Single Highpass for 2 identical drivers. Bad idea? Or good cost saving?

[Frostman96]

I have a solid response curve that I am happy with in a 2 way MTM design using 2x DA RS180 8ohm in parallel, and 1 DA RST28F 4ohm. I initially had each woofer set with its own Highpass filter (2nd order) to obtain the identical FR. However, I was able to a nearly identical FR with both drivers using a shared 2nd order Highpass. Doing this will save close to $20-40 per speaker (assuming using mid grade components) just by eliminating that one Highpass, and I intend to make 3 of these.

Impendence, and all other modeled values seem the same. I have heard that doing this can cause issues with "vertical directivity". If it means what I think it does, why would that be an issue considering you are essentially only ever sitting in the same couch at the same vertical level the whole time? Any other concerns I am missing here to make me reconsider?

TIA,

Evan

 

[Frostman96]

Some screenshots of both designs for reference.

 

[kevinkr]

The single XO shared between both of the drivers is exactly how I would do it. My assumption is that the drivers are very similar, and load into the same shared box volume.

 

[Nelson Pass]

Smaller coil that way...

 

[AllenB]

Answering your question directly, I agree with Kevin.. however, I suspect you're talking about something else.

The context of your question makes me think you've been reading about MTM vs 2.5 way. The difference is that you low-pass one woofer at the baffle step frequency and then you only have one woofer working at the crossover frequency. MTM has a challenging tendency to produce a narrow directivity below and around the cross, compared to above it. This makes it difficult to get right if at all possible with a given configuration, making the 2.5 way a safer bet.

 

[ScottG]

-yes, it's mostly a matter of looking at the vertical polar results around the crossover point. MTM's often have more "suck-out" vertically and can make the "stand-up/sit-down" results quite different.

 

[Frostman96]

MTM has a challenging tendency to produce a narrow directivity below and around the cross, compared to above it.

Other than doing a 2.5 way design, is there other way to help reduce this? I tried modeling a 2.5way crossover with these RS180 drivers, and it's proving pretty difficult to to get an acceptable FR without a pretty (to me) complicated crossover. However, I would rather only build these once, so I want them to be as good as possible, so if need be, Ill do it. I calculated the baffle step to be 570hz (8inch wide baffle) and this is what I came up with. Does this seem appropriate for a 2.5way design? Only real issue I am finding is the notch filter power dissipation is a tad nhigh when I am feeding it reference level power, but more reasonable when its typical listening volume, so maybe a non-issue.

(Just as an FYI, other baffle step considerations I am going to try and save that for DSP attenuation so I can consider room correction as well. SO, my goal is simply as flat as possible for now.)

 

[AllenB]

There is much to sort out here. I'd mention that you shouldn't cut the lower woofer (D2 above) second order, it has to stay first. You shouldn't high pass the other woofer, they should both run all the way down.

However, you said you want to do DSP baffle step compensation instead. So...

What do you hope to get out of the MTM, is it right for you? D'Appolito ran his woofers in quadrature with the tweeters (slightly out of phase) and this gave an improvement in the lobing situation. However many people run them in phase. This produces the same lobing pattern as a single TM. You gain nothing doing it that way.

 

[kevinkr]

This is where you might want to consider a MiniDSP 2x4 or similar device to manage the crossover from the woofers to the tweeter, this would allow you to easily run the tweeter in quadrature to the mids/bass if you want to run both of the mids/bass across the same range .

You can also run both woofers in the bass and low pass one of the woofers (the bottom one?) at a few hundred Hz in the box - just first order with a high quality air core inductor..

Regardless of options DSP will provide you with more flexibility.. (As Allen can attest I am using DSP for crossovers, EQ, and delays as well as room correction [Dirac3] )

Digital in/Analog out: https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd

Analog I/O Dirac3 option: https://www.minidsp.com/products/minidsp-in-a-box/flex

The one problem with this approach is it requires 4 channels of power amplification, and overall it is a significantly more expensive

 

[Frostman96]

You shouldn't high pass the other woofer, they should both run all the way down.

I'm sorry, I definitely misunderstood what you meant. Below is a reworked version of what I think works as a 2.5way crossover.

As for what I am looking for out of my speakers, I am primarily looking to use them as a LCR in home theater for probably 60-70% of listening, while isolating the LR for stereo music listening the rest of the time. I intend to run a subwoofer with a crossover between 70-80hz for the lower frequencies for home theatre and some of my music tastes. MTM to me is very aesthetically appealing and is able to easily create LCR as identical speakers for better tracking and intelligibility with home theatre.

So with that said, I do want a jack of all trades here. Clean, comfortable, accurate, and good depth and imaging for solo music listening, while also providing the dynamics, power, imaging/low directivity for home theatre. I understand more and more that with speaker building there are always trade offs. So, if I can get pretty close to my goals while sacrificing maybe some minor perfections, I will likely be happy. I am very new to this DIY community, so I am sorry for my lack of knowledge, but I appreciate your willingness to humor me. Any other feedback or suggestions is much appreciate.

 

[Frostman96]

This is where you might want to consider a MiniDSP 2x4 or similar device to manage the crossover from the woofers to the tweeter, this would allow you to easily run the tweeter in quadrature to the mids/bass if you want to run both of the mids/bass across the same range .

Interesting... This is a whole new approach I am unfamiliar with! So are you suggesting you don't use a crossover inside a 2 way speaker and use these in there place? Or do you still need a basic crossover and use this to augment it? Do you have a resource that shows how you do that beyond their website?

I initially had my eyes on the Anthem MRX740 due to its room correction capabilities (ARC) and its flexibility as acting as a preamp for some channels and not for others while also being able to remap those unused channels enabling it to run up to 11.2.4 systems. In the near term, my plan was to use the MRX740 running the surrounds and as a preamp to the Emotiva BasX A3 from the LCR. But this whole minDSP with Dirac is very interesting to say the least!

This is where you might want to consider a MiniDSP 2x4 or similar device to manage the crossover

Oh, I found the section on their website. This is very interesting, I definitely need to read into it a bit more. thanks!

https://www.minidsp.com/applications/digital-crossovers/two-way-crossover

 

[AllenB]

I like to experience things as I learn about them, I know I'm not alone in this thread in that regard. The good thing in all this is you'll be able to try both, and more, using the same boxes.

 

[GM]

Hmm, for HT it's about controlling the horizontal polar response, the vertical not so much, so better to mount them horizontal and if need be, 'V' the baffle to get the desired results same as is done with some studio monitors.