Cheap 'n' simple 2x10 top?

[Brian Steele]

Looking at this project again...

I'm thinking of starting with one of the basic pre-built x-overs and modding it if necessary. Given my choice of drivers, which of these two would you recommend?


http://www.parts-express.com/pe/show...number=260-142

http://www.parts-express.com/pe/show...number=260-140

They're not "pro-audio" xovers like the Eminence ones offered by PE, but they are designed to accommodate either 4 or 8 ohm woofer impedance (the two 10s wired in parallel will result in a 4 ohm impedance). I'm thinking of starting off with one of these, and modifying it if necessary.

[mdocod]

Intended use: a pair of tops (>100 Hz) to be driven off a car audio amp, open-air venues small (<100 persons) crowd. Looking for high output and 4-ohm load. Doesn't have to be "hi-fi", but should be reasonably clean and clear.

Don't limit yourself to "4ohm" loads just because the car amp is designed around it. Any higher impedance load is also fine.

Ideas so far:
http://www.parts-express.com/pe/show...-030&FTR=pa255
Dayton PA255 x 2

These drivers are great used in multiples if you need some bass extension without any bottom end support from other systems, but will be a source of impractical inefficiency, and, when use in multiples for midrange, a source of unnecessary destructive coupling problems. You mentioned specifically, that you need response from these above 100hz. A single midbass driver solution of better quality, is the better choice in this case, even being 8 ohm. There are several 8 ohm midbass 10-12" drivers, specifically designed for this sort of application. Use anywhere from the same to half the power, half as many drivers, achieve better off axis response capabilities and equal or better maximum SPL capabilities from a very small, easy to handle, portable box. The PA255's are not the driver you are seeking IMO.

Eminence BH410 exponential Horn
http://www.parts-express.com/pe/show...Number=290-549

Having thrown out the reasoning for the dual 10's, I wouldn't bother with this smaller horn. Something larger, that will be more apt to carry to a lower x-over should be used if possible. I'd consider something wider horizontally. Small crowd applications often involve a lot of near-field listeners may benefit from this approach.

1" compression driver (haven't decided yet, but leaning towards one of the Seleniums)

The 220, easy choice.

Build - 44 l (net) sealed box, with the two 10's positioned vertically as close together as possible with the BH410 located between them. ~2.5 kHz x-over point, nothing too fancy. Pole-mounted to bring the BH410 up to ear level.

Pole mount the smaller boxes up higher, with them inverted for better "reach" out over crowd, and easier crossover design (better chance of hitting phase alignment). Putting a PA horn at the front rows ear level is a bad idea unless you want to cause hearing loss for the people in front, and inaudibility for the people in back.

Sounds good?

It will sound great when you do it right

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Yes the 202 has a better highend response but less power handling, if you want to utilize all the potential in those 10s then you'll need a CD that can handle upwards of 150w assuming 6-7db of padding,

I'm not sure where you are coming up with these numbers, after padding, a typical CD will only need to deal with perhaps tens of watts maximum, to keep up with a "single" compact flying PA unit that may be operating at "hundreds" of watts worth of effective "gain" (in fact, these "watts, will only be present at high levels where the system has impedance dips, it's really a voltage based gain, and the CD will be behind a ton of padding to get down to the level of the midbass drivers, especially of flown, and properly designed to pick-up their step loss)

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Thanks for the feedback. Some rough calcs suggest that my DIY "tops" (if SWMBO gives the go ahead to build them) are likely to see 100W or less from my system, so I think I should be OK with the D202. In any case, I'll be investing in some protection for them, just in case someone decides to be stupid with the volume control. I've seen some circuits online for this that look a lot more involved and useful than a simple inline lamp or polyswitch, and I'm tempted to give them a try.

I would just go with the 220's regardless... $7 more, 3dB better thermal headroom, flatter response, technically able to reach down to lower x-overs.

One more question - the horn I've chosen seems rather small. Is it really useful down to 2kHz, or am I worrying too much?

Keep in mind, that the response of a CD in various horns will change a fair bit, the bottom end reach especially... This is where "DIY PA" really turns into a mess- a complete lack of publicly available response data for particular CD's in particular horns. I'd suggest going larger, and hoping for the best in this case.

Eminence says the lowest recommended crossover freq is 1.2khz so you're plenty safe.

I wouldn't trust that particular claim without measurement. I suspect that could be a -10dB point, but I really don't know for sure.


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Looking at this project again...

I'm thinking of starting with one of the basic pre-built x-overs and modding it if necessary. Given my choice of drivers, which of these two would you recommend?


http://www.parts-express.com/pe/show...number=260-142

http://www.parts-express.com/pe/show...number=260-140

Neither. Sorry

They're not "pro-audio" xovers like the Eminence ones offered by PE, but they are designed to accommodate either 4 or 8 ohm woofer impedance (the two 10s wired in parallel will result in a 4 ohm impedance). I'm thinking of starting off with one of these, and modifying it if necessary.

I'll help you design a speaker and crossover, if you are serious about getting it "right" or at least, a lot closer to "right."

The biggest issue for me, from affar, i won't have any way to figure on the actual response of the CD in a particular horn, so aside from a potential blip around the x-over, I believe it's possible, to get these things very good sounding, especially through the critical midrange from your midbass, which, if flown, should be step loss compensated.

Regards,
Eric

[Paul O]

I'm not sure where you are coming up with these numbers, after padding, a typical CD will only need to deal with perhaps tens of watts maximum, to keep up with a "single" compact flying PA unit that may be operating at "hundreds" of watts worth of effective "gain" (in fact, these "watts, will only be present at high levels where the system has impedance dips, it's really a voltage based gain, and the CD will be behind a ton of padding to get down to the level of the midbass drivers..

Yes technically.. but real world experience suggests that low power(20w-40w) compression drivers are easily blown when the speaker system is subjected to 400-500w of music power, these "lightweight" CDs are common in entry level commercial PA speakers like the JBL JRX and they're notorious for letting out the magic smoke under what is considered normal operation at well under amp clipping levels, so the fix is to replace it with something more robust like a Selenium D210/D220.


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Keep in mind, that the response of a CD in various horns will change a fair bit, the bottom end reach especially... This is where "DIY PA" really turns into a mess- a complete lack of publicly available response data for particular CD's in particular horns. I'd suggest going larger, and hoping for the best in this case.

I wouldn't argue with that at all.. it's one of the reasons I chose the Dayton H110 for my 12+1 project.

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especially if flown and properly designed to pick-up their step loss

Also agree there the mains have to be jacked up overhead, though my suggestion for step loss compensation is not the method commonly used for home audio projects where you "throw away" 6 or more db of sensitivity across the whole spectrum with padding just to achieve flat response with the step loss below 200hz. My suggestion is to use a ported allignment with a high tuning freq so that bass response is boosted and then use EQ or better yet active processing(ie: DSP) to further flatten response where necessary. With PA systems top priority should be given to achieving the highest system sensitivity and therefore power efficiency over everything else.. within reason, super high sensitivity with a stupid peaky response is no good either so you still want to target flat response, but this means active system processing isn't optional it's manditory.

[mdocod]

Hi Paul O,

I like your approach to step loss for Pro Sound. Excellent. Going to keep that in the foreground of thought for awhile till it sticks.

Regards,
Eric

[AMC]

My suggestion is to use a ported allignment with a high tuning freq so that bass response is boosted and then use EQ or better yet active processing(ie: DSP) to further flatten response where necessary.

I like the idea, but that suggestion should also come with a warning statement. If you port it high enough to make a real difference, they will no longer be usable as a 'full range' box, and would ONLY be suitable as tops.

[Brian Steele]

Hi Paul O,

I like your approach to step loss for Pro Sound. Excellent. Going to keep that in the foreground of thought for awhile till it sticks.

Regards,
Eric

Are we talking about the baffle step here? Because, according to my calculations, that "step" should occur just above 1kHz for a box with a 10" driver (assumed baffle width of around 12")...

[mdocod]

Hi Brian,

Here's a couple of simulations to get get a feel for the baffle step and diffraction thing, and Paul O's suggestion:

Simulation of something like a 2x10 MTM cab, from a listening position 5 meters away back and 2 meters over. Cab flying 1M over the audience "ear" level:




Simulation of something like an inverted TM configuration with 1x12" driver, same listening position as before:

This is a better example of baffle diffraction, because it doesn't include the destructive coupling of a pair of drivers... (more on this in a moment)... Notice, how the peak is actually greater than 6dB, this is a culmination of step transition, and edge diffraction coming together to actually produce a peaking response greater than the "6dB" transition. If viewed on axis, this peak would appear even higher and you would also see subsequent dips and peaks in response through the high frequency as wavelengths work constructively and destructively as they reflect and wrap the baffle.


Simulation of Paul O's suggestion, based on using the 2x10" drivers you have in mind. 1.5ft^3, 100hz tune, HPF set to 90hz resulting in -6db@100hz.


Regarding my prior suggestion against the idea of the dual 10" drivers... Notice the response in that first simulation, never even reaches a fully effective step transition of +6dB because from that listening position, the difference in distance to the drivers is already out of sync enough not to couple effectively. By the time 1200hz comes around, it would need to be crossed to a high frequency driver just to prevent this listening position from having a potentially deep null in response. Notice, in the second simulation, the single 12" driver, even being larger diameter (more directional), effectively behaves better off axis because it's not fighting another driver, not getting back down to the 0db mark until more like 2200hz, making this configuration useful for 1500-2000hz crossover options.

The MT configuration suffers from a narrow band potential for lulls in the x-over for listeners off axis, the MTM suffers from potentially wider and deeper nulls in more critical frequency ranges for off axis listeners.

Regards,
Eric

[Brian Steele]

The spacing of the 2x10 example looks a bit wider than what I'm expecting it to be with the horn I was planning to use like if the 10s are a bit closer together, let's say 2/3 of that separation in the example? The lobing seems to start above 1kHz - what if I x-over to the horn at around 1.6 kHz instead of higher. Would this reduce the audible impact of that lobing? One other possibility that might be worth considering is rolling off the lower woofer early, say around 500 Hz. Bear in mind this is likely to be a sealed environment, so venting at a higher frequency to make up for the step loss is not an option.

[mdocod]

Hi Brian,

Spacing can be deceptive when looking at that program, because it is only a simulation of the piston area. The "dotted" area that appears to be the driver, does not include where the frame would be as well, so there is actually less space between those drivers than their appears to be. That simulation assumes a ~14" center to center spacing of approximately 8" pistons... If you get it any tighter than that I would be amazed based on a visual on that horn you have in mind. Furthermore, the effective piston size of a typical 10" is probably a tad bigger than I simulated there, which would actually result in the response looking even slightly worse at that listening position.

All this aside, if you are absolutely set on going sealed (I can't imagine why), then you will probably need either a pair of drivers, or a larger driver with similarly "low reaching" characteristics in order to get down to ~100hz. The trade offs of the multiple drivers is something you may just have to live with if you aren't willing to live with the trade off of venting, which, IMO, isn't a tradeoff at all in this case.

Regards,
Eric

[Sydney]

Perhaps there is a difference in philosophy of implementation or a difference in the meaning of:

Cheap N' Simple

Considering the fact that a Pro cab is likely to be set up and used in a wide variety of acoustic settings not a fixed location. These could be outdoors away from external reflections or indoors very close or conversely far enoughaway from reflective boundaries. With this in mind; to implement BSC or fixed compensation might be inappropriate and incur more expense.
Most guys I know use EQ; Not to correct for speaker non-linearity but for general tonal adjustments. Many consider acoustic non-linearity to be a design flaw.