When you say "two mono amps to each woofer," do you really mean one mono amp to each woofer?
Assuming that is the case, the connections can be made by using a pair of xlr y-adapters, such as these:http://www.bhphotovideo.com/c/search?ci=4173&N=4289288134+4289360554
However, several things have to be kept in mind:
1)The gains of the amps have to be equal or very close to it, or else you will introduce a level mismatch between the woofers and the mid/hi's.
2)The preamp outputs will see a load impedance which is a considerably lower value than the input impedance of each amp. If the input impedances of all of the amps are the same, the preamp will see a load impedance equal to that value divided by 2. If they are not the same, the combined input impedance on each channel is the product (multiplication) of the two input impedances, divided by their sum. That number should be at least 10 times greater than the preamp output impedance, at the frequency at which the preamp output impedance is highest (which is usually 20Hz, especially in the case of a tube preamp). Otherwise a frequency response imbalance, such as deep bass attenuation, will probably result.
3)Low capacitance cables should be used, especially if the runs are long. The signal getting to EACH amp will be affected by the sum of the capacitances of the cables that are connected to BOTH amps. If that total capacitance is high, and if the preamp output impedance is high at high frequencies, upper treble rolloff will result.
The Y adapters are a great idea. What if I used four Bryston Powerpacs, two 300's for the woofers and two 120's for the heads?
If I mounted these to the back or bottom of each speaker it would keep the cable runs short.
The gain difference of the amps would be my primary concern, how would I figure that out? I guess I could could use four 300's to make things easier, but I don't need that kind of power to the heads.
Or I could do two 7B-SST2's @ 600W per. Would that be better than trying to bi-amp each speaker?
The two Bryston Powerpac models you mentioned fortunately have the same gain, either 23 or 29 db for their balanced input, as selected by a rear panel switch. That is indicated on page 8 of the manual
I should have stated more explicitly that the reference to low capacitance cables and/or short cable lengths referred to the interconnects between preamp and amps, not to the speaker cables. The reason for that is that the sum of the capacitances of the two interconnects that are branched off via the y-adapter will form an RC low pass filter in conjunction with the preamp output impedance. The same thing occurs with any preamp to amp connection, but it becomes more critical using the y-adapter and two sets of cables because the capacitances of the two cables sum together, with respect to that effect.
What preamp will you be using? The lower its output impedance, the greater the cable length, and the cable capacitance per unit length, that can be tolerated. As a rough rule of thumb, with most cables and most preamps up to 10 or 15 feet would be ok.
Re bi-amping vs. using two higher powered monoblocks non-biamped, I can't say. Perhaps someone else who has experimented with biamping the Matrix 801's will comment.
I plan on using the new Wyred4Sound Dac2 with volume control. It uses the newest Dac chips from ESS so I think it's as good as any other high-end Dac.
The xover point (380 Hz?) makes passive biamping difficult. However, if you're determined, consider a "pro" amp like a Carver ZR1600 for the bass, which has variable gain. Then again, you could just get a capable amp to start with.
If you're going with the W4S DAC, why not buy the amps also? Why spend more for Bryston? The ST500 on my N802's easily surpasses the Classe amps I had in the past. I suspect the Bryston would fare no better.
I thought about a pro amp for the bass, a QSC or MC2. The Powerpac 300 really isn't much power for the woofer, when you consider good powered subs typically have 1K Watt amps.
The 7B-SST2 would be plenty of power @ 600W, maybe I'll do that. Would that be as good as trying to bi-amp them?
Bryston has been around a long time and has a stellar reputation. That's really the only reason.
I've been looking at ICE amps and other class D amps. I just don't trust them. Bryston is a known commodity.
Specs don't appear to be available for the output impedance of the W4S DAC2.
If you biamp, regarding the cable capacitance issue I mentioned I would feel pretty confident that you'll be ok if the length of each interconnect is say 10 feet or less. Just avoid interconnects that have high capacitance, more than say 50 picofarads per foot (pf/ft).
However, the other impedance-related issue I mentioned may be a more significant concern. The Bryston amps have an input impedance at their balanced inputs of 20K ohms. Which means that if you were to connect each channel to two amps via y-adapters, the DAC2 outputs would see a load of 10K ohms. That is low enough to be a concern, and I would not want to opt for that approach without first somehow verifying that the DAC2's output impedance is less than 1K, and preferably well under 1K, at all frequencies.
I can ask W4S about this if I decide to go in that direction. Thanks for the help and I have a better idea now of my options.
Seen it on here so many times that I cringe every time I see "biamp" in a title. It can be done but, in general, if you're asking, you shouldn't. Conversely, I encourage this learning experience for everyone and wish them luck. It's rarely simple and easy except if you're using identical amps, which defeats the purpose to a degree. Unless you're dealing with a really nasty load, like Kappa9's, the only benefit over adequate amplification is reducing the electrical reactance of the woofer affecting the other drivers which, to these ears, has only been theoretical.
Won't get into active biamping because that wasn't the question and I'm not writing a book. Not sure if I would tackle the 801's xover without DSP.
Why do B&W and hundreds of other manufacturers provide two sets of binding posts on their products?
Consumer demand, to sell more amplifiers, for bi-wiring?
Ideally, all speakers would either be active or come with an external crossover for use at line level. Westlake Audio and Naim provide external x-overs with their speakers, to name a couple.
In the pro market they use monitor controllers with adjustable x-over points. Most studio monitors are active these days but the bigger stuff typically isn't.
I assume that's a rhetorical question.
No, I'd like to hear what you have to say on this.
Ten or fifteen years ago it was popular to bi-amp large speakers by using a SS amp for the LF and a tube amp for the HF.
In better active designs such as ATC's studio monitors one amp is used for each driver. Meridian also does this.
I'm not suggesting that I will get the same results by bi-amping through a passive x-over, but I do think I will achieve better results than a single amp would give me.
Don't remember that ever being popular. Probably more talked about than done, much less accomplished successfully. That said, a friend biamped with Manley Snappers and a Coda System 100 on Dunlavy SCIII's with fairly good luck, although it wasn't all luck for a guy that builds his own DAC and preamp.
There was a time when low impedance, 12 and 15 inch woofers ruled the land and were inexpensive enough to tempt partnering with cheaper amplification.
There was a time when many big amps were clumsy.
That time was called "The 80's" and the world has evolved, except B&W ;). Not all for the better. Perhaps the 80's should best be remembered for life before HIV.
As I said, it can be done. I've done it and learned a lot. Even with MOSFET amps of similar character but different qualities and a xover at 90Hz, matching was difficult. Tried a decent pot on the bass amp but was constantly fiddling with different volume settings and recordings and it never did sound right. Custom made an attenuator (23 step, 2 to 4 Kohm, SE additive) and that was leaps better. How's your soldering?
I've evolved to Genesis 350's with factory servo bass amp/adjustable active low pass (avoids passive inductors). Added an active high pass (NHT X2, simple, fixed 12dB/octave) at 30 Hz higher than stock passive. Next step would be full DSP, 3-way active with real time analysis and removing the passive completely and intact.
So you don't recommend bi-amping because of gain problems between the two amps and/or because each amp will perform differently based on the passive crossover points?
In the context of this discussion and for 95% of the situations: Unless you happen to have amps laying around, one competent amp (or pair of monoblocks) is a better investment in time and funds than two "half" amps.
There's more complexity than has been mentioned. Ground loops, impedance, reactance, .... It's a journey that will let you know where you stand and where you're going.