What’s the Right Power Amp Ratio For Bi-Amping?

Is there a “golden rule” or rule of thumb when selecting amplifier power in a bi-amp setup? It seems to me that the power should be apportioned according to the demands. Since most of the energy consumption  in sound reproduction is by lows, it stands to reason I should use a much more powerful amplifier for lows than highs, but what ratio of power? 2:1? 10:1? Is there a wrong answer?
You get better results using one good stereo amp rather than bi-amping with lesser ones.
It depends on the design of the loudspeaker. It is my understanding that Magnepan recommended the two power amps used to bi-amp their old Tympani T-IVa be of equal power rating (and gain, of course), and chose the speaker’s cross-over frequencies (low pass: 3rd order @250Hz, high pass: 2nd order @400Hz) partly in relation to the speaker’s power "center" (the frequency at which equal amounts of power would be required to achieve the same SPL on each side of the x/o frequency created by the above two filters).
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If there is no active crossover, your amp still has to produce the same voltage swings, and therefore needs to have the same voltage rails and rated for similar power, however it may not need a lot of current/low impedance capabilities.

Using an active crossover 5:1 would not be unreasonable.
Erik, I’m using conventional magnetic/voice coil speakers (not electrostatic) operating in bi-amp configuration with the woofer separated, but the crossover still engaged, limiting the band pass to the mid/high drivers. The upshot is that the mid/high frequency side will have an impedance to frequency curve that presents no load to the amplifier below the crossover point. That is why I want to devote my “big” 150 watt per channel amplifier to low frequencies, driving the woofers, and my “modest” 60 watt per channel amplifier to driving the mid/high drivers. The 150 watt per channel amplifier has input level trim controls so I can match its level to the smaller amplifier. I just need an idea of the ratio of power demand so I can be sure I’m not under-powering the mid/high drivers. 
A rule of thumb I’ve seen stated, which seems to me to make sense as a very rough approximation, is that music tends to require similar amounts of power at frequencies above and below 350 Hz. That should be considered in the context of the crossover point of the particular speaker, as bdp24 alluded to in his post. So if the crossover point of the speaker is considerably lower than that figure the high frequency amp will probably have to supply more power than the low frequency amp much of the time, and if the crossover point of the speaker is considerably higher than that figure the converse would be true.

The upshot is that the mid/high frequency side will have an impedance to frequency curve that presents no load to the amplifier below the crossover point.
That is an oversimplification, of course, as you probably realize. Below the crossover point the load impedance presented by the speaker to the high frequency amp will gradually increase as frequency decreases, at a rate roughly corresponding to the slope of the crossover network (e.g., 6 or 12 or 18 db/octave).

Also, Erik makes a good point about the fact that if the two amps are properly gain matched, and a passive biamp configuration is being used (i.e., there is not an electronic crossover "ahead" of the amps), both amps will have to output voltages corresponding to the full frequency range of the signal. As a practical matter what that usually means is that in a passive biamp configuration there should not be a huge disparity between the power capabilities of the two amps. Otherwise the power capability of the low frequency amp that can be utilized, without driving the high frequency amp into clipping, may be limited by the voltage swing capability of the high frequency amp.

-- Al
The amps should be the same , and using an active crossover .   Without a crossover , you would want amps with output trim.  I agree with the one good amp rather than 2 lesser or different amps.  Personally I think bi amping is a waste.
Al, thanks for your insight. As always it gets the mind looking at different angles. The theory I’m going on with significant differences in power requirements is that low frequencies have longer wavelengths and the woofer cone has a much larger  surface area driven to much greater excursions. The effect is moving significantly greater volumes of air. I theorize that the power to drive a 10” woofer over an excursion of 0.5” must be an order or two of magnitude greater than the power to drive a 3/4” soft dome tweeter 1/100”. 
Hi all, Here's something to think about. If you have two identical amps you will have the option to vertically bi-amp as well as horizontally as you guy's are discussing. I don't know the advantages of each way, maybe ALMARG will chime in.Tish
Racamuti makes a good point. If identical amps are used a vertical configuration (one amp per speaker, one channel powering the lows and the other channel powering the highs) is considered to be preferable to a horizontal configuration (one amp for the lows of both speakers, the other amp for the highs of both speakers).

One advantage of a vertical configuration is minimization of sonic effects that may result from inter-channel crosstalk within each amp, since both channels would be processing the same signal. Another advantage is that a vertical configuration would typically allow the amps to be positioned closer to the speakers, making it possible to use shorter speaker cables, potentially reducing both their sonic effects and their cost.

Also, of course, using identical amps eliminates the possibility of loss of coherence that might result if the two amps have differing sonic characters.

-- Al
As Al points out, it doesn’t work that way.

While the speaker crossover will reduce the current, the amp will still produce the full spectrum voltage, minimizing overhead.

If you are passionate about doing this though, there may be a tweak. IF and only IF all three conditions apply:

  • Bi amping a 2 way
  • There is a resistor in series with the tweeter
  • This resistor is first then you can short it, and reduce the tweeter amp volume.

If you are bi-amping a 3-way however, or that resistor comes after a cap or coil, this won’t work.

This will allow your tweeter amp to produce less output, and eliminate wasted heat in that padding resistor.
Erik, power is the product of current and voltage. If the hi frequency side of the bi-ampable speaker is driven with the same signal (voltage) spectrum, it will only dissipate power where current is drawn. In this case, the crossover presents a relatively high impedance to frequencies below its crossover point, and a relatively low impedance to frequencies above its crossover point. This (impedance that varies inversely with frequency) is the mechanism that prevents low frequency energy from being applied to the mid and high frequency drivers, otherwise they would become damaged. 
One advantage of a vertical configuration is minimization of sonic effects that may result from inter-channel crosstalk within each amp, since both channels would be processing the same signal.
Hi Al, just to add, the biggest advantage of vertical bi-amping is that the whole power supply joule storage of one amp, is dedicated to just one bass driver and not shared, so the bass/upper bass should be better on big dynamic transients. (unless the amps have true dual mono power supplies) which are rare.

Cheers George
Hi Sleepwalkwer,

As some one who builds his own speakers and worked in the motion picture sound industry for years, and got to sit in on lectures at Georgia Tech on the subjects we are covering, please let me walk you through the math you are missing.

You are absolutely correct that when playing to a high pass limited load you won’t have a power issue. What you’ll have is a voltage issue.

Amplifiers don’t amplify watts, they amplify volts. Think of them as an electronic spreadsheet. Some value comes in on the input, and we have to multiply that value by a fixed amount to get the result. That’s the output that is presented to the speaker.

The typical amplifier provides about 26 dB of voltage gain, regardless of power rating. Stereophile routinely measures this in their amplifier reviews, like here:


Simplified, this is about 20x.That is, feed it 1Vrms, and you should get 20Vrms out. This will continue to be true so long as the load impedance is >= 8 Ohms AND you do not exceed the maximum voltage at the power rails.

For an amplifier rated for 100 Watts amplifier these rails are at about +- 26 Volts (if I have done my math right, and it’s been decades since I was tested on this). No matter how much you increase the load impedance, that is the physical limit of the amplifier. Due to losses in the transistors, etc, it will swing less than the rails, maybe +- 20 Volts. This is what the amplifier’s published power rating is really based on.

When you look at the spec rating, that’s the limit you see superbly illustrated in figures 5 and 6, here:


That brick wall is essentially the voltage rails being hit, and while increasing the load impedance helps, you can't beat the power supply rails.

So, back to the original problem. The amp will attempt to apply it’s gain to the input, regardless of how much you increase the load impedance. Now, one benefit of course, is that you won’t have any current flowing at 100 Hz, so the amp power supply will be stiffer. That is, typically, when playing hard and loud, amp voltages sag. The power supply can’t maintain it’s maximum rating. So, yes, little benefit. However, you cannot exceed those voltage rail maximums, and without a line level crossover, you are almost exactly as limited in terms of gain and maximum output as you were before hand. Both amps still have to reproduce the same voltage swings and will apply the same gain as before.

Hope this helps, and apologies in that I’m 90% sure my power supply to amplifier power rating math is probably off, but the principles are not. :)

How will it sound? Well, depends on how stressed out the original setup was. If the amp was never stressed, and it’s voltage rails remained constant and stiff, bi amping won’t show much improvement.

However, if the amps were soft/weak, biamping will keep the mid/tweet amp stiffer, and less subject to the turbulence and weakening power supply of the bass amp.

So, toss-up. :)
The phrase I should have used is "clipping."

When reviewers talk about an amp clipping it means it has reached the limits of the voltage swing, and the sine waves get clipped, or shopped off.


And as discussed above, from 8 Ohms upwards to infinity, the voltage at clipping does not vary that much, and since gain is fixed, we must maintain this :
Vin  < ( Max Vout / Gain )

I think I should have said:
A 100 W/ch amp produces 26 Vrms = 40 Vpeak = 80 Vpeak to peak . So the internal DC voltage rails would be that plus a little more, +- 43-46 Volts.

Anyway, the story still holds. Whatever those rails are are the absolute limits of your output. Since gain is fixed, your maximum input is limited before you clip.

Thanks Erik. The thing that I’m looking to leverage is the impedance curve with a modest amp powering the high frequencies. I have a NAD 2700 for the low frequencies, with 150 watts per channel into 8 ohms, it’s quite capable. The other amp I have is a NAD 3225PE with 40 watts per channel, an integrated amp with external pre-amp in/out jumpers so i can use just the power amp section. The previous setup had that amp acting as the system preamp and low range bi-amp powering the same speakers, and a 3220i for the highs - that combo worked very nicely. The game changer was finding the 2700 amp for a bargain. 
It doesn’t sound like you are getting advice from anyone who has tried bi- amping. I have. First-You just need to try it and see if it sounds good to you. No rules will help guide you in my opinion.  I use two Bedini class A amps to bi-amp. The 25 watt is used for mids and tweeters. The 100 watt is used for woofers. These are excellent sounding amps individually. Bi-amped gives an amazing  improvement. I’ve used them successfully with passive and active crossovers. Fire up the amps you were thinking of using. You can’t hurt anything if you don’t drive the speakers into distortion. 

If the woofers are closer to the floor (as they typically are) you'll probably get some room gain there, so that might need to be considered as well.

The cross-overs can play quite a part in this, just think of how much overlap there can be with 1st order cross-overs. The least complicated way to do this is with identical amps.

I think the OP is asking about the brain's perception of lower frequencies. If I'm correct what the OP really wants to know is how much more SPL does a low frequency tone require to be perceived as equal to a relatively higher frequency tone. I don't have an answer to that but I've had good luck feeding my lower frequencies around 5x to 8x the watts of the higher ones. I use towers that have an active bass section and dsp. My amp is 50 wpc and the bass is a few hundred wpc. 
Throw a 1,000 wpc x 2 on each side and see if you like it. Buy some old  $300 class D's and see if you are happy. Cost $1,200. 
The question boils down to this: if you had the ability to measure power consumption of white noise (equal amplitude across the audible spectrum, not necessarily 20Hz to 20kHz, if you could do it for 50Hz to 10kHz that would be a sufficient demonstration) by the two halves of a bi-amped speaker, what would you see? 
@vinylfan62, actually, I have bi-amped, in the past (first time in 1972) and currently. It's nice that your Bedini 25/25 (I ran one for years with my old QUAD ESL's) and 100/100 work great for you (knowing Bedini, they probably have the same gain), but the pairing of a 25w amp with a 100w may or may not work with anyone else's loudspeaker. They certainly wouldn't with my Magneplanar Tympani T-IVa's!
I have a B&K Sonata 5ch amp,  150W×5 which I used with an active crossover, it has outboard trim but I used the gain on the X-over to set the levels and the x-over points
That's how it's done in a pro-audio environment (for good reason)
Sounded super clean and strong
I would sell it if you're interested 
The question boils down to this: if you had the ability to measure power consumption of white noise (equal amplitude across the audible spectrum, not necessarily 20Hz to 20kHz, if you could do it for 50Hz to 10kHz that would be a sufficient demonstration) by the two halves of a bi-amped speaker, what would you see?

Since the crossover point between the low frequency section and the mid/hi frequency section of a speaker will almost always be in the lower part of the 50 Hz to 10 kHz range, most of the energy of white noise occurs at frequencies that would be above the crossover point. So most of the energy of white noise would be delivered to and reproduced by the mid/hi section of the speaker. The spectral composition of music is very different, of course.

With music, per my earlier comment, if the crossover point of the speaker is 350 Hz both amps would typically have to supply approximately equal amounts of power. If the crossover point is significantly less than 350 Hz the high frequency amp would typically have to supply more power than the low frequency amp. If the crossover point is significantly higher than 350 Hz the low frequency amp would typically have to supply more power than the high frequency amp.

With music this relationship will of course vary somewhat from recording to recording and from moment to moment. Which is why I used the word "typically," meaning that it would hold true the majority of the time.

-- Al

I am using one stereo amp on each speaker. Left for mid highs and right for bass cabs.  The meter is always higher on mid highs.  Same amp.  Crossdover point between upper bass driver and lower mid driver is 230hz.
Thanks, Pauleladue.  That's an excellent data point, which most of us wouldn't be able to provide and which is consistent with what I said.

-- Al
I’ve located and purchased another NAD 2700 amplifier, so I’ll have a pair of identical amplifiers.

I’ll now have 3 options: vertical bi-amp, horizontal bi-amp, or bridge and power left and right separately without bi-amping. Vertical bi-amping seems to be the preferred way, as mentioned by @racamuti , @almarg and @georgehifi, so I’ll try that first.

Bridging seems too extreme by the way, as I’d end up with the capability to produce 400 watts per channel, probably ten times what I’d ever need during normal listening sessions with my relatively efficient Energy Reference Connoisseur 30 speakers in my 12’x12’ listening room.

I will experiment with shorter speaker cables, from the 8ft 12 gauge cables I’ve used with the 3225PE/3020i setup, I’ll be able to go to 3ft 12 gauge cables with the 2700s sitting beside the speakers. I’m not expecting any audibly noticeable improvement, but maybe I’ll be pleasantly surprised.

Thank you everyone for your input! This is is going to be fun!
Bridging seems too extreme by the way, as I’d end up with the capability to produce 400 watts per channel, probably ten times what I’d ever
Not only that, but also sound quality is worse. As everything measured parameter that makes an amp a good one takes a hit when you bridge. The only plus you get is the extra wattage when you bridge, and if you don’t need it, then you actually go backwards in sound quality.

EG: Bridging Amps
Higher distortion
Lower current
Lower stability
Inability to drive as low impedances
Lower damping factor
Higher output impedance.

Higher wattage.

Cheers George

I have bi-amped a100 watt Yamaha receiver with a 100 watt Kenwood separate amp with very satisfying results. The sound warmed up nicely IMO. The Kenwood had trim control so I could match the levels. I connected the receiver to the high side of my bi-amp able speaker posts and the separate amp to the low side speaker posts.

Your power ratios sounds workable to me 40/150 or the 150/150, with the trim to help match.

I have also bi-amped other combinations of amps with no improvement.

I would try bi-amping then compare to only using one 150 watt amp and see if there is improvement.

Agree with you could try bridging but would not expect that to sound better. However, it would not take much time to try it and satisfy the question and I have been surprised before.

Well, I received the second NAD 2700 amplifier this week, and wired it up with the other 2700 to vertically bi-amp my speakers.

Love the newfound benefits of not being power-deficit anymore. There seems to be substantially more detail in the midrange, which is handled a little differently by my Energy Reference Connoisseur 30 speakers. They have a three way design even though they have two identical woofers and one tweeter, designating one of the woofers as a midrange driver. 

Now getting back on topic, checking heat output of the two halves of each amplifier, the sides that drive the woofer / midrange is *much* warmer than the sides that drive the tweeters. That unscientific evaluation suggests there is a distinctly greater amount of power dissipated by the low frequency side of the bi-amp split. I haven’t yet figured out how to empirically measure this, but the first stab will be using my infrared thermometer. 

In the meantime, I’m no longer power hungry 😋 
There is not one. Just go for quality, best low end, best treble etc. You like tube high end, but maybe you like SS bass. Forget about power although the more you can afford the better.

Im bi-amping a pair of Tyler Acoustics Halo Extreme with an Parasound A21+ into the low end and an Prima Luna Evo 400 feeding the top. Rocket 88 carbon x speaker wire. The gain on the amps are within 3 db of each other. The sound and imaging space between the instruments is phenomenal. Im using an Cambridge Audio Cxn v2 into a Audio GD r7he mkii dac and Audio Research LS 28se preamp. Im loving it. 

With speakers that have seperate terminals for LF and HF, doesn't that remove the problem with both amps sharing the same load? Or does it not matter because the amp "sends out" the same signal regardless of what it is connected to?

For example, Tannoys have separate terminals, and Tannoy say they are biamp ready. Would they bother if they didn't feel there could an advantage? 

From what I understand the total out put gain needs to beof equal value or very close. If not the under powered will be drowned out. 

Why vertical bi-amp? You don't need the same power for the highs as you do for the lows. Horizontal bi-amping is much better because the low frequencies are not robbing power supply current from the high frequencies. The actual ratio depends on the efficiency of the drivers. Are you still going to use the internal speaker crossovers? With or without an external active crossover? Multi-amping has been in my blood for 44 years. 

I am using the horizontal bi amp method. Like you said all the low end is being supplied by my ss amp and top end by the tubes.I keep the tubes in ultra linear mode and it sounds fantastic.  Not using an external cross over.