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Drive-Ins: a small, high efficiency speaker

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  • Drive-Ins: a small, high efficiency speaker

    This is a case of a bunch of things coming together in an unplanned manner. I'm interested in your thoughts on several fronts, both component selection and design.
    Goal: Drive-In speakers - DC-powered 105dB SPL, low distortion, FM radio/MP3 player
    Approach: Dayton Point Source drivers and Chinese digital amp in Triska-sized split box

    A recent thread had a link for a Chinese "car/motorcycle" (i.e. 12VDC) FM radio, USB card reader amp. I have no idea why I was moved to order one, a Lepai LP-V9, and fit it, a battery and speakers into a box I already had.
    - the amp:
    UPDATED LINK (same unit, different vendor)
    - the box was for an oversize Triska sub. Built 2 for my kids, and parts for a third cut out of the same sheet.
    - the Singularities showed that a simple shelf filter makes the PS220 a very nice, high sensitvity speaker
    - a high senstivity speaker would get quite loud with 10-20W, a realistic output of these DC amps
    - the PS220 was a drop-in replacement for my pre-cut wood, but modeled poorly at 10-12 liters
    - the PS180 modeled nicely ported with a 75-80Hz tune; I made inserts (see pics, eventually)
    - for other reasons, I had a 7amp-hr battery available along with maintenance charger
    - PE had a 2A power supply on sale, and it got me free shipping on 1 PS180 driver
    - my 13x13.5x14" oversize Triska design lets me retain 11.4L speaker volume after fitting amp and battery

    So… I submit for your perusal and comment, the Drive-Ins, a PS180-based, high sensitvity speaker with a simple LR contour network. As documented for the PS220's in the Singularities, the smaller drivers are also very senstive, but with rising, ragged and narrow (if extended) high frequency output. Without a contour filter, they squalk on-axis, but smooth right out a few degrees off-axis, about the point where the high's go away. The polar behavior is very sharp.

    I used a UFO202 with my laptop, REW 5.0 and a Xenyx 802 mixer/ECM8000 mic for the measurements. Driver response rises above 3KHz, and is increasingly ragged, so I used 1 octave smoothing to evaluate the results of a simple shelf filter. I show 1/3 octave smoothing, with and without contour filter, and final on-axis, 15 and 30 degrees off axis.

    PCD shows unsmoothed data, rags and all as you would see in the PDF if the file weren't too big to attach. The contour filter is a parallel LR, 0.3mH and 20 ohms at this stage. No attempt to address raggedness, just suppress the peaky 3K-10KHz region without losing too much of the top octave on-axis, given how quickly it goes away off-axis.

    Note that I did the measurements in-box (13wx14h", centered 1" up on long side), and with an active port. I've attached a Unibox plot, but will note this driver comes in with an Fs of 66Hz vs. 48Hz spec and 40Hz Kippel. My WT3 is suspect (1K resistor doesn't always measure 1K ohms), but this design only uses the flat upper part of the impendence curve. Bass output, however, is more consistent with the measured Fs.

    The pair of speakers fit together so that the amp and battery can be mounted in the space between their back panels. Close it up with yet-to-be-ordered butterfly clamps and I'll protect the drivers with metal grills like for my Triska's. Open it at the drive-in, set them 10' apart and tune the radio to the local movie frequency. At parties, hook an iPod up to the RCA jacks. I'll get pictures once I have all the hardware.

    So, comments welcome on the speaker design, and… I've heard good things about Lepai amps, and this one has the right combination of features and capabilities. Anyone have any experience with Lepai? Am I barking up the wrong tree somehow?

    Have fun,

    Listening impressions once I get a pair....
    Attached Files
    Last edited by fbov; 12-15-2011, 12:06 PM.

  • #2
    Re: Drive-Ins: a small, high efficiency speaker

    Here we go... my PCD model but the contour filter got cut off....
    Attached Files


    • #3
      Re: Drive-Ins: a small, high efficiency speaker

      You should make them look like a boombox haha, that'd be sweet.

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      • #4
        Re: Drive-Ins: a small, high efficiency speaker

        When I wrote this up, I realized that at one level, you're right; my goal is a super boom box, but unlike the Podzuma, as separate speakers rather than an integrated unit. Plus, I wanted an FM section for the drive-in loop audio signal.

        I've attached some pics:
        - front view as I'll set them up. Note the insert for the driver, and rebates for the SD215PR (trim ring) and grill (see below)
        - partially assembled, showing battery compartment and space for amp. Butterfly latches will hold things together
        - one of the Triskas showing the grills that will cover the driver.

        The Triska legs are longer as I mounted the SD215 driver downfiring. These will be shorter, so the speakers will lean back and fire toward you when put on the ground in front of you. These drivers really need to be pointed toward the listeners.

        So not really a boom box, but close.

        HAve fun,
        Attached Files
        Last edited by fbov; 07-04-2011, 04:28 PM.


        • #5
          Re: Drive-Ins: a small, high efficiency speaker

          These are done and I'll add some pics and FR data for the finished design this weekend; a folding table in the living room can only be a temporary measurement lab for a married man.

          However, they have been used in public, and did just what they were designed to do. It was a work-related picnic in an enclosed shelter. The speakers were set up outside on a concrete patio, on the ground against the building, roughly 15' apart separated by benchs. This placement did two things:
          - kept the listener off axis
          - reinforced the low end nicely.

          As I noted initially, the PS180's have a certain harshness to my ears, and I didn't completely eliminate that harshness on-axis, in order to retain a degree of high end extension just off axis where the harshness quickly disappears. As I backed away from the patio, toward the sweet spot, they retained balanced bass and the upper treble came in nicely when aimed at ankle level. It'll be interesting to see how they do in this respect at the drive-in....

          The Lepai amp/tuner did very well on 12v. We were 10 miles into the next county and I still got my little BOCES station to tune acceptably, so I don't expect issues with a drive-in loop transmitter. The appropriate volume setting only just blinked the LED display; you could hear the music but it didn't interfere at all with conversation.

          One thing I have noticed is that there's no distortion at full volume running from USB input on a 12v 2A source power supply. This input power supply rating tells you the amp's power rating (18w/ch RMS) must be 1 channel driven, so a 24W supply isn't going to drive 2 channels to clip. Conversely, a 14v source, as from the a vehicle's alternator, will also give you more power. I'm not sure I need it, but max volume SPL is a measurement I've yet to do... this weekend, I promise.

          No complaints so far....


          • #6
            Re: Drive-Ins: a small, high efficiency speaker

            I promised a pic at the drive-in, so here it is, sunset and all. (Saw Cowboys & Aliens, and Harry Potter. One was worth it....)

            And some of the finished project. The cube is heavy, as much for the battery as the use of 3/4" MDF for the original sub box. There's an AC power supply (below amp) and a maintenance battery charger (next to battery) all Velcro'ed in. Wire to the left speaker stores in the cavity, above the battery (just barely). I've managed to lose the remote and a thumb drive full of music, but it'll turn up.

            I got lucky catching a flash reflection in the phase plug, showing the bronze color I tried to pick up by using hammered copper paint for the accent ring around the drivers and inner surfaces. Top and bottom are Ebony Star counter laminate (left over from telescopes) and I don't think I'm adding the feet I expected; it's easy to find something to tilt them at the drive-in.

            With the pair of them running off the battery, listening to the radio, I'm hitting low 90dB range at ~2M in a bright 15x22' room. However, the volume control is maxed, so there may be more available from a higher-level input and/or higher voltage source. Regardless, I couldn't turn them up near that high at the drive-in. The advantage was improved sound quality sitting outside.

            HAve fun,
            Attached Files