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  • New 10" Small Sealed Sub Project Start

    Hey all, hope everyone is enjoying a nice Memorial Day weekend. Thought I would share my latest sub project that just got underway (after sitting in the design stage for way too long). I finally committed and started cutting wood and made some good progress just yesterday. The genesis for this sub idea started maybe late last year, been seeing lots of hype for really small-form factor subs and thought it would be fun to build a smaller sub and see what kind of performance can be had when you ditch the big boxes for something a bit more petite. The idea of an ~11" cube that could bump was suddenly appealing and I started throwing ideas together. I modeled a bunch of the smaller subwoofer drivers from PE, starting with the new Epique drivers and Ultimax to Peerless and Tang Band and eventually landed on the classic Dayton RSS sub series. I bounced around between the 8" RSS210's and the 10" RS265's and after a little consideration with the box volume, decided to go for the 10" HF-4 driver and upping the cube dimensions to just over 13".

    Clearly there are a thousand ways to get bass from a small box and this is by no means the best way, but cost being a consideration too, I opted to go with a sealed cabinet and planned to use active EQ to boost the lower end to get better extension. And by active EQ, I mean using a sleazy bass boost circuit found in most of the plate amps here on PE. Fundamentally I think bass boost gets a bad rap because it's arbitrary and in most cases ill-suited to the design application. The proper way to provide low-end correction to a sealed sub would normally be with some kind of DSP, by measuring the sub response and designing a filter to counter it to achieve the desired final response. No disagreement there. But when can a basic +6 dB of boost at 30 Hz be the perfect filter? Especially if the driver and box volume were designed around that exact bass boost? Well, we're about to find out.

    Then just when I was about to pull the trigger on either one of the Yung or Bash 300W plate amps "with bass boost" I was checking out what's new at PE and ran across the Dayton Audio SPA300-D. This amp hit all the high points and best of all the built-in bass boost was defeatable with the flip of a switch. Thank you Dayton Audio! This was all it took for me, that one feature missing from all the other plate amps, not to mention 300W Class D, adjustable low-pass filter, phase switch, on/off/auto and no speaker-level inputs or heat sinks to clutter up the rear panel. Just a nice and clean look. I hit the pre-order button and waited. I had no idea when it would be in stock, but about 3 weeks later, it showed up and shipped. Then quickly went out of stock again. Curious if anyone else picked up one of these while they were in stock? It was supposed to arrive yesterday (hence the sudden jump to get started on the box) but is now delayed until Wednesday. I'm excited to check it out. I've got some simple measurement gear (REW, load box, soundcard, the usual) so I'm hoping to get some raw power measurements, frequency response measurements, bass boost response measurements, etc. as soon as this thing arrives. So stay tuned for that.

    Anyway, so that's where this project sits as of right now. The box is half done and the amp is somewhere between Dayton and Tucson. I had purchased the HF driver a few months back so that's just been sitting waiting for me to get my act together. I'm actually glad I waited as long as I did because that new plate amp looks like it could be a sweet little gem, balancing performance and price, without the exotic DSP but just some good old fashioned analog bass boost to take any small sub project to the next level.

  • #2
    Quick follow-up with additional pics and design details:

    10" RSS265HF-4 Subwoofer
    25L (0.88 cu.ft.) Sealed Enclosure
    Modeled in Unibox using measured T/S params (REW)
    13.5Wx13.5Hx14.5D
    Qtc = 0.760
    Fb = 42 Hz
    F3 = 39 Hz (no boost)
    F3 in-room with +6 dB/30Hz boost = TBD
    Dayton Audio SPA300-D 300W Plate Amp
    3/4" MDF and 1/4" HDF (1" total wall thickness on 4 sides)
    Internal cross bracing/front-to-back bracing
    4-Pack Rubber Cabinet Feet 1.57" Dia. x 0.61" H

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    • #3
      So the SPA300-D arrived today! First thing I notice is how small this thing is. I know the specs said it was like 7"x8" but I guess I didn't realize just how compact that is. This could fit into a sub box that is basically a 9" cube and fit perfectly. Overall impressions are very good. This amp is built very well and feels really solid, lots of epoxy/goop holding things in place, should be free of rattles. The volume and frequency knobs feel really nice and the little switches for power, phase and bass boost all click nicely and feel really good.

      The overall design is made up of just two boards, a pre-amp board and the main board/power supply board. Didn't really care for the zip-tie holding the mains to the audio line from the pre-amp to the amplifier board. Thankfully looks like it is shielded. It looks tidy but isn't necessary and should be clipped and tied elsewhere. I might do a more detailed breakdown of this amp later. It's definitely no chip-amp, this is all discrete Class D. A pair of G20N50C MOSFETS take care of the *edit* switching power supply stage of the amp. A pair of International Rectifier IRFB5615 MOSFETS handle amplifier stage duties. A pair of ST074C op-amps appear to work the pre-amp stages (volume and low-pass filter). The bass boost circuit looks like it's using a TI TL064 quad op-amp and the phase reversal is using a TL072C dual op-amp *edit*. Lots of DECON electrolytic caps throughout and plenty of QC PASSED stickers slathered on for good measure. Check out a few of the pics below and I'll do a follow-up post with some of my initial measurements of this thing. But overall there's a lot of good going on here.
      Last edited by danmarx; 06-04-2022, 11:24 AM.

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      • #4
        So the first thing you probably want to know is...does this little guy deliver its rated output power or not? And I am happy to report that YES, this amp delivers on exactly what it claims. I ran a bunch of spectral analysis (RTA) measurements using REW from 20 Hz up to 80 Hz, from 1W to 300W with and without the boost enabled. Basically at 40 Hz, this amp will deliver a cool 300 watts RMS at ~1% THD into 4 ohms. That's enough juice to take a stack of 4 ohm loads from room temp to 200°F in a matter of seconds. The spec is 300W RMS into 4 ohms at 1.5% THD so I would say the specification was definitely met. At frequencies below 40 Hz the THD goes up a bit, but above 40 Hz it goes down some.

        Additional tests include frequency response measurements from 1W to ~300W, with an without the bass boost enabled. Bass boost is specified for +6 dB at 30 Hz, measured boost looks more like +5 dB at 30 Hz. Which is actually a little better in my opinion. Sometimes a full +6 dB can be a bit much, so no one is going to miss a single dB less boost. With and without the bass boost circuit the response is down about -6 dB at 20 Hz. This is a little disappointing to have the high-pass filter so aggressive. For small sealed subs this doesn't present much of a problem. But for ported subs this filter will reduce some of the lower-end extension. In contrast, will also help with over-excursion below tuning for both enclosure types.

        I exported the various measured FR plots from REW and imported them into Unibox and we can see that the response is in fact down another -6 dB at 20 Hz. The amp's high-pass filter looks to be about 12 dB/octave, combined with the 12 dB/octave roll-off of the 2nd order sealed cabinet and we get a combined response that is 24 dB/octave. Once you switch on the bass boost, the response is down 10 dB at 20 Hz but flattens the simulated response to be nearly dead flat from 30 Hz to 100 Hz. This is the obvious main benefit of the bass boost circuit and for this subwoofer setup it looks like it's a good match. Modeling a comparable but larger ported enclosure the bass boost exaggerates the response significantly and may not be as desirable. Which is why it's pretty great that this amp has the ability to simply switch on and off the boost. No more messing with de-soldering a bunch of resistors and hoping you get something that sounds decent. Or if you decide to use this amp in another enclosure, it can be done easily. It's likely possible to change the bass boost circuit here as well, but note that all parts on the pre-amp board are SMT.

        I'd like to explore pushing the high-pass filter down a few Hz. It looks like what they did was create a high-pass filter with a corner of about 25 Hz that is critically damped with the boost disabled and then under damped with the boost enabled. An underdamped high-pass filter is how most bass boost circuits work as they are simple to implement and require few components. Plus they act double-duty as a subsonic/rumble filter to prevent over-excursion at frequencies below the filter. If we pushed the filter corner down to maybe 20 Hz, the bass boost would probably go with it, creating a bump at around 25 Hz. We can model this and see what it does to the combined sub response, but you can quickly start to run into too much boost at too low of frequencies and over-excursion becomes a concern.

        Anyway, below are just a couple of plots (why does this forum limit uploads to only 5 pictures?) of the many I've grabbed so far. It's been fun measuring this amp too, it stays nice and cool and so far hasn't given me any issues during these bench tests. Next up will be measurements with the the real subwoofer in the enclosure and we can see how it actually sounds. Does the the real-world performance match the sims? I'm betting that they do.





        Comment


        • #5
          Cool tests! Don't see a lot of amps getting the measurement treatment, thanks for sharing.
          Electronics engineer, woofer enthusiast, and musician.
          Wogg Music
          Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

          Comment


          • #6
            No problem wogg! Thanks for commenting too. It's another weird part about this hobby that I've suddenly become obsessed with. It's just about as much fun as building speakers themselves.

            So this morning I mounted the RSS265HF-4 into the enclosure (will have to paint and finish it later) and ran some initial tests as well as just played some music to get a feel for what this thing can do. Initial thoughts, very impressed! This is the 3rd time I've build a subwoofer out of Dayton's RSS sub lineup and they have never disappointed. This sub is no different. And the combo with the SPA300-D is darn near perfect. This amp can drive this sub hard, probably just above its limits, or at least very close. And the bass boost circuit provides the perfect correction and pretty much matches what the Unibox simulations showed.

            Near-field response without the bass boost rolls off around ~35 Hz but with the bass boost there's a +5 dB increase at 30 Hz, which flattens the overall response providing an estimated f3 of around ~26 Hz. See the graphs attached. This about as perfect as it gets in terms of transfer function between amp gain and enclosure roll-off, especially for something as simple as just a basic bass-boost circuit. All plots attached are near-field without smoothing. I'll have to do some in-room measurements at some point, plus it would be fun to compare it to my UM15-22 sub which is 3 times the size. I bet it can keep up, for a little while at least!

            I also shot a quick video of this sub playing, just to show the massive amounts of excursion this little thing has. Haven't had a chance to really play with it much yet, but so far I'm super happy with the overall outcome. It's a small sub that packs a punch. The new Dayton Audio amp can definitely deliver and makes a good match to the RSS driver. Sure it's not going to do 20 Hz at any significant level, but for what it is, it's pretty amazing.

            Click here for a short video
            https://youtu.be/SNLt-qE0JSg

            Dan

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            • #7
              And here's just a few of the subwoofer measurements. Setup is REW 5.20, an ECM8000 mic and a Presonus Firestudio audio interface.

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              • #8
                So the sub is officially complete. Here's the last few pictures of the rest of the build process. After the handful of test runs, I basically committed to keeping the SPA300-D amp and cut the hole in the back for it to fit. I made a little router jig so I could recess/flush mount it for a cleaner look. Then I completely botched the paint job per usual and lined the box with some good ole' fashioned fiberglass insulation. For a sealed sub I think this stuff works great. And Home Depot sells a small pack of it like this for only $6, so it's cheap too.

                I decided to replace the thin 18 gauge speaker wire on the amp with some 12 gauge wire just for good measure. It's attached with a set of .205 inch female disconnects which were pretty easy to remove and replace. I dropped in the amp and hooked up the RSS265HF-4 and basically that was all she wrote. Sub done. And everything is held in place with those generic black #8 x 1" pan head screws that PE sells. Straight into MDF they work fine as long as you keep the disassembly steps to a minimum and if you undersize the pilot hole by a 32nd.

                Anyway, this was a fun build, and only took two weekends to complete. You could do it in one, but I spent most of last weekend measuring the amp and the sub and just playing around with it. Of course now that it's done I can really play around with it, get it paired up with some decent LR speakers, try out some more music, movies, see what this thing has to offer. It was intended to be part of a 2.1 audio setup. It's sitting as a computer sub right now. But will probably end up in a bedroom 3.1 HT setup.

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                • #9
                  And here's just a few more final pics.

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