No, You Cannot Bi-Amp


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The new Magnepan 20.7 is not bi-ampable. The prior model 20.1 allowed bi-amping.

What sonic benefit if any, would any would a speaker gain by removing the capability to bi-amp?

I understand the big Wilsons are no longer bi-ampable either.

I have always been a huge fan of bi-amping.
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Convert?fit=crop&h=128&rotate=exif&w=128mitch4t
The designers of these speakers do not believe that bi-amping is superior to the as designed implementation.
The .7 maggies have a series crossover that they feel superior to any form of biamping. Wilson from the start has always felt that a single amp gives the best overall sound. I have tried bi-amping my 3.6's but have always gone back to a single amp
Take the money you save on extra amps, cables, and jumpers and buy a nicer amp, probably with more power so you don't give up any dynamic range.
In my mind biamping is a 'teenage' phenomenon.
If one is good, two MUST be better.
Yeah a few folks are able, with effort to make it work for themselves.
In general it is a waste of time, if as much effort was made to have one amp work well.
The worst side of it is all the folks who know nothing about it wanting to do it as if it is some easy magic panacea.. Gee I could biamp,,Cool!.... NOT.
Sonic benefits:

Transparency
Dynamic Headroom is not jeopardised by either descrepancies of amplifiers or decreased input amplification impedance
Tonal balance

Negatives:

1. 2 cheaper lower powered amps can have more power than one nicer more pricey amp so there are limitation on available options that should IMHO be present in complexed speakers
2. If two amps are absolutely identical, the benefits could be identical to a single amp only with increased power.
3. Y-connector on preamp output may be necessary that decreases an input impedance of amplification again limiting the dynamic headroom.
Ahendler's comment is right on target. The series crossover is not amenable to biamping. My opinion of the crossover in the 3.7's is that is a pretty significant improvement over the stock crossover in the 3.6. I drive mine with Cary 500 MB's, which delivers sufficient current to drive the speakers without biamping. I have heard neither the 20.1's or the 20.7's so I can't comment on those two. Let your ears be the judge.
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I wouldn't know a crossover from a crossbow. I want to get myself a pair of the 20.7 and I'm glad to know that the new design allows you to power the speaker with one amp without having to worry about whether it would sound better with two amps. I'll just make sure the my amps have plenty of muscle at 4 ohms in order to dominate the speaker.
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01-18-12: Elizabeth
In my mind biamping is a 'teenage' phenomenon.
If one is good, two MUST be better.
Yeah a few folks are able, with effort to make it work for themselves.
In general it is a waste of time, if as much effort was made to have one amp work well.
The worst side of it is all the folks who know nothing about it wanting to do it as if it is some easy magic panacea.. Gee I could biamp,,Cool!.... NOT.
OMG, there you are again spewing BS on a topic that you have no technical knowledge of (& once again you have to be reminded of the same). Like in many other similar posts of yours,I have earnestly requested you to not write garbage on a topic esp. if it calls for a technical explanation. This only continues to spread "old wives tales" on the subject matter which disallows this audio community from getting more informed. This request is being made once again....

Mitch4t,
here is a nice (& lengthy) article on Series vs. Parallel Crossovers.
http://sound.westhost.com/parallel-series.htm
You can see (immediately) that the series crossover has only 1 input hence bi-amping is not possible (the author states this himself as well). From this article & the technical measurements it's quite clear to see that (iffffff) the series & parallel x-overs are done right, they are nearly identical (& the author even states this in the concl para) in their performance so this hyperbole of the Maggie 20.7 x-over being much better than its prev revisions due to it being a series x-over is pure BS! The 20.7 xover might/must be better (I've not heard the 20.7 myself) maybe because Magnepan decided to spend the time & patience to do it right after all these years?? (speculation on my part but who knows the exact reason for it being better - certainly not due to it being a series xover...)
Biamping made sense when amplifiers exhibited lots of IM distortion and a high powered amp was 60 watts. Not the case today.

True biamping means that you use a line level crossover. Such a crossover is usually superior to a passive crossover, and much less costly if the passive uses high quality parts. IMHO, the crossover is the chief advantage.

Subwoofers are almost always biamped.
So folks who use monitors with powered subs are idiot children? Its the exact same thing as those who say choose to power woofers with a beefy SS amp and power top end with warmer tube sound (for example).
Bombaywalla, I agree with your take (I think). It's not so much about the theoretical considerations as it is the execution. Maggies have perpetually been subjected (with good reason) to modifications of their crossovers. Peter Gunn made a pretty nice business out of his crossover mods. Others have gone to the trouble to use external active crossovers, and swear that takes the Maggies to a whole new level. If the new crossovers are better than the old stock ones, it is likely due to better parts and better execution, rather than the inherent advantages of the series design. All I know is, the stock 3.7's sound a whole lot better to my ears than the stock 3.6's, which leads me to hope for a similar improvement in the 20.7's over the 20.1's. But the advice remains, go listen, let your ears be your guide.
Regardless,
It's not worth to rant especially the post that starts with
In my mind
Re Elizabeth's statement: Bombaywalla, I know that you have considerable technical expertise and relevant experience, but upon careful reading I don't see anything unreasonable or technically implausible about Elizabeth's statement. In its essence, at least, which as I read it is:
A few folks are able, with effort to make it work for themselves. In general it is a waste of time, if as much effort was made to have one amp work well. The worst side of it is all the folks who know nothing about it wanting to do it as if it is some easy magic panacea.
I have seen (and answered) more than a few posts here that would justify the last sentence of that statement. (In saying so, I am not implying that the OP in this thread is one of them).

Regards,
-- Al
Hi Al,
Like-wise, you yourself have a lot of technical expertise & your many answers to forum posts written in a congenial manner have been appreciated by me & many others as well.

Where I was coming from was:
* Elizabeth's post, to me, definitely seems like a rant (look at the way it starts off) & it seems to show that she has some grudge against this technique for reasons best known to her.
* she goes on to say that it's not "cool" to biamp but does not say why. In my mind that's where the technical implausibility comes in by omitting the technical reasons for not doing it. Thus, the forum only knows that it's not "cool" but is no better in knowing why not. Since this topic has been brought up by the OP why not discuss the merits & de-merits so that all visiting members can walk away educated. Aren't we in this audio forum to learn & share the knowledge? I don't think we are here to make blanket statements & walk away leaving everyone wondering...
Anyway, that was my angle FWIW. Best Regards.
The problems of newbies asking how to biamp is a regular feature of this and other sites.
Those folks are the ones I am writing about.
As for the reasons, that IS my reason for making it into a rant.
The folks who know what they are with biamping do not ask questions about biamping, as least not on Audiogon.

As for going through all the bother as to why it is not so great an idea.. plenty of posts on the benefits vs downside of biamping have been posted. Are we all supposed to explain the history of the world in every post?

As for Bombaywalla, I realize his need for clarity and precise arguments is bothering him when he reads my posts.
I tend to be oblique, and assume folks have some basic sense. Also perhaps he is not really getting some of my subtle humor?
Anyway, the fact of Bombaywalla not liking my style does not bother me in any way whatsoever. Sorry Bomb.. but you bombed IMO. And I look forward to years of you shaking your head in disgust at my writings.. LOL
(enough folks have appreciated my writing to make me able to easily just ignore bombaywalla's rantings.)
Bi-amping provides more flexibility in circumstances where more flexibility is warranted. While that flexibility MIGHT provide improvements it can provide problems. The Nigels of the world read this forum and will take advantage of the fact that the electronic crossover goes to eleven - or twelve.

As I read it, Elizabeth was telling the OP not to get involved if he doesn't know what he's doing -- and, if he's asking here, he plainly does not.

I have owned and used actively bi-amped systems at least 3 times over the years. To me, they have held great potential that I never felt I fully realized. I even went so far this last time around to hire a professional sound man to come in and set it up for me. The results were very good -far better than what I was able to accomplish on my own, but in the end, I commissioned Bill Woods to design passive crossovers for my system. I'm much happier knowing the crossovers were custom designed for my cabinets and drivers and not having so many cables and components cluttering my life.

My experience supports the KISS principle. There is way too much fantasy out there about DIY and mods. Some of us have the skills to know what's an improvement and what's just a way to spend money kidding ourselves. Most don't.
Elizabeth - I have always found your comments to be informed, opinionated and reasonable. In short, you are one of the better, more entertaining contributors. Perhaps Bombay's problem with you is not in your commentary but in your pants. You got to remember you're just a girl and keep your place instead of butting into important man stuff. So why don't you just trundle yourself off to the kitchen and whip him up a mess a grits and eggs and quit trying to act like you know something about man thangs? Sheeesh!!
Liz, I like my eggs over easy and I prefer link sausage, some buttermilk pancakes would be nice also. Would you mind putting on a Monk LP whilst you prepare my vituals? :)
BTW, I think your posts are highly entertaining.
Thanks guys! Unfortunately my kitchen skills are pretty much non-existant. My oven gets a hard workout making a frozen pizza. LOL.
The top burners are good for popcorn, a can of beans,
My most complicated dish is spagetti.. with bottled sauce.

I detest the smell of cooking in general. (meat is really the worst smell)
My personal Hell would be stuck cooking in a really hot dirty, smelly greasy spoon resturant serving 'baby on a stick" (in Hell remember..) LOL!

I am no model housewife. Thank You.

((Maybe Bombaywalla would like a bit of my "Baby on a stick" dish???)) I wonder???
As for Bombaywalla, I realize his need for clarity and precise arguments is bothering him when he reads my posts.
I tend to be oblique, and assume folks have some basic sense. Also perhaps he is not really getting some of my subtle humor?
To Elizabeth & all the other men: yes, this is the issue that I have w/ many(but not all) of Elizabeth's posts where I've written a rebuttal aimed at her.
Sometimes I get your humour but sometimes I do not.
I'm not totally rankled or totally mad but the lack of precision does get me off & on (I do not always rebutt Elizabeth's posts & she knows that).
As far as a woman knowing about audio - that has never been an issue for me. I welcomed it when I first started out on A'gon & I welcome it now. Au contraire, I wish that even more women would join the audio hobby & share their experiences. It might make for a more balanced exchange of ideas.
So, you men can stop making these massive leaps in your conclusions about my having issues with women knowing a lot about audio. I do not have any such issues.

And I look forward to years of you shaking your head in disgust at my writings.. LOL
yeah, LOL indeed. Sorry to disappoint you Elizabeth I won't be doing that now that you have stated your angle of writing such posts.

Natural desire of being right even if being wrong very often is subconcious.
That's humor?

I'd hate to see you pissed off.
"Eldartford:
...True biamping means that you use a line level (sic) crossover."

WRONG. There are at least 2 types of biamping, and both are 'true'. Each has advantages and disadvantages.

"Subwoofers are almost always biamped."

WRONG again. In the last few decades, subwoofers are usually powered, but a powered SW is NOT biamped. Did you mean that by adding a powered SW to a system, the SYSTEM then becomes biamped?
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"Macrojack:
Bi-amping (sic) provides more flexibility in circumstances where more flexibility is warranted. While that flexibility MIGHT provide improvements, it can provide problems. The Nigels of the world read this forum and will take advantage of the fact that the electronic crossover goes to eleven--or twelve.

...

I have owned and used actively bi-amped (sic) systems at least 3 times over the years. To me, they have held great potential that I never felt I fully realized. I even went so far this last time around to hire a professional sound man to come in and set it up for me. The results were very good--far better than what I was able to accomplish on my own, but in the end, I commissioned Bill Woods to design passive crossovers for my system. ...

My experience supports the KISS principle. There is way too much fantasy out there about DIY and mods. Some of us have the skills to know what's an improvement and what's just a way to spend money kidding ourselves. Most don't."

VERY well said, jack. Designing passive crossovers or even configuring active crossovers is NOT easy, at least not if you want to get it corrrect. The best passive crossovers include impedance- and phase-correction networks that are required for the system to sound correct, and when some amateur eliminates them to use an active crossover, the relationships between/among drivers is left uncorrected.

I built an open-baffle line-array system that used active filtering and biamping; the whole project turned out to be too complicated for this amateur to get right, and I simply gave up.

What's a 'Nigel'?
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Elizabeth, you crack me up! Though I got to be honest, your comment about your "top burners being good for popcorn" got me thinking some unpure thoughts. James Brown needs to take it the bridge before I get myself in trouble.
Jeffreybehr - Nigel Tufnel is (was?) the immortal lead guitarist from the mythical British rock group, Spinal Tap. He reached an amazing level of fame for being the owner of a guitar amp that went to ELEVEN. As he pointed out repeatedly, other guitar amplifiers only went to ten and were therefore inferior to his.
Last November we celebrated Tufnel Day on 11/11/11.
Isn't that Guy Fawkes day?

I don't remember....remember
I don't see why people object to people who know nothing about bi-amping asking questions about it.
I know nothing about bi-amping.

I think I have a pretty good ear for music and have played instruments for a long time.

I recently replaced (for my Maggie 3.5's) my old Levinson 332 with two Magtech amps and I bi-amped those.
The improvement was spectacular.
I attribute that improvement to the fact that I am bi-amping.
This could wrong.
I was so looking forward to getting the new 20.7's and being able to use that bi-amping ability.

Now I learn that you cannot bi-amp these new speakers.

So why isn't it Ok to ask about bi-amping and what differences it makes and if it is that important maybe I should look for a good used pair of the 20.1's instead?
Nottop - I gather from your description that you simply removed the jumpers from your speaker binding posts and bi-amped your Maggies with the passive crossover already onboard. That approach maintains the integrity that Magnepan built into them.

If you had bypassed the integral passive crossovers and introduced an active crossover between your preamp and your two amplifiers, you would have opened a can of worms because of all the variables you would have had to match to restore the original balance. This is why amateurs are warned against jumping into bi-amping too eagerly. It is extremely difficult to get it right.
Hi macro -

yes - that's exactly what i did.
I'm not using an external crossover (although many tell me it's the way to go).
It sounds so good with the bi-amping and the crossover that came stock.
I think one of the reasons that bi-amping is a lightning rod is that there is much misinformation or opinion masquerading as fact.

People also mix non-compatible gear, like a fast SS amp with a 'slower' tube amp on the highs. (strongly held opinion) While not taking into account the latency of the amps being used, can introduce phase effects. (maybe testable)

The endless combinations of filter choices, points, slopes and type just feed the fires.

I'd suspect (again, opinion) that a properly setup (yet more opinion) bi-amp system could yield positive results.

I couldn't imagine my Panels NOT benefiting from a pair of same amps, given the close to 50:50 power distribution crossover point of 600hz.
Some good points have been made above. With respect to passive biamping, another commonly overlooked point is that in that arrangement both the high frequency amp and the low frequency amp have to output the voltage range of the full range signal. Which means that if a low powered tube amp is used for the highs and a much higher powered solid state amp is used for the lows, most of the power capability of the solid state amp will be unusable and wasted, because the maximum volume setting that can be utilized will be limited by the clipping point of the low powered amp. Also, of course, gain matching is likely to be an issue in that situation.

Regards,
-- Al
Relative power needed is almost entirely governed by crossover point.

If you crossed over at 10khz, you could make due with a 30 watt tube amp on top of a 300 watt SS.

Than, other issues would come to the fore.
01-20-12: Magfan
Relative power needed is almost entirely governed by crossover point.
If you crossed over at 10khz, you could make due with a 30 watt tube amp on top of a 300 watt SS. Than, other issues would come to the fore.
Right! A major "other issue," if the two amps are being used in a passive biamp arrangement (i.e., without an active crossover ahead of the power amps) being that some number approaching 270 watts of the 300 watt capability of the SS amp could not be utilized, without clipping the 30W amp. Essentially the 300 watt amp would be reduced to not much more than a 30 watt amp in that scenario (the exact value depending on the voltage swing capability of the 30 watt amp). Which was exactly my point.

Best regards,
-- Al
01-20-12: Magfan
Relative power needed is almost entirely governed by crossover point.

If you crossed over at 10khz, you could make due with a 30 watt tube amp on top of a 300 watt SS.

Than, other issues would come to the fore.

Right! A major "other issue," if the two amps are being used in a passive biamp arrangement (i.e., without an active crossover ahead of the power amps) being that some number approaching 270 watts of the 300 watt capability of the SS amp could not be utilized, without clipping the 30W amp. Essentially the 300 watt amp would be reduced to not much more than a 30 watt amp in that scenario (the exact value depending on the voltage swing capability of the 30 watt amp). Which was exactly my point.

Best regards,
-- Al

if you are going to use the in-cabinet speaker x-overs as-is then the best way to do passive biamping is the method done by member 'jefferybehr' - he's used 4 identical amps that are driven by his preamp. Take a look at his system pix.
I'm sorry, but the power relative power needed IS governed by the crossover frequency. At least for music with a 'normal' distribution of frequencies.

With the crossover at 10khz, per my example, you really would need more than 90% of the power below crossover, with only 10% above.

http://sound.westhost.com/bi-amp.htm

The 50:50 point is about 350hz. I don't know that active or passive makes much of a difference. The amp will only have to produce power for those frequencies for which the speaker has a need.

The 'other' factors I was thinking about would be amp latency. How long it takes a siganl to pass thru the amp.
Also, gain and how linear the gain is will play a part in freq. balance thruout the volume range.
I have a straightforward question for Elizabeth...

... who has a most strong negative opinion of actively bi-amping, e.g. Magnepans, and seems to have an even stronger negative opinion of the people ("adolescents") who engage in the practice:

Has she ever actually heard a pair of Magnepans, e.g. the 3.5s, 3.6s, or 20.1s, that were bi-amped with two excellent amps and an excellent line-level external active crossover, e.g. a Pass Labs XVR-1?

Has she ever done anything close to an A/B comparison of a pair of Magnepans thusly bi-amped and the same pair single-amped with the stock, or even a "Gunned," passive crossover?

If so, what did she hear to cause her to demean so vehemently active bi-amping?

If not, where does she get her confidence in such an ungrounded opinion?

Respectfully (up to a point),
David Zimmerman
01-21-12: Magfan
I'm sorry, but the power relative power needed IS governed by the crossover frequency.... The amp will only have to produce power for those frequencies for which the speaker has a need....
Hi Magfan,

Somehow my point isn't coming across.

Yes, the amp will only have to produce POWER for those frequencies for which the part of the speaker it is connected to has a need. However, in a passive biamp arrangement, where there is no active crossover ahead of the power amps, both the high frequency amp and the low frequency amp will need to produce the SAME output VOLTAGE, corresponding to ALL of the frequency components of the music.

And therefore if the low frequency amp has a rating of 300W and the high frequency amp has a rating of 30W, corresponding to the maximum power requirements above and below a crossover frequency that is very high, the 300W amp will not be able to produce 300W, or anything close to it (in a passive biamp arrangement), without the 30W amp (which presumably has a far smaller voltage swing capability) being driven into clipping. Therefore how far the volume control can be turned up without severe distortion occurring (and perhaps a blown tweeter as well) will be limited by the output voltage range of the 30W amp, and most of the power capability of the 300W amp will not be usable.

As I said earlier, that is a point that is commonly overlooked. I have often seen questions about passive biamping that refer to the possibility of purchasing amps such that the low frequency amp has a power rating several times greater than the rating of the high frequency amp, which makes no sense.

Best regards,
-- Al
Als right. For me best bi tri amping comes from matching ea amplifier to drivers it will run. This is a pain to pull off but once done worth while. Had a horn system with class d on bass horns, PP on midbass horn,SET on midrange horn,low power class A on horn tweeter worked very well though I had to try many amplifiers ;) Today I mostly biamp using Class D or SS on bass horn and SET on main horns. So 250-500 watts on bass and 12 watts on mains...
It is entirely possible that some speaker manufacturers discourage or outright deny bi-amping because of the warranty problems and damage to their products and their reputations that result from misunderstanding of the process. Our hobby and our websites are alive with unqualified opinions from self-appointed experts who start rumors and create confusion either mischievously, maliciously or naively. It all works out the same, however, with sometimes irreversible rumors perpetrated to no good effect.

The key to this discussion is the crossover itself. If that isn't right, amp matching remains meaningless. A good pair of properly designed crossovers can cost a couple of thousand dollars. When these are included in a repeatable speaker model and sold in volume the costs are amortized, but in my case the designer spent over 50 hours getting it right. He used the same horn and driver that I have and the same woofer in the same size cabinet. This was all done outdoors to provide anechoic conditions with microphones, instruments and a computer. There is a lot of trial and error involved if you have 30 years experience as he does. What would it take for you to pull it off?
I ask that question because when you try to set up your digital speaker management system in your own little bi-amp arrangement, you will need to create all the alignment and compensation that Bill worked out for me on your own. Not only is it not easy, it will prove to be impossible for most of you.
So, we go back to Elizabeth who indicated that it is a fool's errand, too often taken lightly by duffers and conceited neophytes. Some of them will arrive as I have with something they think sounds pretty good, Some will, as I did become frustrated and some will do real damage. None will get it right. Even if you use the thousand monkeys with typewriters logic, the odds of a happy accident are too extreme to be calculated and would require a much longer life than any of us can reasonably anticipate. It is your money and your gear, though , so have at it.
Have to agree with others here who dont see the need for a rant when someone admits they dont know something. They're asking those of us who do have the knowledge and experience to share that knowledge, which is how they become knowledgeable> Did all of us start off knowing everything there is to know about audio? Come on guys and girls,I admit I dont know everything there is to know and I've been involved in this hobby since 1986. Ok, my rant about ranting is over.
JohnK, thanks for your comment.

To be sure it's clear to everyone, I'll repeat that the issue I cited, involving a large disparity in power capability between the high frequency amp and the low frequency amp, just applies to PASSIVE biamping, in which the only crossover that is present is the one in the speakers.

I'll add that my comment was intended to apply to the situation where a single pair of speakers is being passively biamped, with the single pair of speakers having been designed to provide reasonable results (in particular, some semblance of flat frequency response) when driven by a SINGLE amplifier.

John's comments about combining Class D and SET, etc., apply to a different situation altogether.

Regards,
-- Al
Almarg's arguement makes sense to me. I believe that using two amps via the passive crossover ought to be called "dual amping" to distinguish it from true biamping using a line level crossover.

I performed testing to determine how powerful an amp is needed to drive my MG1.6 after lows are removed to an auxillary large cone driver system which I use as a subwoofer. I tested for a wide range of crossover frequencies. What I found was that the high frequency signal needed a lot more "power" than I expected. "Power" is in quotes, because what I was measuring was voltage peaks when playing music at high volume. Voltage swing is what is necessary to avoid distortion. Because the peaks are very brief there is no doubt that the power is actually low. But, because of the way audio amps are designed you have to buy power to get voltage.

Bottom line... don't skimp on the high end amp.
A long time ago I had new Mirage M3si speakers which could be biamped.

So I did what anyother audiophile would do, I tried biamping.
I bought two Brytson 4B amps.

I tried them every which way, and then even as mono blocks.

No magic solution.
Perhaps it was the fault of the speakers, but back then the amps and speakers were well respected.

What I eventually settled for was a one amp solution and I even turned my back on the bi-wiring option.

The best sound I got was with one Bryston amp in stereo, and an easy rewire of the Mirage speakers(just re-arranging the internal speaker wires at the posts, no soldering required)so that one pair of speaker wires was all I needed.

Perhaps bi amping and biwiring do make a difference in other situations, but in my case they weren't worth the expense or bother.

Now I have Ref 3A Grand Veena which are also bi wireable(not biamp as I understand )and so far I prefer to run single wire (Nordost Heimdal) with the Nordost jumpers in the diagonal position.

I tried various speaker wires and none of the biwiring configurations I tried were any better than the way I have them wired now.
I ran my Maggie IIIas with an Audio Research D400 for years, then bought another D400 and used the XO1 passive crossover but quickly got myself a Marchand electronic crossover which cross to a pair of subs below 50hz then uses same slopes as Magnepan suggests bass to mid/high

To my ears the difference was pretty significant and and I haven't been a teenager for decades (sorry just being flip)

I thought one advantage was the quality of the crossover itself and limiting the bandwith each amp must produce

Supposed to audition the 3.7s in a couple weeks so will find out 1st hand what they sound like "out of the box".

Audio Research says that the D400s will drive the low impedence load presented by bridging the amps and driving Maggies so can use both amps still but again my ears will be the judge

I doubt I will pull the trigger until I can hear the 20.7 and check out some other models at the Capital Audi Fest this summer but "once you go Maggie you seldom/never go back" :-)

MikeH
Mike, I have to agree with you. I bi-amped my Apogee Duetta Signatures using a pair of totally rebuit and upgraded Threshold Stasis 2 amps and a highly upgraded/highly-modified Marchand XM-9 electronic crossover a few year ago and have never looked back. The upgrade to bi-amping was the most major improvement that I ever made to this system and I've made more than a few.

Is bi-amping a simple solution for the person who just wants to power-up the system a take a listen and not think about the details? No, not at all.

As many know, when you bi-amp a system, in most cases, you totally by-pass the existing passive crossover in your speakers. Now that your amps are connected directly to each driver - with no capacitor in series to block DC - you now need to be concerned about your power-amps' DC offset. Too much DC offset and you will cook your more delicate drivers over time - even with no music playing. Woofers and bass panels can absorb a good amount of DC without damage. Even some ribbon MRTs can survive 200mVDC without damage over time; however, most anything over 50mVDC will move the driver's diaphragm or ribbon away from its "home" position which negatively impacts the driver's design performance. Employing amps with a low DC-offset and the regular use of a multi-meter is a must with a bi-amped system.

Now, once you decide to bi-amp, not only will you need to be concerned about finding the correct crossover point, you will need to be attentive to the slope of the existing passive crossover which you will be bypassing verses the slope of the electronic crossover. If the slope is different, you may need to invert the phasing of your mid and/or tweeter drivers in order to have correct overall phasing. A lot of information about crossover slope phasing is available with a search on the Internet. At least this is a one time thing.

Anyway, what I’m saying is in addition to the added cost, is that bi-amping is not for everyone for the reasons stated above. Bi-amping does improve amp-to-driver efficiency through the elimination of the speaker's power-consuming internal passive-crossover system.

Somehow, I get the impression that some folks think bi-amping is like bi-wiring but with two amps and some sort of "Y" cable, but nothing could be further from the truth.

Bi-amping takes some effort to make sure everything is properly set up and requires some maintenance attention in order to make sure that your mid/tweeters doesn't become toast!

BTW, my guess is that Magnepan made a business decision as to not allowing bi-amping of the 20.7. Maybe to reduce ribbon warranty claims?
This thread makes me appreciate the coherent design of the Linn Aktiv system. Amps, active crossovers and speakers all designed by the same company, made specifically to work with each other. I can tell you that my Linn Katans sound a heck of a lot better in Aktiv mode (Linn active crossovers placed into my Linn amps) than they did in passive mode or passively bi-amped mode with the exact same amps.