Its me again - SFrounds, Need help on biamping questions.

Stupid question#1
Why is it that an amp rated at 125 wpc can deliver 400 watts mono, why wouldn't it be 250?

Now for the rest of these questions, lets say I have two of these identical amps, and a pair of speakers that can be biwired.

Hopefully - not as stupid question#2
What is the best way to wire these amps?

(A)Have one amp wired to supply the low frequencies on each of the right and left channel speakers, and one amp wired to supply the high frequencies on each of the left and right channel speakers.(at 125 wpc each)

(B)Have the amps wired mono at 400 wpc with one amp biwired to the right channel speaker and the other amp biwired to the left channel speaker.

(C) none of the above or other recommended option.

Thanks again, (Don't worry I can Take abuse very well)
Half an answer - Using your 125W example, assume a 5 ohm nominal speaker impedance for convenience. If the amp can supply the 125W to the 5 ohm load, it is supplying 5 amps. P=I*2 x R > 125 = I*2 x 5 > 25 = I*2 > I=5. To do this, the output must swing 25 volts. I = E/R > 5 = E/5 > E = 25. Now, bridging essentially puts the two amp outputs in series. Therefore there is a possible swing of 50 volts. E/R = I > 50/5 = 10 amps. P = I*2 x R > P = 10*2 x 5 > P= 100 x 5 > P = 500 watts. Apparently, though, the amps can't quite supply the 10 amps because of component limitations (transformer, transistors, etc), but can only supply about 9 amps, so 9*2 x 5 = 400 watts limit in that configuration. PS - a single one of those amps could also supply 9 amps, but that would be about a half ohm load, which it not where it's rated! :)
It looks to me like you have the following options:

1) Run each amp bridged mono, wired in to each speaker.
2) Run each amp stereo, one for highs, one for lows
3) Run each amp stereo, high and low of one speakers in one amp and v.v.

I went through this scenario fairly recently, and there are things to be set for each of the three setups. I preferred using scenario 2, using one amp for left and right mids, the other for left and right bass. It just seemed to create an energy and bass clarity that the other combinations did not.

(I did this with a set of NHT 2.9 speakers I own, and two ML331's I had on loan.)
Hmmm...perhaps an important point to make here... If the speaker just has dual connectors without some sort of shorting bar you can remove, the speakers could be BI-WIRABLE but NOT BI-AMPABLE. I don't know if there are any speakers to which this would apply, but if you have no shorting bars you can remove, be extremely careful! Tying the outputs of two amps together is almost certain cause of serious amp damage...
More answer - in the example, if you can't play your system loud enough (at least on peaks) then perhaps you are output voltage limited and need the higher voltage swing to drive your speakers (and ears) to their potential. Voltage limitation like current limitation current limitation can result in clipping and distortion or damage. That is the argument for bridging as long as the amps can supply the additional current. You must also know that the speakers can accomodate the extra current/power; that is, they were meant to play louder. If your system plays loud enough, even on peaks, you would just be running the level control lower with the bridged amps and not accomplishing much. Now comes the counter responses, I guess. :)
Hi Sfrounds; You may be interested; there is passive vertical biamping and passive horizontal biamping. To do vertical biamping you need a pair of identical stereo or dual mono amps (ideally). In vertical biamping, one channel drives the bass of the left speaker, and the other channel drives the mid/tweeter of the left speaker. To do this, the speakers must be bi-wireable, and an important advantage is that the amps can sit right next to the speakers and very short speaker cables can be used.

Horizontal biamping differs in that one amp drives the bass of both speakers, and another amp drives the mid/tweeters of both speakers. Some use this with a tube amp for the upper frequencies and a solid state for bass. In horizontal biamping the shortest speaker cables can be achieved by placing both amps right between the speakers.

About bridging and power; I've recently been on the 'phone with Steve McCormack discussing this very thing as I'm going to vertical biamp my speakers with McCormack amps. According to SMc when the DNA.5 (100 wpc, 8 Ohms) is bridged, it will put out 400 wpc, but when the DNA-1 (175 wpc, 8 Ohms) is bridged it will only put out about 370 wpc. Further, when the DNA-2 (300 wpc, 8 Ohms) is bridged it will put out an incredible 1200 wpc. So, the bridged power level depends on the original configuration of the amps power supply. I have a pair of DNA2s, but decided not to have the channels bridged as I don't need that kind of power. SMc is going to upgrade the amps and match them for me for biamping. Good Luck. Craig.
These are good yet basic questions that many audiophiles do not know or understand. As such, it makes a great platform for all to learn and share. Thanks for posting your questions. Hopefully we can answer them in a manner that you and others can understand.

When you "bridge" a stereo amp, you are combining the two channels together electrically. As such, an amp rated for 100 wpc @ 8 ohms and 200 wpc @ 4 ohms would be rated at 400 watts into a single channel @ 8 ohms.

Due to how the amplifier circuitry works ( i'm keeping it simple here ), it sees half the impedance of what the speaker is really rated at. Since an 8 ohm speaker would look like a 4 ohm speaker to a bridged amplifier and you've tied both channels together, you get two times its' rated power into that 4 ohm load. Hence, your 2 x 200 @ 4 ohms is now 1 x 400 @ 8 when bridged.

Keep in mind that most amps don't "double down" ( double power as impedance is cut in half i.e. our 100 wpc @ 8 & 200 wpc @ 4 example). Many amps rated at 150 wpc @ 8 will do appr. 225 wpc @ 4. As such, it should do appr. 450 wpc bridged @ 8 ohms, or what is two times the 4 ohm rated power.

Since most bridged amps do not want to see a speaker with a low impedance rating to start off with, you have to be careful. A speaker that is nominally 4 ohms would look like a 2 ohm load to a bridged amp. Since most speakers DO dip measurably lower than their average rating, this could go VERY low and become quite close to what looks like an electrical "short circuit" to an amp. This can cause it to run very hot and / or thermally shut down. There ARE amps that can deal with being bridged and driving very low impedances though. This is a sign that the amp is both powerful and has a very sturdy power supply.

Another factor that most people don't realize is that as impedance of a speaker is lowered, the damping factor of the amplifier is reduced. The closer the impedance of the speaker is to the output impedance of the amplifier, the more likely the bass is to become "sloppier" or "bloated". That is why some amps have a hard time with or sound "loose & flabby" into low impedance speakers. The amps output impedance may be higher AND the power supply may be current limited when really "gittin' it". Since tubes amps typically are high impedance designs on their output section AND current limited, this explains why SOME tube amps suffer from "round" or "ill-defined" bottom end, especially with low impedance speakers.

Your questions about amplifier configuration is a matter of personal choice. Running one amp for the highs and the other for the lows is called horizontal bi-amping. This can be done using two identical amps or two different amps. One benefit to this is that you can run a "soft, sweet & airy" amp for the mids and treble and a "big brute" for the bottom end. Another benefit to this is that when large bass notes pull on the amps' power supply come into action, it would only sag the woofer amp without affecting the output of the mid and tweeter amp. This can result in more stable imaging and soundstage. Of course, this would only happen if the amps were not quite powerful enough to deal with the demands being presented to them in the first place.

While running two specialized amps for each frequency range can give you the best of both worlds, there is more work involved since you now have to worry about making sure that each amp puts out the same amount of power with the same amount of "drive" or input signal applied. This requires either gain matching of the individual amps or an active crossover with individually adjustable channel output control levels.

The method that uses both channels of a stereo amp to individually run the high and low section of one speaker is called vertical bi-amping. You would obviously need two identical amps to do this if running in stereo. This avoids the need for gain matching in most instances ( as long as the amps are built to specific standards during production ) and can also be done passively ( no active electronic crossover ).

Either way, an electronic crossover with a relatively steep slope can be drastically advantageous IF properly set up and the speakers are optimized for it. Since the amp would only see a portion of the frequency range, it can now concentrate on reproducing ONLY that area. As such, it becomes more efficient. The smaller the range that it has to reproduce, the more efficient it becomes. In effect, a 100 wpc can seem as powerful as an amp rated for almost twice its' power output if the system is well assembled and carefully tweaked. This is why you can get away with running a "wimpy" amp on top in comparison to what the bottom end requires. The "wimp" doesn't have to supply or even get to see any of the high current, big power demands of low frequency reproduction.

Your other option was bridging the amps into monoblocks and running one for each speaker. Many amps do not sound as good when running bridged for some reason while others cope with this in stride. This gives you very high voltage potential compared to the other amps but your still stuck with the current limitations as if you were running only one channel of the amp. THIS is why many bridged amps get sloppier with low impedance or reactive speaker loads. Another common complaint with bridged amps is that high frquency reproduction becomes "harder" or "grainy" sounding. Of course, this is all up to individual designs and components.

As you can see, there are pro's and con's to each method. Much of what works best will depend on the specific amps and speakers in question and what you're trying to achieve. Either way, the expense of the system has drastically risen, as you now need TWO amps and FOUR sets of speaker wires AT THE MINIMUM for any of the scenarios that you presented.

Do your homework very carefully before jumping into any of the above set-ups. It can get very involved and is definetly more work to get things dialed in. Make sure you know what your dealing with BEFORE shelling out the cash. Hope this helps and clears some things up for you. Sean
This is an excellent question that I have often pondered.

I will soon have B&W Nautilus 802's with 2 Bryston 4BST's.

Anyone out there familiar with which setup would work best?
-vertical bi-amp
-horizontal bi-amp
-bridged mono to each

I realize there are differences as Sean and others have stated. My question is with this setup, which might be the best solution?


I run a tri-amp system using an electronic crossover. I have found that the tweeter gets by nicely with 50 watts per channel but the midrange improved dramatically when it was replaced with a 250 wpc amp from a 100 wpc amp. The mid range runs from 90 to 4000 Hz. My subwoofer uses another 250 wpc amp but it may be more conservatively rated. My impression is that my ears are very sensitive to any clipping in the midrange but not in the low bass and tweeters being more efficent don't need much power to be driven. If the led readouts can be believed, at very loud levels, the tweeter amp is putting out 5 watts while the peak meters on the midrange amp say 50 to 100 watts. I have no meters on the bass amp but I find myself attenuating my bass levels to placate the spouse and neighbours. So go stereo mode and triamp!
Sfrounds: Njonker has raised the most important question for
you to answer. Are your speakers truely biamp-capable to
begin with? Check with the dealer/manufacturer. The speaker inputs MUST allow you to connect to (typically) the
woofer independently, and the midrange+tweeter units
independently (usually there is a passive crossover inside
the speaker to split between mid and high). If so, then
separate amps can drive the two "halves" of the speaker, each amp delivering a restricted frequency range specific
for that driver(s). This is done by putting an active
crossover device between your preamp and the amps, to "feed"
each amp a limited Hz range. This active device should at
least approximate the crossover frequency/slopes that the
manufacturer originally built into the speakers (i.e., the
original internal division of bass from mid/hi signal). In
my opinion, the resulting "vertical biamping", which uses 2
stereo amps, gives a much more open sonic presentation. The
speaker/amps are less stressed too. And as Sean has nicely
outlined above, you can choose different "styles" of amps
for bass and mid/hi. If the 2 amps have different GAIN, then
your active crossover will need level controls to match the
loudness resulting from the separate amplification. Active
crossovers can be obtained from Bryston, Marchand, and some
other companies. You may want to consider a 3-way crossover
which would do all the above, but also route the lowest Hz
to an active subwoofer, to relieve your main woofers of having to produce the lowest octave; this in a sense would
be tri-amping.
Good luck... it's a lot of work/money but worth it.
In my last post, I should have said that the use of 2 stereo
amps with an active crossover gives "horizontal" biamping.
Anyway, there is a detailed disussion of this type of
biamping at

if yer speaks are truly bi-ampable, and ya *do* have two identical amps, the best ting to do is yust try the 3 different methods - bridged, wertical, or horizontal bi-amping. it's easy to tell if yer speeks w/two sets of binding-posts are *really* biamp-able: yust hook one amp to only one set of binding-posts w/o the yumper in place, and only that driver should play - the other should be silent. (but, it's hard for me to imagine why a speaker mfr wood have two sets of binding-posts, if all drivers play when only one is used, w/o a yumper).

i have had good luck horizontally biamping w/an electrocompaniet aw100 driving the woofs & an aw75 driving the tweets of my monitors. but, yust this week, i sent my aw100 to a guy who wanted to trade his aw75 for a bit more power. his aw75, w/serial # only 5 digits away from my aw75, should give me the perfect opportunity to try bridging & wertical bi-amping, as well as horizontal bi-amping. i also have a pair of gnu aw60's, bought from europe, waiting for me to conwert from 220 to 120v. (i had given up on finding an aw75, & the trade offer happened shortly after i bought the aw60's!). so, i may also try bridging all 4, so each driver has a mono amp. but, this may really be over-kill in my system, so i may end up using 'em in another system, trying to sell 'em & use the money for a pair of melos mono-toob amps, or...??? ;~)

re: ral's suggestion of using an active x-over, i'd recommend this *only* if ewe can completely bypass the speakers' internal passive x-over - i wood *not* use an active x-over *and* the the speek's passive x-over - this is sure to muck-up the signal, imho. and, an amp run to one set of binding-posts of a bi-amped speaker only sees the the load above or below the speakers' passive x-over point, anyways...

good luck, doug s.

There are some speakers which actually sound better if you use an active xover to externally separate the bass from the mid/hi (prior to the amps), then let the speaker's internal passive
xover split the mid and hi's within the remaining signal. E.g. the larger Magnepan speakers (I
own and biamp the MG3.6 in just this manner). Also, you certainly don't want to run a full-range
amp signal into each individual driver of a biamp-ready speaker; this would send high-freq. info.
to the woofer, and bass info. to the midrange/tweeter, which could cause distortion or
damage the drivers.
ral, there should be no problem w/running a full-range amp signal into each individual driver of a biamp-ready speaker - it wood *not* send hi-freq info to the woofer & bass info to the mid/tweet, unless ewe also went inside & disconnected the passive x-over, but why wood anyone do that, unless they're using an active x-over before the amps?

it's interesting hearing your success w/using *both* the passive x-over *and* an outboard active x-over w/yer maggies - i guess this is yust another example of what seems logical on-paper isn't always best in the real-world. i guess ya yust gotta experiment! ;~)

regards, doug s.

I want to thank you guys for this indepth education (especially you Sean). I've read over all your post several times, and I think I get it. However I have some clarifications and a final question.

Given that I have these two identical amps, and lets say I have speakers that are bi-ampable.

(A) I can vertical bi-amp without any modifications, and do not have to use any active x-overs?

(B) I can configure both amps mono without any modifications, however, I should have speakers rated at 8 ohms (which are really 4 ohms when using amps mono blocked?

(C) If I wanted to horizontal bi-amp, I could use these two identical amps or one smaller amp for the mid/high frequencies.

My question regarding horizontal bi-amping is; My duplicate amps have gain controls, is it necessary to use active x-overs? Can the gain control on the mid/high frequency amp be adjusted to achieve a match between the the low frequency amp?

PS - Remember, I did say I think i get it!!
regarding (b), the rating of the speakers really wood depend on the capacity of the amp to deliver current. my electrocompaniet amps, for example, put out serious current & are stable down to at least 0.5 ohm. so, if bridged, they'd still be stable down to at least 1 ohm, & wood still drive most any speaker.

re: your question regarding horizontal bi-amping, assuming identical amps, no difference in gain should be needed - the speakers' built-in x-over wood be seeing the same amp for all intents. but, i guess ewe could experiment... as far as needing active x-overs, again, ya won't need 'em w/identical amps. but while it sounds illogical to me, ral sez ya mite get even better sound if ya *do* use 'em - at least he did w/his maggies...

re: (c), this *is* where having a gain control on at least one of the amps wood be useful - different amps may have different gain characteristics, especially if ya wanna, say, run toobs on the top & solid-state on the bottom.

hope this helps, doug s.

Sean's answer is excellent overall, but the fact that you double the voltage applied to the speaker and therefore double the current (if the amp has the ability) through the SAME load accounts for the 2*2 times power. (*2 mean to the power 2, or squared).
I've heard recommendations from 'experts' for both Bi-amp techniques, as well as not even bi-wiring (to round out the ridiculous to the sublime). What I've heard, and I am no expert, is to bi-amp across (one multi-channel amp doing lowering amplification) another multi-channel amp doing the high side. Oddly, like getting two psychiatrists to agree which one is crazy, there are only questions, no dead correct answers.
Time for fairness. A question on another thread about stability/load rating when bridging made me realize the point of Sean's perspective on "effective" load impedance. While the two amps being in series with the same physical load does account for (up to) twice the current in the circuit, indeed each amp is supplying twice the current that it alone would into the nominal load impedance of the speaker. Thus, each is operating as though it were seeing an impedance half that of the speaker. If I may simplify (dangerous), think of the other amp as a negative impedance because it is supplying current rather than impeding it. My apologies for being focused on my point.