AVS Forum banner
Status
Not open for further replies.

Learn WHY passive bi-amping isn't actually more powerful:

43K views 183 replies 21 participants last post by  markrubin 
#1 · (Edited)
If you have another 2 amp channels and mean passive aka fool's bi-amping, then no as well. This has been discussed to death. Use the search function.
I'm not so sure that would help the OP because just as many if not more people, including some with electrical engineering degrees, fall for the myth that passive bi-amping improves output power. It doesn't. And I don't mean "by much", I mean the traditional FTC 5-rule power rating spec of 20-20kHz, at X% distortion or less, continuous, into an X ohm load (often 8 or 4), X number of stated channels driven, does not increase by even a fraction of a dB!
---

Please keep an open mind and examine the evidence I will present for yourselves, everyone. It's not all that hard to understand with the following graphic I have made of an actual consumer speaker impedance load with and without the tweeter to woofer jumper bars (straps) removed (to allow a bi-amp connection). Let's look at a typical bass frequency which is easy to read on both of the charts I am about to produce, 200 Hz, found in nearly all normal music.

As we all know when passive bi-amping the two amps [the "bass" amp and the "treble" amp] in truth receive the exact same incoming full range signal. So people need to dispel this notion that the bass amp is less burdened because it doesn't "see" the full frequency range. Baloney. BOTH AMPS SEE THE FULL RANGE coming in. So in determining what an amp can do and how loud it can play before the onset of clipping we need to know two simple things:

A) The incoming signal content including its frequency, duration, and level [there's no change there]

and

B) The load the amp sees when reproducing that frequency. [OK, that one does change a bit but not in a significant way for the hardest part of the audible spectrum the amp has to drive, the low bass, as I will now demonstrate.]

Here's the animated GIF I made of a fairly typical 2-way, bi-amp capable speaker [a KEF Q100 IIRC] from another poster (AJ in Florida) . This measurement was made with good, but affordable equipment and there is a little bit of measurement slop (minor inconsistencies) which should be ignored [even how tightly you screw down your speaker wire posts, reading to reading, which alters the electrical connection's contact area, can make this sort of change] plus my Photoshop skills of combining the two images into a single, perfectly overlapped one which alternates as an animation is not so great, but I think people will get the picture.

The two alternating images show the impedance load change in ohms [0-100] on the vertical axis vs. frequency [20-20kHz] on the horizontal axis, measured at the woofer section's speaker terminals with, and then without, the jumper bars to the tweeter in place [i.e. the load the amp will see when using traditional mono-amping vs. the load when passive bi-amping]:

Notice the major changes are in the high frequencies but the low bass stays pretty consistent.

OK, that's what the load appears as to the amp under the two scenarios. Now let's use an Audio Precision analyzer [thanks to Audioholics for posting this detailed graph], to measure what a typical AVR amp channel, in this case from a Denon AVR-4310CI, can do at any given frequency in terms of maximum, continuous power output, measured in watts, in this case into the 4 ohm load the speaker poses in much of the bass range:


OK, now that way have the raw data in front of us, let's do some analysis!

Question 1: When the jumper bars to the tweeter are disconnected so we are just driving the woofer section from this amp by itself, what is the maximum continuous output level in watts this amp channel can produce at 200 Hz, into the speaker's 4 ohm load found at this frequency, before it will start to clip?

The Correct Answer: about 267 watts into the 4 ohm load

Question 2:
When the jumper bars to the tweeter are connected so we are driving both the woofer section and also the tweeter section, i.e. conventional mono-amping, what is the maximum continuous output level in watts this amp channel can produce at 200 Hz, into the speaker's 4 ohm load found at this frequency, before it will start clip?

The Correct Answer: also about 267 watts into 4 ohms!

So if our music contains the exceedingly common frequency of 200Hz, how many dB louder can the bi-amped speaker play this frequency before the onset of distortion compared to the same speaker reproducing the same music content but through mono-amping?

The Correct Answer: 0 dB louder, none, i.e. the bi-amped scenario will have clipping problems at the exact same point: that is to say it can not play normal, full range music any more loudly before it will distort (clip).

For those of you who might not know, bass is the hardest part of the frequency range for systems to reproduce, not the treble, which is why using weaker amps for the tweeters in active bi-amp setups can be a useful cost cutting move that usually won't compromise performance. But suddenly having huge amounts of reserve power for the treble, by using an equally powerful tweeter amp when bi-amping, doesn't help us for normal music reproduction because our bottleneck of how loud we can play before we start to clip is pretty much always due to the bass, such as the 200Hz example I just gave.

Passive bi-amping's claim to "improve maximum power output" is baloney, folks. You can neither measure a greater output [with normal full range music] nor hear it. People who claim otherwise are suffering from expectation bias, which is common to every single human on the planet, without exception, including myself.
 

Attachments

See less See more
3
#107 ·
Rod Elliott covered this topic nearly nineteen years ago:

Benefits of Bi-Amping (Not Quite Magic, But Close) - Part 1

Benefits of Bi-Amping (Not Quite Magic, But Close) - Part 2

Those two articles cover almost everything that has been discussed here (and elsewhere).

It's a good write-up with diagrams and pictures for those visual learners out there. Some of the visuals have been posted upthread, but everyone should read both articles to understand the context as well as gaining a greater understanding of the topic. Doing so will probably answer many questions and allay many doubts.

I read those articles and I implemented it on my system. I'm satisfied with it. Others here have done the same and are probably as equally satisfied with it.
 
#110 ·
Rod Elliott covered this topic nearly nineteen years ago:
Rod was covering active bi-amping, not passive bi-amping which is what this thread is about. I just borrowed his graphics earlier as they showed what I wanted to without having to do my own.
 
#115 · (Edited)
Passive bi-amping gets a giant boost in credibility because so many people, and consumer owners manuals, refer to it as "bi-amping" and when they consult a pro and say "Do you use bi-amping?" the pro often responds, "Yes, it is quite common and useful in pro settings despite the complexity." but what the consumer doesn't realize is the pro has never even heard of "passive bi-amping" and assumes the questioner meant active bi-amping, you know, using electronic crossovers and such, the legit kind of bi-amping.

Conflating the two purposefully is also a move used by deceitful salespeople. Watch out for it.

This is why it is important to always use the word passive or active in front of it, or better still "fool's bi-amping" if you fall in my camp and are referring to passive.
 
#119 ·
Jady, are clear on what is meant by passive biamping? There is no crossover before the amplifiers. Both HF and LF amps receive the same full range signal. The voltage peaks are clipped regardless of the output load connected or not connected. Current at 70Hz may not flow to the tweeter, but the HF content obviously will, and it is clipped all the same.
 
#124 ·
I have assumed we are all talking about separate power supplies as well.

And even in that case, the HF amp will still clip when driven with LF signal that requires higher voltage swing that it is capable of producing. It doesn't matter that the supply is independent, or that there is little to no LF current output.

The amplifier still clips the voltage peaks, which includes clipping (first) any HF waveforms present, as well as adding bad HF distortion products due to the clipping.

The only benefit is that the supply rails don't sag in the HF amp because of the LF current demands, but this is minimal. It may add 10% or whatever headroom to the HF amp, which didn't need it at all, as HF content is lower in demand than the LF content to begin with.

Total system output is still limited by the LF amp. Reduction in voltage sag in that amp is neglible as HF demands aren't that great to begin with.

End result is tiny fraction of a dB increased potential output from the system. Clipping point for BOTH AMPLIFIERS will have trivially changed.
 
#131 ·
As much as people complain about underrated avr power supplies, poor continuous or FTC output vs "rated" power output combined into all channels, it isn't typically an issue if sub duty current demands are removed. This is due to the high crest factor of real music content, requiring average power levels of perhaps 1/8 the peak levels. So long as sufficient capacitor reserves are present to handle the peaks, a weak (on paper) supply may be sufficient. Again, this is when freed from sub duties. While LF content can also have a decent crest factor, it isnt unusual to see passages of several seconds or more of sustained high signal levels. This sustained current demand can easily drain reserves and overburden a modest power supply. Doesn't matter if its just 10 seconds out of an hour... when that 10 seconds of bass comes around, you want to hear it without the system going to pot.
 
#134 · (Edited)
If you feed the amps a content with some clipped signals, well you feed the amps some clipped signals , and if loads demands it, they will strive to reproduce the input signal, as inputted, at the outputs, etc., this is completely obvious. If an amplifier isn't presented with a significant load at the frequencies at and around the clipped inputted signal, they won't acoustically materialize.

If clipping occurred within a discrete output channel, it is in fact isolated to that output. Again this is obvious.

What is all the attempts to pervert my clear context?:confused:

Adding more discrete channels of discrete amplification, will always result in sonic betterment within the same system. Always, that is unless, you don't know what you're doing, and as a result, make poor product selections and set things up incorrectly.

Why, because of improvements in amplification. The system will at a minimum, play cleaner, and therefore a little louder before THD, IMD, TIM, etc., become audible. That will increase the FTC power score, fact not fiction.

Subjective and objective reality!
 
#135 ·
It seems like this thread keeps bouncing around different situations. In passive bi-amping, the topic of this thread, the same signal is applied to woofer and tweeter amp channels. When one clips, both clip, due to the output exceeding the supply rails. It doesn't really matter that the load is lighter for one or the other, the voltage clips regardless. A lighter load may gain a tiny bit of headroom but insignificant in the real world. And for this particular case you could have perfect power supplies with zero IR drop anyplace else and, if one amp clips, the other still clips, since the voltage demands exceed the available voltage of the amplifier. The only solution is to buy a bigger amp.

Active bi- (or whatever) amping is a different situation. In that case you do relieve the treble amp of the bass signal (and vice versa) so, if the bass amp clips, the treble need not (unless you overdrive them both, natch). The advantage of independent power supplies is still a secondary issue unless they are really bad, and ditto crosstalk and so forth since the speakers, room, and ears largely dictate the sound we hear.

The IHF/AES (I forget and don't have the paper in front of me, my memory is it was in the AESJ) measured the peak-to-average level in music at around 17 dB, a power factor of 50. Some a little more, some a little less, but that is what was reported decades ago. I have read it can exceed 20 dB up to as much as 30 dB for movies, which sort of jives with the 85 dB reference and 105 dB (mains)/115 dB (LFE) levels for THX etc. Bottom line is the average power level is much, much lower than the loudest peaks.

I can hardly wait for page 6...
 
#141 ·
I now regret having used an multi-channel AVR (albeit a very capable one with a hefty singular power supply) in my opening example because it does technically introduce the potential for small changes based on how many channels are taxing that singular power supply at once, and this introduces a tangential issue that diverts us from the true main topic [20-20kHz maximum output capability] however my overall point remains exactly the same had I used monoblock amplifiers, each with their own, independent power supplies:

Using four identical monoblock amps in passive bi-amp configuration does not allow one to play music (a full range signal containing both treble and the much harder to reproduce bass range) any more loudly than using just two monoblock amplifiers in conventional mode, when looking at the entire 20-20kHz range seen in toto. This is because the first thing that will cause clipping, the bass content, will do so at the exact same level in the two scenarios. How do we know this? Because the load the amps see under these two conditions at this taxing 200/250 Hz bass note I'm using as an example does not change in the two scenarios:


If you can't play the bass any more loudly (and it is nearly always the limiting factor, not the treble) then what about the ability to play music in general? Obviously it can't increase either since the bass range is a subset of the music's range.
 
#142 · (Edited)
Here are the numbers using "monoblock" amps.

Note: In the following scenario you are buying four of these Denon AVRs and using just one channel in each of them, as if they were monoblock amps, because you are afraid the maximum output capability may be diminished if asked to drive any more than one channel at a time. [A silly thing to do if you ask me but work with me on this.]

Maximum continuous clean power output into a non-inductive 4-ohm load at 200/250 Hz, for conventional amp configuration: 267 continuous watts

Maximum continuous clean power output into a non-inductive 4-ohm load at 200/250 Hz for just the speaker's LF section (i.e. now using passive bi-amplified configuration) by using double the number of these "monoblock" amps: 267 continuous watts

Increase in maximum, unclipped sound level output for music, i.e. looking at the full range 20-20kHz spectrum: NONE


If you can't play 200/250Hz any more loudly then you also can't play the full range spectrum 20-20kHz any more loudly either.

It doesn't matter if you are using state of the art monoblock amps or an all in one AVR: passive bi-amping will not increase the level you can play normal music at yet it is doubly expensive. This is why it is called "fool's bi-amping".

P.S. I posted two examples of speakers which indeed pose a 4 ohm load with a zero degree phase angle at 200/250 Hz earlier.
 
#145 ·
In the real world I would expect a few volts from IR drop and supply sag, maybe a little more, so say 10% as mentioned earlier. That's about 1 dB. And only when everything is at its loudest limits. And so forth and so on...

This thread has gotten so convoluted I am losing track and have other things to deal with, enjoy guys. - Don
 
#146 · (Edited)
In the real world I would expect a few volts from IR drop and supply sag, maybe a little more, so say 10% as mentioned earlier. That's about 1 dB. And only when everything is at its loudest limits. And so forth and so on...
I disagree. An amps's continuous output capability at a specific lone frequency is dictated by the frequency/level of the signal coming in and the speaker load the amp sees at that frequency. If you don't change either of those two things then its maximum output capability at that frequency doesn't change. There is no increase in "headroom" [I prefer the term reserve power] at that frequency.

If there is no increase in headroom at that frequency then when looking at the entire full range 20-20kHz signal the "headroom" can't go up either since 200Hz (or 250Hz) is a subset of that range. It is one of the frequencies which would need to increase in order to say that the overall 20-20kHz reserve power increases, and it hasn't.
 
#148 · (Edited)
I was speaking of the overall system...
I was speaking of a specific, continuous, bass frequency, 200/250Hz. Did I not make that clear?

Do tell, in my example above at 200/250Hz with a 4-ohm load and zero degree phase angle, how much would that "tiny bit" of headroom be exactly?:)
 
#150 ·
What? The crossover changes the load over frequency. I was not speaking of your example but of the actual usage. The tweeter amp will not "see" the woofer, and the woofer amp will not "see" the tweeter. That is what matters for bi-amping, passive or active. The load current changes and that changes the output, however slightly. That is a real and measurable effect. Looking at one specific frequency is not really relevant and does not cover the entire system over all frequencies.

At any given frequency the load the two amps see depends upon the crossover. The amps will clip at roughly the same point irregardless of the frequency assuming voltage limits are reached. There will be a small difference due to the difference in loads since current output differs and that changes the output voltage slightly. As has been said over and over in this thread...
 
#151 · (Edited)
. Looking at one specific frequency is not really relevant and does not cover the entire system over all frequencies....
Yes it is relevant if the goal is to increase full range output, from 20-20kHz, as would be found by buying a more powerful amp in substitution of the first, underpowered one. [The correct way to approach an underpowered amp situation instead of this silly passive bi-amping approach so many fall for.]

I think you might have missed this underlined and bold text part from my opening post:
. . .the myth that passive bi-amping improves output power. It doesn't. And I don't mean "by much", I mean the traditional FTC 5-rule power rating spec of 20-20kHz, at X% distortion or less, continuous, into an X ohm load (often 8 or 4), X number of stated channels driven, does not increase by even a fraction of a dB!
.
Sorry everyone, I couldn't fit all of these many caveats into the title of the thread: They don't provide enough room.

Yes, for people that listen to music without bass content (such as this exceedingly common 200Hz/250Hz frequency I've selected) bi-amp systems can indeed play a tiny bit louder, at some very limited frequencies, under the right conditions (not that I think it would be noticeable in any real world use even after cherry picking a specific song to try to optimize the audibility). I've never met any such individual that listens to only music without bass content however. :)
 
#154 ·
Hevi, any problem with me saying: "Passive bi-amping will not play typical, full range music any more loudly before the onset of clipping, not even by a fraction of a dB." ?

Or are there still problems with that version for you?
 
#157 · (Edited)
Yikes.

If I was in the other camp and wanted to prove passive bi-amping, um, "matters" (at least for some frequencies, which is not the premise of my thread), I'd wear ear plugs and crank the singular amp to the point of (just barely on rare peaks) clipping, using a speaker with a specific crossover frequency, trying to play solo piccolo music (which has no bass), and keeping my fingers crossed there would be a note here or there that didn't clip the passive bi-amps in the test with their ~1dB or so advantage at some very limited frequencies:


I would even allow this song if I was doing a blind test challenge as long as the listener had to set their volume knob by ear alone, no instrumentation allowed [as a normal consumer does, of course]. :)
 
#160 ·
Either. It doesn't play the music any more loudly to the ear/brain nor at a higher level to test gear, not even by .1 dB (for the full spectrum that is).
 
#171 ·
Jady Jenkins;51397745[U said:
].1 dB[/U] is the accepted threshold of human perception of deviations in loudness.

.1dB doesn't match any sources online that a quick Google search revealed. Please provide references supporting .1dB.

Per a Wikipedia article on loudness, "A complete model of the perception of loudness will include the integration of SPL by frequency." Which makes the question more interesting, but no sources claimed that .1dB was the threshold.

Here is a good piece on hearing and loudness from the Physics Department at the University of Illinois:

https://courses.physics.illinois.edu/phys406/lecture_notes/p406pom_lecture_notes/p406pom_lect5.pdf
 
#174 · (Edited)
You have to pay for the AES papers. I am going to engage in idle speculation based upon my memory of testing performed long ago. In BT/DBT, 0.1 dB is the accepted threshold for level matching, and has been for a long time (I read the papers and used that threshold back in the 1970's and 1980's when I was doing that sort of thing). That is NOT the just-detectable level for normal listening; that is more like 1 dB or so, and may be higher depending upon the source (music etc.) In other words, when switching between two components, or (as in the tests) the same component with an adjustable pad (attenuator) on one side, 0.1 dB was not detectable by the majority of listeners. 1 dB was readily detected, and the slightly louder signal was determined to be "better" by listeners even when it was the same source. Most people fell off around the 0.5 to 0.3 dB level and 0.1 dB was essentially undetectable by all in the study. That became the accepted DBT level matching requirement back then and AFAIK has not changed.

To reiterate: One must be careful to note that the difference (deviation) in level detectable in a test scenario switching between two sources is not the level detectable if you just e.g. walk out of the room, have somebody bump the volume and walk back in and try to see if it changed. Or stay in the room and have them do it when you aren't looking. I can sometimes tell ~1 dB with test tones or pink noise, but often (usually) not with music or a movie playing. Bump it 3 dB and it's obvious. Two different test conditions.

FWIWFM - Don

Edit: In the context of this thread, I would not expect the difference to be noticeable, especially since it would occur only on the loudest peaks. I can't imagine you'd hear anything unless something was really strange/wrong.
 
#175 ·
That is NOT the just-detectable level for normal listening; that is more like 1 dB or so, and may be higher depending upon the source (music etc.)
+1. The decibel didn't just land on our collective doorstep. Originally called the Transmission Unit, it was literally invented by Bell Telephone, with 1TU/1dB quantified as the smallest attenuation detectable to the average listener. Different sources, frequency content and variations in the sensitivity of various listeners will cause the figure to shift a bit higher or lower, but that's where the term 'average' comes into play.
 
  • Like
Reactions: fill35U
#182 ·
A bit of googling led me to this thread after reading Bruce Hofer’s “Notes from the Test Bench” in the latest Audio Precision Tech Support Tips newsletter. In my opinion, based on the above and other threads he has contributed to, Mr. “Jady Jenkins” is clearly a plagiarist as well as a con artist and a flim flam man. He needs to be banned from the forum, not only to avoid legal liability but also to avoid the continuing spread of misinformation and outright pseudoscience.

-Mark S.
 
Status
Not open for further replies.
You have insufficient privileges to reply here.
Top