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Bi-amping 101

Thanks for the very informative posts Don. Regarding your last paragraph in the first post:

So, there are some potential benefits using passive bi-amping, but I suspect they are inaudible (I have not tried passive bi-amping so cannot say). And a lot of drawbacks. The major benefit is mostly mental, IMO; users can now use their “extra” amp channels. Whether this benefits anyone other than the electric company I cannot say, but I suspect not… It does eliminate signal “bleed” through the crossover, so if the amplifier has high output impedance (like a tube amp) then passive bi-amping could benefit. If the amplifier is near clipping, or current-limited, then the reduction in current by a passive bi-amp scheme could reduce distortion. Again I suspect that is an insignificant improvement.

I can think of one good case to passive bi-amp. Since the power needed for the highs is nowhere near the power needed for the lows, it's very feasible to stick with a class A/B amp for the lows and upgrade to a class A for the highs. That is exactly what I'm planning on doing with my NHT 3.3 speakers. These speakers are very, very power hungry because of the 12" subs they house. Plus they only have a sensitivity of 87db. Currently they're being driven by an ATI-1502, which is a beast of an amplifier in my opinion and is doing a great job, but I suspect most of my listening is at class B level.

Prior to considering bi-amping, I had been looking into doing a pair DIY Pass Aleph 2, but I can't justify a 600W heater in my room. So I am trying to figure out if I can bias the Aleph 2 differently so that I end up with 50W/amp and a lot less heat, or if I go with another class A amp that'll give me 50W/channel. I think the end result may be worth the effort.
 
I can think of one good case to passive bi-amp. Since the power needed for the highs is nowhere near the power needed for the lows, it's very feasible to stick with a class A/B amp for the lows and upgrade to a class A for the highs.
That is running under the assumption that a Class A amplifier will produce better sound quality than a A/B amp, or a modern high efficiency Class D amp. These are all misconceptions, a properly designed amp in any of the classes can be fully transparent to the source with the only basic difference being the amount of AC power they use..
 
Thanks for the very informative posts Don. Regarding your last paragraph in the first post:



I can think of one good case to passive bi-amp. Since the power needed for the highs is nowhere near the power needed for the lows, it's very feasible to stick with a class A/B amp for the lows and upgrade to a class A for the highs. That is exactly what I'm planning on doing with my NHT 3.3 speakers. These speakers are very, very power hungry because of the 12" subs they house. Plus they only have a sensitivity of 87db. Currently they're being driven by an ATI-1502, which is a beast of an amplifier in my opinion and is doing a great job, but I suspect most of my listening is at class B level.

Prior to considering bi-amping, I had been looking into doing a pair DIY Pass Aleph 2, but I can't justify a 600W heater in my room. So I am trying to figure out if I can bias the Aleph 2 differently so that I end up with 50W/amp and a lot less heat, or if I go with another class A amp that'll give me 50W/channel. I think the end result may be worth the effort.

No, because with passive bi-amping, the output voltage available from each amplifier has to be identical, as does the gain. Each amplifier gets the same input voltage, so the output voltage must be the same, or the lower output one will clip first. One way to destroy tweeters.

S.
 
Thanks for the very informative posts Don. Regarding your last paragraph in the first post:



I can think of one good case to passive bi-amp. Since the power needed for the highs is nowhere near the power needed for the lows, it's very feasible to stick with a class A/B amp for the lows and upgrade to a class A for the highs. That is exactly what I'm planning on doing with my NHT 3.3 speakers. These speakers are very, very power hungry because of the 12" subs they house. Plus they only have a sensitivity of 87db. Currently they're being driven by an ATI-1502, which is a beast of an amplifier in my opinion and is doing a great job, but I suspect most of my listening is at class B level.

Prior to considering bi-amping, I had been looking into doing a pair DIY Pass Aleph 2, but I can't justify a 600W heater in my room. So I am trying to figure out if I can bias the Aleph 2 differently so that I end up with 50W/amp and a lot less heat, or if I go with another class A amp that'll give me 50W/channel. I think the end result may be worth the effort.

Not with passive bi-amping. See posts just above, and look at the article again. It does not matter that the treble amp actually needs (delivers) much less power; the problem is that the voltage swing is the same for both amplifiers, so you cannot use a low-powered treble amp in a passively-bi-amped system. That is a major drawback IMO. You need a crossover that only sends the desired frequency range to the amp.
 
No, because with passive bi-amping, the output voltage available from each amplifier has to be identical, as does the gain. Each amplifier gets the same input voltage, so the output voltage must be the same, or the lower output one will clip first. One way to destroy tweeters.

S.
That's basically just matching the levels between the two amps, and that's something of concern for sure, but easily overcome with separate passive volume controls for each amp. The NHT 3.3s are very finicky in regard to their placement and room response to bass, so it wouldn't be out of the realm of possibilities to attenuate the lower signal more if the physical placement of the speakers don't get the desired bass output.
 
That's basically just matching the levels between the two amps, and that's something of concern for sure, but easily overcome with separate passive volume controls for each amp. The NHT 3.3s are very finicky in regard to their placement and room response to bass, so it wouldn't be out of the realm of possibilities to attenuate the lower signal more if the physical placement of the speakers don't get the desired bass output.
The passive crossovers in the speaker are designed around the assumption that the same signal voltage will be fed to the input of the HF and LF crossover sections, so altering the gain of one of the amps will upset the balance that the speaker manufacturer designed in. It is far, far, far, far better to do the bandpass splitting before the respective amplifiers (aka active crossovers) and eliminating passive crossovers from the speakers entirely.

Passive crossovers somehow became the norm in the early days of HiFi and in my opinion are one of the stupidest possible ways to approach the problem. I'm frankly surprised that 'high end' audio never latched onto active crossovers since it would mean them selling twice as many amplifiers, speaker cables and interconnects - in addition to an insanely overpriced crossover box. Morons. :facepalm:
 
Not with passive bi-amping. See posts just above, and look at the article again. It does not matter that the treble amp actually needs (delivers) much less power; the problem is that the voltage swing is the same for both amplifiers, so you cannot use a low-powered treble amp in a passively-bi-amped system. That is a major drawback IMO. You need a crossover that only sends the desired frequency range to the amp.
I fail to see that being an issue. When you decouple the highs and lows on the 3.3 by removing the the plate that joins them at the connector on the back of the speaker, you're essentially left with 2 satellites and 2 subwoofers, each retaining their own passive crossover. Powering them would be no different than powering satellites with one amp and having a powered subwoofer. The only issue I see beside the one sergeauckland mentioned (signal matching) would be possible phase difference between the bass and treble, but reversing the cables on either would resolve that issue. The following is from the manual for the NHT 3.3:

To biamplify the 3.3, you will need two amplifiers: a main amplifier for
the upper range drivers (top pair of binding posts) and a second amplifier for the subwoofers (bottom pair). The main amplifier can be a
stand-alone stereo amplifier, dual monoblock amplifiers, or the amplifier section of an integrated amplifier or audio/video receiver that features "pre-out" and "main-in" jacks. The second amplifier may be a full
range amplifier connected parallel to the main amplifier ...
 
I fail to see that being an issue. When you decouple the highs and lows on the 3.3 by removing the the plate that joins them at the connector on the back of the speaker, you're essentially left with 2 satellites and 2 subwoofers, each retaining their own passive crossover. Powering them would be no different than powering satellites with one amp and having a powered subwoofer. The only issue I see beside the one sergeauckland mentioned (signal matching) would be possible phase difference between the bass and treble, but reversing the cables on either would resolve that issue. The following is from the manual for the NHT 3.3:

Please look at the first post again and see the pictures and explanation under "Passive Bi-amplification". The signal to the sub and the signals to the speaker terminals are not the same. The difference is that there is a crossover inside the AVR before the signal is sent to the subwoofer's amplifier so the subwoofer only sees a low-pass filtered signal (only the lows are sent to the sub). With passive bi-amping as implemented by an AVR there is no crossover to the main speaker channels' high and low sections. The same signal is sent to both (high and low) amplifiers, and that signal includes everything (bass and treble). Power is the product of voltage and current. Even if no power were delivered to the tweeter terminals, the voltage is exactly the same as sent to the bass terminals, and thus voltage clipping will occur at the same point no matter how much (or little) power is delivered. That is the fundamental problem with passive bi-amping; both amps provide the same (voltage) signal, and thus both will clip at the same point (voltage), even though they are not delivering the same power.

If you have or add a crossover before the power amplifiers to separate the high and low signals to the amp then you have conventional ("active') bi-amping and the amps need not be the same. That is what is shown in the "Conventional Bi-amping" section of the article.
 
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Passive crossovers somehow became the norm in the early days of HiFi and in my opinion are one of the stupidest possible ways to approach the problem. I'm frankly surprised that 'high end' audio never latched onto active crossovers since it would mean them selling twice as many amplifiers, speaker cables and interconnects - in addition to an insanely overpriced crossover box. Morons. :facepalm:
There are exceptions:
  • In the 1990s Linn offered the Kaber in passive and active version, with the idea to activate the passive version later by getting more power amps with additional matching crossover modules. The Isobarik speaker may have been always active.
  • Backes & Müller produces active speakers since more than 40 years now. Prices are highend.
 
There are exceptions:
  • In the 1990s Linn offered the Kaber in passive and active version, with the idea to activate the passive version later by getting more power amps with additional matching crossover modules. The Isobarik speaker may have been always active.
  • Backes & Müller produces active speakers since more than 40 years now. Prices are highend.
I was referring to the conventional idea of 'active crossovers' and in that context the crossover is an outboard box inserted before the power amplifiers. Of course active speakers use active crossovers (I sure as hell hope so!), but the wider acceptance of active speakers is a more recent development although I'm sure somebody could be found who did this much earlier - I had Mackie active speakers in the late 1990s. As early as the 1950s, passive 'active crossovers' were being inserted before the power amplifiers which were then directly connected to the speaker drivers, so this is not a new concept. This was limited to hobbyists, unfortunately, and did not become the norm.
 
Passive bi-amping, when I first read about it a decade or so ago, confused the heck out of me because what people were telling me made no sense. To me, passive bi-amping meant a passive line-level crossover (RC and/or RLC) before the power amps. Active bi-amping meant the crossover used active devices (tubes or transistors). When I was finally set straight, it still made no sense, as there was no crossover as I understood it save the one already in the speakers, and I could not understand the benefit. The only benefits I could see were a reduction in current-related and thermal distortion, via isolating back-emf and intermodulating speaker (driver) currents by separating the amplifiers, which meant the single amplifier and wiring was not up to the task. The benefits, then and now, seemed so slight as to not be worth the effort and additional wasted power.
 
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Passive bi-amping, when I first read about it a decade or so ago, confused the heck out of me because what people were telling me made no sense. To me, passive bi-amping meant a passive line-level crossover (RC and/or RLC) before the power amps. Active bi-amping meant the crossover used active devices (tubes or transistors). When I was finally set straight, it still made no sense, as there was no crossover as I understood it save the one already in the speakers, and I could not understand the benefit. The only benefits I could see were a reduction in current-related and thermal distortion, via isolating back-emf and intermodulating speaker (driver) currents by separating the amplifiers, which meant the single amplifier and wiring was not up to the task. The benefits, then and now, seemed so slight as to not be worth the effort and additional wasted power.

I had 4 mono blocks., each 2 just behind the speakers for years and switched back to a single one per speaker.
Cannot differ the pleasure other than my brain wishes me to believe in
 
I had 4 mono blocks., each 2 just behind the speakers for years and switched back to a single one per speaker.
Cannot differ the pleasure other than my brain wishes me to believe in
And the savings on your electric bill. ;)
 
So, there are some potential benefits using passive bi-amping, but I suspect they are inaudible (I have not tried passive bi-amping so cannot say). And a lot of drawbacks. The major benefit is mostly mental, IMO; users can now use their “extra” amp channels. Whether this benefits anyone other than the electric company I cannot say, but I suspect not… It does eliminate signal “bleed” through the crossover, so if the amplifier has high output impedance (like a tube amp) then passive bi-amping could benefit. If the amplifier is near clipping, or current-limited, then the reduction in current by a passive bi-amp scheme could reduce distortion. Again I suspect that is an insignificant improvement.

FWIWFM - Don

I really like the way this material is presented. A bit of a crash course but not entirely impenetrable for a layman/hobbyist such as myself. It seems like a wasted opportunity that this thread isn't more active, but I'm happy to take advantage.

So, given the best way to find the right answer to a Q in the 21st century is to post the wrong answer on the internet, let me know if my proposed bi-amp scenario (based on equipment I own) accurately reflects your recommendation?

Monolith 11ch amp (3x200W, 8x100W), Kef R3's, Denon 8500.

Kef lists the crossover points at 400Hz and 2.9KHz, but both Amir and Erin's reviews show response far exceeding that. How much is usable/relevant during normal operation, I'm not certain.

To give myself wiggle room, I'd get a 500Hz LPF,a 500Hz HPF (both @12dB/octave, HarrisonLabs) and a Y-cable for each L/R/C channel. The LPF go in-line toward the 200W channels and the HPF on the 100W channels. I would also use a standard 80Hz crossover In The Denon.

Do I have that right?

if so, lingering questions in my mind:

will the Denon pre-outs push more V to achieve same level as if I did NOT use y-adapter?

typically it's accepted that doubling W = 3dB of usable volume. I'm only increasing by 50%, but I'm "steering" it toward mid/tweeter so would I still only expect 1.5dB of gain altogether?

the most common argument against AVR bi-amping is "there's only one power supply.". You've gone into more detail here which is helpful, but in my scenario the Monolith has 2 transformers. Does that matter at all? Would I be better served by trying to split the load across the 2 transformers?

ARe the 500Hz FMODs the right choice given there's a lot more extension from the individual drivers?

thank you again for the write-up
 
I really like the way this material is presented. A bit of a crash course but not entirely impenetrable for a layman/hobbyist such as myself. It seems like a wasted opportunity that this thread isn't more active, but I'm happy to take advantage.

So, given the best way to find the right answer to a Q in the 21st century is to post the wrong answer on the internet, let me know if my proposed bi-amp scenario (based on equipment I own) accurately reflects your recommendation?

Monolith 11ch amp (3x200W, 8x100W), Kef R3's, Denon 8500.

Kef lists the crossover points at 400Hz and 2.9KHz, but both Amir and Erin's reviews show response far exceeding that. How much is usable/relevant during normal operation, I'm not certain.

To give myself wiggle room, I'd get a 500Hz LPF,a 500Hz HPF (both @12dB/octave, HarrisonLabs) and a Y-cable for each L/R/C channel. The LPF go in-line toward the 200W channels and the HPF on the 100W channels. I would also use a standard 80Hz crossover In The Denon.

Do I have that right?

if so, lingering questions in my mind:

will the Denon pre-outs push more V to achieve same level as if I did NOT use y-adapter?

typically it's accepted that doubling W = 3dB of usable volume. I'm only increasing by 50%, but I'm "steering" it toward mid/tweeter so would I still only expect 1.5dB of gain altogether?

the most common argument against AVR bi-amping is "there's only one power supply.". You've gone into more detail here which is helpful, but in my scenario the Monolith has 2 transformers. Does that matter at all? Would I be better served by trying to split the load across the 2 transformers?

ARe the 500Hz FMODs the right choice given there's a lot more extension from the individual drivers?

thank you again for the write-up

Thanks.

A lot to cover... I suggest opening a thread to deal with your specific situation. The result of this whole effort would be to gain a little headroom in each of the amplifier channels, and to isolate the high and low sections of your speakers. Whether that provides any audible benefits I could not say, and of course the passive crossovers within your speakers would remain with their impact on performance (good or bad). From here on the answers are more guesses about your situation since there are too many unknowns about the various parameters (e.g. all the component and speaker impedances, how loudly you listen, room effects, etc.) Don't bother trying to find all of that as this is not meant to be a tutorial for preferences. I personally would not bother to bi-amp in this situation as it seems like a lot of effort for something unlikely to provide audible improvement, at least in a blind test.

Short answers: The connections seem OK and 500 Hz is a reasonable choice for the filter corners given your experiment. The Denon's preamp outputs will probably be unaffected by the splitter and I would not be concerned. The maximum power gain available in dB will be 10*log10(Pamp/Pavr) for you amps and AVR, and different for each amp since they have different power ratings. I doubt the two transformers is a significant factor. The additional filters will change the effective crossover response and you must use the AVR's room correction program to compensate, if it can, otherwise you will likely have severe frequency response problems.

IME most people who try even passive bi-amping report improvement, and most (perhaps all) would be unable to prove those improvements in a controlled blind listening test. The theoretical benefits are small but as always overridden by our innate bias (something we cannot do anything about).

A 500 Hz crossover (HPF+LPF) is barely above the speaker's crossover so will contribute significantly to the response, both amplitude and phase. Hopefully the Denon's room correction program will compensate for that so the speaker still sounds good in the end. It is probably as good a choice as any. True bi-amping eliminates the internal speaker crossovers but that is not practical for most of us, and more pain than it is worth for a home system.

One thing to consider is how loud each frequency band may be and how much power each will require, bearing in mind you are still using a sub (inferred from your 80 Hz crossover for LFE and bass management). 500 Hz is still in the midrange, so chances are the LF amp will benefit from greater power than the HF amp, though the HF amp covers a much wider frequency range.

How much the Y-adapter, and filters, decrease the Denon's output depends upon its preamp output impedance, the impedance of the filters, and the input input impedance of the amplifier. The usual assumption is that the preamp outs are low-impedance and amp inputs high-impedance so a Y splitter does not significantly decrease the signal level. I don't know the impedances of any of those components but suspect the answer is that the Y splitter does not change things enough to worry about, and in any event your Denon+amp combination should be able to nudge the volume a hair to compensate.

Doubling the power is a 3 dB increase and corresponding increase in SPL. You are offloading the power demands from the AVR so it should run cooler and draw less power from the wall outlet, and perhaps the external amplifier provides other benefits like lower output impedance and the ability to handle longer peak (transient) demands due to more robust power supply and output stages. Whether or not the extra power improves performance I could not say (too many unknown variables); perception bias says you will hear an improvement even if it is not "real".

The power supply is one of many factors, and whether a single transformer or multiple transformers is the better choice depends upon their capability. Personally I wouldn't worry about it in this scenario. The overall power supply capacity is probably much greater for the external amp.

HTH - Don
 
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Sorry Don. I don't think @tifune's idea is a good one.

We need to careful about correct summing at the cross-over region. Linkwitz-Riley was invented for that reason. Below is my simple simulation of the (electrical responses of the) filters.

The top graphs are the responses using the standard LR4 with cross-over at 400 Hz. The bottom graphs are the responses when both the low-pass and high-pass filters are cascaded with 2nd order Butterworth filters with corner frequencies at 500 Hz.

The phase mismatch from adding the Butterworth filters resulted in a deep suck out.

filters.PNG
 
This article provides a quick look at how bi-amplification works. The conventional scheme is to split the signal into two frequency bands before the power amplifiers that drive the speakers. The bass (LF) amp sees only LF signals and drives only the LF driver (woofer). The treble (HF) amp sees only HF signals and drives only the HF driver. This reduces the signal for each amplifier, improving headroom and potentially allowing amplifiers to be chosen for their target frequency range without the burden of supplying the full-range audio signal.

Here is a regular single-amplifier system and passive speaker crossover. The crossover comprises a low-pass filter (LPF) for the woofer and high-pass filter (HPF) for the tweeter. The crossover is inside the speaker; the dashed outline shows there can be a single or split (high/low) set of speaker input terminals, but the connection is the same as far as the amplifier is concerned.

View attachment 126273

For this experiment both crossovers are second-order (12 dB/octave) Linkwitz-Riley designs and the same whether passive or active. The crossover frequency is 1 kHz. The speakers are represented by 8-ohm resistors in this simplified analysis. This is the transfer function (frequency response) of the crossover:

View attachment 126274

For analysis I am using two signals at 300 Hz (LF) and 3 kHz (HF) which on a log scale sit on either side of the crossover:

View attachment 126275

The output voltage from the amplifier includes both signals, of course:

View attachment 126276

You can see how the low-frequency signal and high-frequency combine to create the modulated output signal. Since both signals have the same amplitude, the combined signal is twice the level of the individual signals. While this makes it easy to see, note in practice HF signals are typically much lower in amplitude than the LF signals (see Equal Loudness Curves in Wikipedia or wherever).

The relative voltage, current, and power is in the table below. The numbers are low since I am using a small signal; everything is relative. The combined RMS voltage for this case is simply sqrt(LF^2 + HF^2). The power is the average power, the product of RMS voltage and current (RMS power is a meaningless term though often used mistakenly).

View attachment 126281

Now repeat the experiment, but place a line-level crossover before the power amps. The crossover is typically active but could be passive; the usurpation of the term “passive bi-amplification” the way AVR marketing defines it is an on-going source of confusion.

View attachment 126282

There are now two amplifiers but each only handles a portion of the signal divided into two (low and high) frequency bands. Since the crossover is not ideal, that is does not drop instantly to zero on either side of the crossover frequency, a little bit of the HF signal is still seen in the LF output and vice-versa. It is a little hard to see in the time-domain plot, but the frequency-domain plots clearly show this.

View attachment 126283
View attachment 126284
View attachment 126285

Now we can create a new table showing the parameters of each amplifier in the bi-amplified case:

View attachment 126286

The voltages are similar to the previous table except now they are at the output of the amplifiers and, with the filters, the RMS value is a little lower. The RMS current is reduced more significantly since the load is split between the two amplifiers, and power is also significantly reduced (by about 60%). Note power is not a linear function, but rather the product of voltage and current (a nonlinear function), so splitting the signal this way does not reduce the power by exactly one-half (it is better than that). You could use two amplifiers each one-half the rated power if the single amplifier and deliver the same power to the speaker. The amplifiers also directly drive the speakers so there is no crossover loss, and no chance of the signal from one speaker modulating the other through the crossover circuit, potentially reducing distortion.

Now consider “passive” bi-amping as implanted by most AVRs and shown below. In this scenario both amplifiers are driven by the same signal, so their outputs have exactly the same voltage. There is no benefit in voltage and, since most amplifiers are essentially voltage-mode designs, no increase in system headroom. Current is reduced from each amplifier, since the passive crossover in the speaker “blocks” current that is out of band, and there is a corresponding reduction in power (not as much as when an active crossover is used, however).

View attachment 126287

The voltage output of each amplifier is the same:

View attachment 126288

The current is lower since the load seen by each amplifier is only part of the speaker. The LF amp has almost no HF current in its output, and similarly the HF amp has little LF current, even though the voltage is the same.

View attachment 126289

Here is the parameter table for the “passive” bi-amping case:

View attachment 126290

And a comparison of all three examples:

View attachment 126291

A ratio less than one represents an advantage (decrease) in the parameter the amplifier is delivering to the speaker. In a conventional active bi-amp system, each amplifier delivers only 65% of the voltage and current and only 42% the power of a single amplifier providing the same signal to the speakers. A “passive” bi-amp system the way an AVR does it has no voltage benefit but does require less current from each amplifier, resulting in the same 65% reduction in current, but due to the higher voltage must deliver 65% of the power. A reduction, true, but less dramatic than for an active bi-amp system, and since there is no reduction in the voltage that must be delivered there is no (voltage) headroom advantage. Where an active system would allow you to replace a 200 W amplifier with two 100 W amplifiers, the passive scheme requires two 200 W amplifiers to maintain the voltage headroom required.

One other significant consideration is how we hear as reflected in the equal-loudness curves (see e.g. https://en.wikipedia.org/wiki/Equal-loudness_contour). It take much higher levels of low-frequency signals to sound as loud as midrange signals. At 80 dB SPL, a 100 Hz signal must be about 10 dB louder (10x the power) to sound as loud as a 1 kHz signal, and must be about 20 dB louder (100x the power) by around 60 Hz. That is one reason bi-amplified systems (of the non-passive kind) often use a much larger bass amplifier than for the tweeter.

A few key points about “passive” bi-amping, from the original version of this roughly ten years ago:
  1. Each amplifier requires the same voltage output as a single amplifier since they each have the same signal. There is no voltage headroom benefit.
  2. Because the speaker load is essentially an “open” in the unused frequency band, less current output is required from each amp.
  3. There is no net system power increase at the speakers assuming the amps have the same voltage rails (e.g. inside an AVR or multichannel amplifier with the same power voltage rails to all amps). If you had a 100 W amp before, and bi-amp with two 100 W amplifiers, passive bi-amping does not give you 200 W to the speaker. You have split the load into two frequency bands, but the maximum power is the same to the speaker. That is, 100 W to the lows and 100 W to the highs is the same as having a 100 W amp that covers the entire frequency range. It is not the same as driving the speaker with a 200 W amplifier; to increase the power, you need to increase the voltage rails. There is not an effective increase in power headroom as there is for an active approach.
  4. In fact, there is more power lost, since the amps are not 100% efficient. That is, it actually take more energy from the power supply to passively bi-amp than if you used a single amp. This is also true for active bi-amping, but in that case we can choose lower-power amps for the highs (which rarely need the same power as the lows) and realize net power savings. That does not happen with (typical) passive bi-amping.
  5. There is no damping factor improvement over a single amp since the speaker crossovers are still in-circuit. One of the benefits of active bi-amping is direct connection from amp to driver, providing better driver control; this is not true in passive bi-amping.
  6. There is no longer electrical interaction among drivers with passive (or active) bi-amping. (There may still be mechanical coupling if the drivers are not isolated from each other.) That is, if the woofer starts to distort the input signal through electromechanical forces, it no longer modulates the HF amp’s output. One plus for bi-amping, active or passive.
  7. If the amps share a power supply, as do most AVRs and many (most?) multichannel amps, then modulation between high and low amps can still occur through the power supply. This can also happen with active bi-amping, although separate amps are the norm in the pro world. At least when I have done it…
  8. There may be some distortion reduction since power output is lessened in the amps. I suspect this is not significant, but it should happen due to the lower current draw. The catch is that the voltage swing of each amp is unchanged, so any distortion related to voltage swing is not changed. Only distortion components depending on output current may be reduced. That is design-dependent, but since most amps are primarily voltage-mode amps, I suspect any distortion reduction is small.
  9. You have two amps now so presumably noise is a little higher since you have two uncorrelated noise sources. At the speaker outputs I suspect it’s a wash since only a reduced frequency band gets through the drivers to hear.
  10. Thermally it is a loss since no amp is 100% efficient. There is always a little “waste” power that gets turned into heat, both standing bias current (especially if not class D amps) and losses through the components in the amp. Thus passive bi-amping will cause your AVR/amp to run hotter than if using a single amp (assuming unused channels). It is worth noting that amplifiers are typically most efficient at maximum output; the HF amp is probably loafing most of the time and thus wasting power and generating heat.
So, there are some potential benefits using passive bi-amping, but I suspect they are inaudible (I have not tried passive bi-amping so cannot say). And a lot of drawbacks. The major benefit is mostly mental, IMO; users can now use their “extra” amp channels. Whether this benefits anyone other than the electric company I cannot say, but I suspect not… It does eliminate signal “bleed” through the crossover, so if the amplifier has high output impedance (like a tube amp) then passive bi-amping could benefit. If the amplifier is near clipping, or current-limited, then the reduction in current by a passive bi-amp scheme could reduce distortion. Again I suspect that is an insignificant improvement.

FWIWFM - Don
Thank you Don,

Is DENON addressing your point (#6) in their user manual?
1630077384515.png
 
Sorry Don. I don't think @tifune's idea is a good one.

We need to careful about correct summing at the cross-over region. Linkwitz-Riley was invented for that reason. Below is my simple simulation of the (electrical responses of the) filters.

The top graphs are the responses using the standard LR4 with cross-over at 400 Hz. The bottom graphs are the responses when both the low-pass and high-pass filters are cascaded with 2nd order Butterworth filters with corner frequencies at 500 Hz.

The phase mismatch from adding the Butterworth filters resulted in a deep suck out.

View attachment 149891

I do not think so either, but he can try. The crossover will be completely mucked up unless the AVR compensates. I said that in my post but your pictures provide an excellent explanation of the "why" behind my statement. Basically it all depends on the AVR's room correction program (Audyssey, I think, for Denon?) to compensate the additional amplitude roll-off and phase change through the crossover region. I've no idea how well (or even if) that works and probably did not emphasize the problem enough in my reply. My experience is all with conventional bi-amping.

I added another line to help highlight the issue.
 
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