preamps, especially tube preamps, cannot properly drive that low an impedance.

If the preamp provides two pairs of outputs that are individually buffered (as opposed to simply being wired together inside the rear panel), that would not be a concern (although the 30K/20K is still low enough to be a problem for some preamps).

Regards,
-- Al
almarg's avatar
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almarg
9,670 posts
 
What would the sound become if the impedance drops to 15K balanced or 10K unbalanced?
The rule of thumb for proper matching of preamp output impedance and power amp input impedance is that the power amp input impedance (or the combined impedance of two amp inputs, if they are driven by the same preamp output stage) should be at least ten times the output impedance of the preamp, at the frequency within the audible frequency range for which that output impedance is highest.

That requirement will be met by most (but not all) solid state preamps under most circumstances. However, many tube preamps have output impedances of a few hundred ohms at mid and high frequencies (which is also ok under most circumstances), but which rise substantially at deep bass frequencies, often reaching a few thousand ohms at 20Hz. That rise occurs because of the coupling capacitors that most tube preamps (and some solid state preamps) have between their output stage circuits and their output jacks. That rise in output impedance at low frequencies will roll off the deep bass significantly when connected to a load impedance as low as 10K or 15K.

If the 10x rule is not satisfied, frequency response irregularities can also occur in other parts of the spectrum, to the extent that the impedances vary as a function of frequency. Under extreme circumstances, distortion can also worsen significantly.

If the preamp has been reviewed by Stereophile, John Atkinson's measurements which are usually included with the review will indicate the preamp's worst case output impedance, and whether or not this may be an issue.

Regards,
-- Al
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