The 3-Way Retro, revisit 1.
Usually it only take days after I finished a pair of speakers until I start to think about a new build. New concepts, a new look, another project. I am therefor quite surprised by the 3-Way Retro. I have had them for quite some time now and I am amazed by how much I like them. I would say it is the first pair of speakers I have made that supersedes all my expectations. I therefor would like to sum up my experiences with them in this post and provide enough information in one place for the ones wanting to build something similar.
Lets start with a short review of my listening experience. The tweeter has always been excellent and I don't find the mid a bit lacking as was my fist impression. In this build with the DSP its excellent. But it is the woofer that impresses me the most. I have used these elements in several speakers before. The bass has been good but not excellent. Without EQ an alignment of 0.707 would give you a volume of around 37 liters for a closed speaker. But it will only take the speaker down to 50 Hz. It left me wanting a subwoofer for some added bass extension. You may try a ported box but I didn't like it. I usually prefer the sound from a closed box but they are also much easier to get right (especially if using EQ to extend bass).
My advise to anyone using this woofer is to put them in a closed box and EQ them down to 30 Hz. The bass quality is amazing and it eliminated my need for a subwoofer (your milage may vary). I used 16 mm MDF for my build but I would recommend at least 18 mm birch plywood or 19 mm MDF because my box resonates a bit when driven hard. I did not use a Linkwitz transform to extend the bass, I used a DSP to EQ a target line with a slope of -6 dB from 20 Hz to 20 kHz. I let the bass roll off from 30 Hz to protect them a bit and to keep the EQ < 10 dB.
The design goal was to make a 3-way speaker that did not stand as tall as most of them (100-120 cm). And something that doesn't have to be dragged out in the room. I wanted something squat that would work pushed up against the walls. I also wanted to use a set of elements that would perform well without costing a fortune and the option of cost efficient passive filters. I didn't expect to actually meet all these targets and maybe thats why I am so surprised by the 3-Way Retro. Because it is all I wanted and even more. I give the DSP some credit for it but not all. This speaker can be built in several different configurations. It will perform better in some but it really depends on what you want it to do.
The basis for this build is the following set of elements from SB Acoustics (links to the latest versions of these elements):
- SB21RDC-C000-4 (3/4 inch ring dome tweeter)
- SB12MNRX25-4 (4 inch midrange)
- SB23NRXS45-8 (8 inch woofer)
Note: I used the 8 Ohms version of the woofer, there is a 4 Ohms version too. Pick the one that suits your needs.
The enclosure can be made super easy or a bit more fancy as I did in my build. It doesn't matter from a sound quality point of view. Just make a box with a volume of 1.8 to 2 liters for the midrange and 32 to 37 liters for the woofer. I used the following dimensions based on aesthetics but a closed speaker design is quite forgiving so feel free to change the dimensions into something that pleases you.
- Height: 54 cm
- Width: 36 cm
- Depth: 24 cm
Click on the drawing below for a blown up version:
The speaker is supposed to sit close to the floor (and walls) so make sure it is leaned a bit backward, I went with 8 degrees and it works great in my room. I initially made a pair of feet in ash but I later redid them in black MDF. I like the contrasting color of the blonde ash veneer vs. the black baffle and feet.
The following crossover frequencies works well with this speaker:
- HF (tweeter to midrange): 3700 Hz
- LF (midrange to woofer): 600 Hz
The filters can be second or fourth order, just pay attention to phase if using second order (you might have to swap polarity on the tweeter or mid depending on your combination of filters). An all active build and a DSP is making it easy to use any order and to compensate phase shifts. I personally think a DSP is overkill if only used as an active crossover. I have used Rod Elliotts P09 with great success in the past and it is a great alternative with no need for an AD/DA conversion. Rod Elliotts P71 can be used to EQ the speaker down to 30 Hz. P48A and P197 are other options. A DSP not only makes it easy to create complex filters, it is also useful as a PEQ to fine tune the speaker response and to make up for any flaws in the crossovers and response.
Another option is a semi-active build. A passive filter is used in combination with an active filter. The following second order filter works great as a passive HF-filter (the 0.82 Ohms resistor in series with the tweeter can be removed for some extra treble):
I am planning a fully passive version of this speaker. The filter will look something like this (I will make HF and LF into two separate boards so I still got the option to go semi-active using one or the other):
A fully passive build will probably not perform as good as a fully active or the semi-active version. I intend to use a DSP and try to EQ it into shape. That is in it self probably enough for another update...
