Mini cum laude 3-way project thread

[max_andrews]

PE's "magna cum laude" project is fairly popular, and for good reason. Those things can play loud! The downside is that they are quite large, making it difficult for some people (like me) to justify and/or construct, and also fairly expensive which can be intimidating to first time builders. My solution was the "mini cum laude," a more budget oriented project with a similar layout (WWMT) and smaller footprint. The basic design uses dual SD215 8" DVC subwoofers (vented), a dayton PA130 as the midrange, and the hivi SD1.1 tweeter for the top end. It has an F3 of about 30hz, and a max volume of over 110dB (with about 100 watts) before woofer xmax becomes an issue. At no extra total cost, you can also use dual SD270 10" dvc subs sealed, which will give you another 3dB of sensitivity at the expense of a bit less extension (48hz F3), but with the floor and room gain I doubt this will be too big of an issue. In fact in smaller rooms, I'd wager the 8" version sounds too boomy due to small room gain. Also, since I have a feeling many will use this as part of a HT setup, if placed on either side of the TV and next to a wall the bass will be further magnified, making the sealed a better choice in this situation.

Cost for a pair if these is under $250!

Here's a rendering showing overall dimensions, layout, bracing etc. Since I did this one, I decided to build the midrange chamber onto one of the sides, so imagine the mid and tweeter shifted and moved over toward one of the sides. This simplifies construction and lowers baffle diffraction effects. Box is approx. 2.25 cubic feet after bracing and the mid chamber are taken into account, overall size is 11"X16"x36" which can be cut from a single sheet of 3/4" birch ply or MDF if you're careful about the cuts. Mid is open-backed t-line similar to the mini statements. I intend on heavily damping it , the lack of a rear wall is to reduce lower frequency standing waves, not to vent midrange into the room.



So I posted about this and did some sims and renderings, and a few people also wanted to see a more hifi version using a better mid and tweeter. Well there are hardly any reasonably priced midranges with high sensitivities, save the RS150, which seemed like it might work. For the tweeter I immediately chose the seas 27tdfc. I've used this tweeter several times and it simply can't be beat for the price. Low FS, low distortion, and $30. These two driver choices keep the sound quality high but the price pretty low. A bit (ok, a LOT) of crossover simulation later...

This simulation is for the SD270 woofers sealed, deeper extension can be had if using the SD215 vented (30hz F3), but at the expense of ~3dB of sensitivity. I'll have a simulated crossover for this version soon, but here's the SD270 version:



Price for drivers and XO parts is $350/pair, which is pretty cheap for a WWMT. This assumes using the jantzen p-core inductors for large values, NPE caps for large values, and poly caps and 18 guage inductors for the smaller values. I have a parts list I will post soon, everything but the tweeters (madisound) can be ordered from parts express. I know the parts count could be slightly reduced by being smarter about the notch filters, but it was flatter this way and not all that much more expensive so I think I'm going to leave the overall topology untouched.

I'll be building all four versions of this design, and I'll post my process and progress in this thread. I'll also be getting down and dirty with a measurement setup, so I'll be able to see how the results stack up against the simulations!

Please don't hesitate to offer any comments of suggestions, I hope this design can become popular to those with a somewhat limited budget or workshop, but who still crave a large, full sound.

[John L]

Max, you mention two 8" or two 10" DVC drivers for each cabinet. To clear up something, on your crossover, it does not acknowledge the DVC. Are you using just one coil per driver, or are you using both?

[max_andrews]

The coils for each woofer are wired in parallel and then the drivers are wired in series, yielding a 6-8ohm nominal load. Nice to see you here

[tinitus]

I cant say why, and not sure if it will make much difference, but I would wire the voicecoils in series and then the drivers in paralel...I think its normally done that way

[wigginjs]

Originally posted by max_andrews

Here's a rendering showing overall dimensions, layout, bracing etc.

I can't see the rendering you mentioned. Is it possible for you to attach it?

[max_andrews]

Yes indeed I must have made a mistake with the URL for the rendering, I can't seem to edit the first post...
Here it is again:

[max_andrews]

I think I'm going to ditch the PA130 as the midrange in favor of something with (likely) lower distortion and a smoother response curve, at least for the vented 8" version where I don't need the extra sensitivity. The problem is mostly that the PA130 has a pretty large dip around 1khz and it's made worse on this baffle size, so although really cheap it might not be the best choice here.

I've mocked this up using the RS125 and HiVi SD1.1
This is about $300/pair, $50 more than the PA130 original concept but a lot smoother. The 8" drivers have lower sensitivity than the 10" drivers so we're at 86dB here as opposed to 90dB. The crossover point here is fairly high between the mid and woofers (450hz, a.k.a 2.5foot wavelength), but phase alignment is excellent. I'm not going to be able to go lower than 350 any way I slice it, will I see any benefit with a 350 xo rather than a 450? The woofers and tweeter already have very little interaction.

* Vented impedance not modeled here, assume a standard vented twin peak below 100hz...

[John L]

Max, could you go into more detail the reason for your leaving the back of the midrange open to the back? I'm curious about the science behind this.

[John L]

One other thing. The RS125, while having a smooth response curve, is much less sensitive than the PA130. Won't using the RS125 be 'dumbing down' the SPL curve a bit?

[max_andrews]

Sure John,
I was thinking about an open back transmission line mid for two main reasons. The biggest is to eliminate the standing waves caused by a typical closed back cabinet, as the distance from the driver to the rear wall (15.25"), is smaller than the lowest wavelengths it will be producing, so a standing wave would be created that would lead to either a peak or dip in the vocal range, which is the last thing I want . This is typically what leads to "boxy" sounding mids, as the rear energy re-enters the cone delayed in time and leads to muddy sound/

This open line is intended to be lined and filled with some damping material, I'll have to actually experiment and see what the right amount is. The goal is to damp the rear energy, but still let some escape for an ambient "open baffle" type sound, which should lead to larger sounding vocals and strings.

As for the RS125 lowering the SPL curve, the limiting factor with the 8" version is the woofers, which are 86dB. I won't be using the RS125 for the 10" version as the RS150 is a better match for sensitivity. I'll still try the PA130 version (I'm going to build a whole set of removable baffles so I can try all these combinations), but I want to make sure I have a backup if that doesn't quite work out as a midrange.