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Post by creimes on Jul 16, 2019 23:34:14 GMT -5
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Post by Deleted on Jul 17, 2019 0:01:51 GMT -5
No, we got side tracked on other topics. Perhaps in the next call.
Thanks for your advice Chad.
Does anyone want to try bridging an Emotiva Gen 3? I'm so tempted:  There isn't a bridge switch but couldn't the channels be bridged out of polarity by reversing one input channel?
1 Bridged 860 watts ** 8 ohms
Just wondering whether an Emotiva Gen 3 8 channel amp could be bridged to output 4 channels into 8 ohm loads? There's no bridge switch on the Gen 3 amp. But couldnt' the inputs be inverted and a bridge be made?
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Post by AudioHTIT on Jul 17, 2019 5:41:29 GMT -5
No, we got side tracked on other topics. Perhaps in the next call.
Thanks for your advice Chad.
Does anyone want to try bridging an Emotiva Gen 3? I'm so tempted: There isn't a bridge switch but couldn't the channels be bridged out of polarity by reversing one input channel? 1 Bridged 860 watts ** 8 ohms Just wondering whether an Emotiva Gen 3 8 channel amp could be bridged to output 4 channels into 8 ohm loads? There's no bridge switch on the Gen 3 amp. But couldnt' the inputs be made out of polarity and a bridge be made?
Even if you could bridge Gen 3’s you’d be back to an unstable situation into 4 Ohms (bridged amps see half the impedance). If you really want the most clean, stable power power you’d be better off with a pair of DR-2’s vertically bi-amped — 1 stereo amp giving you 1600 watts into each 4 ohm speaker — 800 watts for highs, 800 for lows, they would also be fully balanced/differential (do we know yet if the Onkyo has fully balanced L/R?). If your setup allowed you could place them close to the speakers (like monoblocks) having the additional advantage of short speaker cable. Speaking of monoblocks, you could do the same thing with 4 DR-1’s, giving you 2000 watts per 4 ohm speaker (it would be good to know Tekton’s thoughts on bi-amping). With either of these options you could also grow into them, starting with one DR-2 or two DR-1, then doubling if you want to bi-amp. |
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Post by foggy1956 on Jul 17, 2019 6:09:25 GMT -5
If I remember correctly, Eric is a big fan of Parasound amps. |
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Post by davidl81 on Jul 17, 2019 7:40:47 GMT -5
This has escalated quickly from is the XPA-5 G3 good enough to what if we take 6 XPA-1's bi amp all the main speakers!
What's funny is that there are multiple solutions to your speakers just inside on the Emotiva lineup, and then of course amps outside of Emotiva. I only suggested the DR-3 because its a damn good amp, and a simple single amp for your main LCR speakers. But honestly many of these post are just as good, and maybe sound wise better solutions than the DR-3. It just depends on how elaborate you want your set up to get.
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Post by creimes on Jul 17, 2019 8:58:35 GMT -5
I still recommend Pallpoul's XPA-1's for sale here, a pair of those would be great for these speakers, as per most things in audio we start to overthink it, heck I do it myself haha.
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KeithL
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Post by KeithL on Jul 17, 2019 11:08:37 GMT -5
With the exception of a few specialized models our amps cannot be bridged (channels can be bridged on the A-800; and the output on older generations of the XPA-2 could be bridged). (On fully differential amplifiers, like the old XPA-1 Gen1 and Gen2, and all the new XPA-DR models, the output already consists of two modules operated as a bridge.)
In theory, on any amplifier whose outputs aren't already bridged, you could bridge a pair of channels into a single more powerful channel. You need a way to invert the signal going to one side, but that's relatively simple, especially with balanced inputs (just swap the input wires on one input)
However, when you do so, each previous channel sees a load impedance equal to one half of the actual load. (So, if you connect a 4 Ohm speaker to a bridged amp, each side of the output sees a 2 Ohm load, which is going to be difficult to drive.) Differential amps, like the XPA-DR series, operate from lower supply rails, to enable them to operate efficiently into these lower-impedance loads. In addition, in practice, some amplifier designs may not be perfectly stable when operated in a bridged configuration. (Most will work, but some may oscillate, or do something nasty - because, well, they simply aren't designed to be used that way.)
What you should take away from this is that it really isn't a good idea to experiment with bridging amplifier outputs unless they're designed to be used that way. (There's a good chance it would work fine - but a small chance that something very bad could happen - so better safe than sorry - unless you really know what you're doing.)
The XPA-1 gen 2 is (unofficially) stable at 2 ohms - but not below it. It produces 1700 something watts at that rating - though likely at increased distortion. The reason Lonnie the designer gave to not listing that as a feature was because somebody would then connect it to something that dips below 2 ohm and complain that the amp went in to protection. Interestingly since the XPA-1 gen 2 is a fully differential amp it sees a 4 ohm load as 2 ohms. So for it to be stable with a 2 ohm speaker load, it would be pushing output at 1 ohm. So it is pretty impressive.  The Emotiva XPR-1 (which was an older amp) was so powerful that it produced over 2000 watts per channel at 4 ohms at 0.1 % THD. It had so much beef that both Emotiva and Audioholics were not able to actually measure its maximum power due to the need for large resistors in the measuring equipment and also they were unable to keep the voltage from sagging from the house power. At 1% distortion in a 4 ohm load it produced 2200 watts. I couldn't even begin to imagine what kind of power it could push at 2 ohms. Thanks for your input garbulky. I edited my post and found a possible solution. Do you know whether Emotiva amps are bridgeable? Can I bridge 2 channels in a multiple channel Emotiva Gen 3? |
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Post by Deleted on Jul 17, 2019 14:00:22 GMT -5
With the exception of a few specialized models our amps cannot be bridged (channels can be bridged on the A-800; and the output on older generations of the XPA-2 could be bridged). (On fully differential amplifiers, like the old XPA-1 Gen1 and Gen2, and all the new XPA-DR models, the output already consists of two modules operated as a bridge.)
In theory, on any amplifier whose outputs aren't already bridged, you could bridge a pair of channels into a single more powerful channel. You need a way to invert the signal going to one side, but that's relatively simple, especially with balanced inputs (just swap the input wires on one input)
However, when you do so, each previous channel sees a load impedance equal to one half of the actual load. (So, if you connect a 4 Ohm speaker to a bridged amp, each side of the output sees a 2 Ohm load, which is going to be difficult to drive.) Differential amps, like the XPA-DR series, operate from lower supply rails, to enable them to operate efficiently into these lower-impedance loads. In addition, in practice, some amplifier designs may not be perfectly stable when operated in a bridged configuration. (Most will work, but some may oscillate, or do something nasty - because, well, they simply aren't designed to be used that way.)
What you should take away from this is that it really isn't a good idea to experiment with bridging amplifier outputs unless they're designed to be used that way. (There's a good chance it would work fine - but a small chance that something very bad could happen - so better safe than sorry - unless you really know what you're doing.)
Thanks for your input garbulky. I edited my post and found a possible solution. Do you know whether Emotiva amps are bridgeable? Can I bridge 2 channels in a multiple channel Emotiva Gen 3? Thanks for clarifying Keith!
I'll resist the temptation to try bridging the XPA Gen 3.
I was really delving into this topic about Emotiva's capabilities as a curiosity point. If I am correct, biamping these speakers would result in 2 8 ohm channels each. If it were possible to bridge two channels of the amp then they'd see half the impedance, a 4ohm bridged output.
It would be easy indeed to switch two of the pins on one side of a XLR Y splitter to invert one channel:
Just clarifying.
Many thanks, William
P.S. My order was placed for the Ulfberhts L/C/R.
Can't wait to hear my favorite song "Tin Pan Alley" by Stevie Ray Vaughn!
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Post by Deleted on Jul 18, 2019 0:48:03 GMT -5
Shimei, I enjoyed several conversations with Eric prior to taking delivery of the DIs. Very entertaining gentleman that was most gracious with his time. Must not sleep much. I'm very happy to see him succeed. My room is about 14x20. Open to another larger living space. I would suggest building a mock up of the Ulfberht to appreciate it's size. As far as flooring and room interaction, the wife recently opted to replace the sofa. Sold the old one in two hours. Tossed the rug. The hardwood floors are a tad bright to my ear without the furnishings. To each his own. Once you have everything installed, you can always tweak. Boomzilla had some outriggers from Soundocity on a pair of Pendragons he reviewed last year. I was tempted to discern what impact they might have on my Impacts  Steve Had to share these sound dispersing/absorption panels. They are visually appealing. Haven't heard them yet. But thought some may get a kick of some high end sound absorption ideas:
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Post by tropicallutefisk on Jul 18, 2019 5:46:13 GMT -5
Shimei, I enjoyed several conversations with Eric prior to taking delivery of the DIs. Very entertaining gentleman that was most gracious with his time. Must not sleep much. I'm very happy to see him succeed. My room is about 14x20. Open to another larger living space. I would suggest building a mock up of the Ulfberht to appreciate it's size. As far as flooring and room interaction, the wife recently opted to replace the sofa. Sold the old one in two hours. Tossed the rug. The hardwood floors are a tad bright to my ear without the furnishings. To each his own. Once you have everything installed, you can always tweak. Boomzilla had some outriggers from Soundocity on a pair of Pendragons he reviewed last year. I was tempted to discern what impact they might have on my Impacts Steve Had to share these sound dispersing/absorption panels. They are visually appealing. Haven't heard them yet. But thought some may get a kick of some high end sound absorption ideas:
Those panels are awesome. Very modern and very visually stimulating |
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Post by AudioHTIT on Jul 18, 2019 12:27:45 GMT -5
... If I am correct, biamping these speakers would result in 2 8 ohm channels each. If it were possible to bridge two channels of the amp then they'd see half the impedance, a 4ohm bridged output. ... I don’t believe this is true, at least with passive bi-amping, however if you heard it from the designer, or someone well versed in crossover and speaker design then I’ll stand corrected. You may know all this but, one parameter speaker designs ‘try’ to accomplish, is to present a constant impedance across the frequency range, this allows the amp to work with ‘similar’ efficiency and stability at all frequencies, and can help the speaker achieve the more important goal of flat frequency response. However it’s not usually accomplished very well and most speakers have pretty erratic impedance curves (some to the detriment of many an amplifier). So the ‘nominal’ impedance a speaker specs is somewhat of an average for all frequencies. So if your speaker is bi-amp capable, and removing the ‘jumpers’ are the method for exposing that capability, then each set of terminals will present its portion of the nominal rating to the amp — as made up by the crossover, drivers, back EMF, etc. Assuming that the ‘nominal’ rating for the speaker somewhat represents the entire frequency range, then the individual impedance presented at each crossover entry will be similar to the nominal rating of the entire speaker. I believe you are assuming that the 4 ohm rating of the Tekton is created by running two 8 ohm loads in parallel, and while this would be true for resistors, it is not true for the complex impedance presented by a crossover and speaker load. So I believe your bi-amp capable speakers will present (roughly) two 4 ohm loads, not two 8 ohm loads. As I said I’m willing to fall on my sword if someone more knowledgeable corrects me, but that’s my understanding of the situation. Edit: I should have said ‘true for two identical speakers or drivers’ instead of resistors. |
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Post by Deleted on Jul 18, 2019 13:45:01 GMT -5
... If I am correct, biamping these speakers would result in 2 8 ohm channels each. If it were possible to bridge two channels of the amp then they'd see half the impedance, a 4ohm bridged output. ... I don’t believe this is true, at least with passive bi-amping, however if you heard it from the designer, or someone well versed in crossover and speaker design then I’ll stand corrected. You may know all this but, one parameter speaker designs ‘try’ to accomplish, is to present a constant impedance across the frequency range, this allows the amp to work with ‘similar’ efficiency and stability at all frequencies, and can help the speaker achieve the more important goal of flat frequency response. However it’s not usually accomplished very well and most speakers have pretty erratic impedance curves (some to the detriment of many an amplifier). So the ‘nominal’ impedance a speaker specs is somewhat of an average for all frequencies. So if your speaker is bi-amp capable, and removing the ‘jumpers’ are the method for exposing that capability, then each set of terminals will present its portion of the nominal rating to the amp — as made up by the crossover, drivers, back EMF, etc. Assuming that the ‘nominal’ rating for the speaker somewhat represents the entire frequency range, then the individual impedance presented at each crossover entry will be similar to the nominal rating of the entire speaker. I believe you are assuming that the 4 ohm rating of the Tekton is created by running two 8 ohm loads in parallel, and while this would be true for resistors, it is not true for the complex impedance presented by a crossover and speaker load. So I believe your bi-amp capable speakers will present (roughly) two 4 ohm loads, not two 8 ohm loads. As I said I’m willing to fall on my sword if someone more knowledgeable corrects me, but that’s my understanding of the situation. Edit: I should have said ‘true for two identical speakers or drivers’ instead of resistors. You're probably right. I haven't directly heard about the bi-amp impedance and don't want to bother Eric about this. I can easily test the ohm rating when the speakers arrive.
I'm probably going to just test initially with my XPA-5. I'll run the amp to provide four channels to the Ulfberhts in bi amp configuration. And the 5th channel to the center Ulfberht. Possibly running the system in a L/C/R and 2 Subwoofer configuration. The Onkyo PR-RZ5100 does support a wide range of configurations including a 3-0-2 setup.
I'll add another amp if need be later. Until then I'll sacrifice the Atmos height speakers. I have no idea as of yet how I'm going to run the Atmos. The ceilings are cathedral and too tall in distance to do an upward firing Atmos speaker arrangement which I have now. And the ceiling has all kinds of angles which will take some thinking to install in ceiling units.
And then there's the sound dampening. I was actually thinking about doing something like this hanging down level from the ceiling. Again, it is going to take some thought to get Atmos in the room:
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KeithL
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Post by KeithL on Jul 18, 2019 16:00:56 GMT -5
It's not quite that simple (with bi-amping).
I'm going to skip the detailed explanations..... but here's the result when you bi-amp that speaker......
The amplifier driving the tweeter sees the same 8 Ohm load as before AT THE FREQUENCIES BEING REPRODUCED BY THE TWEETER. However, the amplifier driving the tweeter sees almost no load at all AT THE FREQUENCIES BEING REPRODUCED BY THE WOOFER.
The amplifier driving the woofer sees the same 8 Ohm load as before AT THE FREQUENCIES BEING REPRODUCED BY THE WOOFER. However, the amplifier driving the woofer sees almost no load at all AT THE FREQUENCIES BEING REPRODUCED BY THE TWEETER.
Therefore, in terms of the load it sees, each amplifier sees the same load as before over the range of frequencies it is driving. But it sees almost no load over the range of frequencies that the OTHER amplifier is driving.
So, assuming a perfect speaker, with a crossover at 1 kHz..... The amplifier driving the woofer sees an 8 Ohm load from 20 Hz to 1 kHz ... but almost no load above 1 kHz. The amplifier driving the tweeter sees an 8 Ohm load from 1 kHz to 20 kHz ... but almost no load BELOW 1 kHz. As a result, each amplifier is being asked to do less work, because it only sees a load for some frequencies.
And, since the loads at the different frequency ranges are being powered by different amplifiers, there's also less chance that loading one heavily will affect the sound of the other.
(So, technically, both amplifiers see the same load impedance as before, but are only "loaded" over a narrower frequency range, and so don't have to work as hard.)
(So, when you bi-amp an 8 Ohm speaker, each half is still "an 8 Ohm speaker", but each one only operates over a limited frequency range.)
I'm not really considering this but if the Ulfberhts are bi-amp ready then wouldn't each speaker connection be 8 ohms? 8 Ohms paralleled wired would equal the one 4 ohm rating of the Ulfberhts. If the Tektons are bi-amp ready, running two 8 ohms in series would equal 16 ohm load and the mono blocked amps bridged or otherwise would probably make half the output. The only option would be to bi-amp and run a mono block or bridged amp on each speaker channel which would be the correct 8 ohm rating. That means I'd need 4 bridged (8 channels) or 4 mono block amps which would all be running at 8 ohms each. Are Emotiva Gen 3 amps bridgeable? If so I'd only need to order one more 8 channel amp. Then in total 13 channels with 8 bridged together to power up the Ulfberhts, 1 channel for the center, and 4 channels for the Atmos. If the Emotiva amps are bridgeable then puzzle solved.
(I doubt the Ulberhts if bi amp ready are 2 ohm running in series to produce the 4 ohm rating.)
Shame home audio isn't like car audio where pro level amps are stable at 1 or 2 ohms.
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Post by AudioHTIT on Jul 18, 2019 16:20:28 GMT -5
... Therefore, in terms of the load it sees, each amplifier sees the same load as before over the range of frequencies it is driving. But it sees almost no load over the range of frequencies that the OTHER amplifier is driving. ... I think that more or less confirms what I said, but to clarify, when you say “no load” would that actually be an ‘infinite load’, thereby the amp generates no current at the frequencies outside the range of the driver. Edit: I guess it’s semantics, I think of no load as in the impedance approaches zero, whereas an amp with nothing on its terminals has “no load” yet the impedance is approaching infinite. |
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Post by Deleted on Jul 18, 2019 16:52:15 GMT -5
It's not quite that simple (with bi-amping).
I'm going to skip the detailed explanations..... but here's the result when you bi-amp that speaker......
The amplifier driving the tweeter sees the same 8 Ohm load as before AT THE FREQUENCIES BEING REPRODUCED BY THE TWEETER. However, the amplifier driving the tweeter sees almost no load at all AT THE FREQUENCIES BEING REPRODUCED BY THE WOOFER.
The amplifier driving the woofer sees the same 8 Ohm load as before AT THE FREQUENCIES BEING REPRODUCED BY THE WOOFER. However, the amplifier driving the woofer sees almost no load at all AT THE FREQUENCIES BEING REPRODUCED BY THE TWEETER.
Therefore, in terms of the load it sees, each amplifier sees the same load as before over the range of frequencies it is driving. But it sees almost no load over the range of frequencies that the OTHER amplifier is driving.
So, assuming a perfect speaker, with a crossover at 1 kHz..... The amplifier driving the woofer sees an 8 Ohm load from 20 Hz to 1 kHz ... but almost no load above 1 kHz. The amplifier driving the tweeter sees an 8 Ohm load from 1 kHz to 20 kHz ... but almost no load BELOW 1 kHz. As a result, each amplifier is being asked to do less work, because it only sees a load for some frequencies.
And, since the loads at the different frequency ranges are being powered by different amplifiers, there's also less chance that loading one heavily will affect the sound of the other.
(So, technically, both amplifiers see the same load impedance as before, but are only "loaded" over a narrower frequency range, and so don't have to work as hard.)
(So, when you bi-amp an 8 Ohm speaker, each half is still "an 8 Ohm speaker", but each one only operates over a limited frequency range.)
I'm not really considering this but if the Ulfberhts are bi-amp ready then wouldn't each speaker connection be 8 ohms? 8 Ohms paralleled wired would equal the one 4 ohm rating of the Ulfberhts. If the Tektons are bi-amp ready, running two 8 ohms in series would equal 16 ohm load and the mono blocked amps bridged or otherwise would probably make half the output. The only option would be to bi-amp and run a mono block or bridged amp on each speaker channel which would be the correct 8 ohm rating. That means I'd need 4 bridged (8 channels) or 4 mono block amps which would all be running at 8 ohms each. Are Emotiva Gen 3 amps bridgeable? If so I'd only need to order one more 8 channel amp. Then in total 13 channels with 8 bridged together to power up the Ulfberhts, 1 channel for the center, and 4 channels for the Atmos. If the Emotiva amps are bridgeable then puzzle solved.
(I doubt the Ulberhts if bi amp ready are 2 ohm running in series to produce the 4 ohm rating.)
Shame home audio isn't like car audio where pro level amps are stable at 1 or 2 ohms.
Awesome and thorough response!
While I agree lemme point out that even a single speaker rated at 4 ohms will have different impedance at various frequencies.
Correct me if wrong! Doesn't speaker impedance change? If I take an ohm meter and measure a woofers impedance and push in or out the woofer at various points of excursion does it not change? Not meaning to overthink this but I was referring to nominal impedance.
Thank you Keith! You're a treasure of knowledge!
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Post by leonski on Jul 18, 2019 17:31:33 GMT -5
I see these are fairly HIGH sensitivity speakers. I'd buy Better watts, not just MORE.
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Post by AudioHTIT on Jul 18, 2019 18:48:09 GMT -5
... If I take an ohm meter and measure a woofers impedance and push in or out the woofer at various points of excursion does it not change? ... You can’t measure impedance with an ohm meter, it measures resistance and your mostly seeing the resistance of the woofer’s voice coil with some crossover components adding to the mix. Pushing the woofer generates some small voltage which affects the reading. If you try to measure most high end (tweeter) inputs, you won’t get a reading because the high pass filter from the crossover is blocking the DC the meter is using to measure. Measuring impedance is more complex. |
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Post by AudioHTIT on Jul 18, 2019 19:06:46 GMT -5
... I'm probably going to just test initially with my XPA-5. I'll run the amp to provide four channels to the Ulfberhts in bi amp configuration. And the 5th channel to the center Ulfberht. Possibly running the system in a L/C/R and 2 Subwoofer configuration. The Onkyo PR-RZ5100 does support a wide range of configurations including a 3-0-2 ... I think this is a great idea, if you have all 5 speakers you should try that too. I’ve always wondered if those who have 5.x setups with overhead surrounds, might try them as 3.x.2. In your case you could could try 3.2.2 or 5.2 (with subs), or as you say 3.0.2 without. Try everything you can with your existing gear. The pictures of room treatments you’ve posted are interesting too. |
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Post by leonski on Jul 19, 2019 1:00:49 GMT -5
If you REALLY want to biamp, and with the overall VERY high sensitivity of these speakers, I can't figure the need, your best bet, especially to avoid what Keith wrote about is to GO ACTIVE.
In Short:
First? Find out about the speakers built-in crossover. Frequencies, 'order' and 'type' for a start.
Buy a MiniDSP to cover your needs. Go to their website and check it out.
You'll end up with an amplifier connected to each 'way' of the speaker AND no out of band signal TO the speaker so Keith's objections are off-the-table.
For this type of setup? You could EASILY use the PA-1 amp at 3 per speaker. This would give you incredible power AND ultimately more control than you'll know what to do with.
The MiniDSP has a good software setup which gives you more power than a speaker designer of maybe only 10 years ago could Dream About.
The MiniDSP as crossover goes between the PREAMP and the AMP. And it's all done at LINE LEVEL......
I'd avoid bridging, too. In decades in this 'hobby', I've heard of only a few examples of bridging actually helping. Most often, you go backwards as far as sound quality.
What makes you even THINK you need so much power? How much do you think you need? Keep in mind that doubling power only results in a net of 3db increase. So If you have
100 watts per speaker NOW, a bump to 200 watts is really minimal. Assuming all watts are 'equal', which is certainly NOT the case. 10x the power to an even KiloWatt would buy you
a more meaningful 10db increase. You see where this is leading, right? If the speakers I looked at in the Tekton line are what you have, and at 99db sensitivity? 20 watts would put you
out of the house. And 150 would get you evicted from the neighborhood. About a 9db increase.
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KeithL
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Post by KeithL on Jul 19, 2019 9:25:46 GMT -5
You're quite right - the impedance of most speakers varies significantly over their operating range (in terms of frequency). The so-called "nominal impedance" is just a sort of general approximation. There are, however, a few "guidelines" that are usually followed.
For example, it is lower impedance that requires more current from an amplifier, and so makes it work harder. Therefore, it's not considered to be important if a certain speaker has an impedance that goes very high at a certain frequency, but having an impedance that dips especially low is considered to be "bad form". The general guideline is that a "well behaved" speaker shouldn't dip much below about 75% of nominal impedance... So, for example, a 4 Ohm speaker shouldn't dip much below 2.75 Ohms or perhaps 3 Ohms.
It's also less of an issue if a speaker has an impedance that dips very low at very high frequencies - because there is less energy at those frequencies (so the amplifier isn't being strained as much).
Now... as for impedance and driver excursion. In theory a driver should have a consistent impedance over its full range of excursion. (If not, that would suggest that the force driving it will vary as it moves, while reproducing a waveform, which would cause distortion.) In practice, this is never perfect, and some drivers are designed this way as a way to protect themselves. (Instead of whacking on the end stops if you overdrive them they just sort of stall instead... which doesn't sound quite as bad and doesn't actually break anything.)
It also relates to driver efficiency. (As a designer, you're trading low distortion over a long excursion, against better efficiency during short excursions, and a bigger and more expensive magnet.)
I should also point out two flaws in what you said about measuring a speaker with an Ohmmeter. First, a normal Ohmmeter uses DC current to measure resistance, so it will be measuring DC resistance and not the impedance of the voice coil. In order to measure impedance you need to use an AC test signal.
Second, there's a lot more going on when you push on a woofer. Yes, by moving the position of the voice coil relative to the magnetic field, you may alter the impedance (by pushing it to a less linear region of its travel). (This really shouldn't be the case and would normally be considered to be a design flaw... unless it's being done deliberately as a sort of protection.)
However, the impedance of a speaker is largely determined by its mechanical characteristics... like the moving mass of the cone and the spring force provided by the suspension. By pressing directly on it, you are restricting its movement, which is going to have a major effect on those other parameters. It would be the equivalent of measuring the characteristics of an electric motor - while holding the shaft so that it can't spin...
The results will be quite far from those you would see "under normal operation".
(It's also rather dangerous to push on a woofer cone because some speakers can be damaged rather easily that way.)
It's not quite that simple (with bi-amping). I'm going to skip the detailed explanations..... but here's the result when you bi-amp that speaker...... The amplifier driving the tweeter sees the same 8 Ohm load as before AT THE FREQUENCIES BEING REPRODUCED BY THE TWEETER. However, the amplifier driving the tweeter sees almost no load at all AT THE FREQUENCIES BEING REPRODUCED BY THE WOOFER.
The amplifier driving the woofer sees the same 8 Ohm load as before AT THE FREQUENCIES BEING REPRODUCED BY THE WOOFER. However, the amplifier driving the woofer sees almost no load at all AT THE FREQUENCIES BEING REPRODUCED BY THE TWEETER.
Therefore, in terms of the load it sees, each amplifier sees the same load as before over the range of frequencies it is driving. But it sees almost no load over the range of frequencies that the OTHER amplifier is driving. So, assuming a perfect speaker, with a crossover at 1 kHz..... The amplifier driving the woofer sees an 8 Ohm load from 20 Hz to 1 kHz ... but almost no load above 1 kHz. The amplifier driving the tweeter sees an 8 Ohm load from 1 kHz to 20 kHz ... but almost no load BELOW 1 kHz. As a result, each amplifier is being asked to do less work, because it only sees a load for some frequencies.
And, since the loads at the different frequency ranges are being powered by different amplifiers, there's also less chance that loading one heavily will affect the sound of the other.
(So, technically, both amplifiers see the same load impedance as before, but are only "loaded" over a narrower frequency range, and so don't have to work as hard.)
(So, when you bi-amp an 8 Ohm speaker, each half is still "an 8 Ohm speaker", but each one only operates over a limited frequency range.)
Awesome and thorough response! While I agree lemme point out that even a single speaker rated at 4 ohms will have different impedance at various frequencies.
Correct me if wrong! Doesn't speaker impedance change? If I take an ohm meter and measure a woofers impedance and push in or out the woofer at various points of excursion does it not change? Not meaning to overthink this but I was referring to nominal impedance.
Thank you Keith! You're a treasure of knowledge!
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There isn't a bridge switch but couldn't the channels be bridged out of polarity by reversing one input channel? 
