Potentiometer + Subwoofer; hookup?

Discussion in 'The Projects Forum' started by Godzilla, Jan 3, 2010.

  1. Godzilla

    Thread Starter New Member

    Jan 3, 2010
    3
    0
    Hey guys. I'm new here.

    A friend of mine recently gave me his old computer speakers. They're in great shape, only one problem: The sub doesn't have a volume control, and it's loud! After searching the internet I found a post someone left on Amazon.com about just hooking up a "variable resistor".
    I know absolutely nothing about electronics.
    So my question is: How can this be done?
    ----------------------------------------------------
    After figuring out that a "variable resistor" is another name for a Potentiometer (Rheostat?), I picked one up at radio shack. Here's the specs:

    Resistance: 25 ohms. 20% Tolerance
    Rated Power: 3 Watts
    Sliding Noise: <47 mV
    Mounting hole: 10mm
    ----------------------------------------------------
    The speakers are; Altec Lansing 221 computer speakers:

    -50 Watt total system power.
    -25 Watt RMS
    -Sub: 10 Watts @ 4 ohms @ 10% THD @ 270 - 22000 Hz2
    ----------------------------------------------------
    I've taken the back panel off and can only tell for sure that there are 2 wires (red & black) coming from the back of the speaker.
    I figured this would be easy, and I'm sure it is, but like I said, I know nothing about electronics so before I start cutting wires and maybe burning the house down or electrocuting myself I thought I'd run it by some experts.

    Thanks for any help you can give. Matt.
    P.S. I can post pictures if anyone needs them.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A volume control is 10k ohms to 100k ohms and is connected as a voltage divider at the input of an amplifier, not as a 25 ohm rheostat at the output of an amplifier in series with a speaker.

    The little 3W rheostat will burn out soon and will ruin the damping of the resonance of the sub-woofer.

    Sub-woofers go down to 20Hz or 30Hz. Yours goes down to only 270Hz which is not a low frequency.
     
  3. nikonian

    New Member

    Jan 2, 2010
    2
    0
    Insufficient data. How do you intend using them? Are they multi speakers with Xover network or just single speakers.
     
  4. Godzilla

    Thread Starter New Member

    Jan 3, 2010
    3
    0
    Ok, so... get a better rheostat? I understood about 12% of what you said. Sorry, like I said, I'm new to this. Looks like the guy at Radio Shack knows about as much as I do when it comes to electronics.

    Basically you're telling me that the rheostat he sold me is not strong enough and also that I'm trying to attach it in the wrong spot? I'm glad I asked before I destroyed something.

    Thanks for the quick reply Audioguru.

    If it's too much trouble to Hook up a Rheostat, can I just cut the wires leading to the sub? The satellites sound great on their own, plus I live in an apartment building so really I'm just hooking up a rheostat to the subwoofer to turn the volume all the way down.
    ---------------------------------------------------------
    Nikonian: Thanks for the reply as well.

    I intend on using them for my computer. Like I said the subwoofer is uncontrollable and extremely loud. All I want to do is turn the volume down, but it has no such controls and the equilizer in any given computer program is insufficient for bass control. So it's a hardware issue. Here's a link to amazon.com, Altec Lansing 221 Amplified speaker system:

    http://www.amazon.com/gp/product/B000068IGQ/ref=cm_pdp_rev_itm_img_2

    ...and a link to a random review:

    http://www.tweaknews.net/reviews/altec221/

    ....also, here's a review where someone sort of describes how he pulled this off, if it helps (scroll to the bottom):

    http://www.amazon.com/gp/pdp/profile/A2WOZ5EUCIDOM3/ref=cm_aya_bb_pdp

    I tried emailing this person directly, but was unsuccessful. Thanks again. Matt.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Maybe you know where the amplifier for the sub-woofer is located but i guess it is inside the sub-woofer.
    Somebody who knows a little about sound sytems would be able to add a simple volume control to the input of the sub-woofer's amplifier but not you and not a NOOB from Radio Shack.
     
  6. Godzilla

    Thread Starter New Member

    Jan 3, 2010
    3
    0
    Of course the amplifier is in the subwoofer. It's computer speakers. Did you not check the link? How difficult could it possibly be to attach a potentiometer? Clearly I know nothing about electronics, but I would expect a little more help from a guy with the name AudioGuru. I came here to ask a question to people more capable than I, not to be told that I cant do it because I have no experience. I already know I cant do it. That's why I asked you.

    Anyone here know anything about audio?
     
  7. nikonian

    New Member

    Jan 2, 2010
    2
    0
    The simplest way is to purchase a few low value high wattage wire wound resistors, say 10 to 25 ohms 15 to 20 watts. Put one in series with one of the leads of the speakers,and pick the best. As it is a solid state amp there is no probs with matching.If you get ambitious you could purchase a constant impedance pot, but I do not think high wattage types are made.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
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    What you are trying to do requires some electronic knowledge. If this was my project I would have modified the circuit board before the subwoofer power amplifier. I think your best option is the suggestion from Nikonian. Go to Radio Shack and pick up item 271-130 and 271-131 (0.47 ohm and 1 ohm 10 watt resistor) and test in order to Nikonian suggestion. I you do not how to solder pick up terminal block at the same time. I would recommend the Euro-style terminal strip
     
  9. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    With these speakers, hooking up a potentiometer at the subwoofer output should be ok, since this is not a "real" high power system, and the sub is only 10 watts so connecting a linear potentiometer to the sub output will work, I have done a similar modification to another set of Altec of Lansing speakers + sub.

    I used this potentiometer from Radio shack >> http://www.radioshack.com/product/index.jsp?productId=2062358

    and connected it to the sub leads like this....

    Bass Control.png
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    I think the 25-Ohm 3-Watt RheostatModel: 271-265 will be better choice
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    A resistor in series with the 0.04 ohms output impedance of an amplifier in series with a speaker ruins the damping of resonances of the speaker provided by the very low impedance of the amplifier. The speaker might sound very boomy with a resistor in series with it.

    The 3W rheostat is rated to dissipate 3W only at max resistance. At half resistance its max allowed dissipation is 1.5W and at less resistance its max allowed dissipation is less which will cause it to burn up.

    If a detailed service manual is available then I could show how to connect a proper volume control to the input of the sub-woofer amplifier.

    It might be simpler to make a new power amplifier with its own proper volume control for the sub-woofer speaker fed from the old amplifier.
     
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