Using Adcom GFA-555s as monoblocks to power magnepan 3.3s in active biamp

I miss my GFA-585 LE, what a beast!  

@allenf1963 You’re asking questions about a topic on which I’m a bit fuzzy. I studied the design of filters for ‘high pass,’ and ‘low pass,’ performed the basic calculations, and became conversant in ‘first order,’ ‘second order’ and the like; ‘notch filters,’ etc. but I don’t have much practical experience with building, or tuning them. I would have go to my textbooks to get a refresher to go into detail. I’m much more comfortable discussing the sort of filtering that takes place in the power supply, trying to convert AC to DC and eliminating ripple.

That said, I can describe the basic function and components. For a passive crossover, the high pass filter takes advantage of the properties of a capacitor to ‘pass AC.’ The amount of capacitance determines the frequency at which the the ‘highs’ pass to the tweeter. Stages of filtration can be built up and at each stage (or ‘order’) the ‘pass frequency’ becomes more and more selective. That is, the slope of the frequency response curve gets steeper and steeper. I think a first order slopes off at 6 bB per octave, and a second order filter slopes off at 12 dB per octave, but I’m not sure, it may be 3 dB and 6 dB. The inductor is used for a low pass filter, and to be honest, I’m not sure how to describe its operation, but again, first and second order filters can be made quite simply. Resistors are also employed in the design of cross-overs, if you find a ‘vintage’ speaker, there is often a knob or two offering to attenuate either the highs or lows: the potentiometer is serving to ‘trim’ the frequencies at which the filter (cross-over) is effective.

So, yes, cross-overs serve to direct frequencies to the appropriate driver within the speaker (bass frequencies to the woofer, high frequencies to the tweeter, those frequencies that are nether high nor low, to the midrange driver.

The issues with passive cross-overs are at least twofold, (1) the fact that they are passive means that some of the signal’s energy will be absorbed by the cross-over components: the capacitors will want to hold onto their voltage; the inductors will want to hold onto their current, the resistors will restrain current flow and will maintain a voltage across them with a polarity opposing the source as long as they are in the circuit (it’s what they do). An alternative is ‘active’ cross-overs, which must have their own power supply, which ‘massage’ the frequencies without using the signal’s energy, but which may also add to the sound a coloration of their own. I’m not sure what components are used in an active cross-over, but I’m assuming transistors and ICs like op-amps are involved.

If you want to pursue this topic further, you probably need to pm me, as I fear we’ve hijacked this thread. We’re not all on the thread’s stated topic.

((((Wow, thanks.  So, another question: is there any harm in running 1 channel of each Adcom?)))

that's the ok way if your going to do it.

While the Adcom is at its best controlling woofers like some of the infinity speakers

In my experience i cant thing of a worse combination.

These amps are way over damped for any of the Magnepans.

Try any vintage Bryston and you will quickly see what works.

 listen to The Planets, Gustav Holst 4th movement with the opening Acoustic Basses you will appreciate the proper dampening once you experience the difference.

 JohnnyR Magnepan Dealer

Don't forget when you take part of the crossover out  the passive box and replace that part with the active cross over the speaker impedance  is likely  different  than what the manufacturer  has published.  When I had my 3.3 actively  blamed I had the horizontal  that way I was able to ture the base separately  form the mid and top end. By using different  powercords and interconnect s. 

I find it perplexing  what you really are wanting. On paper your amps are bigger than my w3 amps the moons were known to be able to swing large current.  But that being said Maggie's need that and not only was there never a need at any volume  the need of more power there was the sense of ease  that comes with a hi-fi  with big power. You don't need a power number to accomplish  great sound in fact the numbers game is a mid fi low fi game. In horizontal  configuration  you have separation before  the amps so that each amp only receives  the signal it is going to drive.. I should say that the room I was I n at the time wasn't  huge but not small either. Around 3200 cubic feet. 

Adcom GFA 555 and a Nak PA-7...that's a blast from the past ! We were selling both of those brands w-a-y back in the early 1990's when I was running the Harvey Electronics chain of audio stores in the NYC/NJ area ! We sold a ton of the Adcom, fewer of the Nak as it was pretty expensive and though built well some people felt it was a little lean sounding...I think Nelson Pass may have had a hand in the design of that but not certain...