I was reading the Stefano Bertoncello's post about his new multi-amping again options.

http://twogoodears.blogspot.com/2010/09/multi-amping-again.html

.. and I feel that it would be a good idea to brief summarize multi-amping crossovering. Being myself multi-amping advocate and seeing a lot of multi-amping advantages I still feel that multi-amping crossovering is not well understood.

Since the multi-amping crossovering need to be implemented before amplification, or what we call at line-level then there are 2 main options: use active devise and use passive devises.  I would like to stress the difference the difference between active and passive crossovering.

Active crossovering imply use of active gain elements (tube or solid state) and the crossover curve is written around the aliments with active gain. This has a lot of advantages: might allow regulation of gain for each crossover channel, eliminate any input and output impedances variations, might even permit to wary the order or Q of the filter, stabilize the filter with respect of load. However, the active crossovering also has just one minor disadvantages - it put a collection of active stages into you signal path. Do you remember how much you suffered until you found a preamp that is neutral and transparent enough? There are plenty of preamps that cost $10K-$40K and that claim some sort of neutrality. The requirements to active crossovers are absolutely identical as the requirements to preamps. I would say that the requirements to active crossovers even higher than the requirements to preamps. Some people use passive inductive preamps or one stage buffer preamps and fell that only they can do “transparency”. However, active crossovers almost always imply more than one active stage – to far from minimalistic approach of a simple buffer.

For any active crossover, or a phonostage at this mater, it always was good test to disconnect the filtering part, to set the unit in unity gain to perform the “insertion” or “bypass” test.

http://www.romythecat.com/Forums/ShowPost.aspx?PostID=2589

Only if a unit demonstrated transparency being “inserted” then the filter might be introduced (it might be a bit complicated if filets are written in feedback). My experience with line level electronics suggests that it is VERY difficult, practically imposable to have line level active devise with gain to be “inserted” and to be absolutely transparent. Still, many people run active crossover with multiple active gain stages and do not realize that their active crossover is also acting a very pure sounding line level preamps. This way I have a stand against active crossovers and feel that additional active stages might be used only if a crossover needs to be higher than second order.

Now we enter the second option of line-level crossovering – the passive filters. There are two types of line-level passive crossovering: out-of-devise and in-devise. Out-of-devise implies that a filter located between line-level components.  This is very effective filters that do minimal damans to sound but they practically always are very fixed and in many ways they are tunes to the output and input impedance of line-level units. The out-of-devise passive filters have a disadvantage that they are environmentally dependant and in some ways even change of interconnects might hugely affect the performance of the filter. To cure the problem of environmental dependency exist the “in-devise passive crossovering”.

In-device passive crossovering are the same passive filters but they are imbedded into line level or power –level electronics. Since Multi-amping implies multiple amplifiers then it always there is a huge amount of locations where passive crossovering filters might be implemented. The sonic damage of in-devise passive filter is absolutely lessen of any topology and in very many cases the in-devise passive filter in fact make sound even better. A very simple example – you need to implement a high pass filter. In case of out-of-device passive crossovering you run a capacitor in series with your power amp. However, your power amp has 2-3 stages with capacitive couponing and by you might use one of the couponing caps as your filter. This way you not only eliminate one extra capacitor from a signal path but will be able to use the remaining capacitor of much less value, which most likely would lead to a better quality of capacitors.  Sure, the examples might be more complicated but the key is in following: the in-devise passive crossovering is a crossovering topology that permits an active a devise to perform it’s main duty while using the active stages of the devise to write the necessary filters. By definition the in-device passive crossovering imply no damage to signal as in-devise passive crossovering is always subtractive.

I hope the folks who use multi-amping review their playback configurations under the light of this article and see how and if they can take own crossovering a bit further in the “interesting realms”…