diy mtm using Dayton pro drivers - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 31 Old 05-09-16, 09:57 PM
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Re: diy mtm using Dayton pro drivers

No you do not want to go from a 12in woofer to a tweeter (dome) but you are on the right track with a compression driver with a 12in waveguide.
The stated 104dB sensitivity of the compression driver is not what you are going to achieve with a 12in waveguide but even then you will have to pad the HF driver down to match the woofer's sensitivity (once you add BSC).
2dB does not seem much but it will take the Dayton 317W to match the 200W output of the Eminence.
I think you figured any pre-made 2-way crossover is not going to make a great speaker... it is simpler to measure drivers in the cabinet and design the XO after.
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post #22 of 31 Old 05-10-16, 12:18 AM
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Re: diy mtm using Dayton pro drivers

I'm going to bring up something that is a consideration, but in all likelihood a relatively minor effect. You can probably choose to disregard this with very little adverse effect.

There's a phenomenon known as High Frequency Beaming that may become an issue. It's the tendency of wave propagation to become more directional as frequency increases. Where it comes into play here is in the size of the LF driver and the crossover point to the HF driver. With a 12" driver, waves will start to beam around 1100Hz. The end result in a two-way box with a large woofer that is crossed over relatively high in comparison to the same size woofer in a three-way design is that mid-range response will get a bit dodgy.

Since frequencies below the start of the beaming phenomenon appear omnidirectional, and frequencies significantly above the crossover point are subject to the dispersion pattern of the tweeter, the response of the system in the range between the start of beaming and an octave or an octave-and-a-half above the crossover point becomes highly axial in comparison. Basically, mid range will sound very different depending on whether your LP is on-axis or off-axis. There are two ways to combat this problem:

1) Smaller woofer, thus raising the SOB (start of beam) Frequency to a point at or above the crossover point of the system. Remember that SOB is dependent on the diameter of the woofer... so a larger woofer will beam at a lower frequency than a smaller woofer.
2) Lower the crossover point to a frequency at or below the SOB for the chosen woofer. This requires a HF driver that has a relatively low Fs, and higher power handling capacity.

Ultimately, the best method is to combine the two approaches and choose the largest woofer that you can accommodate with the HF driver's crossover point. An 8" driver nominally starts to beam around 1700Hz, and a 6" driver will beam around 2250Hz. Granted, these numbers are where the effect theoretically begins to appear. As you ascend in frequency from any one of these points for a given driver, the effect will become more apparent. The larger a bandwidth one allows to exist between these two points (start of beaming and crossover to HF driver) unchecked, the harder the midrange of the finished product will be to control.

Fun Fact: A 15" woofer with a 1" compression driver mated to some sort of wave guide is probably the most common setup for low-price pro-audio speakers. It's also the main reason why most of them have muddy midrange response and why you have to pay so much more for quality speakers. Better designs fix this problem with three-way orientations or smaller woofers that are intended to have subwoofers providing supplemental coverage on the bottom end.

I just want to reiterate that this effect will likely be fairly minor under the circumstances that govern your usage, and you will likely be able to get away with a 12" woofer without more than an acceptable amount of funk in your midrange response. But if I were designing these boxes, I'd lean towards the smaller drivers for their better HF extension.
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post #23 of 31 Old 05-10-16, 12:57 AM Thread Starter
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Thanks for that. It certainly makes me realize why quality hifi speakers cost so much.
Hmmm...Lots to think about.
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post #24 of 31 Old 05-10-16, 03:05 AM Thread Starter
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What about a balanced mindsp as active crossover? Seems like there may be benefits. Measure adjust measure and adjust till close and then some eq? ??.
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post #25 of 31 Old 05-10-16, 07:09 PM
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Re: diy mtm using Dayton pro drivers

Well, if you have the ability to alter your crossover frequency on the fly, you can approach the design from a standpoint of having a large overlap in frequency responses between the HF and LF drivers. At that point, it would mostly be a question of matching sensitivities so you don't have to do too much to balance high against low.

This is one of the reasons MTM or MMT designs are popular... the second woofer effectively increases the sensitivity of the system on the low end, and provides the potential for a lower F3 if the cabinet can accommodate the additional volume requirement.

Also, if you're designing for a large overlapping bandwidth between HF and LF drivers, this will still nominally push your design requirements toward a smaller woofer. One thing to consider though: If you're using an active crossover, you're very likely to need to bi-amplify each speaker. So, 4 amp channels for L/R instead of 2. Food for thought.
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post #26 of 31 Old 05-10-16, 10:03 PM
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Re: diy mtm using Dayton pro drivers

I fully agree with DqMcClain's comments about directivity and beaming and the design tradeoffs related to matching cone drivers to dome tweeters.

However with the waveguides we are controlling the directivity better with less "taper" of the polar pattern vs. frequency (more directive than the dome in the crossover region but not so higher in frequency). This not only allows but requires a larger LF driver to match the narrower dispersion of the waveguide at the crossover frequency.

I've experienced the same issues with PA systems. Versus horns, modern waveguides trade ultimate efficiency for better directivity control and reduction of internal reflections within the waveguide, which are partly responsible for the typical horn sound. Both horns and waveguides are affected by axial cavity resonances which require notch filters to flatten the response (something the cheap PA obviously does not have). So, in these circumstances, yes a MiniDSP could be a great crossover design tool (especially if you don't have a stash of L,R,C's) or a permanent crossover in a active system.
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post #27 of 31 Old 05-11-16, 01:30 AM Thread Starter
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Then learning how to build a crossover. Or changing the parts on a pre made crossover.?
I think I am still leaning towards the Dayton pro 12 with Dayton compression because of availability and budget.
A 3 way is not an option because I need a small speaker. (not to tall) but high spl.
It needs only play down to 80hz but I want decent punch.
Could use minidsp to design the crossover and then try and build the board to match.???????
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post #28 of 31 Old 05-11-16, 08:43 PM
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Re: diy mtm using Dayton pro drivers

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mikmaz wrote: View Post
Then learning how to build a crossover. Or changing the parts on a pre made crossover.?
I wouldn't consider changing parts on a pre-made crossover as your final crossover will likely require more components (series resistors, parallel notch filters etc...) than the original board allows. The result will be more messy and probably even more time-consuming when you consider de-soldering etc... You are better off with screw terminals and/or alligator clips to test your circuit before you actually solder it.

Quote:
mikmaz wrote: View Post
Could use minidsp to design the crossover and then try and build the board to match.???????
That's a good idea especially if you don't have the software to simulate the crossover from its components. That will give you a good starting point. However not that the transfer functions you obtain for the active crossover will have to be duplicated accounting for the driver's impedance which is not constant with frequency.
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post #29 of 31 Old 05-13-16, 03:15 AM Thread Starter
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Can a receiver be used for tweeter amplification? And external 2 channel amp for woofer
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post #30 of 31 Old 05-14-16, 08:11 PM
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Re: diy mtm using Dayton pro drivers

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Can a receiver be used for tweeter amplification? And external 2 channel amp for woofer
If you had some way to route the crossed-over signal from the miniDSP back to the receiver, and then use it to amp the tweeters, then yes... at least in theory. In practice, it's highly unlikely you'll be able to route signal the way you'd need to in order to pull this off. If you're taking the pre-output from the receiver and feeding it to the miniDSP for processing purposes, you'd then have to feed the processed signal back into the receiver to amplify it... but that would require you to switch inputs to the one receiving signal from miniDSP. Obviously, this would disconnect the original signal and defeat the entire purpose.

If you were using a completely separate AVR, then you'd be just fine. If the separate unit was a spare and you had the space to incorporate it, this would be a reasonably practical approach. But if you have to buy equipment, I'd say you're better off just getting 4 channels worth of amplification.
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