Hello,
I am new here and fairly new on my journey on building a high quality speaker project. I am a residential architect that recently rekindled my love for great sound and I am diving into the world of product design with my first attempt at designing an auditorily and visually beautiful enclosure. Before I begin ordering drivers and start the actual build, I am trying to select components that will serve as a great foundation for driving various iterations as I go.
My end goal is to build a pair of 3-way speakers for audiophile grade music listening with a frequency range between slightly below 20Hz to 40khz. I would like to drive each tweeter, mid-range woofer and subwoofer individually with either a single 6 channel amp or most likely discrete amps for each pair depending on the power demands of the individual drivers.
I found various drivers (from SEAS, Scan-Speak, Mark Audio and Tang Band)
One possible pairing might be:
-Scanspeak Illuminator D3004/6640-00 1" Tweeter Beryllium Dome - 4 Ohms
-Scanspeak Illuminator 12MU/4731T-00 4.5" Midrange - 4 Ohms
-ScanSpeak Revelator 26W/4867T 10" Woofer Black Aluminum Cone, 4 Ohms
Another might be:
-SEAS E0121-04/06 C16NX001/X - 4 Ohms - 100W (coaxial mid and tweeter)
-SEAS XM001-04 L26ROY - 4 Ohms - 250 Watts
For amplifiers, Emotiva seems to fall within my budget and criteria. (open to suggestions on this and everything else)
Now I am on the hunt for a pre-amp DSP with at least 6 channel output capable of the wide frequency range I am looking for. It would be great if it also includes room correction features.
So far I found the miniDSP Flex Eight ($549) and the Octo dac8 Pro ($1,300+/-). I might be wrong, but I believe those both are limited to between 20Hz and 20kHz.
Being new at this, I am very open to suggestions, tips and tricks on the best practices for equipment matching. In my case I'd like the dsp/amp setup to be somewhat flexible for various driver configurations I might try along the way. As for budget, I would like to keep it reasonable, but open to spend a little more for valuable features and high quality.
Thanks and looking forward to the journey!
-Chris
I am new here and fairly new on my journey on building a high quality speaker project. I am a residential architect that recently rekindled my love for great sound and I am diving into the world of product design with my first attempt at designing an auditorily and visually beautiful enclosure. Before I begin ordering drivers and start the actual build, I am trying to select components that will serve as a great foundation for driving various iterations as I go.
My end goal is to build a pair of 3-way speakers for audiophile grade music listening with a frequency range between slightly below 20Hz to 40khz. I would like to drive each tweeter, mid-range woofer and subwoofer individually with either a single 6 channel amp or most likely discrete amps for each pair depending on the power demands of the individual drivers.
I found various drivers (from SEAS, Scan-Speak, Mark Audio and Tang Band)
One possible pairing might be:
-Scanspeak Illuminator D3004/6640-00 1" Tweeter Beryllium Dome - 4 Ohms
-Scanspeak Illuminator 12MU/4731T-00 4.5" Midrange - 4 Ohms
-ScanSpeak Revelator 26W/4867T 10" Woofer Black Aluminum Cone, 4 Ohms
Another might be:
-SEAS E0121-04/06 C16NX001/X - 4 Ohms - 100W (coaxial mid and tweeter)
-SEAS XM001-04 L26ROY - 4 Ohms - 250 Watts
For amplifiers, Emotiva seems to fall within my budget and criteria. (open to suggestions on this and everything else)
- For Tweeter and Midrange woffer: Emotiva BasX A4 Four-Channel Power Amplifier 130 watts per channel - 4 channel @ 4 Ohms ($549)
- For Subwoofer: Emotiva BasX A2 Stereo Amplifier 250 watts per channel - 2 channel @ 4 Ohms ($579)
Now I am on the hunt for a pre-amp DSP with at least 6 channel output capable of the wide frequency range I am looking for. It would be great if it also includes room correction features.
So far I found the miniDSP Flex Eight ($549) and the Octo dac8 Pro ($1,300+/-). I might be wrong, but I believe those both are limited to between 20Hz and 20kHz.
Being new at this, I am very open to suggestions, tips and tricks on the best practices for equipment matching. In my case I'd like the dsp/amp setup to be somewhat flexible for various driver configurations I might try along the way. As for budget, I would like to keep it reasonable, but open to spend a little more for valuable features and high quality.
Thanks and looking forward to the journey!
-Chris
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Not sure why you want 40kHz. 16/44 CD audio only goes up to around 20kHz. So most dacs have a hardware filter after the dac chip to work with a 44.1kHz sample rate. Making the hardware filter frequency tunable may take relays to swap filter capacitors and or resistors for each sample rate. Could be RME dacs can do that, you might check. Of course it adds to cost.
Other than that, the Octo dac you mention likely sounds better than the miniDSP, but so far as I know Octo doesn't have onboard DSP. Also, running a DSP chip at a higher than necessary clock frequency in order to reproduce ultrasonics may require a more powerful DSP chip, or else maybe more computationally demanding PC resources. Also, DSP filters may need to have more taps to have sufficient frequency resolution at lower audio frequencies, etc. Also IIUC, miniDSP is clocked at 48kHz, not enough to give you 40kHz audio bandwidth.
OTOH, very little of practical value to reproducing audio above 20kHz. IME, usually the problems affecting perceived SQ of reproduced music are problems in the audio band. Again IME, some problems simply do not show up that well in standard FFT spectral measurements, so they can be pretty easily missed. The solution to those problems is not to be found in ultrasonics. And problems in audio reproduction below 20kHz tend to be more audible than ultrasonic content produced by physical musical instruments (electronic instruments don't necessarily have that limitation).
In regard to physical instruments, I once did a test striking a small cymbal very hard and recording the ultrasonic output. There was some very brief content up to 40kHz or possibly a bit higher, but it lasted only milliseconds, and it was at a much lower level than the below 20kHz content.
Regarding amplification, low cost class-D probably isn't going to sound like the best of class-D. Again, tending to problems in the audio band below 20kHz tends tend to have the most practical benefit. Low cost class-D and or use of SMPS can cause problems that aren't necessarily easy to see on typical FFT spectral measurements. The reasons why are technical, but can be discussed if there is interest.
Other than that, the Octo dac you mention likely sounds better than the miniDSP, but so far as I know Octo doesn't have onboard DSP. Also, running a DSP chip at a higher than necessary clock frequency in order to reproduce ultrasonics may require a more powerful DSP chip, or else maybe more computationally demanding PC resources. Also, DSP filters may need to have more taps to have sufficient frequency resolution at lower audio frequencies, etc. Also IIUC, miniDSP is clocked at 48kHz, not enough to give you 40kHz audio bandwidth.
OTOH, very little of practical value to reproducing audio above 20kHz. IME, usually the problems affecting perceived SQ of reproduced music are problems in the audio band. Again IME, some problems simply do not show up that well in standard FFT spectral measurements, so they can be pretty easily missed. The solution to those problems is not to be found in ultrasonics. And problems in audio reproduction below 20kHz tend to be more audible than ultrasonic content produced by physical musical instruments (electronic instruments don't necessarily have that limitation).
In regard to physical instruments, I once did a test striking a small cymbal very hard and recording the ultrasonic output. There was some very brief content up to 40kHz or possibly a bit higher, but it lasted only milliseconds, and it was at a much lower level than the below 20kHz content.
Regarding amplification, low cost class-D probably isn't going to sound like the best of class-D. Again, tending to problems in the audio band below 20kHz tends tend to have the most practical benefit. Low cost class-D and or use of SMPS can cause problems that aren't necessarily easy to see on typical FFT spectral measurements. The reasons why are technical, but can be discussed if there is interest.
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Maybe he wants to scare dogs away from the house? 🙂
If you do not need a frequency of 40 kHz, then pay attention to this solution.
https://www.diyaudio.com/community/threads/3-way-dsp-amp.415065/#post-7734938
These boards can be connected in parallel via the SPDIF port. If the output power per channel is not enough for you, then I have similar boards for two bands of 200 W per band. Purely theoretically, I can configure these boards to reproduce 40 kHz, but I consider this a very unreasonable action, since a person who does not hear 18 kHz cannot hear the 40 kHz component of a complex signal. Are you sure that you hear at least an 18 kHz signal? This is easy to check, there are many software frequency generators on the Internet, with which you can find out the frequency hearing threshold.
And yes, if you want to assemble a three-way speaker, then you won’t be able to do it effectively without a microphone.
On the 40kHz question, while I agree that reproduction of that highest octave is probably not necessary, having the DSP hardware running at a 96kHz sample rate helps move the analog, post DAC, low pass filters up in frequency, reducing their pass band effects. So I agree with choosing the hardware to go that high!
As for the DSP solution, the one that most interests me is these guys: DEQX They seem to be a little slow getting their latest products off the ground and into the market place, but I like the approach and general design of the gear.
As for the DSP solution, the one that most interests me is these guys: DEQX They seem to be a little slow getting their latest products off the ground and into the market place, but I like the approach and general design of the gear.
DACs are different, and yes, increasing the sampling frequency simplifies the task of filtering the output analog signal from aliasing components, but there is no need to increase the cutoff frequency of the filter at the DAC output, it is enough to simply increase the sampling frequency of the signal and the aliasing will decrease. And DACs have long used digital filtering of the output signal, since it is more effective than analog filtering. In general, DACs are different and if the DAC reproduces 40 kHz of an analog signal, this is not a guarantee of its high quality.
Others have already alluded to the problem of getting DSP/xover to 40 kHz. I'd point out that you also need to consider the maximum volume you want to achieve, especially at the low end.
The ScanSpeak 26W/4867T has a max SPL of ~90dB (at 1m) at 20Hz. Maybe this is good enough for you but it isn't very loud, imo. Our hearing sensitivity at the low end is pretty poor. Raise the low limit to 30Hz, and it can achieve 96.6dB.
The Seas L26ROY does a bit better -- 94dB max SPL @ 20Hz, or 102dB @ 30Hz.
With a pair, you'll get +3dB, not to mention possible room gain, which makes the Seas more suitable if you sometimes want to crank it.
A 12" version of those woofers makes it easier to get high (realistic) volume at such a low frequency. A Scan-Speak Revelator 32W/4878T00, for example, would give you 97.8dB max SPL @ 20Hz.
See the SPL curves here:
https://loudspeakerdatabase.com/ScanSpeak/32W-4878T00
https://loudspeakerdatabase.com/ScanSpeak/26W-4867T00
https://loudspeakerdatabase.com/SEAS/XM001-04_L26ROY
Thank you everyone for the input. Honestly I feel like I've jumped into the deep end with the amount of knowledge you all shared. I humbly have much to learn.
In regards to the 40Khz question, I simply noticed that some high-end drivers and amplifiers show they are capable of this. My impression was that even though we can't hear much above 18 to 20Hz, we may still perceive it in other subtle ways beyond my knowledge as to how. I did stumble upon this Wikipedia page stating "Under ideal laboratory conditions, humans can hear sound as low as 12 Hz and as high as 28 kHz" https://en.wikipedia.org/wiki/Hearing_range I'm sure my hearing is not all that good, but I want to make something to share, so it would be nice to extend beyond my personal hearing for those that might perceive it.
That said, I thought I would give it a try with a system capable of this range to see if there are any real benefits. I can find drivers and amplifiers, but was just having trouble with the DSP and of course the system is only as good or capable as the weakest link.
miniDSP did get back to me and said:
----------------------------------------------
"Thanks for your interest in our products. Flex Eight effective bandwidth depends on the input signal oversampling rate .
The Flex Eight non-Dirac Live version features an internal oversampling rate of 96kHz, allowing for high-quality audio processing. With an upper limit that extends beyond 20kHz, this device is capable of delivering crisp and detailed sound across a wide frequency range.
We are currently limited to a DSP operation range of 20Hz to 20kHz due to the complexity of the algorithm. This range allows us to effectively process and manipulate audio signals within the specified frequency range."
----------------------------------------------
So if I use their DSP feature, it looks like I am limited to 20Hz to 20kHz. For those wanting to go beyond for whatever reason, it seems there is a hole in the market for a DSP that is as capable in terms of frequency range as the drivers and amplifiers that are currently available. I'm assuming this is for a small segment, so it's probably not a priority.
As for the amplifiers, thank you for the input @Markw4. I'm new to this so I'm having fun trying to find my way. Do you have a recommended alternative to the Emotiva BasX. I noticed their XPA Gen3 seems like a step up and can be customized. https://emotiva.com/products/build-your-own?variant=6293371256875 With this being my first go, it would be nice to get something that performs well at a reasonable price of around $2,500 USD or less.
As for the custom dsp thanks, @uriy-ch, is that something you make? How does the interface work? The schematics are beyond my comprehension for the moment but that does look interesting as I dive in deeper.
@DCtoDaylight, Thanks for the link to DEQX. Their HDP-5 (https://www.deqx.com/products/hdp-5/) looked nice but has been discontinued and as you mention, everything else is on pre-order with no pricing listed.
@mikessi, thanks for the note about speaker loudness. I admit, it would be nice to blast the music occasionally, but with our neighbors houses only 5 to 10 feet away on either side, that would not be the usual listening scenario. We actually have a rather small family room (15'x12') where these will be going, so I'm actually thinking of down sizing to the SEAS XM011-04 L22ROY2 4 Ohms 250/500W (8.7") paired with the SEAS H1602-04/06 L12RE/XFC 4 Ohms 80/250W. Happy to hear your thoughts on those.
Thanks again,
-Chris
In regards to the 40Khz question, I simply noticed that some high-end drivers and amplifiers show they are capable of this. My impression was that even though we can't hear much above 18 to 20Hz, we may still perceive it in other subtle ways beyond my knowledge as to how. I did stumble upon this Wikipedia page stating "Under ideal laboratory conditions, humans can hear sound as low as 12 Hz and as high as 28 kHz" https://en.wikipedia.org/wiki/Hearing_range I'm sure my hearing is not all that good, but I want to make something to share, so it would be nice to extend beyond my personal hearing for those that might perceive it.
That said, I thought I would give it a try with a system capable of this range to see if there are any real benefits. I can find drivers and amplifiers, but was just having trouble with the DSP and of course the system is only as good or capable as the weakest link.
miniDSP did get back to me and said:
----------------------------------------------
"Thanks for your interest in our products. Flex Eight effective bandwidth depends on the input signal oversampling rate .
The Flex Eight non-Dirac Live version features an internal oversampling rate of 96kHz, allowing for high-quality audio processing. With an upper limit that extends beyond 20kHz, this device is capable of delivering crisp and detailed sound across a wide frequency range.
We are currently limited to a DSP operation range of 20Hz to 20kHz due to the complexity of the algorithm. This range allows us to effectively process and manipulate audio signals within the specified frequency range."
----------------------------------------------
So if I use their DSP feature, it looks like I am limited to 20Hz to 20kHz. For those wanting to go beyond for whatever reason, it seems there is a hole in the market for a DSP that is as capable in terms of frequency range as the drivers and amplifiers that are currently available. I'm assuming this is for a small segment, so it's probably not a priority.
As for the amplifiers, thank you for the input @Markw4. I'm new to this so I'm having fun trying to find my way. Do you have a recommended alternative to the Emotiva BasX. I noticed their XPA Gen3 seems like a step up and can be customized. https://emotiva.com/products/build-your-own?variant=6293371256875 With this being my first go, it would be nice to get something that performs well at a reasonable price of around $2,500 USD or less.
As for the custom dsp thanks, @uriy-ch, is that something you make? How does the interface work? The schematics are beyond my comprehension for the moment but that does look interesting as I dive in deeper.
@DCtoDaylight, Thanks for the link to DEQX. Their HDP-5 (https://www.deqx.com/products/hdp-5/) looked nice but has been discontinued and as you mention, everything else is on pre-order with no pricing listed.
@mikessi, thanks for the note about speaker loudness. I admit, it would be nice to blast the music occasionally, but with our neighbors houses only 5 to 10 feet away on either side, that would not be the usual listening scenario. We actually have a rather small family room (15'x12') where these will be going, so I'm actually thinking of down sizing to the SEAS XM011-04 L22ROY2 4 Ohms 250/500W (8.7") paired with the SEAS H1602-04/06 L12RE/XFC 4 Ohms 80/250W. Happy to hear your thoughts on those.
Thanks again,
-Chris
It looks like the Emotiva XPA Gen3 Modular Amplifier System Six-2-2S might be a nice option at $1699. I welcome anyone's thoughts.
Flex 8 is the obvious best choice for you. Forget 40kHz; imo, it's a goose chase.
Woofer -- interesting you're looking at subwoofer drivers only. Most sound best crossed below 300Hz. Which means higher load on the mid in a 3-way. That Seas is very low sensitivity, BTW, & you won't get down to 20 Hz with it. Imo, the best of all the woofers you mentioned is the 10" SS.
It seems to me you might want to clarify your priorities for this system. Rather than technicalities, think about what you want it to do for you. Your use model is important to think through. The small room will dictate limitations.
Woofer -- interesting you're looking at subwoofer drivers only. Most sound best crossed below 300Hz. Which means higher load on the mid in a 3-way. That Seas is very low sensitivity, BTW, & you won't get down to 20 Hz with it. Imo, the best of all the woofers you mentioned is the 10" SS.
It seems to me you might want to clarify your priorities for this system. Rather than technicalities, think about what you want it to do for you. Your use model is important to think through. The small room will dictate limitations.
Thanks again Mikessi,
Here is my current idea for the system:
The H1602-04/06 l12RE/XFC is a coaxial driver with a small woofer and tweeter. I was hoping this would work well with the two L22ROY2 8" in a transmission line design for the low notes. The 10" would be great, but the enclosure starts to get large for a TL design and my small family room. I could just go ported, but from my limited understanding TL seems to be a nice compromise for low frequency response while still remaining somewhat accurate, where a ported design is best for low frequency, sacrificing accuracy and a sealed design is best for accuracy but limits how low the driver can go.
All advice/suggestions/comments are welcome. 🙂
Thanks,
-Chris
Here is my current idea for the system:
The H1602-04/06 l12RE/XFC is a coaxial driver with a small woofer and tweeter. I was hoping this would work well with the two L22ROY2 8" in a transmission line design for the low notes. The 10" would be great, but the enclosure starts to get large for a TL design and my small family room. I could just go ported, but from my limited understanding TL seems to be a nice compromise for low frequency response while still remaining somewhat accurate, where a ported design is best for low frequency, sacrificing accuracy and a sealed design is best for accuracy but limits how low the driver can go.
All advice/suggestions/comments are welcome. 🙂
Thanks,
-Chris
You really want HDMI ARC for ideal TV sound control integration. miniDSP Flex HT & bigger Flex HTx have it.
Your driver choices seem OK, but I'm not sold on either TL or coax overall. The former because of size requirements & latter because the FR & directivity of the 2 drivers seem often to be negatively affected by each other. Examine the factory spec sheet of the coax closely. What made you choose that driver? Has it been used in an successful design already?
You also want to think about both speaker placement & seating. Obviously, there's a TV involved, so it will likely be against a wall. What about speakers? Best close to wall or can it be placed 3-4' out? The latter most often gives best soundstage effects, but it can be inconvenient/impractical. Best to choose & design the speakers accordingly. Also leave $ in your budget for room acoustics improvements. The room is such an important component!
With the EQ & other controls available on the minidsp Flex devices & the high power available from modern amps, all kinds of things are possible. For example, a sealed box for those woofers + in-room EQ of the bottom end can give you deep powerful bass in a small box. 0.7 Qts or lower can be achieved in a 1' box; with the woofer positioned close to the floor floor in a cabinet against the wall for +6dB bass lift (vs anechoic) would likely extend bass down below 30Hz to a reasonably high volume.
Much cheaper drivers can achieve similar performance, btw -- 8~10" models from Dayton, SB Acoustics. I recently installed a Dayton RSS265HF-8 in a ~1.3 cf closed box & with less than 10 dB EQ boost ~25Hz, achieved flat response to well below 30Hz in room. W/ 300W available for each woofer, the bass extension & impact in a 14x20x9' room was amazing. (See pics in my post here )
But the coax has an effective diameter of <4", which means it needs to cross probably at... 300Hz or higher? The sub might not be so good that high. 200Hz might work. You'd have to experiment, measure, listen.
Lots of complexity & choices in speaker design/construction. Maybe too many to make full DIY a practical first speaker project. You might consider building a known successful kit or clone a DIY system by a well-regarded enthusiast here. First one that comes to mind -- @hifijim has very well documented build threads well worth studying and/or copying.
PS -- I suspect a Hypex nCore amp would outperform the Emotiva. Try looking up Buckeye amps or similar.
Your driver choices seem OK, but I'm not sold on either TL or coax overall. The former because of size requirements & latter because the FR & directivity of the 2 drivers seem often to be negatively affected by each other. Examine the factory spec sheet of the coax closely. What made you choose that driver? Has it been used in an successful design already?
You also want to think about both speaker placement & seating. Obviously, there's a TV involved, so it will likely be against a wall. What about speakers? Best close to wall or can it be placed 3-4' out? The latter most often gives best soundstage effects, but it can be inconvenient/impractical. Best to choose & design the speakers accordingly. Also leave $ in your budget for room acoustics improvements. The room is such an important component!
With the EQ & other controls available on the minidsp Flex devices & the high power available from modern amps, all kinds of things are possible. For example, a sealed box for those woofers + in-room EQ of the bottom end can give you deep powerful bass in a small box. 0.7 Qts or lower can be achieved in a 1' box; with the woofer positioned close to the floor floor in a cabinet against the wall for +6dB bass lift (vs anechoic) would likely extend bass down below 30Hz to a reasonably high volume.
Much cheaper drivers can achieve similar performance, btw -- 8~10" models from Dayton, SB Acoustics. I recently installed a Dayton RSS265HF-8 in a ~1.3 cf closed box & with less than 10 dB EQ boost ~25Hz, achieved flat response to well below 30Hz in room. W/ 300W available for each woofer, the bass extension & impact in a 14x20x9' room was amazing. (See pics in my post here )
But the coax has an effective diameter of <4", which means it needs to cross probably at... 300Hz or higher? The sub might not be so good that high. 200Hz might work. You'd have to experiment, measure, listen.
Lots of complexity & choices in speaker design/construction. Maybe too many to make full DIY a practical first speaker project. You might consider building a known successful kit or clone a DIY system by a well-regarded enthusiast here. First one that comes to mind -- @hifijim has very well documented build threads well worth studying and/or copying.
PS -- I suspect a Hypex nCore amp would outperform the Emotiva. Try looking up Buckeye amps or similar.
Just realized I need to upgrade to the miniDSP Flex HT to get 8 channel Dirac Live capability versus the 2 channel you get with the miniDSP Flex Eight.
Thanks again @mikessi ,
For this project, with my limited understanding, it seems coaxial designs have better stereo imaging and reduced interference patterns versus a discrete design with separate drivers. The subwoofer with its low notes I believe is more forgiving in this regard. I also have designs for discrete drivers, but thought I'd give the coaxial a try first.
As to why the SEAS drivers. When searching audiophile grade options, SEAS, Scanspeak, Mark Audio and a few others came up as options. In looking at Dayton Reference series, the specs and price seem like it could be a winner, but with a limited budget, I can't buy multiple drivers to play with, so I figured I'd go with what seems to be high quality an hope it works. Curious at what the community thinks about the various brands out there in regards to fidelity, build quality and durability.
As for room placement, I can probably float these off the wall 18" or so, but beyond that they will be too much in the way. And point well taken regarding acoustic treatment of the room.
As for the sealed versus TL, I will take a deep dive into the options. Even though a sealed enclosure seems more manageable, part of the reason for the direction I am going is that I am simply intrigued with the challenge of a TL design and look forward to the design process.
As for the cross-over points, If I'm reading the graph correctly, I think that could happen between 200Hz and 300Hz
Also, thanks for mentioning @hifijim. I will definitely take a look at his builds.
Lastly, thanks for the recommendation on amps. What do you think of this one? https://www.buckeyeamp.com/shop/amplifiers/hypex/nc502mp/6_channel
For this project, with my limited understanding, it seems coaxial designs have better stereo imaging and reduced interference patterns versus a discrete design with separate drivers. The subwoofer with its low notes I believe is more forgiving in this regard. I also have designs for discrete drivers, but thought I'd give the coaxial a try first.
As to why the SEAS drivers. When searching audiophile grade options, SEAS, Scanspeak, Mark Audio and a few others came up as options. In looking at Dayton Reference series, the specs and price seem like it could be a winner, but with a limited budget, I can't buy multiple drivers to play with, so I figured I'd go with what seems to be high quality an hope it works. Curious at what the community thinks about the various brands out there in regards to fidelity, build quality and durability.
As for room placement, I can probably float these off the wall 18" or so, but beyond that they will be too much in the way. And point well taken regarding acoustic treatment of the room.
As for the sealed versus TL, I will take a deep dive into the options. Even though a sealed enclosure seems more manageable, part of the reason for the direction I am going is that I am simply intrigued with the challenge of a TL design and look forward to the design process.
As for the cross-over points, If I'm reading the graph correctly, I think that could happen between 200Hz and 300Hz
Also, thanks for mentioning @hifijim. I will definitely take a look at his builds.
Lastly, thanks for the recommendation on amps. What do you think of this one? https://www.buckeyeamp.com/shop/amplifiers/hypex/nc502mp/6_channel
Doesn't most DSP these days run at 96kHz? So 40kHz analog bandwidth should not be a problem (if the recording even has it)
The Lab Gruppen LMX48 (Lake DSP) does it at least. MiniDSP is not clear, but i also thought they process at 96kHz 24bit. I heared both working (i have a MiniDSP Flex and calibrated the LMX48 for someone recently) and don't hear a difference. The MiniDSP is way more suited for hifi and has DIRAC, what the LMX does not have.
You can get sampling rates to 192kHz, but I've not seen an "audiophile" audio interface that is actually spec'ed for a frequency response beyond 20kHz. Not sure about DSP boards. I looked on ASR (Audio Science Review), and saw one post asking about response above 20kHz, but no one else cared.
This is not borne out by the vast number of successful speakers -- commercial & DIY -- that don't use coax. Either approach can give you great (or poor) end results.
18" is not bad, but this means you want to think of these speakers as close-to-wall designs & proceed accordingly.
I generally prefer to keep it simpler. Even though I've built several TLs before, I'm less keen about them these days because the benefits just don't seem worth the effort with the sheer ease & excellent results of closed-box + DSP. There's much less to go wrong. YMMV.
As for your amp choice, 502s for mids & tweets is overkill. You could do 252 for all or 502 for bass + 252s for mid/high. You could even do a 3-ch DSP plate amp and integrate one into each speaker. That would eliminate the minidsp... though you might still want a HDMI ARC control unit. ie -- Wiim Ultra, etc.
ps -- minidsp allowed 96kHz internal processing even in much earlier generations. My old 4x10HD does it; ditto OpenDRC-DA8, 2x4HD.
BTW, don't underestimate the wire chaos of multiple speaker and high level cables for multi-amp active systems. My wife rolls her eyes and sighs every time she want to vac behind the Orions system, even with 8-conductor stage cabling & Speakon connectors at the speaker end, at the amp/minidsp stage, there are 8 pairs of speaker outputs + 8 phono cables. The DSP plate amps eliminate almost all of this.
Going back to the SEAS coax, I don't like the peakiness of the woofer response at 1~2Khz. That's smack in the mid, no way the tweeter could cross lower, no way not to hear this unless you employ sharp notch filters, and even then... It doesn't look like good cone behavior to me. I'd avoid it myself. Look for smoother response out to 3kHz if you want to cross at 2~2.5 khz.
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Many other similar size drivers have much flatter FR, including the Scanspeak Illuminator 12MU/4731T-00 you mentioned in your first post. And the SS 10F/8424. Check the measured data at hificompass on these and other comparable size drivers.