simple technical question about resistors?

Discussion in 'Live Sound' started by Exsultavit, May 12, 2005.

  1. Exsultavit

    Exsultavit Active Member

    Jan 5, 2005 I have a cheapo set of computer speakers that use a subwoofer that is a bit too loud. There is no volume control for the sub, but I imagine I could add a couple of resistors to the line that feeds it to lower the level.

    Will this work? If so, what value might you smart people suggest to lower the level, say, 3db? 5db?


  2. When I think resistors, I think in Ohms and not dB. Are we on the same page?
  3. Zilla

    Zilla Active Member

    Mar 29, 2005
    WY / CA
    Home Page:
    You need to know the output and input impedances of the circuit you are going to pad down with that resistor. Then calculate the resistor value needed for the voltage divider's desired attenuation.

    Or you can just use a potentiometer (say 10Kohms). Then you can vary the attenuation by hand.
  4. Exsultavit

    Exsultavit Active Member

    Jan 5, 2005
    Thanks for the fast responses!

    Clearly, I need an ohmage that will equal the attenuation (in DB) that I desire. So we ARE on the same page.

    Zilla- I will try the potentiometer idea first. Good idea, as I do not know yet if the system will sound 'good' even after this fix.

    I suppose I could just throw some resistors in there and see what happens. Unfortunately, I do not know the I/O impedances to this system. Could I find out with a standard Radio Shk ohm/voltage meter? If, so, can you advise?

  5. ghellquist

    ghellquist Guest

    Hmm. It is not at all clear if you want to change the volume before or after the power amplifier. It does a bit of difference, as the resistor inbetween will get warm.

    Anyway, first version, is before the power amplifier. Running at line level for a consumer unit is a few Volts maximum. I would go the local electronics store or mail-order house and buy a potentiometer. It would be something from 10kOhm up 100kOhm as the value is not really that important. More important is to get the logarithmic version (instead of a linear). These pots you could probably find at quite a few different places.

    Now solder this into the line level signal and you can easily modify the volume. On line level you have two cables, live and ground. Solder the 10k pot between live and ground. And fetch the output signal between the viper of the pot and ground.

    Second alternative is if you are running after the power amplifier and before the speaker. In this case the resistor or pot you put in will have to burn some energy and will get hot. So if you put a normal potentiometer into you 1000W PA sub, expect some smoke. There are variable solutions here as well, probably you could find them at some DIY speaker house. Or possible at a DIY car radio house, they tend to do this kind of things.

    Generaly, you should really never generalize, speakers are from 4 to 16 Ohms in impedance. If you measure them with an Ohm meter though, the resistance is much less. That is all the way it is supposed to be, as the speaker coil will have higher "resistance" at music frequencys.

    So what you could do is to get a few small resistors and place them in series with the loudspeaker. Try 1 Ohm, 2Ohm, 4 Ohm or 8 Ohm to see which comes closest (the values are not important, and you will see that standard values comes in a specific series). These are not the easiest to find, especially as you want them to be able to burn a bit of power. On a typical home PC system though, there is not really very much power to burn from the beginning, I would simply look for 1/2W resistors and monitor how warm they get. If they get too hot, go for a larger value.

    Tinkering and experimenting as big fun so go ahead. It does help if you know the basic laws in the area. By analogy, Ohms law and the power laws are very good helps in designing this kind of things.

    DonĀ“t burn your fingers on the soldering iron though, smells real bad.

  6. chriscavell

    chriscavell Guest

    If you're trying to dissapate the power to the speaker to effectively lower it's sensitivity, the aforementioned solutions aren't what most speaker designers would the amount of heat dissipation is enormous, and could easily lead to a fire in the case of a sub. They also dramatically alter the impedance of the circuit, and can force an amplifier to operate outside of its designed range. What you need for this application is referred to as an "L-pad" by speaker designers. It consists of 2 resistors, one in series, and one in parallel to the woofer, and when properly designed, maintains the original impedance of the woofer (so the amplifier continues to operate in it's most efficient...and safest...range). The relative values of these resistors determine the level drop in dB. There are numerous online calculators to determine the two values for your purpose, just do a google for it. I suggest you use 20watt or greater resistors (they are large brick like white resistors, and usually have their resistance as well as power handling printed right ontheir side...typically ~50 cents a pop).

    While this would work, I strongly suggest you simply build an in-line attentuator for the line level signal to the amplifier instead. It's much can find schematics for these all over the internet, but a simple pot setup as a voltage divider could do the trick.

  7. ghellquist

    ghellquist Guest

    I second the suggestions of chris if you want to do it he right way.


    PS: half of the fun is seeing the smoke from the experiments that went wrong. Just make sure that what you burn is the cheap stuff.
  8. JoeH

    JoeH Distinguished Member

    Jun 22, 2004
    Philadelphia, PA/ Greenville, DE
    Home Page:
    I had an assistant a long time ago who, when trouble-shooting, would replace blown fuses with higher ratings in power supplies that had shorted out.

    He'd then look/smell for the smoke, and find the trouble that way.

    Terribly inefficient way to fix things, but it was fun for a while, watching things blow up. Had to let him go eventually....just a bit TOO crazy. :twisted:

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