# Passive biamp with different amps?



## Hakka (Sep 10, 2006)

I currently have my mains powered by a Rotel RB-1070, I was planning to add another 1070 and biamp. I've been thinking and I'm leaning towards getting a RB-1080 instead. Is it possible to biamp using these 2 amps or will I get level matching issues. These amps dont have gain controls.

The Rotel website lists the RB-1070 (2x120w)input impedance/sensitivity as 1.0v 33kohms, the specs for the 1080 (2x200w)are 1.5v 32kohms. I'm not sure if these specs are relevant or not.

I guess i could always use the 1070 to biamp the centre if I cant mix amps on the mains.

Thanks.

Harry.


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## Josuah (Apr 26, 2006)

I'm thinking you might have issues. But also, from a technical standpoint, everything I've read seems to indicate passive biamping will not improve the sound quality.


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## Hakka (Sep 10, 2006)

Josuah said:


> I'm thinking you might have issues. But also, from a technical standpoint, everything I've read seems to indicate passive biamping will not improve the sound quality.


I have read conflicting reports on passive biamping, but I would eventualy like to upgrade to an active biamp setup but thats a little while off. 

My options are:

1) Add another 1070 to biamp mains

2) Add a 1080 to power mains, use 1070 to biamp centre. Add another 1080 to biamp mains at some point in the future.


*I currently have a RMB 1075 powering my centre and 4 surrounds.


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## Josuah (Apr 26, 2006)

The only reason I think you might have issues with mixing is because the gain will not be uniform.

Have you read any technical statements that are in favor of passive biamping? I've only seen subjective statements.


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## Hakka (Sep 10, 2006)

Josuah said:


> The only reason I think you might have issues with mixing is because the gain will not be uniform.
> 
> Have you read any technical statements that are in favor of passive biamping? I've only seen subjective statements.


Thats what I was thinking too. 

I have mainly read subjective reviews which range from 'no difference' to 'huge difference', I have some tech articles on my laptop at home. There's the obvious power increase if nothing else.

Maybe I wont notice a difference by going to the passive biamp setup, but it is a step on the road to an active biamp setup, which I'm confident will be a huge improvement. 

Harry.


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## Josuah (Apr 26, 2006)

There's no power increase though. You're not going to be sending 400W to your speaker instead of 200W. Your "top" binding posts will still see the same signal they did before, and so will your "bottom" binding posts. Only difference being that your amp sees a slightly different LCR than it did before. And I think that has a negligible effect. But I'm not sure. I haven't done the math.

Also, unless your speakers are designed for active biamping, you'll probably end up putting in a bad crossover which will actually be a huge negative rather than improvement.


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## Otto (May 18, 2006)

What are the gains of each amp? I think if the gains are the same, then your general increase in signal should be the same. The power rating of the amp doesn't tell the whole story.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Otto, the amps don’t have any gain controls (Hakka mentioned that in his opening post). On top of that, they have different input sensitivities, which is probably going to make either the tweeter or woofer louder than it would normally be, depending on which amp he uses for which driver.

Hakka,


> I have mainly read subjective reviews which range from 'no difference' to 'huge difference', I have some tech articles on my laptop at home. There's the obvious power increase if nothing else.


I’ll bet that input sensitivity situation has a la lot to do with the widely-varying feedback you get with passive bi-amping. For instance, if the amp someone uses for their tweeter ends up boosting it a smidge, they would probably be describing the sound as “more detailed, ”opened up,” “air,” etc.

Also, take note of what Joshua said:


> Also, unless your speakers are designed for active biamping, you'll probably end up putting in a bad crossover which will actually be a huge negative rather than improvement.


If and when you do get around to active bi-amping, make sure you use a high-quality electronic crossover. I’m interested in bi-amping my speakers someday, too.

Something else: If you ever do go with active bi-amping, you’ll find out pretty quick how much power passive crossovers soak up. For instance, you’ll probably get more output with a 60-watt amp for the woof and 30-watt for the tweeter than you did from a 200-watt amp running them full range. As such you might want to plan your second amp purchase accordingly, ‘cause a pair of 1070s is going to be way overkill, not to mention a waste of money because you’ll only be using a fraction of what they’re capable of putting out.

Regards,
Wayne


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## Josuah (Apr 26, 2006)

For active biamping of speakers not designed to work that way in the first place, you need to figure out the crossover that is being implemented internally (which is usually secret, i.e. you'll have to figure it out from the parts) and implement that externally using your own parts or with a high quality crossover unit.

The crossover not only affects a frequency amplitude change, but will also have an impact on the phase and impedance, from what little I know. You might match the amplitude with a crossover unit, but not the other things. I don't know enough.


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## Hakka (Sep 10, 2006)

Wayne A. Pflughaupt said:


> Also, take note of what Joshua said:
> If and when you do get around to active bi-amping, make sure you use a high-quality electronic crossover. I’m interested in bi-amping my speakers someday, too.
> 
> Something else: If you ever do go with active bi-amping, you’ll find out pretty quick how much power passive crossovers soak up. For instance, you’ll probably get more output with a 60-watt amp for the woof and 30-watt for the tweeter than you did from a 200-watt amp running them full range. As such you might want to plan your second amp purchase accordingly, ‘cause a pair of 1070s is going to be way overkill, not to mention a waste of money because you’ll only be using a fraction of what they’re capable of putting out.
> ...


When I go active I will use a high quality electronic crossover.

Thats an interesting point you make about the power. I was considering maybe 2x RB1080s just to be safe but seems like that would way way way overkill.

Something else that occurred to me, if I remove the passive crossover network wouldn't I then need to somehow balance the relative levels of the drivers again as the mid and tweeter would have different sensitivities? If this is the case then there wouldn't really be an advantage to using identical amps in the active setup. So I'm thinking maybe get 1 1080 now to use for my mains, move my 1070 to the center, and when I go active use both the 1080 and 1070. It might be overkill but I will have a bigger better amp for my mains in the meantime. It is also possible that I might upgrade my speakers before going to the active setup so the extra grunt might come in handy.

Harry.


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## Wayne A. Pflughaupt (Apr 13, 2006)

> if I remove the passive crossover network wouldn't I then need to somehow balance the relative levels of the drivers again as the mid and tweeter would have different sensitivities?


 Yes you will. Your electronic crossover should have gains for its outputs. If it doesn’t you’ll have to have amps with gains.

Regards,
Wayne A. Pflughaupt


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## F1 fan (Jul 6, 2006)

Wayne A. Pflughaupt said:


> Something else: If you ever do go with active bi-amping, you’ll find out pretty quick how much power passive crossovers soak up. For instance, you’ll probably get more output with a 60-watt amp for the woof and 30-watt for the tweeter than you did from a 200-watt amp running them full range. As such you might want to plan your second amp purchase accordingly, ‘cause a pair of 1070s is going to be way overkill, not to mention a waste of money because you’ll only be using a fraction of what they’re capable of putting out.
> 
> Regards,
> Wayne


Yes I would agree,I recently changed my system to a completely active setup (I previously had a passive mid /tweet xover)and can say that not having several db of driver sensitivty being robbed by a passive crossover means that amplifier power is used much more efficiently.I now use a 60watt amp on my both my mids and tweets and I have yet to see the clipping LED's light up on the tweeter amp and only seldom on the mid amp and that is only when I am listening at very high levels. I could not reach the same spl's cleanly with the passive biamp setup.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Fred,

Did you modify factory speakers for active bi-amping, or are they DIY?

Regards,
Wayne


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## F1 fan (Jul 6, 2006)

Wayne A. Pflughaupt said:


> Fred,
> 
> Did you modify factory speakers for active bi-amping, or are they DIY?
> 
> ...


Hi Wayne ,it was a DIY design.


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## Guest (Jan 7, 2007)

Personally I feel that if it was 'passive' in the beginning and that was what the speaker design was based on then that is how it should remain.
Changing to 'active' would change everything about how those midrange and tweeter drivers will sound in the existing cabinet they came in.

The midrange and tweeter drivers would be coupled directly to amplifier outputs instead of the passive dividing network. This will cause the midrange and tweeter drivers to 'see' a different impedance, which in turn will affect frequency response of the midrange and tweeter drivers. So just 'matching' levels is not the answer here.
The x-over points maybe close with your new electronic x-over, but the charactor (x-over 'knee') of the sound coming from those midrange and tweeter drivers will be a challenge to tame.

I'm not saying that making this kind of change is a bad thing, it just that this kind of change comes with a bit of a challenge at the other end.


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## F1 fan (Jul 6, 2006)

Bryan Nemecek said:


> Personally I feel that if it was 'passive' in the beginning and that was what the speaker design was based on then that is how it should remain.
> Changing to 'active' would change everything about how those midrange and tweeter drivers will sound in the existing cabinet they came in.
> 
> I'm not saying that making this kind of change is a bad thing, it just that this kind of change comes with a bit of a challenge at the other end.


 Brian,I agree that just blindly removing the passive crossover from a properly designed speaker and going active is no guarantee that you will improve the sound of it.In fact there is a good chance of making it sound worse if it is not done correctly.If it was a good sounding speaker with the stock crossover I would be very hesitant to mess with it.(There are a couple of companies like Linn and PMC that offer properly engineered active upgrades to some of their models.)

Designers of good loudspeakers spend considerable time engineering the passive crossover to try and achieve a good blend between woofer/mid/tweeter.They have access to detailed measurements of the drivers (such as frequency plots) and use sophisticated CAD programs to aid in there development.
If you do not have this same kind of info on the drivers and the transfer function of the individual sections of the crossover then duplicting it actively will be impossible,so it will definately sound different.

However with a DIY design I think that the active approach is superior and offers many benifits if it is done wisely.You still need to have good info on the drivers to make the proper choices on crossover slope and frequency etc.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Those are all good things to keep in mind, but I don’t think I’d let it deter me if I wanted to experiment with my speakers – depending on what I paid for them, of course. I’d just make sure I didn’t make any alterations that couldn’t be reversed!

While it’s true that crossovers typically contain elements to properly blend the tweets and mids, and even do some minor equalizing, all that can be done more effectively electronically. For instance, blending the tweeter to the woofer typically involves power resistors to pad down the tweeter’s output – not a terribly efficient way to accomplish that task, since it wastes power. 

While it’s indeed possible that bypassing the stock crossover and going active can result in poorer sound, it’s equally likely that the drivers will perform much better once they are unencumbered from all those capacitors, chokes and resistors. With programs like REW it’s not hard to do “before and after” response plots to help correlate what you’re hearing.

The prospect of downsizing amplifiers was risen before, but something else important there occurred to me: Since you’re connecting the drivers directly to amplifiers, it probably is a very good idea to downsize to lower-powered amps. Don’t know if all amps are like this, but my 250 wpc Adcoms power up with a surge substantial enough to momentarily dim the lights on that circuit. I imagine that could easily damage or fry a tweeter.

There was a guy over at Home Theater Talk a few years ago who experimented going active with (I think it was) some Adire speakers, it was pretty interesting. If I recall, he got immediate improvements, and things only got better from there as he tweaked the crossovers. I’ll see if I can dig it up and post a link.

Regards,
Wayne


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## Josuah (Apr 26, 2006)

The initial surge should be going to fill up the capacitors in your Adcom, and not be seen by the speakers attached. You can either turn things on in a delayed sequence, or what you should really do is put that amp on its own circuit.

Here's an article that goes a bit into what effects the passive crossover has on the speaker.


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## Hakka (Sep 10, 2006)

Wayne A. Pflughaupt said:


> Don’t know if all amps are like this, but my 250 wpc Adcoms power up with a surge substantial enough to momentarily dim the lights on that circuit. I imagine that could easily damage or fry a tweeter.



I notice a similar thing with my Rotels, when they are powered up the protection circuit is active for a few seconds. When the protection cuts out it causes the lights to dim. There is never anything audible through my speakers at power up though.

There was an interesting point raised about using DIY speakers designed for active biamping, one of the projects on my to do list happens to be a set of DIY speakers, so this might be a good place to start experimenting with the active setup.

Harry.


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## F1 fan (Jul 6, 2006)

With my own personal expierience and everything I have read on the subject I have no doubt that active crossovers have a clear performance advantage over passives if it is implemented properly.
If you can do accurate measurements of the system and have something like the Behringer DCX2496 to play with then you might very well improve the sound of a speaker.But it might not be a simple task to improve the sound of an expertly engineered passively crossed over design like a Revel F52 or Vandersteen 3A just by converting it to active. (Although I sure would like to hear the Vandersteens with 4th order Linkwitz Riley crossovers instead of the stock 1st order design.)

About the amp turn on thump,it is common practice when using an active crossover to connect a hi quality film capacitor to the tweeter to protect it from any DC offset from the amp.It must be large enough to not have any affect on the crossover.


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## F1 fan (Jul 6, 2006)

Here is a very informative article about the benifits of active bi and tri-amping.

part 1 http://sound.westhost.com/bi-amp.htm
part 2 http://sound.westhost.com/bi-amp2.htm


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## Chris in Dallas (Apr 4, 2007)

Regarding the OP, I am, for the brief moment, driving my Energy C-8's in a passive bi-amp configuration with two different Rotel amps. Lows are handled by the RB-981, highs by the 951, at 130 and 50 watts, respectively. They have the two sensitivities your reference: 1.0 V for the 981, and 1.5 V for the 951. They sound magnificent. But, I haven't done any real listening tests to see if I could hear a difference were those sensitivities matched. I don't know how different those input voltages really are. 

I'll be getting a 991 this evening, and it has a 1.0 sensitivity, like the 981, but I don't want to run my highs with the 981 because 130 watts is just ridiculous for powering tweeters. Maybe I should look at swapping my 991 for a 1080 so I can match the sensitivity of the 951. 

Also, I think I'll try a test to see if I think passive biamping is superior to not biamping with my system, just because Joshua believes there is no difference. Maybe he's right, maybe not, maybe my ears will be too tinny to tell.

On the subject of active biamping, I have a few equipment tips for saving moulah. I have accumulated 5 Rane AC-22 electronic stereo crossovers off ebay and craigslist for an average price of about $85 plus shipping. These are very high quality 4th order crossovers with gain for all channels, plus delay if you need it. I am using the previous generation with 1/4" or TRS inputs. 
Welborne Labs should still have some black anodized aluminum binding post plates in there clearance section. These are specifically for active biamping. They measure 4" x 4". I picked up 12 of them for $5 each. Finally, for a good deal on dual binding posts, you can go with Exodus Audio, which is a WBT knock-off, from DIY Audio (google it). They run about $9/ pair and will fit the Welborne plates. Or, you can go with the plates and binding posts that come in a variety of colors from Dayton Audio through Parts Express.

As for the sound quality of industry-built speakers converted to active biamp, I'm also going to explore that. I have 5 pairs of Polk LS-50's to play with. These were the smallest of the LS line, beneath the LS90 and LS70. They were the only of the three that could not be biamped. When I have a minute or 360, I'll pull the crossovers out of a pair and biamp them. I'll be using a Rane MA-6 for the lower drivers, and a Yamaha M35 for the tweeters. Both of these amps have gain controls. I'll use an Adcom GFS-3 speaker selection switch wired to the Rane, and reroute the low level connections by hand, to run another pair of stock LS50's in the same room. That should be a good way to tell if active biamping on industry speakers can sound superior to the stock passive crossover.


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