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Ohm passive subwoofer SB-4A - $20 (Westchester, NY)

Discussion in 'Dollars and Sense' started by swechsler, Jun 12, 2018.

  1. swechsler

    swechsler Frog Whisperer

    Messages:
    5,131
    Location:
    7th row center
    I've never seen this before, and the *internet (including Ohm's website) has no knowledge of it (at least that I could find in a brief search). I suspect it was part of a 5.1 sub/sat system (where the center was completely separate) based on the number of connection posts on the back and the inclusion of an AOL address on the manufacturer plate (IOW, it probably dates to the early-mid 90s and was designed for Dolby Pro-Logic). Whatever, it looks pretty hefty and since it's Ohm it's probably well made. What it lacks in style, it makes up for in sheer mass. Based on the shape of the surrounds, it looks like the foam on the woofers is going to give up the ghost soon. If I was closer I'd take a shot at it for 20 bucks, but it's not worth the drive for me.

    *It's a shame that the founders of Ohm Acoustics didn't predict the invention of the Internet, the WWW and webcrawling when they chose a name for their company, since combining the words ohm and loudspeaker in a Google search gives absolutely useless results. :)

    No affiliation.

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    Last edited: Jun 12, 2018

     

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  2. toxcrusadr

    toxcrusadr AK Subscriber Subscriber

    Messages:
    32,888
    Location:
    Central Missouri
    Wow look at all those banana jacks. Must be dual VC woofers. I have a similar beast made by M&K in the early 70s, it was possibly the first consumer subwoofer.
     
  3. swechsler

    swechsler Frog Whisperer

    Messages:
    5,131
    Location:
    7th row center
    Yeah, I had that thought as well. Note that it says that either top or bottom connection can be used for input or output, which means no high-pass crossover built in.
     
  4. toxcrusadr

    toxcrusadr AK Subscriber Subscriber

    Messages:
    32,888
    Location:
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    Perhaps there is a low-pass crossover built in, that's the only way it would work without running the subs full range...
     
  5. swechsler

    swechsler Frog Whisperer

    Messages:
    5,131
    Location:
    7th row center
    I would hope so.
     
  6. Freddymac2

    Freddymac2 Super Member

    Messages:
    1,825
    Do you see any controls? That is two dual voice coil speakers in a box. Much better to get a complete powered sub with 200 watt amp and crossover.
     

     

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  7. nyhifihead

    nyhifihead Well-Known Member

    Messages:
    771
    Location:
    long island NY
    I picked this up on the way back from hudson valley brewery.. still don't know how to connect it properly or if it even has an internal summing/crossover network

    I hook up power amp to 2 leads and both woofers play, so I'm really at a loss for how this thing is hooked up. I emailed ohm and I will crack it open later tonight

    as far as the foams, they are actually in great shape. and this thing moves air, very respectful output with -6db around 25hz. For someone used to 2x high excursion 18s these 12s really do deliver.

    ohm makes great stuff
     
  8. swechsler

    swechsler Frog Whisperer

    Messages:
    5,131
    Location:
    7th row center
    You had me second-guessing my decision not to head out to get these, glad I don't have to worry about that any more. :) Where on LI are you? We could be neighbors. I'm in West Hempstead.

    I suspect that each set of binding posts (top+bottom) are connected in parallel, and then to a low-pass crossover which goes to a single coil of each dual coil woofer. If you have an external crossover (or HT receiver with sub out) and an extra amp. then you could just override the internal crossovers, and decide the best way to connect to your amp. I'd guess that the voice coils are 8Ω apiece, so you could parallel them and have 4Ω for each channel.
     
  9. nyhifihead

    nyhifihead Well-Known Member

    Messages:
    771
    Location:
    long island NY
    I'm out in port jeff. grew up in nassau though, and I work in queens, this is not the first time I've hit the road after seeing your posts! (thanks for the RBHs LOL)

    no low pass, which works out fine for me since I run active xover, been using both channels of a hafler p7000, this thing is pretty sweet.

    right now I'm only using half of the voice coils, will hook up the other 2 tonight.

    from ohm

    "The woofers each have four voice coils, that are 8 Ohms nominal impedance each. They are variously connected to the different inputs you see on the back. It doesn't mater which pair of binding posts you use as input and which as output - but the input/output combinations are always on the same cup. They are just pass-throughs in/out with no crossover.

    We recommend using one each for front left/right and rear left/right channels, and not using a center channel. To use the subwoofer in this configuration from an A/V receiver, you should configure the speaker setup in the receiver to call those four channels LARGE and to tell it that you're NOT using a center channel speaker or a subwoofer. If you're running from a subwoofer out to a separate power amp and you're NOT using it as a pass-through for the satellite speakers, we recommend running the four inputs in series/parallel with nothing hooked up to the second set of binding posts on each input cup."

    quad voice coil is pretty obscure.. I will take the drivers out tonight and see the build before I give these things some serious power
     
  10. swechsler

    swechsler Frog Whisperer

    Messages:
    5,131
    Location:
    7th row center
    Looking forward to seeing some pics...
     
  11. Beatnik

    Beatnik What's this ?

    Messages:
    4,549
    Location:
    DFW
    So, it's a sub for all four channels front and rear, cool. Kind of confused on the 4 coils each, does that mean you could add even more channels, or maybe they used two for each channel ?
     

     

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  12. EngineerNate

    EngineerNate AK Subscriber Subscriber

    Messages:
    3,114
    They're probably dual voice coil woofers. You can wire them in series or parallel depending on how you want to hook them up to your amp to get different impedance loads. Then for some reason they have outs for every single voice coil as well.

    Seems weird to me to have outs on a passive sub.
     
    Beatnik likes this.
  13. Beatnik

    Beatnik What's this ?

    Messages:
    4,549
    Location:
    DFW
    Until reading around last night, I never knew there was such a thing as quad coil woofers, the above says four each ?????

    "The woofers each have four voice coils, that are 8 Ohms nominal impedance each."
     
  14. EngineerNate

    EngineerNate AK Subscriber Subscriber

    Messages:
    3,114
    That doesn't make sense to me in combination with the "Either set may be used for in/out" verbiage on the Ohm label.
     
  15. Beatnik

    Beatnik What's this ?

    Messages:
    4,549
    Location:
    DFW
    Maybe it's wired in some way that in combination with the another speaker added would give the correct ohms ?

    I'd like to have a single woofer sub that would take lfe/front/rear/center all in one box ! That would be one heavy ass box with amps and individualy adjustable crossovers.
     
    Last edited: Jun 19, 2018
  16. nyhifihead

    nyhifihead Well-Known Member

    Messages:
    771
    Location:
    long island NY
    so I used this for about a week in a secondary system.. I started to play some bass heavy material and I realized I heard some distortion- thinking I cooked one of the VCs, so I took it apart. All VC are 4 ohm and it's not blown, perhaps some kind of mechanical noise I was experiencing

    The woofers are actually quad voice coil, interesting.. and there seems to be some kind of xover network.. here's some pics.

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  17. swechsler

    swechsler Frog Whisperer

    Messages:
    5,131
    Location:
    7th row center
    Check for loose surrounds and dust caps, that can cause what you're describing.

    How were these wired? Was it one voice coil from each woofer to each connection plate, or did they double 2 VCs from one woofer to each?
     
  18. nyhifihead

    nyhifihead Well-Known Member

    Messages:
    771
    Location:
    long island NY
    each 8 ohm VC is paralleled to 4 ohm, since there are 8 VC total in the enclosure its 4x inputs at 4 ohm

    I could not find a source of mechanical noise from the dust cap or surrounds, for an older subwoofer the surrounds are actually mint, no signs of dry rot or weakness. I will have more time to play around with them this weekend. The boost in sensitivity compared to my 18" and the vented box added a lot of punch in my smaller listening room. The subs sound clean, powered by a hafler p7000, 525wpc @ 4 ohm but they don't extend much below 30hz
     

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