Summary

This project is similar to the Dayton BR-1 kit.  While I think there are other similarly priced projects of equal value, this design provides excellent bass and good treble performance at a very low price.  While its primary weakness is in the midrange, it is an excellent project for the DIYer on a budget, looking for a quality bookshelf-style speaker.

Specifications

Response: 42hz-20khz (-3db)
Impedance: 8 ohms
Sensitivity: 82.5 db

On and off-axis response of the Dayton 295-305 woofer


Dayton 295-305 woofer impedance and Thiel-Small parameters

Impedance profile for the Dayton 275-070 tweeter

Frequency response for the Dayton 275-070 tweeter

Calsod simulation of system response

Woofer in-box impedance

System impedance

System response

Final Crossover

This project uses the 295-305 6 1/2" woofer with the 275-070 1 1/8" silk dome tweeter and was designed was designed in parallel with the Parts Express BR-1 kit, which was designed by my good friend Paul "Peppy" Holsopple (inside joke, but since it makes me laugh, it made it in this write-up).  Paul was designing the BR-1 while I designed this project and we collaborated extensively during the process.

Since the time I originally posted this project, it's become clear to me that the revised Dayton Budget Project provides superior performance to the this design, at a slightly lower cost.  While the DBP doesn't have the same depth in its bass as the this design, it does come close, and offers superior midrange performance.  Nevertheless, I still offer this project to show the design exercise involved in its creation and because I think that the "D2" (what this project has come to be known as) is still a good good alternative to the DBP, for those looking for more full-range performance at the expensive of midrange performance.  Also, the crossover provided for the D2 is worth trying, for tinkerers looking to extract a bit more performance from their BR-1's.

The design objective was to extract the best performance possible out of inexpensive drivers without "choking the life" out of them or using such an expensive crossover that the benefit of the inexpensive drivers was lost. The "final" speakers have a very warm, clean sound with an impressively wide soundstage and reasonably good image depth. The highs are very detailed and smooth, the lows are impressively quick and detailed, and the midrange is more than acceptable (although you should beware the "anti-Dayton" crew and their limp-wristed obsession with trash-talking the Dayton 6 1/2" woofer -- if you run into them, ask them to find you another <$20 driver that has usable bass extension into the 30 hz range and see what they say). Overall, the final cost of the drivers and crossover for this design is about $125/pr, making it an exceptionally good value.

The following details generally cover the design process and response of the final speaker. The recommended enclosure dimensions are described, as well.

Driver Information

The Dayton 295-305 woofer has very good bass response but suffers from a nasty peak in the midrange performance. You can view its raw frequency response curve here. Once the Thiele Small parameters and raw impedance have been obtained, as shown below, the vented enclosure was design and I decided that a zobel filter was necessary to help attenuate the midrange peak.

The Dayton 275-070 silk dome tweeter is one of the best low-cost tweeters on the market. It genuinely performs as well as much more expensive tweeters, but it is not as easy to work with as I had originally thought. As you can see from the frequency response curve below, it has numerous peaks and dips to make finding an optimum crossover solution difficult. This is not the response curve I used for the final design, as it was obtained before I acquired a "Mitey Mike II" (professional measurement microphone, designed and calibrated by Joe D'Appolito) to use with CLIO, which I used to obtain much more accurate data.

The impedance for the tweeter is shown below. The average Fs for the tweeter samples was around 900 hz. The final design uses a 4th order LR slope for the highpass rolloff at 2khz. After optimization, the acoustic crossover occurs at about 1750hz, somewhat below the 2xFs guideline. However, since the tweeter is attenuated about 8db, there is no danger of distortion or damage.

The Enclosure

The box is .44 ft^3 total internal volume with a net of .42 ft^3 with a tuning frequency in the low 40's (about 42hz). this gives the proper balance (to me) between bass response and transient response/group delay. The internal box dimensions are 7"Wx9"Dx12"H. The woofer/tweeter center-to-center spacing on the front baffle is 5.75", and both drivers are mounted along the vertical center-line of the front baffle. The tuning port is 2"x7" (angled to keep the end away from a wall and braced in the center of the enclosure where there is a 1/2" birch plywood H-brace.) The top/sides/back are covered in acoustic foam and the crossovers should be mounted on the bottom of the enclosure. Alternatively, the vent tube can be 1 1/2" diameter PVC, 4 3/4" long.  I used the smaller diameter vent during the design process.  In either instance, you should avoid placing the vent directly behind the woofer.

The Crossover

After about three weeks of mucking about with various topologies that, at one time, led me to a 14 component crossover, I eventually tweaked it down to a 2nd order lowpass with a zobel for impedance compensation and a parallel notch filter to attenuate the peak in the woofer's upper midrange (that's where this design differs from the BR-1, btw, which did not include a peak filter, due to cost constraints), giving the speaker a much cleaner, slightly laid back, and more open sound. The final tweeter is a 2nd order electrical filter with a two-resistor "L-Pad" for attenuation, to achieve the target 4th order acoustic rolloff. Aside from off-axis performance and impedance considerations, I tried to design a crossover that would highlight the speaker's strong points -- good high end and excellent low-end punch -- without neglecting the midrange. The most time, therefore, was spend choosing a response in the crossover region to get a relatively smooth response while maintaining a good sound.

In the process, I also tried a 3rd-order highpass with a single series resistor for tweeter attenuation. This sounded very good and was going to be the final design, but I felt it didn't blend well enough with the lowpass filter that provided good baffle step compensation and acceptable woofer rolloff. Also, the 3rd order filter required that the drivers be connected with reverse polarity for smoothest on-axis performance. This led to good overall traits, but the tweeter sounded just a bit too "controlled". As a result, I went to a 2nd order highpass with the L-Pad. The tweeter seems very sensitive to topology and does not like a lot of components in the signal path. The 2nd order filter opened up the tweeter a little and also allowed for the drivers to be connected with positive polarity. The other problem to overcome was dealing with about 110μS of delay between the acoustic radiating planes of the woofer and the tweeter. I thought about angling the baffle to better time align the drivers, but I think this causes excess localization of the "sweet spot" for listening. Since one rarely ever sits in the "sweet spot", I thought it better to deal with the offset of the drivers' acoustic centers in the crossover.

All crossover capacitors are polypropylene (I used all dayton caps to save money) except the 47uF cap in the zobel -- use a non polar electrolytic for that with a .47uF film and foil bypass capacitor. All inductors are 18awg except for the .82mH series inductor in the lowpass which requires 16awg. Please note that in the schematic, 100uH=0.1mH.

The individual response of each driver is relatively smooth and adheres fairly closely to the 4th order LR target slopes. The acoustic crossover occurs at about 1750hz, instead of 2000hz, because I decided to underdamp the tweeter slightly to smooth out the on-axis performance. The off-axis and polar performance are very good, but I recommend that the speaker be listened to on the tweeter axis for best performance, i.e., place the speakers in your room, so that the tweeters are at ear-level. Also, the design compensates for all but about 1.5db of the baffle step rise, so they should be placed with 10"-18" of a wall. Stand mounting with the tweeter about 40" off the ground would be excellent.

The impedance shows that the cabinet, when completed, will have an Fs of around 42hz (with the crossover mounted internally). The nominal impedance of the speaker is about 8ohm with a local minimum of around 6 ohms in the crossover region. The phase angle of the impedance is good and the speaker should provide an easy load for any amp.

Final Words

Settling on a final design is just that -- settling. It was time to reach a decision on all the compromises that go into designing a speaker. I'm very pleased with the results and look forward to getting a set of these finished in a nice white oak veneer with a hand-oiled finished. I have also built a test cabinet for an MTM design based on these drivers (ed: now known as the D3). If there is any particular drawback with the 2-way design, it's the 82.5db average sensitivity in the passband. This means that you need an amp with at least about 75 watts to drive it comfortably, although that will be room dependent. My 30W tube amp gives it a pure, clean sound, with just a hint of "grunge" in the upper midrange -- some of that due to the non-polar electrolytic cap in the zobel, some due to the inherent midrange performance of the Dayton woofer. I may experiment with a polypropylene cap to see if it's worth the extra cost, but adding a $15 cap to a crossover for a $16 woofer just doesn't make sense to me. Overall, the sound is much better than I expected when I first started the project and will recommend this design to any of my friends looking for a great set of speakers for under $150/pr (estimated price in finished, painted enclosures -- add a few bucks for wood veneer, if you want to make your wife happy enough to let you build something better down the road).

A Parts Express parts list for building this project