AudioCircle
Industry Circles => GR Research => Topic started by: Danny Richie on 5 Jan 2019, 10:52 pm
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This is another topic that I wrote about and posted in one of the Facebook pages for audio stuff.
It's a long post, but follow through and study the measurements and it will all make sense.
I see lots of guys here running wide band (or full range) drivers on their open baffle speakers. Most go for large diameter drivers as well. Unfortunately most large diameter drivers struggle to play the top octave. They also have very poor off axis response because of the size of the diaphragm. So the rooms are loaded un-evenly. Lower frequencies and even mid-range areas are somewhat omni direction minus the 90 degrees off axis cancellation of the open baffle design. But upper mid-range areas and all the higher frequencies beam the signal forward only, and as frequency increases the coverage area gets smaller and smaller. You hear the upper ranges only if your ear is directly in line with the drivers.
Off topic but another issue is that most of the wide band drivers are run without a filter at all. I have yet to measure one that did not need some sort of filter to control a rising response, break up, ringing, and in some cases a baffle step loss. Don't worry guys, high quality parts in a filter won't degrade the signal, but they can solve or help control amplitude and break up issues. And those issues are much worse and much harder to listen to.
Okay back to adding a super tweeter. Most guys think that going for some type of ribbon or air motion device is the way to go. It's not. Most are too tall and limit dispersion. You are trying to add the lost upper ranges in a universal pattern and not a limited one.
Another issue is that often the acoustic centers are way too far apart. So the two drivers will always sound like too different drivers playing from two different sources. To accurately blend to drivers crossed at 10kHz then the acoustic centers of the two drivers need to be within 1.25" apart. This is because the wavelengths are so short. And that is not possible.
You are much better off allowing the tweeter to cover as wide of a range as possible and cross as low as the tweeter will allow. And it doesn't matter how good you think your full range driver is. Any tweeter from 2kHz and up will outperform it in every way. It is just flat out going to have better dispersion, speed, and resolution due to its smaller size and much less moving mass.
But many insist on running the wide band driver full range and dropping in the tweeter in the top octave. This doesn't work like most think it does. It really doesn't fill in the upper range as much as it causes huge suck outs over a really wide range. This is especially the case when a single capacitor is used on the tweeter as a filter, and it is especially worse the farther the acoustic centers are from one other. I will illustrate exactly what happens in the measured responses that I will follow this with.
So to illustrate what happens. I used a FR-125 full range driver. It has an exceptionally smooth response compared to most drivers. And it sounds really good too. It is a 4" diameter driver. The cabinet is a small ported box. The front baffle is 9" tall by 6" wide and the driver is mounted in one end of it. To show the effects of the distance differences I have taken measurements with the woofer farther from and closer to the added tweeter that I will sit on top of the box.
First lets try what everyone does. I have oriented the woofer to near the bottom of the box (flipped it over) and added an Aurum Cantus G-2 ribbon tweeter to the top of it. To minimize cancellation effects I used only a 1.0uF capacitor on it. The green line is the FR-125 driver alone and the purple line is the ribbon tweeter alone. Note how low in range the ribbon still plays even with a tinny 1.0uF cap on it. Since the full range driver is the primary driver I took the measurements on the full range driver axis (in line with it).
(http://gr-research.com/measurements/1%20FR-125%20and%20G2%20ribbon.jpg)
So we should get green plus purple and equal additional output right? Okay, here is the sum of them together in red. What happened? This isn't a better response. There is almost a 20db swing in response. Well what's going on is that the short wavelengths arrive out of phase with each other from the two sources due to distance differences. This is the affect most people get when doing this.
(http://gr-research.com/measurements/2%20FR-125%20and%20G2%20ribbon%20combined.jpg)
Now lets move up to the tweeter axis. Note how the top end picked up on the tweeter. That is because of the height of the tweeter. It will have a limited vertical dispersion due to its length that is about two and a half inches tall. So getting in line with it will bring the top end up and it changes the time delay to the lower full range driver.
(http://gr-research.com/measurements/3%20FR-125%20and%20G2%20ribbon%202.jpg)
Now let's combine them again. We still have multiple cancellation points where the output of both drivers is less than the output of one driver. It's still a big mess. And this is just what is happening at one point in space. In the off axis it gets much worse.
(http://gr-research.com/measurements/4%20FR-125%20and%20G2%20ribbon%20combined%202%20flipped%20polarity.jpg)
Now lets trying varying the offset. So for this measurement I pushed the tweeter forward by 3/8". Yes, three eighths of an inch! The new measurement in the orange line. It moved a lot didn't it, and it is still a mess.
(http://gr-research.com/measurements/5%20pushed%20forward%20three%20eights%20of%20an%20inch%20of%20an%20inch.jpg)
Okay, now I am going to push the tweeter forward to 1/2". Yes just one half of an inch forward. It is now the yellow line. Keep in mind that it is just pushed forward only one eighth of an inch further than the measurement that is the orange line. That's what happens in those shorter wavelength ranges. It requires very close center to center spacing and steep filters to avoid these effects.
(http://gr-research.com/measurements/6%20pushed%20forward%20one%20half%20of%20an%20inch.jpg)
Now let's try flipping the polarity of the tweeter to see what happens. Now we have coupling above and below the crossing frequency and cancellation at the crossing frequency.
(http://gr-research.com/measurements/7%20FR-125%20and%20G2%20ribbon%20combined%202.jpg)
This time I moved them around to find a distance to where they coupled the most. The red line is the combined output again.
(http://gr-research.com/measurements/8%20Red%20line%20on%20woofer%20axis%20best%20response.jpg)
Now pay attention. Watch what happens when I move the microphone up just 4". Keep in mind also that the microphone is 1 meter away. The response moved a huge amount. You see, even if you use a digital filter to correct the response in one spot, moving just out of that spot nullifies the whole thing.
(http://gr-research.com/measurements/9%20Orange%20line%20up%20four%20inches.jpg)
Now I am moving the microphone up another 4". It made the yellow line. Now it is coming back up again, but still showing cancellation effects.
(http://gr-research.com/measurements/10%20Yellow%20line%20up%20four%20more.jpg)
Let's try something else. Let's use a 1" dome tweeter. It will have a more even off axis response in all directions. Here the full range driver is in green again. The purple response is the dome tweeter with a 1.0uF cap on it. The level was a little high. Most people think they can just add an inline resistor to balance the output. So I added an inline resistor to bring the level down. Note though, that the level did not come down evenly. It brought down the top, but left the bottom end. It is in light blue. That didn't help at all. These were measured on the tweeter axis.
(http://gr-research.com/measurements/D1%20resistor%20added%20to%20dome%20tweeter%20on%20tweeter%20axis%20tweeter.jpg)
Now here is the measured response of both drivers on the full range driver axis.
(http://gr-research.com/measurements/D2%20with%20dome%20tweeter%20on%20woofer%20axis.jpg)
Now here in red is the sum. Not too bad for a first shot, but it didn't help the overall response any.
(http://gr-research.com/measurements/D3%20dome%20tweeter%20and%20woofer%20combined.jpg)
Let's flip the polarity of the tweeter and see what happens. That is it in red. This made them in phase across the board for this point in space. This is not an easy spot to find either. The response can easily fall apart anywhere else that you can measure from.
(http://gr-research.com/measurements/D4%20tweeter%20polarity%20flip%20purlpe%20line.jpg)
Now here is just the summed response in red. Let's see how fragile this is now.
(http://gr-research.com/measurements/D5%20dome%20tweeter%20and%20woofer%20combined%20on%20woofer%20axis.jpg)
So I pushed the dome tweeter forward three eighths of an inch and this happened (in orange).
(http://gr-research.com/measurements/D6%20dome%20tweeter%20moved%20forward%20three%20eights%20of%20an%20inch.jpg)
Now the tweeter has been pushed forward a total of just over one half of an inch. See it in yellow. Moving the microphone up or down also changes the time delay and has the same effect. Quite the mess huh? But a lot of people think they can just mount a tweeter on the baffle somewhere and it will fill in just fine. The really bad part is that many people really don't hear the upper octaves very well and don't know any different.
(http://gr-research.com/measurements/D7%20dome%20tweeter%20moved%20forward%20one%20half%20of%20an%20inch.jpg)
Another really bad part is that this negatively affects the imaging and sound stage layering by the cancellation effects and holes in the response. This creates a very unbalanced room response.
Now let's look at a solution. Some people resort to rear firing the tweeter. This helps in that it doesn't effect the on axis response but in an open baffle design it does affect the rear wave output just like in the illustrations that you saw already. So the in room response is still negatively affected.
The real way to implement a super tweeter without any of the negative effects that you've seen here is to face the tweeter upward. And ideally line the tweeter up (tweeter voice coil) so that it is inline with (or centered on) the voice coil of the full range driver. This will keep the drivers time aligned facing forward and backward. So the full range drivers limited off axis response won't allow it to overlap to the limited (90 degrees off axis) off axis response of the upward facing tweeter. No overlapping.
The great part is that this adds the low output, upper octaves, back to the room response, and balances out the room load. This more balanced sound and upper range extension greatly improves the imaging and sound stage layering that is present in those upper octave harmonics. All of those spatial cues in the music are up there in those upper ranges. Bringing them back can be a revelation.
So let's see. Here is the on axis measured response of both drivers. Note the tweeter is very down in output over 90 degrees off axis.
(http://gr-research.com/measurements/S1%20dome%20tweeter%20facing%20up.jpg)
The red line is the sum of the two. Looks pretty good huh?
(http://gr-research.com/measurements/S2%20dome%20tweeter%20facing%20up%20combined.jpg)
Now let's improve them further by flipping the box over to get the drivers acoustic centers closer together. This will also help make the driver blending better.
(http://gr-research.com/measurements/S3%20dome%20tweeter%20and%20woofer%20combined%20closer%20acoustic%20centers.jpg)
So here is the new on axis response. And yes a little filter on the full range driver to compensate for baffle step loss would bring the level above 1kHz down to match the level below that, and balance out the sound. See I told you guys they always need a filter.
(http://gr-research.com/measurements/S4%20with%20up%20firing%20dome%20on%20axis.jpg)
Now let's test the concept and measure in the vertical off axis again. Here is a new measurement (in orange) of the response with the microphone moved up 4".
(http://gr-research.com/measurements/S5%20with%20up%20firing%20dome%20four%20inches%20up.jpg)
Now up 8" (yellow line).
(http://gr-research.com/measurements/S6%20with%20up%20firing%20dome%20eight%20inches%20up.jpg)
And up 12" (green line). So even in the vertical off axis the response remains clean and free of coupling and cancellation errors.
(http://gr-research.com/measurements/S7%20with%20up%20firing%20dome%20twelve%20inches%20up.jpg)
For you guys running a full range driver try adding an upward facing tweeter and hear what happens.
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How would proximity to the back wall change the upward firing tweeter response?
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I have no knowledge whatsoever of technical measurements but I know what sounds good to me, and that's what I care about.
For many years now I've used Cain & Cain Abby single driver horn-type speakers and love the sound they produce, but for my tastes they needed more bottom end so I added a pair of subs that filled in the missing deep bass. On someone's suggestion I also added a pair of Fostex T900A super tweeters to bring in some high end sparkle and I've never looked back. I hadn't expected it, but they also added considerable width and depth to the soundstage
In fact a couple of months ago one of my super tweeters failed and I immediately bought a replacement, that's how much I missed it. The T900As aren't inexpensive at $500 apiece (at least) but in my system they are very addictive, an essential ingredient in the sound I love.
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How would proximity to the back wall change the upward firing tweeter response?
It will not.
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I have no knowledge whatsoever of technical measurements but I know what sounds good to me, and that's what I care about.
For many years now I've used Cain & Cain Abby single driver horn-type speakers and love the sound they produce, but for my tastes they needed more bottom end so I added a pair of subs that filled in the missing deep bass. On someone's suggestion I also added a pair of Fostex T900A super tweeters to bring in some high end sparkle and I've never looked back. I hadn't expected it, but they also added considerable width and depth to the soundstage
In fact a couple of months ago one of my super tweeters failed and I immediately bought a replacement, that's how much I missed it. The T900As aren't inexpensive at $500 apiece (at least) but in my system they are very addictive, an essential ingredient in the sound I love.
The addition of the tweeter can easily give you what you have noted. But at the same time a forward facing super tweeter can and will cause on axis and off axis cancellations. You may or may not notice the effect. It also could be no worse on that speaker than the response was already. Personally, I notice the cancellation effect quite quickly. Try facing it upward and see (or hear) if the overall response improves.
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I added a pair of LCY 100K-MK2 top mount ribbon super tweeters to a pair of Omega Super 3 High Output monitors. I’ve positioned ‘em to the very rear of the monitors. Adds a subtle layer of sparkle...albeit...time will tell if it becomes fatiguing. I don’t hear any comb filtering or out of phase issues. I also have a Decware ZROCK2 in my desktop system to round out the lower (bass) octaves. So far it all works...to my ears...or...I have a horrible sense of imaging. :lol:
(https://www.audiocircle.com/image.php?id=216654)
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You will still get out of phase cancellation easily especially if you move up or down. It is a guaranty due to the simple physics of it.
You can face it up or turn them towards the back of the room to elevate the problem.
Also if those two woofers are covering the same range then there will be significant out of phase cancellations with those as well.
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Danny,
If an upward firing super tweeter is used, placed on top of the speaker, is it's the optimum position towards the front, middle or back of the speaker's top?
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Ok. I’ll fiddle with placement to see if I hear/gain a notable difference (improvement). As far as the dual drivers go...the LF driver is cut off at 200Hz by design. The HF driver functions/running @ full range (a 1.5 design).
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Danny,
If an upward firing super tweeter is used, placed on top of the speaker, is it's the optimum position towards the front, middle or back of the speaker's top?
I'm no expert at all, but im gonna day to line up the voice coils of the drivers.
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I'm no expert at all, but im gonna day to line up the voice coils of the drivers.
The ST will be lined up with the voice coils.
I'm referring to the front to back positioning of the ST upon the speakers top...
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Yes, you position the super tweeter above the center voice coil for the main driver.
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Danny,
If an upward firing super tweeter is used, placed on top of the speaker, is it's the optimum position towards the front, middle or back of the speaker's top?
Try moving it forward and aligning it to the voice coil of the driver below.
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It's all a compromise. I don't think using a tweeter from 2kHz is ideal for many reasons either. Nothing is going to be perfect.
IMO/IME... I have had dozens of folks listen to my horn speakers with super tweeter filling in over ~12kHz and NOBODY has ever mentioned a lack of coherence or uneven frequency response, and I've had a lot of very experienced people over to hear it. However, if the tweeter was brought in lower it could be an issue. At 2kHz the effects of interference is far more obvious, both ears and microphones show this to be the case. So IMO, it's valid to cross over ~2kHz, but it must be close to perfect, including avoiding discontinuities in polar response or off-angle response, so now the tweeter needs a waveguide or a small midrange below it along with a 3+ way design. A wideband midrange driver avoids these issues but has other issues.
I think if you value complex music the most then the 3+ way speaker without a wideband midrange driver can offer better separation and clarity, if the xo is done well. If you listen primarily to less complex music then a speaker with a wideband midrange that has no crossover in the sensitive frequency range has some advantages. I think some of the best modern multi-way speakers such as YG have basically negated many of the advantages of a wideband driver, but it's very hard to drive and very expensive so it has it's own set of compromises.
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This post obviously caught my attention, and I love cheap tweaks. I have Omega SAMS and Townshend Supertweeters (with just a touch of dual 10 inch stereo sub for some of the bottom octave; I've struggled to integrate them and have gotten really close after years of experimentation.). I've always loved the combo, but I catch my looking at other speakers, b/c well brain tells me there's something missing. NTL, I've heard many more expensive setups that don't come close to this combo. Sweat equity in setup, I guess. Lots of experiments.
So this post led me to conduct a few more experiments. I tried the Townshends 4 ways, 1) pointing up (yes it was not easy to do), 2) forward, 3) facing backwards and 4) I'll get to it. Options 1 and 2 did not make much of a difference compared to the default of #2; in some cases it was significantly worse.
Number 4 was to slowly move the supertweets towards the back of the cabinet to align with the voicecoil (facing me). As I slowly did this, the supertweeter and the woofer seemed to "pick up" on each other. It was very audible and surprising. I measured the distance of each from the edge of the speaker cabinet. Wouldn't you guess it...they were each about the same distance in from the edge. This is shocking given the initial adjustment was made by ear. Thanks for the idea.
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There was probably also a lot of reflection and diffraction spraying the room with HF, that's often best with a super tweeter. Trying to measure what's happening with one microphone at one point in space is a waste of time with a super tweeter, you can find peaks and nulls all over the place which you won't hear.
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Also if those two woofers are covering the same range then there will be significant out of phase cancellations with those as well.
https://omegaloudspeakers.com/products/super-3-high-output-monitor?variant=34755858700
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There was probably also a lot of reflection and diffraction spraying the room with HF, that's often best with a super tweeter. Trying to measure what's happening with one microphone at one point in space is a waste of time with a super tweeter, you can find peaks and nulls all over the place which you won't hear.
I actually find those peaks and nulls to be very easy to hear. Those upper ranges really affect the imaging and sound stage layering. And there really is a big difference in the level of perfection that can be achieved verses how badly it can be disrupted. And the measurements go a long way to explain what I am hearing.
Of course of this this goes back to the room setup, treatment, and speaker placement. If all of that is a disaster then the rest of it won't really matter. Those are the things that allow all to be heard.
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For some reason I can't see your measurements, are you using the tweeter from 2k up?
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For some reason I can't see your measurements, are you using the tweeter from 2k up?
When Danny retited his old server, it unfortunately removed a lot of his pictures.
I believe he's been working on a way to restore them, but it may take some time.
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Of course of this this goes back to the room setup, treatment, and speaker placement. If all of that is a disaster then the rest of it won't really matter. Those are the things that allow all to be heard.
It's more likely that those who don't hear much or any improvement already have systems with good power response.
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This topic is of particular interest to me as I have recently "completed" a horn based speaker that combines a Peavey FH1 bass bin (which is a slightly improved La Scala bass bin), with an Altec 511B horn driven by a Renkus-Heinz SSD-1800 compression driver. The two drivers are time aligned. The crossover is at 650 HZ and has a HF compensation segment to drop the mids and boost the highs as is required for this horn/CG combo. The crossover is also separated for bass and high frequencies so that each can be on a separate amp with its own volume control. This speaker set sounds great. It sounds more natural/real, and far more cohesive than the La Scalas that were previously my primary speakers. However, when I switch between the La Scalas and my new speaker, I miss the top end sparkle and air of the La Scalas. I have a selection of tweeters, some dome and some horn loaded, and I am excited to try Danny Richie's suggestion. Any advice on how to do it properly given my particular setup would be much appreciated.
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This topic is of particular interest to me as I have recently "completed" a horn based speaker that combines a Peavey FH1 bass bin (which is a slightly improved La Scala bass bin), with an Altec 511B horn driven by a Renkus-Heinz SSD-1800 compression driver. The two drivers are time aligned. The crossover is at 650 HZ and has a HF compensation segment to drop the mids and boost the highs as is required for this horn/CG combo. The crossover is also separated for bass and high frequencies so that each can be on a separate amp with its own volume control. This speaker set sounds great. It sounds more natural/real, and far more cohesive than the La Scalas that were previously my primary speakers. However, when I switch between the La Scalas and my new speaker, I miss the top end sparkle and air of the La Scalas. I have a selection of tweeters, some dome and some horn loaded, and I am excited to try Danny Richie's suggestion. Any advice on how to do it properly given my particular setup would be much appreciated.
Start out trying a rear firing version and see (or hear) how that works out for you.
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I am interested in doing something like this for my OB Spatial M3 Turbo S. As Danny said, open baffle won’t work well with a rear firing tweeter, so I’m going to use an upward firing dome(I didn’t see it in this thread, but he said in a video that a ribbon would/might sag if set in upward position).
I’m not sure where to begin- I would think matching efficiencies of the add on tweeter and mains would be a good start. Then the rated ohms of each. Then I need to figure where to cross it over- I’m doing this to add ‘air’ to the speakers. I love them and feel it’s the one area lacking.
I do enjoy some loud time with my system- I guess crossover frequency and slope will determine how beefy a tweeter I should be looking at. Which brings to mind another question- would a simple capacitor work in this situation in line with speaker wire from the M3’s binding post? I’m also not opposed to some sort of resistor or L-pad if necessary to tame things.
All of the above should have question marks after each sentence. I may have a good idea or not a clue...
Thanks for any thoughts.
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If just to add the "AIR", I would cross at 20k and up with either 1uf cap or 0.68uf.
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Thank you Mr. Bendingwave, and now I have more basic questions- perhaps the ‘air’ that I’m looking for is related to my 61 year old ears, and I wonder if 20k is too high. I don’t know- if I add a capacitor, what does that make the ‘crossover’? Is it then a 1st order where it’s down 6 db at 10k? That might be fine, and I can experiment with a few different values...
Second question- In the video I watched, Danny said a rear firing add-on tweeter wouldn’t work well with an OB speaker, I believe because of cancellations happening. Does this apply even if my OB Spatials only have the woofers firing to the rear? The compression tweeter only fires forward.
Last question- if I do go ahead with the add-on tweeter with inline cap, and I run it directly off of the speaker’s binding post, do I risk changing what the amplifier ‘sees’ as the impedance or other factors I am unaware of.
Thanks for any answers- My knowledge is very basic, and I appreciate any explanations. Also, if this is the wrong forum, please point me in the right direction.
Thanks,
Jim
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Hey!
Yeah, I'm one of those weirdo among audiophiles with Single Driver and super tweeter added. Maybe I've been doing wrong after all these years?
Anyway, I've been using Single Driver speaker that I built back in 2002 based on Fostex 6" drivers and Fostex FT17H for the so called "Super Tweeter" Yes, I've tried facing the super tweeter backward, upward, reversed polarity and different caps for the cut off point. After all that I ended up with the super tweeter with .47uf caps, facing forward and same polarity as the main driver of which I piggy back the super tweeter from.
Without any good measurement tools, beside my Rat Shack analog SPL meter and test CD. I have no idea how accurate or in phase they are. I mostly just go by ears. The FT17H level is very low and don't call attention to themselves, but they add a sense of space. The speaker as a system together gave me that solid imaging and wrap around sound to me. And I think because they are aluminum, they add some bite to brass horns and cymbals. I do like what hear and still do after 19 years now.
Still, I appreciated Danny's writing on this and enjoy his videos on YouTube. It is good to see different ways of doing things.
Buddy
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Danny, I've been intrigued by the LX-Mini for a long time. In this case, the mid range is facing up and the "tweeter" is facing forward reverse to what you've suggested. Any comments on this arrangement?
(https://www.audiocircle.com/image.php?id=223087)
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Second question- In the video I watched, Danny said a rear firing add-on tweeter wouldn’t work well with an OB speaker, I believe because of cancellations happening. Does this apply even if my OB Spatials only have the woofers firing to the rear? The compression tweeter only fires forward.
Last question- if I do go ahead with the add-on tweeter with inline cap, and I run it directly off of the speaker’s binding post, do I risk changing what the amplifier ‘sees’ as the impedance or other factors I am unaware of.
Since that model has only the low frequency drivers in an open baffle then you can add a rear facing tweeter.
If you check the impedance of the speakers then you will likely see a rise in the upper octave due to the inductive reactance of the voice coil in the tweeter. So you are still pretty good paralleling a tweeter in the top octave.
Danny
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Thank you Danny. I appreciate your knowledge and input.
Jim
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Good info! I am looking forward to experimenting with this advice in mind