Do amps waste power in passive biamping?

warnerwh · « **on:** 10 Apr 2006, 11:10 pm »

Due to the fact the amps are only outputting part of what's fed to them does this mean they are wasting power? This is what I read a couple of times but don't see how as the only current going through the amp is what the speaker will allow. If someone could elaborate on this I'd appreciate it. Please keep it simple as I can't even site Ohm's law. Thanks

YoungDave · « **Reply #1 on:** 11 Apr 2006, 02:03 am »

Keeping the explanation simple is a good idea. Here goes (and this is just my take on the theory. Lots of variables can enter into real-life setups, of course):

1. Do amps waste power in passive bi-amping?

yes. But IMO that isn't the critical factor that makes passive biamping a poor choice (see below).

2. Are the amps are only outputting part of what's fed to them?

no. The amplifier bumps up the entire input signal by the 30dB gain that most amps make. That entire signal is sent to the passive crossover. Part of the signal goes to the driver in question, the other part is dissipated in the reactive and resistive components of the "other leg" of the crossover.

Passive biamp is a decidedly sub-optimal solution. Why?

1. You get none of the advantages that can be realized by amplifying only the signal that you will use - in a passive bi system, the tweeter amp is working a signal from 20Hz - 20kHz. Can't be as clean as an amp that is only fed a 6kHz-20kHz signal, all other things equal.

2. You have to use really big, really suboptimal crossover parts that load the amp weirdly and unpredictably, and that must operate in a roughly 35VAC, 5 amp circuit.

Active bi:
1. Cheaper. Transistors, IC's, etc cost pennies. Big expensive caps & inductors in the speakers are no longer needed. Take 'em out and toss 'em.

2. Better phase coherency. No reactive elements in the output circuit (other than the cable & drivers)

3. Much easier to change crossover points, damping characteristics, levels, & slopes.

4. Much more civilized load on the amp - with some of the mariginally stable amps on today's market, this can prevent destructive oscillation.

All the above is just my opinion, but I always welcome lively discussion...

JohnR · « **Reply #2 on:** 12 Apr 2006, 02:10 am »

Bump to recover warnerwh's missing thread.

warnerwh · « **Reply #3 on:** 12 Apr 2006, 04:21 am »

Dave: Can you explain how the amp is wasting power? It seems to me that with all the resistance it sees with the crossover there that it would be unable to put out any significant power at those frequencies. This in turn means no current delivered but to the frequencies where the resistance is low enough to allow the current to drive the speakers.

I know nothing about electricity but using the water hose analogy that has been used I've come to this conclusion.

Thank you

warnerwh · « **Reply #4 on:** 13 Apr 2006, 01:26 pm »

I emailed Blue Circle with this question. Here's an excerpt from the response I received: "you're not "losing" anything. You're simply not using the amp to power the full range of the speakers."

I'm inclined to believe them over all the people on the forums I've read saying you're wasting power when passive biamping. I also found this same answer on Anthem's website last night. So in fact the "wasting power" by passive biamping is nothing more than a myth.

mfsoa · « **Reply #5 on:** 13 Apr 2006, 01:45 pm »

I could be misquoting him, but I seem to remember Jon Risch explaining that when an amp "sees" a crossover, it simply doesn't put out the power at the frequencies that are being filtered out. The speaker hasn't "asked" for the juice at those frquencies, and so the amp just doesn't make it.

Otherwise, in a passive biamp the amp feeding, let's say just a tweeter, would be producing its full output, which is then almost entirely dissipated by the crossover. I don't think that this what's happening, and this is one reason for biwiring having beneficial effects - The high-freq wire is simply not passing any power at the low frequencies.

That's my totally non-technical understanding.

Occam · « **Reply #6 on:** 13 Apr 2006, 01:57 pm »

Warner,

Why in heaven's name did you initiate this thread? What was the point of this (IMO) troll? You knew your response to YoungDave's response before you ever posted the initial query.
I'd actually talked to a technically astute poster and asked if he would respond to the one error in his lengthy response. His response was simply that 'no good deed goes unpunished' and that your post was an obvious troll.
Was its purpose to allow you to debunk this 'myth'?
If you wish to initiate a dialouge in the Lab forum, please post what you do know FIRST as a baseline, so that we can actually have a beneficial thread, rather than provide you a forum as 'myth debunker'.
Dave was nice and gracious enough to respond because he actually thought you wanted to learn something. The fact that he didn't fully think through that one small point in a lengthy response is minor. We all occasionally do this.

[Edit - removed the even more over the top comment]

brj · « **Reply #7 on:** 13 Apr 2006, 02:02 pm »

Check out the article titled Benefits of Bi-Amping (Not Quite Magic, But Close) - Part 1. It will give you a good overview of the benefits of active bi-amping, and how it differs from passive bi-amping and bi-wiring. If you jump up one level on that website, you can see the index listing several related articles.

Note that whether you consider passive bi-amping to waste power or not largely depends on what you are comparing too. Are you comparing to mono-amping, or active bi-amping? Active bi-amping will indeed save you quite a bit of power (though not always monetary cost) vs. passive bi-amping or mono-amping. Passive bi-amping, however, does not waste any more energy than mono-amping. Whether it provides any benefit or not, however, largely depends on the size and quality of the mono-amp you are comparing it against.

Frankly, I wouldn't worry about the power differences as a first priority. I make respectable attempts myself to conserve power, but accidentally forgetting to turn out a 100W lightbulb for a couple of hours will have wasted more power than almost any combination of amps playing music. Ignoring possible sonic differences, the biggest difference between the above mentioned approaches is going to be in the cost and complexity of the system.

konut · « **Reply #8 on:** 13 Apr 2006, 02:12 pm »

Not being an engineer, this is something I've wondered about myself The fact remains that you're inputing a full range signal. The amp is then reproducing the entire range of frequencies, as it was designed to do, and then connects to a 'load'. As the amp is doing the 'work' of increasing the amplitude of the full range signal, its subject to increased IM distortion, compared to a situation in an active set up where the amp is only increasing a limited range of frequencies, as dictated by the active crossover. This effectively lowers IM as there are less multiples of frequency being reproduced. Its not a matter of waste as much as its a lowering of distortion and increasing of dynamic range, as in an active set up, since only a limited range of frequencies amplified, more headroom is available for a given SPL. In the passive bi-amp set up the full range is still being reproduced, thus no headroom increase or potential for less distortion. But where does the discarded frequency range go? How does the 'load' change what the input is feeding?

mfsoa · « **Reply #9 on:** 13 Apr 2006, 02:29 pm »

Konut,
I'm probably less of an engineer for you, but I just don't think that each amp in a passive biamp configuration is really "doing the 'work' of increasing the amplitude of the full range signal" as you say.

The frequencies that don't make it through the driver aren't being amplified by the amp.

I'm not pretending to be right here, but that's my understanding.

warnerwh · « **Reply #10 on:** 13 Apr 2006, 05:50 pm »

Occam: Just how exactly did I know the answer before I posted it? I'd like to know how you figured that out. That is an assumption that is incorrect or I wouldn't have bothered. I merely continued to research the subject.

If you'd like I'll forward the email I got from Blue Circle AFTER I posted this question. Give me your email address and you will see I could not have known before I posted this question due to the fact if you look at the date of my post and the response to my question from Blue Circle there's two days gap.

Please send me your email address and you will see that you are wrong. The reason I questioned Young Daves response is that if you read it he doesn't sound to sure of himself as he doesn't explain "why" just what he's read on forums such as this.

woodsyi · « **Reply #11 on:** 13 Apr 2006, 07:05 pm »

Warner,

I use passive highpass on my mid/treble and active lowpass on my woofers. I have the wherewithall to use an active high pass on the ribbons and compare the wattage used (through a power regenerator that gives percentage of capacity used). I will let you know if passive filter still makes the amp work. My guess is only a little power that is required by the passive filter is used. The same filter attenuates the signal below the high pass frequency, there by eliminating the load. Thus no work.

warnerwh · « **Reply #12 on:** 13 Apr 2006, 08:43 pm »

Woodsyi: Thank you very much. I am very curious as I Googled this question and read alot of reports on the various forums claiming you're wasting power.

Even Al Whyley at Blue Circle's response that I posted earlier hadn't been enough for me and I emailed him again. AL said: A specific answer would be more than I am able to provide. The demands on the amp are determined by the speaker design. The driver details and crossover design will determine the actual draw on the amp. You would need to contact the speaker manufacturer and ask them the question too. I know you feel that this is a simple question but it is not. Now I'm more confused

I guess I'll have to ask Brian. I'm only trying to get a layman's answer but the little I know about electricity tells me that those frequencies that are fed to the amp and aren't being reproduced by the speaker are probably not drawing the current from the amp. It does seem feasible that the power is lost as heat either in the amp or crossover parts.

I do want to know what kind of numbers you come up with. Thank You

Occam · « **Reply #13 on:** 14 Apr 2006, 12:42 am »

Warner,

Sorry, my response to you was over the top. (I injured my foot a few weeks ago and have been favoring one side, resulting in my crushed discs acting up. When I'm in pain I'm even more of a wanker than usual)
Your description of an aswer as a 'myth' as opposed 'incorrect' prompted me to read more into your response than might have been appropriate.

Regards,
Paul

warnerwh · « **Reply #14 on:** 14 Apr 2006, 12:59 am »

Paul: No problem. I'm somewhat ornery myself when I'm in pain, kind of like a wounded bear.

Yes I was just trying to learn here. I'd done alot of research and could not come up with the "why" part of the equation. Even now I'm still confused but will find out from the designer of my speakers and see what he says.

It's interesting that the wasting power rule seems so prevailant on so many various forums with nobody explaining why it works that way. Amplifier design is no doubt way over my head anyway but I wonder about how things work too.

Hope you get to feeling better.

amplifierguru · « **Reply #15 on:** 14 Apr 2006, 01:13 am »

OK I'll bite.

An amplifier powering a half passive crossover will see a low impedance and deliver power only in that range of frequencies in which the driver operates. Outside that frequency range, the impedance of the passive crossover goes high and the amplifier power is greatly reduced - just as an amp that delivers 100W into 8ohms will only deliver 8W into 100 ohms.

This is for a typical splittable series crossover. There are no doubt unusual exceptions of crossover design but this would be the norm.

Cheers,
greg

woodsyi · « **Reply #16 on:** 14 Apr 2006, 04:01 am »

Warner,

My amp does not work any more running full into passive filter (quasi 2nd order high pass with 280 Hz pole frequency) than running through an active filter (4th order with 200 Hz pole frequency). As Ampguru said, high impedance presented below the high pass means the amp driving the ribbons are not seeing the woofer load. The only difference is that attenuation is done at line level for the active filer and at driver level for the passive. The active, however, lets you adjust level for amps of different gain. Passive, on the other hand, may require a passive attenuator on the amp with higher gain. I tried passive biamping when I started with the first RM 40s but my $100 DIY passive attenuator muddied the sound and I went active.

BTW, I just discoverd that whatever I have been doing in the past year, I am getting better sound out of my RM40s now than a year ago. I remember testing my Marchand XM44 crossover to see if it impeded signal and not hearing any difference. Now I do. That chunk of TRTs are giving me airier sound than Marchand active. I wonder if it would be the same with other cap options. A lesser powered amp may also favor the active crossover over the passive one. Anyway, passive high pass on the ribbons driven by 100w tube power and active lowpass on the woofers with 1000w ICE amp is working for me.

warnerwh · « **Reply #17 on:** 14 Apr 2006, 04:40 am »

Thanks woodsyi. That is what my layman brain guessed but I knew I could be completely wrong too. I wonder why there are so many people saying you waste power when passive biamping. I saw this all over after doing a Google research.

I do not and will not use a line level attenuator. Fortunately the RM 40's have the pots that give you quite a bit of room to adjust for different gain.

Another problem I see when using active crossovers is that if you remove the speaker designers crossover you could create other problems. This is because more than a crossover is commonly in there to adjust for phase or significant frequency response variations.

Also using an active crossover is also putting something else into the system and is user adjustable. I don't think I'm smart enough to dial in the crossover the way Brian did. Still most people say active biamping is the only way to go if you biamp so what do I know?

woodsyi · « **Reply #18 on:** 14 Apr 2006, 04:34 pm »

Warner,

I hear you. It is simpler to just plug and play (pots and putty not withstanding). I am too ignorant in speaker building to try my hand at the tweeter/mid exchange. That is why I am sticking with the passive filter on the ribbons. The woofers, however, are not as finicky and using a 4th order minimally affects the phase issue since Brian still uses 1st order highpass on the ribbons. He is also offering powered woofer version with 4th order lowpass adjustable up to 200 Hz. I am thinking Marchand has to be better than those built in crossovers. I like the fact that I can open up the pot all the way on my mid panels. I am thinking the lesser the resistance in the pot the better . Who knows, I may take that one out of the loop completely when I go in there next.

andyr · « **Reply #19 on:** 17 Apr 2006, 10:48 am »

Quote from: Occam

Warner,

Sorry, my response to you was over the top. (I injured my foot a few weeks ago and have been favoring one side, resulting in my crushed discs acting up. When I'm in pain I'm even more of a wanker than usual)
Your description of an aswer as a 'myth' as opposed 'incorrect' prompted me to read more into your response than might have been appropriate.

Regards,
Paul

Hi Paul,

All of us who frequent this forum know you to be a wanker extraordinaire

) ... but that shouldn't stop you from:

a) as an old dog with more than a passing knowledge of these matters, being able to explain to neophytes like me the techernikal basis for certain things; and

b) knocking out-of-line posters into shape!!

)

I've read lots about active crossovering - including all of Rod Elliott's articles. I've also built his crossovers and run my Maggie IIIAs 3-way active. So I personally think active is the only way to fly!!

None of the posters in this thread seem to disagree with the theory that active is better than passive. To me, this is because of two things:
a) in an active setup, the amp is in direct control of the driver ... there are no coils or caps in the way (like in a passive setup) which, typically, "relax" the control which the amp has over the driver; and
b) you can use several lower-powered amps in an active setup instead of one humungous amp. I suspect that high-powered amps sound less refined than lower-power amps!

However, there has been a lot of what I believe to be mis-information spread about passive bi-amping ... not only in this post but elsewhere!!

Fer instance ... if in a passive crossover one is "wasting" amp power then where is this power being "wasted" to? It is certainly NOT going into heat in the passive crossover ... therefore one of the fundamental electrical laws is being broken if, indeed, the power is being "wasted"!!??

Secondly, I have a couple of mates (GPs, so one would generally consider them to be above Neanderthal intelligence levels) who have identical systems - monstrous Dali speakers which stand nearly 5' high, driven by humungous 800wpc Brystons.

They both agree the Dalis sound much better when one mate lends the other his amp - so the Dalis are driven by TWO monstrous Brystons!!

So, here are two examples which refute statements made in this thread ... yet I, personally, cannot figure out what is really happening in a passive bi-amping situation. I've heard it - and it sounds better - but nobody seems to be able to explain what exactly is going on!!

Regards,

Your Aussie mate, Andy