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Choosing the correct potentiometer

  1. Jul 18, 2006 #1
    Excuse me if I'm in the wrong forum for this question, as I'm new here.
    I'm building a set of speakers for my motorcycle and do not know how to correctly choose a volume control. The bike is 12V; the speakers are 4 Ohm; the amp is 150W max; an MP3 player will be the music source. Can anyone help me understand the correct qualities or rating of an appropriate volume control (potentiometer/rheostat). I see some with an Ohm rating or a watt rating, but I do not know how to correlate that to my parts. It seems to be a very large potentiometer for a 150 Watts, so that seems not to be the correct quality to be used as the appropriate criteria. Is Ohms or voltage how to choose the correct volume control part, and if it is either one, am I to look for a potentiometer rated greater than or equal to 4 Ohms (the speaker's criteria), or 12 volts (the bikes electrical system)? Or am I all dazed and confused !?
    Thanks for any help.:confused:
  2. jcsd
  3. Jul 19, 2006 #2
    The output of the amplifier is the wrong place to put the volume control. For one thing, the potentiometer will get very very hot. Usually an amplifier is driven from a low level input, such as from the mp3 player. You may already have everything you need if the signal from the mp3's headphone jack is sufficient to drive the amplfier. Then you can use the mp3's volume control to control the volume of the output.
  4. Jul 19, 2006 #3


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    Yeah, you should just use the MP3 player's volume control. That should work fine.
  5. Jul 21, 2006 #4
    Thanks Hammie and Berkeman. The mp3 player volume control does work but I am going to have to modify its cable to mount it at a stable and convenient location. That aspect is what is driving my thought to add the volume control to a fixed location (plus I've seen two bike speaker builders using an additional volume control on their systems. Not sure why as they won't give out info).
  6. Jul 21, 2006 #5


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    You don't want to put a potentiometer/rheostat in the speaker circuit. As you found out they would be very big and you need multiple ganged unit in order to control for impeadence. Important for amp output.

    What kind of outputs does you player have?
    If it's just an earphone jack then I'd use a potentiometer around 600 ohms with an audio taper. Connect one of the player wires and the amp lowside(ground/shield) to one leg. The other player wire to the remaining leg. And the amp input to the wiper contact.

    If your player has line out then use 10k ohms.
    Wirings the same but connect the two lowside(ground/shields) together.

    Edit: With an audio taper pot it matters which leg of the pot the ground/shield wires are connected to. If you have a problem controling the volume then switch the leg connections. In other words low volume should occur when the pot is in the full counter clockwise position.

    Edit2: If its stereo setup you need a dual ganged potentiometer. Two potentiometers one shaft.
    Last edited: Jul 21, 2006
  7. Aug 2, 2006 #6
    Guys/gals, may I hook on to this thread, and ask basically the same question, although the context is different.

    I am building a control box for two set of speakers and one headphone connecterd to my computer, and I want to control the volume of the speakers and earphones from this box (not softwarewise). Here are the impedances:

    Speaker set 1: >10 kohm (according to spec)

    Speaker set 2: 4 ohm (according to spec)

    Earphones: 75 - 150 ohm (I'm guessing -- these are obviously the normal values)

    I am going to use a double-deck logarithmic potentiometer for each of the three speakers/earphones above. I have scoured the Internet and several lists, but this seemingly simple problems is hard to find a solution to: what should the potentiometer values for the three cases above be?

    I would be most grateful for your comments.

    Hans L
  8. Aug 2, 2006 #7


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    What is your computer audio output rated for? Probably not for 4 Ohms, I would guess?
  9. Aug 2, 2006 #8
    I will have to got to the Soundblaster and Logitech websites to find out what the Line out impedances are. I'll be bakk.

    Hans L
  10. Aug 2, 2006 #9
    Both sound cards were Sound Blasers, but I could not find any data on output impedance. My guess is that, since no matter what I plug in, it works fine, the output impedance adapts automatically to whatever speaker or earphone is connected. Which brings me back to the output impedance and the impedeance of the potentiometer. Any ideas?

    Hans L
  11. Aug 2, 2006 #10


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    I'm surprised that the card can drive a 4 Ohm speaker, but whatever.

    For each pot, connect the CW end to the card audio output, the CCW end to ground, and connect the wiper to your speaker input (other speaker input to ground).

    >Speaker set 1: >10 kohm (according to spec)
    I'd use a 1kOhm pot for starters.

    >Speaker set 2: 4 ohm (according to spec)
    This is harder -- maybe try a 10 Ohm pot (power!).

    >Earphones: 75 - 150 ohm (I'm guessing -- these are obviously the normal values)
    I'd start with a 100 Ohm pot.

    When the pot wiper is fully CW, you get all the output of the amp, as long as the amp can drive the parallel combo of the speaker and the pot resistance. When the wiper is fully CCW, the speaker drive is zero.
  12. Aug 2, 2006 #11


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    For speakers the device you want is called an L-pad.
    It shows a constant load to a power amp.
  13. Aug 2, 2006 #12


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    Cool. Yeah, for his jellybean sound cards, I didn't think the varying impedance with volume settings would make the amp mad. But for bigger setups, this L-pad sounds like a good solution. Do you have any pointers to info on them?
  14. Aug 2, 2006 #13


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    I'd have to say your right about the sound card case.

    Google L-pad and speakers.
    Brought back lots of stuff.
  15. Aug 3, 2006 #14
    Thanks for the responses! While I have subscribed (as it is called) to instant notification, I have only received one e-mail, which I missed, so I did not know there were all these answers.

    I'll check everythig out and get back to you. Thanks!

    Hans L
  16. Aug 26, 2006 #15
    Hello guys!@

    When all was said and done, and i had read everything and gone to all websites, I decided to adjust volumes either softwarewise or with the buttons on the speakes. And I bought a simple in-line volume control for the earphones.

    I am starting to solder my now switchbox, which will the headset between the PC sound card and the Skype sound card (two cards because of conflicting settings). When the headset i connected to the PC soundcard, the speakers and desktop mic are connected to the Skype soundcard, and vice versa.

    Thank you all for some good adivce and links.

    Hans L
  17. Feb 19, 2010 #16


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    Isn't the fact that it will be mounted on a motorcycle relevant?
    I should have thought that the volume control unit would need to be water resistant, too. And what about the problems of operating it in gloves?
    It seems to call for a two button (vol up / vol down) waterproof membrane switch and an electronic fader. Over the top, maybe but anything short of that may not do the job adequately. If it has to be done with a conventional potentiometer then the resistance value must be fairly low so that dampness won't affect the resistance values.
  18. Dec 5, 2011 #17
    I see this is a very old topic, but I have the same kind of question, so here it is.

    I recently took an old tuner case (Sansui TU-717) and made it into my computer case. The case has a 50k Ohm stereo pot meter in it, can this be used as a volume control for my pc, because that would look super awesome. As I read the previous entries I get the sneaking suspicion that 50k is probably way too high for the voltages used in computers, right?

    And part two of the question, where to hook up the pot meter to the motherboard, is there a dedicated connector for volume control, because I can't find it. MoBo is an Asus M4A88TD-M EVO. Or can I somehow hook up the line out to the line in with a pot meter in between?

    Some insight would be apreciated ;)

  19. Dec 5, 2011 #18


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    Audio output on most "IBM-type" computers is software controlled.

    I have seen some computers with mechanical gain controls, but these had an audio module mounted in one of the DVD slots. Most computers would not have these.
  20. Dec 5, 2011 #19
    Ok, but what about HTPC's, a lot of times they come with a big volume control knob on the front, how do they work and could I buy something like that separately from the whole case? I've looked at the Griffin Powermate, but first of all I need one in black metal/aluminium and second of all it connects to an external USB, which is not so pretty.

    I could replace the 50k ohm potmeter with anything, lower resistance, or a rotary pulse encoder, or anything that can be hooked up internally to some motherboard header, as long as I can make the volume dial that came with the tuner case functional. Any ideas?

    PS what kind of resistance are we talking here with computer parts, 1k Ohm or less?

    EDIT: referring to the mechanical volume control, the "better" sound cards often have front bay panel with that function, but that's all digital and is directly connected to that particular sound card right? I don't have a sound card and am working with my on board sound.
  21. Dec 5, 2011 #20


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    Once you start talking about home theatre, it is really not a normal Windows computer any more.

    50 K would not be unreasonable in an audio amplifier, but it would have to be followed by an input that is much higher than that. Like 1 megohm.
    However, FET inputs and opamps can easily achieve such impedances.

    You could take the output sound from a computer via the "line-out" connector and just pass this through a power amplifier with analog gain controls if you like.
    There are some very nice looking audio amplifiers with knobs that have LED lighting on them and which move by remote control.

    You should wander through an up-market computer store and ask a few questions. If you have plenty of dollars, they will be able to provide you with some nice goodies.
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