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post #1 of 56 Old 10-12-2019, 06:56 PM - Thread Starter
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Question AVR passive bi-amping (dead horse but still confused)

Hello all

First post, so be gentle.

I've read quite a few threads on this topic, but I've been unable to extract a reasonable answer so I will try to be as specific as possible:

Denon AVR 2113 7.1 passive bi-amp capable
JBL e100 (4 binding posts on the back)
Dali Lektor Sub (crossover at 90db)

NOTE: ONLY RUNNING A 3.1 SETUP

So:

If I set my receiver for basic 3.1 (no bi-amping) I have no idea what the remaining 4 amps are doing when playing stereo music (Direct mode - and yes the sub is active both in Direct and Pure mode), if anything.

If I set my receiver for bi-amped 3.1 I assume that two more amps (back surround) are helping carry the Direct stereo load.

I've tried both scenarios and believe to perceive (I know, I know...) that the speakers seem noticeably more relaxed overall at moderate to high output when bi-amped.

Most of the arguments I've read are based on the fact that an AVR has the one finite power supply, and therefore can't play any louder when bi-amped. Fair enough.

But to my untrained ears, it does seem that my speakers are able to play cleaner (and hence louder?) when bi-amped.

I understand that using my ears is purely relative, and most responses will probably be; If it sounds good to you, go with it.

But I would really like to know if there is merit to the idea that a 7.1 AVR using 4 of its amps sounds better (maybe even louder?) than the same AVR with only the 2 stereo amps assigned to active duty?

Thanks in advance.
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post #2 of 56 Old 10-13-2019, 08:28 AM
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In your situation, no.

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post #3 of 56 Old 10-13-2019, 09:01 AM - Thread Starter
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Originally Posted by mlknez View Post
In your situation, no.
So back EMF is a hoax in this context?
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post #4 of 56 Old 10-13-2019, 09:06 AM
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If I wear shorts and pretty young ladies look my way I perceive it is due to my youthful 60 year old figure, ignoring the pot belly, and nothing to do with the blinding white legs flashing sunlight in their eyes.

It is not just that there is one power supply in the AVR, it is also that the input voltage to both amplifiers is the same, so they clip at essentially the same level, so there is no practical increase in power or anything else. The "pro" argument contends output current (not voltage) to the speaker from each amplifier is lower/different, and that is true, but in practice irrelevant.

But if you think you hear an improvement, go for it, and don't ask anybody here as you may not like the answer. As for me, I've ladies to impress, and am most certainly not asking the opinion of anyone on AVS.

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post #5 of 56 Old 10-13-2019, 09:10 AM - Thread Starter
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Originally Posted by GlobusDiablo View Post
...
Dali Lektor Sub (crossover at 90db)
...
Can't find an edit function, but obviously I mean hz, not db for the crossover.
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post #6 of 56 Old 10-13-2019, 09:20 AM - Thread Starter
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Originally Posted by DonH50 View Post
If I wear shorts and pretty young ladies look my way I perceive it is due to my youthful 60 year old figure, ignoring the pot belly, and nothing to do with the blinding white legs flashing sunlight in their eyes.

It is not just that there is one power supply in the AVR, it is also that the input voltage to both amplifiers is the same, so they clip at essentially the same level, so there is no practical increase in power or anything else. The "pro" argument contends output current (not voltage) to the speaker from each amplifier is lower/different, and that is true, but in practice irrelevant.

But if you think you hear an improvement, go for it, and don't ask anybody here as you may not like the answer. As for me, I've ladies to impress, and am most certainly not asking the opinion of anyone on AVS.


So back emf is mumbo jumbo? I did a bit more searching and found this explanation from Denon regarding passive bi-amping (practically my setup):


http://manuals.denon.com/AVRX3500H/N...SYcqhsmjdu.php
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post #7 of 56 Old 10-13-2019, 09:30 AM
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I have not heard of back emf in anything but electric motors.

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post #8 of 56 Old 10-13-2019, 09:40 AM - Thread Starter
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Originally Posted by mlknez View Post
I have not heard of back emf in anything but electric motors.
Oh, it's a thing in speakers as well apparently. Apart from the specific Denon reference regarding passive bi-amping advantages that I've already relayed:

https://forum.audiogon.com/discussio...eaker-back-emf

https://www.sweetwater.com/insync/back-emf/
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post #9 of 56 Old 10-13-2019, 09:40 AM
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Like many things marketed in audio, "back emf" is a real effect but its audibility is (greatly) overstated. Think about where back emf can occur and cause problems: through the crossover, or through the amplifier. The amplifier's output impedance is low and with passive bi-amping each amplifier is still sending the full audio range to the speaker. The speaker's crossover is not removed so any cross-current circulating in the crossover will still be presented at each amplifier. The crossover's job is to prevent woofer energy, whether back-emf or direct from the amplifier, from reaching the tweeter, so bi-amping the way an AVR does it cannot really change that. The only place it can help is in the amplifier itself. It is true that woofer current only minimally flows in the treble amp, and treble current only minimally in the bass amp, but the real-world effect is negligible IME/IMO. And the overall current demand (and thus stress on the power supply) from the AVR is actually a little higher, since no amplifier is 100% efficient, so there is a little more wasted power.

The last statement in the manual makes no sense to me, sounds like a bad translation from engineering to tech writers. "This connection enables back EMF (power returned without being output) from the woofer to flow into the tweeter without affecting the sound quality..." The goal is to not have current flowing from the woofer to the tweeter so that sound quality is not affected; that is what the crossover does. The argument is that woofer current is not required from the treble amp (and vice versa); in the real world I cannot imagine you would notice. I have not tried to measure it in years as it never made sense to me; back when I did, there was negligible change in distortion when passively bi-amping the way an AVR does it, and only when the amps were near clipping. Marketing lives for the outliers, however, to convince folk anything possible must possibly be audible.

IME/IMO/FWIWFM/YMMV/my 0.000001 cent (microcent) - Don
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post #10 of 56 Old 10-13-2019, 09:49 AM
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Your speakers have a separation with filters for each speaker and you are using separate amplifiers in the receiver so yea it could sound better. It would take a different calibration than non bi-amped.
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Originally Posted by DonH50 View Post
...I have not tried to measure it in years as it never made sense to me; back when I did, there was negligible change in distortion when passively bi-amping the way an AVR does it, and only when the amps were near clipping...
Should have left this part out of your explanation.

So as I understand it, you're saying that you haven't fiddled with this aspect in years, but that you think it may actually have a positive impact if the amp is played loud (close to clipping)?

Not trying to be annoying, just looking for clarification. It seems odd to me, that so many AVR manufacturers promote this feature if it is in fact mumbo jumbo.

Thanks.
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post #12 of 56 Old 10-13-2019, 09:52 AM - Thread Starter
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Originally Posted by whipit View Post
Your speakers have a separation with filters for each speaker and you are using separate amplifiers in the receiver so yea it could sound better. It would take a different calibration than non bi-amped.
Not quite sure what you are saying...
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post #13 of 56 Old 10-13-2019, 11:52 AM
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Quote:
Originally Posted by GlobusDiablo View Post

Not trying to be annoying, just looking for clarification. It seems odd to me, that so many AVR manufacturers promote this feature if it is in fact mumbo jumbo.

Thanks.
Marketing is all mumbo jumbo, all the time.

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post #14 of 56 Old 10-13-2019, 11:56 AM
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Originally Posted by GlobusDiablo View Post
So as I understand it, you're saying that you haven't fiddled with this aspect in years, but that you think it may actually have a positive impact if the amp is played loud (close to clipping)?

Not trying to be annoying, just looking for clarification.
That's how I read it too and IMHO Don is incorrect.

There is no increase in the measurable, full bandwidth output power [using FTC standards/methodology] when passive bi-amping, regardless if you are close to clipping or not.

In fact in the example of passive bi-amping using an AVR (where all amps rely on the same power supply), it is possible you may actually decrease your maximum output power in passive bi-amp mode because the more channels of amplification drawing from that singular power supply the more taxed it is. This is why the power output specs are better (stronger) when driving just stereo rather than ACD [all channels driven (simultaneously)].

Take for example the measured output per channel (I've placed in bold text) in stereo vs. 5 ch for this similar Denon AVR-2313ci by S&V magazine:

"
Two channels driven continuously into 8-ohm loads:
0.1% distortion at 126.8 watts
1% distortion at 152.1 watts

Five channels driven continuously into 8-ohm loads:
0.1% distortion at 77.7 watts
1% distortion at 90.8 watts "
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Originally Posted by whipit View Post
Your speakers have a separation with filters for each speaker and you are using separate amplifiers in the receiver so yea it could sound better.

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That's how I read it too and IMHO Don is incorrect.

There is no increase in the measurable, full bandwidth output power when passive bi-amping, regardless if you are close to clipping or not.

In fact in the example of passive bi-amping using an AVR (where all amps rely on the same power supply), it is possible you may actually decrease your maximum output power in passive bi-amp mode because the more channels of amplification drawing from that singular power supply the more taxed it is. This is why the power output specs are better (stronger) when driving just stereo rather than ACD [all channels driven (simultaneously)].

Take for example the measured output (I've placed in bold text) in stereo vs. 5 ch of this similar Denon AVR-2313ci by S&V magazine:

"
Two channels driven continuously into 8-ohm loads:
0.1% distortion at 126.8 watts
1% distortion at 152.1 watts

Five channels driven continuously into 8-ohm loads:
0.1% distortion at 77.7 watts
1% distortion at 90.8 watts "
Interesting. Too bad they didn't measure the 8-ohm loads with two channels (stereo) driven continuously while bi-amped. Haven't been able to find any data on this feature.

So you think back EMF and passive bi-amping in AVRs is mumbo jumbo?

But apart from measuring, has someone here spent time listening to their speakers/system, while switching passive bi-amping on and off on their AVR?
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post #17 of 56 Old 10-13-2019, 03:52 PM
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Originally Posted by GlobusDiablo View Post
Interesting. Too bad they didn't measure the 8-ohm loads with two channels (stereo) driven continuously while bi-amped. Haven't been able to find any data on this feature.

So you think back EMF and passive bi-amping in AVRs is mumbo jumbo?

But apart from measuring, has someone here spent time listening to their speakers/system, while switching passive bi-amping on and off on their AVR?
Well that's driving 4 channels simultaneously so I'd assume the power per channel is closer to the spec I quoted where five channels were driven.

Quote:
Originally Posted by GlobusDiablo View Post
So you think back EMF and passive bi-amping in AVRs is mumbo jumbo?
I have worked in this industry for decades and I have never seen any passive bi-amp test that showed either:

A. An effectively increased, maximum output level into the room [FTC guidelines] using even the optimal form of passive bi-amping [four independent mono amplifiers, each using their own, independent power supply] vs. conventional [just two of those same amps].

B. An audible change, good or bad, in the sound character, nor quantity of output, tested under proper, blind, level-matched, listenging conditions.
---

Keep in mind besides cognitive bias potentially clouding results when people apply testing under sighted (not-blind) conditions, there are other forms of error which may occur. Say for example you had a person who attempted to use two stereo amps to fulfill the necessary 4 channels of amplification, in total, for wiring up passive bi-amping. How do we know the two amps have exactly the same measured gain factor? We don't! Just because amp A says it is "100wX2" and amp B is also "100wX2" doesn't mean that the exact same input signal to both will necessarily elicit the exact same output level. If one were to use the higher gain factor amp on the tweeters this will make them put out more sound hence the perception would be that the sound in the room was "brighter". That is, the user would have effectively "EQ'd" their system without realizing it because some drivers were being run "hotter", unbeknownst to them.

Quote:
Originally Posted by GlobusDiablo View Post
But apart from measuring, has someone here spent time listening to their speakers/system, while switching passive bi-amping on and off on their AVR?
I assume there are zillions of posts, conducted under sighted conditions, where people say they heard a difference and I highly doubt any said "and it was inferior". If they've gone through the bother of wiring it all up it shows from the get go they are inclined to think it would be beneficial. The placebo effect is extremely strong in audio, in fact there are instances where it is even stronger than actual changes!

Another reason there is a lot of confusion on this topic is because there is a legit form of bi-amping called "active bi-amping". Here an electronic crossover divides the signal going to each amp and since each amp sees a smaller, limited range of frequencies coming in [unlike in passive bi-amping] it has a lighter load and therefore may play slightly louder. [Anyone using a bass managed system with a powered subwoofer is sort of doing this, by the way.]

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post #18 of 56 Old 10-13-2019, 04:42 PM
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[QUOTE=GlobusDiablo;58681648]

So you think back EMF and passive bi-amping in AVRs is mumbo jumbo? /QUOTE]

It might be, but if your setup is capable of doing it, why not just do it and make up your own mind, instead of looking for data. It shouldn't be that hard for you to run two extra sets of speaker wires to your front speakers.
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post #19 of 56 Old 10-13-2019, 04:51 PM
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Quote:
Originally Posted by GlobusDiablo View Post
Should have left this part out of your explanation.

So as I understand it, you're saying that you haven't fiddled with this aspect in years, but that you think it may actually have a positive impact if the amp is played loud (close to clipping)?

Not trying to be annoying, just looking for clarification. It seems odd to me, that so many AVR manufacturers promote this feature if it is in fact mumbo jumbo.

Thanks.
I have no need to "fiddle with it", it is (very) old news. I have not derived Maxwell's equations recently but am pretty sure they have not changed. All bi-amping I have done, either at home or for professional sound reinforcement (live sound), used line-level crossovers into separate power amps and speakers designed for bi-amping (no internal crossovers). It was probably ten years ago I took a quick look at this (passive bi-amping) but many others have done the measurements (there are some on AVS but I have not searched for them). It is not something I consider worthwhile; my day job is GHz circuits and not audio but the engineering principles are the same. I prefer to focus on more meaningful improvements to my home audio/HT systems.

There are many things that have real, measurable outcomes that are not audible. If you compare two DACs with no changes except one has 120 dB and the other 125 dB SNR, then you can say the one with higher SNR is better, but you'll never hear the difference in the real world. If you look at the actual circuits then yes you can gain a hair in output if current is reduced by (passive) bi-amping. In practice, you are talking about a negligible (fraction of a dB) increase, and that also depends the speaker's crossover frequency since that determines how power is split between the high and low amplifiers. That little a change in volume will go unnoticed, especially if the amps (speakers, etc.) are already right at their clipping point. You'll never hear it. If the max SPL is 100 dB, and you bump it to 100.1 dB, you'll never be able to tell. And there's no difference at 99 dB. Back EMF refers to charge "kick back" from the drivers and is a function of the amplifiers' output impedance and feedback. In practice, IME, it is a non-issue for any decent SS amplifier and speaker combination.

Passive bi-amping provides a reason for selling multichannel AVRs by convincing consumers that the extra channels can be used beneficially to drive the speakers. You'd be better off leaving the unused channels idle to provide additional headroom in the power supply, which likely results in greater gain than passive bi-amping. I have not measured that, however. From an engineering standpoint the idea of passive bi-amping is rather silly; implementing a conventional bi-amp system can be helpful but requires a line-level crossover, amplifier channels, and enough knowledge to set up the crossovers.

IMO making inaudible differences believable is one of the triumphs of audio marketing, albeit at the expense of consumers. You can add bi-wiring to the list if you wish. They prey on real, measurable effects that simply have no impact on the sound you actually hear. Audio is not alone in making such claims but is the area I am most familiar with. A friend says golf also has its share of magic balls and clubs.

At the end of the day, if you think you can hear a difference, you will, and nobody on either side is being harmed assuming the cost of the wires is insignificant.
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As a [former] dealer, I've had technicians from Sonus Faber tell me straight out [but off the record] that they consider both bi-wiring and passive bi-amping to be effectively worthless but according to their cost/benefit analysis the trivially higher expense using four terminal speaker posts instead of just two is worth it, considering the potential loss in sales to people who think speakers must have this, and if they don't, they'll shop elsewhere.
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Originally Posted by m. zillch View Post
As a [former] dealer, I've had technicians from Sonus Faber tell me straight out [but off the record] that they consider both bi-wiring and passive bi-amping to be effectively worthless but according to their cost/benefit analysis the trivially higher expense using four terminal speaker posts instead of just two is worth it, considering the potential loss in sales to people who think speakers must have this, and if they don't, they'll shop elsewhere.
---

P.S. Retailers love both buy-wiring and buy-amplifying because it means they'll be doubling their profits [in wire/amps] to the customers who believe it all.
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Originally Posted by GlobusDiablo View Post
Can't find an edit function, but obviously I mean hz, not db for the crossover.

Hopefully Hz and dB. hz and db are obviously wrong.
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Originally Posted by DonH50 View Post
Like many things marketed in audio, "back emf" is a real effect but its audibility is (greatly) overstated. Think about where back emf can occur and cause problems: through the crossover, or through the amplifier. The amplifier's output impedance is low and with passive bi-amping each amplifier is still sending the full audio range to the speaker. The speaker's crossover is not removed so any cross-current circulating in the crossover will still be presented at each amplifier. The crossover's job is to prevent woofer energy, whether back-emf or direct from the amplifier, from reaching the tweeter, so bi-amping the way an AVR does it cannot really change that. The only place it can help is in the amplifier itself. It is true that woofer current only minimally flows in the treble amp, and treble current only minimally in the bass amp, but the real-world effect is negligible IME/IMO. And the overall current demand (and thus stress on the power supply) from the AVR is actually a little higher, since no amplifier is 100% efficient, so there is a little more wasted power.

The last statement in the manual makes no sense to me, sounds like a bad translation from engineering to tech writers. "This connection enables back EMF (power returned without being output) from the woofer to flow into the tweeter without affecting the sound quality..." The goal is to not have current flowing from the woofer to the tweeter so that sound quality is not affected; that is what the crossover does. The argument is that woofer current is not required from the treble amp (and vice versa); in the real world I cannot imagine you would notice. I have not tried to measure it in years as it never made sense to me; back when I did, there was negligible change in distortion when passively bi-amping the way an AVR does it, and only when the amps were near clipping. Marketing lives for the outliers, however, to convince folk anything possible must possibly be audible.

IME/IMO/FWIWFM/YMMV/my 0.000001 cent (microcent) - Don

EMF is voltage, not current. The analysis should be done based on voltage. Normal amplifiers used for audio operate as voltage sources, not current sources. That said, Denon is likely trying, admittedly not very effectively, to explain the following:

The woofer generates back EMF, a voltage. Some of this voltage contains distortion from the woofer that is low enough in frequency to pass back through the woofer crossover and yet high enough in frequency to pass through the tweeter's crossover. Effectively some of the distortion from the woofer is driving the tweeter.

This effect is most pronounced in two-way speakers with 1st or 2nd order crossovers. Some people actually use small two-way speakers as their L and R speakers in home theater systems, which is likely the worst case. The woofer is likely to produce a lot of distortion since it is too small for the job, and is over driven. Much of this distortion will range in frequency from just below the woofer's crossover point and upward in frequency. The use of 1st or 2nd order crossovers allows a lot of this distortion to pass from the woofer to the tweeter. There was an AES paper a number of years ago on this effect, with calculations and perhaps measurements as I remember. I don't have access to the AES papers at present or I'd reference it.

The measured level of the above effect would obviously depend on many variables, such as the specific of the loudspeaker, the content played, etc. The audible effect will never be known without controlled listening tests, which will likely never happen. Driving the woofer and tweeter crossovers with separate amplifiers eliminate this cross transfer of distortion.


There a many technical benefits to passive bi-amping. Some of these benefits apply to bi-amping an AVR, some with separate external power amplifier channels and some to both. It is late tonight so the length of this post will be limited, but here are a few of the benefits to bi-amping a typical AVR that can be added to the EMF advantage described above.

o Each amplifier channel sees a higher effective load impedance than if one amplifier channel drives the entire speaker,. This difference should reduce overall distortion and crossover distortion.

o The cooling load for the amplifier channels will be spread over a larger portion of the heat sink. Less variation in temperature means better dynamic bias tracking. which should reduce distortion.

o A lower output current requirement, and smaller change in output current, for an amplifier channel should reduce the distortion of the input stage, which is typically a long-tailed pair in say a D/M AVR. Often the input stage is lacking a current mirror and other desirable design features. AVR power amplifier stages typically have only a two transistor output stage (Darlington) with modest current gain. Less current gain is available than the three transistor output stage output, using more capable transistors, found in many external power amplifiers. In addition, this current gain is not constant with changes in currant output due to the choice of transistors. These added, and varying current requirements in an AVR amplifier channel ultimately put more stress on the input stage, which intern clauses more distortion. The less the output current requirement and the less change in current output, the less distortion is caused by this factor. Using two AVR amplifier channels provides this reduction in current requirement per channel.

Again, how these technical factors affect the listeners experience is not quantifiable, and will likely stay that way, but it is nice to know that there are technical factors that support bi-amping, even with AVR's.
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Again, how these technical factors affect the listeners experience is not quantifiable,
Probably because it is not discernible to the ear, at all, in the first place.

In the over half century passive bi-amplification has been available not a single paper in any science journal has ever demonstrated it is perceptible to humans under properly controlled conditions.

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First of all, thank you very much for some excellent responses. Wow.

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Originally Posted by m. zillch View Post
...Keep in mind besides cognitive bias potentially clouding results when people apply testing under sighted (not-blind) conditions, there are other forms of error which may occur...
You assume quite a bit in this post , but the above quote rings very true.

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Originally Posted by DonH50 View Post
...At the end of the day, if you think you can hear a difference, you will, and nobody on either side is being harmed assuming the cost of the wires is insignificant.
They were lying around anyway, and I agree with the gist.

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Originally Posted by m. zillch View Post
As a [former] dealer, I've had technicians from Sonus Faber tell me straight out [but off the record] that they consider both bi-wiring and passive bi-amping to be effectively worthless but according to their cost/benefit analysis the trivially higher expense using four terminal speaker posts instead of just two is worth it, considering the potential loss in sales to people who think speakers must have this, and if they don't, they'll shop elsewhere.
---

P.S. Retailers love and endorse both buy-wiring and buy-amplifying because it means they'll be doubling their profits [in wire/amps] to the customers who believe it all.
Again, as an average consumer, I definitely see the logic in what you're seeing.

P.S. And how the hell are you editing your posts!? I can't find that function anywhere...

Quote:
Originally Posted by bigguyca View Post
EMF is voltage, not current. The analysis should be done based on voltage. Normal amplifiers used for audio operate as voltage sources, not current sources. That said, Denon is likely trying, admittedly not very effectively, to explain the following:

The woofer generates back EMF, a voltage. Some of this voltage contains distortion from the woofer that is low enough in frequency to pass back through the woofer crossover and yet high enough in frequency to pass through the tweeter's crossover. Effectively some of the distortion from the woofer is driving the tweeter.

This effect is most pronounced in two-way speakers with 1st or 2nd order crossovers. Some people actually use small two-way speakers as their L and R speakers in home theater systems, which is likely the worst case. The woofer is likely to produce a lot of distortion since it is too small for the job, and is over driven. Much of this distortion will range in frequency from just below the woofer's crossover point and upward in frequency. The use of 1st or 2nd order crossovers allows a lot of this distortion to pass from the woofer to the tweeter. There was an AES paper a number of years ago on this effect, with calculations and perhaps measurements as I remember. I don't have access to the AES papers at present or I'd reference it.

The measured level of the above effect would obviously depend on many variables, such as the specific of the loudspeaker, the content played, etc. The audible effect will never be known without controlled listening tests, which will likely never happen. Driving the woofer and tweeter crossovers with separate amplifiers eliminate this cross transfer of distortion.


There a many technical benefits to passive bi-amping. Some of these benefits apply to bi-amping an AVR, some with separate external power amplifier channels and some to both. It is late tonight so the length of this post will be limited, but here are a few of the benefits to bi-amping a typical AVR that can be added to the EMF advantage described above.

o Each amplifier channel sees a higher effective load impedance than if one amplifier channel drives the entire speaker,. This difference should reduce overall distortion and crossover distortion.

o The cooling load for the amplifier channels will be spread over a larger portion of the heat sink. Less variation in temperature means better dynamic bias tracking. which should reduce distortion.

o A lower output current requirement, and smaller change in output current, for an amplifier channel should reduce the distortion of the input stage, which is typically a long-tailed pair in say a D/M AVR. Often the input stage is lacking a current mirror and other desirable design features. AVR power amplifier stages typically have only a two transistor output stage (Darlington) with modest current gain. Less current gain is available than the three transistor output stage output, using more capable transistors, found in many external power amplifiers. In addition, this current gain is not constant with changes in currant output due to the choice of transistors. These added, and varying current requirements in an AVR amplifier channel ultimately put more stress on the input stage, which intern clauses more distortion. The less the output current requirement and the less change in current output, the less distortion is caused by this factor. Using two AVR amplifier channels provides this reduction in current requirement per channel.

Again, how these technical factors affect the listeners experience is not quantifiable, and will likely stay that way, but it is nice to know that there are technical factors that support bi-amping, even with AVR's.
Awesome counter point. Thank you very much.

----

N.B. So I guess I'll be sticking to my perceived gains...
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...P.S. And how the hell are you editing your posts!? I can't find that function anywhere...
...
Hang on. I have an edit tab now. Must have been withheld until general approval? Anywho.
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Quote:
Originally Posted by bigguyca View Post
EMF is voltage, not current. The analysis should be done based on voltage. Normal amplifiers used for audio operate as voltage sources, not current sources. That said, Denon is likely trying, admittedly not very effectively, to explain the following:

The woofer generates back EMF, a voltage. Some of this voltage contains distortion from the woofer that is low enough in frequency to pass back through the woofer crossover and yet high enough in frequency to pass through the tweeter's crossover. Effectively some of the distortion from the woofer is driving the tweeter.

This effect is most pronounced in two-way speakers with 1st or 2nd order crossovers. Some people actually use small two-way speakers as their L and R speakers in home theater systems, which is likely the worst case. The woofer is likely to produce a lot of distortion since it is too small for the job, and is over driven. Much of this distortion will range in frequency from just below the woofer's crossover point and upward in frequency. The use of 1st or 2nd order crossovers allows a lot of this distortion to pass from the woofer to the tweeter. There was an AES paper a number of years ago on this effect, with calculations and perhaps measurements as I remember. I don't have access to the AES papers at present or I'd reference it.

The measured level of the above effect would obviously depend on many variables, such as the specific of the loudspeaker, the content played, etc. The audible effect will never be known without controlled listening tests, which will likely never happen. Driving the woofer and tweeter crossovers with separate amplifiers eliminate this cross transfer of distortion.


There a many technical benefits to passive bi-amping. Some of these benefits apply to bi-amping an AVR, some with separate external power amplifier channels and some to both. It is late tonight so the length of this post will be limited, but here are a few of the benefits to bi-amping a typical AVR that can be added to the EMF advantage described above.

o Each amplifier channel sees a higher effective load impedance than if one amplifier channel drives the entire speaker,. This difference should reduce overall distortion and crossover distortion.

o The cooling load for the amplifier channels will be spread over a larger portion of the heat sink. Less variation in temperature means better dynamic bias tracking. which should reduce distortion.

o A lower output current requirement, and smaller change in output current, for an amplifier channel should reduce the distortion of the input stage, which is typically a long-tailed pair in say a D/M AVR. Often the input stage is lacking a current mirror and other desirable design features. AVR power amplifier stages typically have only a two transistor output stage (Darlington) with modest current gain. Less current gain is available than the three transistor output stage output, using more capable transistors, found in many external power amplifiers. In addition, this current gain is not constant with changes in currant output due to the choice of transistors. These added, and varying current requirements in an AVR amplifier channel ultimately put more stress on the input stage, which intern clauses more distortion. The less the output current requirement and the less change in current output, the less distortion is caused by this factor. Using two AVR amplifier channels provides this reduction in current requirement per channel.

Again, how these technical factors affect the listeners experience is not quantifiable, and will likely stay that way, but it is nice to know that there are technical factors that support bi-amping, even with AVR's.
Nice post and I agree with most of it. I didn't try to define EMF given the OP's apparent technical level, and had already introduced current, so just waffled. I thought I had made the comment about audio amps usually being voltage sources but sure enough left that out. This has come up so many times I forget what has been said. I had not thought about input stage modulation; you'd think feedback and loop gain (voltage and current) would be high enough to obviate that from consideration, but I didn't design them. Spreading the heat is a good point, although using more amplifiers means more heat to begin with. Thermal management in AVRs does not seem to be a real strong point.

I am not sure (both) the individual amps see higher impedance; that depends upon the speaker's impedance profile. One of them probably does, tweeter or bass, whichever is not handling the frequency of the impedance minimum. Power per amp is definitely reduced, though not voltage (to any degree), just current flow.

I'm in the same boat for AES papers; let my membership lapse many years ago since my career took a different path. Keep thinking I should renew but I waste enough time on the 'net as it is. Maybe when I retire... Someone sent me a paper, probably same you had in mind, some years ago and IIRC the effect was significant but only really an issue when levels were high (woofer heavily overdriven as you said). I tend to discount that case in my mind by lumping it into the whole "clipping" thing even though it is the speaker and not the amplifier going nonlinear in that case.

Thanks - Don
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post #28 of 56 Old 10-14-2019, 11:50 AM
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Passive bi-amping (as well as bi-wiring when one is using an adequate gauge single run of wire in the first place) are "solutions looking for problems to solve", as they say, in order to justify their existence. In truth neither have any audible effect in real world use and there is no evidence based science published in any scholarly science journal which has found any audible change, whatsoever, under controlled conditions.

Proving that a load an amplifier sees changes slightly or that there is a measurable change in EMF values, distortion, or anything else for that matter, does not prove it is audibly significant and hence consequential. Here's an analogy of how this same sort of marketing trick could be used where the con will be very easy for all to see, assuming you understand that all things being equal, lighter cars have faster acceleration than heavier cars:

Marketing Claim: "Your car will have faster acceleration so you will beat others in races if you buy this device I am marketing."

The device is a floor mat vacuum cleaner. The premise is that by removing the dirt from the floor mats the car's overall mass will be reduced and since we know that the reduced mass of an object reduces the force necessary to make it accelerate, this means, according to science, that after applying the device to the car the acceleration times will be "better". The floor mats are then weighed on a precision scale to show the change in mass after cleaning and the marketer exclaims. "Voila! I have proven my device works exactly as advertised. The car's mass is now lower, I have proved it using instrumentation, hence your acceleration is improved. . . . Will that be cash, check, or charge?"

What's that? "No proof has been shown in makes any real difference!" you say? Well guess what? Zero evidence has been provided that passive bi-wiring (nor bi-wiring) makes any real difference has ever been shown either in the over half century we have had speakers which provide this option: Both of these things are marketing ploys.

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Technically, 'passive biamping' is also biwiring.

So, of course, you still get the biwiring benefits when you passively biamp.




"All men are frauds. The only difference between them is that some admit it. I myself deny it."
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post #30 of 56 Old 10-14-2019, 12:45 PM
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Quote:
Originally Posted by DonH50 View Post
I am not sure (both) the individual amps see higher impedance; that depends upon the speaker's impedance profile.
All that matters is the impedance load (and phase angle) the amp sees at the frequency it is currently being tasked to reproduce. This is how FTC data is generated under the 1974 guidelines we all use and advertisers are held to in magazine ads in the US. [Japan is different.]

As I have already shown you previously, with this animation I prepared of a Kef speaker I once sold [measurements by AJ in FL], the 4-ohm impedance load the amp sees in the most critical, power sapping bass region [in this example 200-350Hz] does not change when passively bi-amping, therefore the amp's ability to reproduce a loud note in this same region also does not increase simply because we have disconnected the tweeter load from its "burden" (passive bi-amping, that is):

[The vertical scale is impedance, measured in ohms, from 0 to 100. The horizontal scale is frequency, from 20Hz to 20kHz.] In both scenarios, conventional and passive bi-amping, the impedance from 200-350Hz stays exactly the same: 4 ohms.

Let's pretend for the moment that the tweeter amp works completely differently and suddenly has, I don't know, let's say 6000% more output capability over all frequencies when used in this passive bi-amp scenario I'm showing [unlike the above woofer measurement curve, this "6,000%" is just a pretend value]. It doesn't matter to the FTC spec, which quite fairly asserts that when we talk about the output of an amplifier we must specify its capability over the full bandwidth, not just some cherry-picked frequencies it happens to be good at. [THANK YOU, FTC.] The bottle neck in maximum music output is nearly always due to bass frequencies unless one cheats and cherry picks some wacky music like solo flute recordings, where there is never any deep bass.

When we talk about a conventional 200 watt amplifier having the capability to play 3dB higher in maximum clean output than a 100w amplifier, under the same conditions, are we talking about cherry-picked frequencies only? NO. We mean all of them over that stated bandwidth, usually 20-20kHz. Using this exact same and fair methodology the increase in effective output when passive bi-amping is 0 dB, none, nada, zilch, because as I've demonstrated in the measurement above, if the most power sapping part of the music's range does not increase in power output, at all, then we also can't fairly say the system's overall 20-20kHz output has increased either, since 200-350Hz is a subset of that full range.
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