Quick Bi-amping question?

Your receiver is rated at 100wpc with 2 channels running(The only "mainstreamers" that rate with all channels running are Marantz and HK).

Typically, 100wpc "morphs" to 70x5 and 60x7.

So, you really don't have 100wpc when running all the channels, and you apparently don't understand how power works in speakers either.

Taking a 90db at 1 watt/meter speaker...

1 watt=90db
2=93db
4=96db
8=99db
16=102db
32=105db
64=108db
128=111db(by now you are so loud you are causing permanent hearing loss)
256=114db

and keep on going.

So extrapolating from above...even if you only counted "in stereo"...

100 x 2.

Then 80x4...which would create...
160 x 2.

The difference between 100 and 160 is negligible.

But as the adage goes...

More power is never a bad thing.

+1 Even the difference betwen 100 and 200 watts is only 3 dB (which I've seen described as sounding about "one notch louder.") And, since the tweeters typically only get something like 20% of the power the woofer gets, with real program material, you probably can only get about a 20% increase with real world content, leaving aside the diminishing power available as we add channels with most receivers.

Oh you had to go there...

That whole "can't they be honest" is an FTC nightmare...

There is so much wiggle room it isn't even funny.

They can state power at a 1khz sine wave burst.

They can state power from a constant 20hz-20khz.

They can state(look at some Sherwood 5/7.1...not the NewCastle, just the Sherwood) with 1 channel running.

Receiver "power ratings" are a mishmash of misinformation, and always will be.

Little tidbit about the FTC...

You know those "1000watt PMPO" HTiB(the one piece that include the dvd/BD player). That 1000watt rating is a complete joke. They take this thing, and with ONE CHANNEL RUNNING, attach it to a 3ohm speaker(that in itself has dubious specs) and pump it with a 1khz test tone to the point where it STARTS TO SMOKE. They back it off a tad and find a power they can send through for a millisecond that won't cause it to CATCH FIRE...

They that take that UNREALISTIC number and say "200watts times 5" making 1000 watts.

Way back in the day, when VHS HTiB were being made(you want to talk about utter disasters waiting to happen)...
We used to call these ridiculous power ratings ILS power. (If Lightning Strikes)

You are oversimplifying this a bit, I think.

Your receiver has a power transformer. It has limitations. If you put a high continuous load on it it's output voltage will likely sag. That limits power to all the amps. A short term load might be handled by the capacitors in the supply, who's job is to even out voltage fluctuations (without those caps, you would be getting voltage swings from 0 volts to some peak voltage.) That won't help for contiuous demand, only for short term demands.

That's not the whole story though. The receiver has circuits to protect itself. One of those is probably a current limiter. The current limiter could be devised various ways. One way is simply to limit voltage to the amp's voltage amplification stage, which will limit the peak voltage it can amplify.

Any of these limiting mechnisms will limit voltage, or clip the output voltage from the receiver's amps.

Say you are using the receiver in a 5.1 setup. You are probably power supply limited (or perhaps power gets limited by the current limiter circuits, I don't know for sure.) Continuous power per channel with all channels driven is limited by how much power the supply can put out. When you connect up two more amps in biamp mode, they are putting more load on the power supply. This will further limit power to all the amplifiers under high demand.

Of course, I could be wrong. So go test it and see if it sounds better in bi-amp mode.

I'm not an engineer, didn't stay at a Holiday Inn Express last night nor am I very technical in this stuff.. (though I DO actively biamp).

So, when I butcher the analogy below, maybe those that view it as imperfect will give me some slack.

Mike, let's say you buy a 21' boat. On this boat you install a 100HP outboard motor. This 100HP motor will allow the boat to accelerate up to a certain speed and will max out at a certain speed.

Now, adding a SECOND 100HP motor will NOT make the boat go twice as fast, will it?

The second motor might allow the boat to accelerate more quickly or perhaps pull a heavier load....but in the end you don't really gain any speed.

That's my take on passive biamping. If both channels are being 'fed' the same signal then you are not dividing the signal properly to instead, allow each channel to reproduce only part of the sound. By allowing each channel to be responsible for less of the load, the amp(s) will have more headroom to drive that specific load.

Make sense?

(It took me a while to get my head around it)

Put differently.... which (of two identical) car might pull away from a stoplight faster?

1. The car with a trailer (full load) being pulled behind it or
2. The car WITHOUT the trailer (partial load) behind it?

Clearly the car without the trailer can get away quicker....because it's not having to pull the full load that the other one is.

So with amps, if you have an amp that is sending for example, 20hz-20K hz to a driver BUT 50% of that signal is getting wasted via passive biamping, then that portion of the amplifer is NO LONGER available available for use by the speaker since it's already spoken for.

If instead, you actively biamp, split the signal and now this same amp is only reproducing say, 2K-20K hz then by NOT having to redundantly reproduce everything from 20hz-2Khz, that portion of the amp WILL be avaiable as additional headroom to reproduce the other part of the spectrum.

Fair warning to you... yes, there will be a test on the boat & car analogy in 60 minutes

OP...

"Now, if that same receiver is cranking out 70 watts per channel with 5.1 channels being driven. Then what happens is, you are TAKING AWAY power from the Center, Surround L, and Surround R, and adding that power to the FL and FR. Is that fair to say? Which I would think, would be a bad thing by the way"

In a nutshell, the answer to your question is "yes".

However, not all receivers are created equal. Would a Pioneer VSX920(have no idea if this SPECIFIC receiver can bi-amp...and don't really care) behave the same way as the Integra DTR-80.2.

Nope, not in the slightest, cause the DTR 80.2 has a whole lot more to work with.

Believe it or not, the "bigger" improvement is taking your main channels off the receiver and handing power duties to a 2 channel amp. If you take a "normal" 100wpc receiver and buy even a 125wpc amp(cause you'd think 100 to 125 would not be a big jump)...but when the receiver no longer has to create the front channel it will still have around 90wpc for the center and surrounds(keeping in mind your rears and surround no longer perform bass duties...the "power hungry" realm of <80hz is no longer a power sap).

So, in that scenario you'd see a HUGE improvement in sound.

Just to be clear, no amp always sends its max power. It only outputs however much is needed (within its limits) based on the input signal.

The "idling" amp sections don't cause any appreciable drain, AFAIK.

As desceribed above, the realities of the power supply limits for recivers mean that you'll get less power per channel as you increase the number of channels.

As I understand it the FTC rule is generally interpreted to require only testing 2 channels at a time. Tests that do that are "legal" and "proper" although they might not be as revealing as many of us would like. It would be contrary to good marketing to state multichannel output for receivers that don't perform well in that sphere. So, no, they'll never do it, but you can find professional reviews that test the amps' power output into different impedances (some can handle 4 ohms better than others) and with different numbers of channels connected (some keep a higher percentage of their two-channel output per channel when 5 or 7 channels are driven.

About diminishing returns. The basic unavoidable physical reality is that every time you double the power, you achieve a 3 dB gain in SPL (all other things being equal). Ten times the power yields a 10 dB increase. So if, like me, you happen to listen to movies largely in the -20dB range, you are using 1/100 of the power that would be neeeded for full reference level.

Which is to say tha while there are lots of reference level listeners on this board (more power to 'em sez me) for many of us, huge power never becomes a real necessity. Although a person MIGHT crank it to reference briefly a time or two just for fun. Jsut sayin'

If it was active biamping, you would gain a clear advantage, because, by splitting the signal into a signal containing the LF and HF portions of the signal, you would lower the voltage of both signals. Because power is voltage squared, this is a big gain.

With passive biamping, you don't reduce the voltage. Both low and high terminals put out the same voltage. This seems to reduce the chances of reducing clipping.

You don't really add power, because power is power supply limited, near as I can tell.

Even if there was an advantage, maybe 80% of the energy in the signal in is the LF portion of the signal depending on crossover (total wild azz guess here.) But you are not doubling power in any way.

As I say, and as I usually say, test it yourself. Speaker cable is still relatively cheap.

In a best case scenerio (passive biamping with seperate amps) You gain 3db. You would generally do better with a single, better amp.


On a 4-channel amp: the power per channel is affected by channels driven. Your results vary from harmful to none to (most likely) negligible, to +3 watts.

To state there is no possible benefit, is incorrect.

In the passive bi-amp situation, the load that each amp is presented with is reduced when compared to the total load on the amp in the single amp scenario. The inputs are paralleled, and that voltage is amplified, however it's where the current demands are different, that's where one attains the small benefit.

The load on the HF amp; it possesses a high impedance at low frequencies, so it is easier to drive. The amp stages are boosting voltage, then the current is determined by the load at the terminals, and in this case the current demands are very low because it's amplifying the upper portion, or the higher frequencies of the signal. Thus, not depleting the current reserves of the amp. High voltages can exist, without current flow, it's the high current flow that saps everything from the output stages, the supply rails, and all the way back to the wall voltage during peak current demands. In this scenario, using a separate amp for the high pass section, that huge current depletion doesn't occur.


Likewise, the amp supplying the low side of the crossover network, is presented with a high impedance above the pass band, so although the voltage amplification remains (parallel input), the current demands are reduced somewhat. Now obviously the dramatic lowering of current demands aren't the same in the LF amp as they are in the HF amp, but a reduction of the load is benefited by the amp, just a lesser degree.

Is it worth it, most likely not....Regardless, to state that there is no possible benefit associated with passive bi-amping, incorrect.

It's just not worth the effort in my opinion.



Good luck