can i use a resistor here?

W

wayne

2[H]4U
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Aug 4, 2003
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hey guys, i have a little project i wanted to get to. Let's say i have a speaker but its producing too high of a volume, even at minimum volume. Can i put a resistor directly on the positive lead to lower the volume? Is that how it works? I figured it should because volume control are controlled by potentiometers and resistors are the same except they cant be adjusted.

Thanks,
wayne
 
If you mean disconnect one lead and insert a resistor in series, yes it will work.
 
But...you have to have an adequate resistor.

If you want to drop the volume by say 3db, it will basically require cutting the speaker power by 1/2, an resistor of 40% of the impedance of the speaker will basically give you 1/2 of the initial power the speaker dissipates, but that resistor will also dissipate 40% of the power the speaker dissipates.

In lay terms, you have an 8 ohm speaker and a 100 Watt amp, insert a 3.2 ohm resistor in series to get 1/2 the power at the speaker (50 Watts, a 3db reduction in sound level) and the resistor will disipate 20 Watts itself.

In short, a 3.2 Ohm 20 Watt resistor is not easy to come by, you have to buy the right part, usually a physically large ceramic sandfilled resistor, or a physically large metal oxide package resistor. You also need to be mindful there is enough heat radiation/dissipation area around the resistor, or it will become hot enough to potentially cause a fire if used at maximum volume for extended periods of time. (basically, think of it as a 20 Watt light bulb, would you let it sit behind your speaker and leave it turned on continously?)

So, to be honest, I would recommend using very low value resistors with high heat dissipation ratings, that or just use a line level volume control and adjust it.
 
BBA, the fact that the amp is rated at 100W doesn't mean that the speaker is drawing 50W. Chances are that if it's a cheap computer speaker, it's more along the lines of 5W, if even that. And putting an 8Ohm resistor in series with the speaker will actually cut the power by a factor of two. (Twice the resistance means half the current).

One other thing to consider: if the speaker has an LED on it (like the speakers sitting next to my laptop at the moment, then the wires are also carrying DC, which will further increase the load on the resistor.

Bottom line, you'll be fine with a 5W resistor. There is one other option, however. If it's possible, you can cut down the signal before it goes to the amplifier, and then you can use nice, cheap 1/4W resistors. Here's a sketch:
Code: 
From
source +:
----+
    |
   (R1)
    |
    +----to amplifier +
    |
   (R2)
    |
----+----to amplifier-
to source -
I wouldn't suggest anything with more than 1kOhm of resistance.
 
thats why you put the resistor in the signal line where the power is jack squat.

BBA said:
But...you have to have an adequate resistor.

If you want to drop the volume by say 3db, it will basically require cutting the speaker power by 1/2, an resistor of 40% of the impedance of the speaker will basically give you 1/2 of the initial power the speaker dissipates, but that resistor will also dissipate 40% of the power the speaker dissipates.

In lay terms, you have an 8 ohm speaker and a 100 Watt amp, insert a 3.2 ohm resistor in series to get 1/2 the power at the speaker (50 Watts, a 3db reduction in sound level) and the resistor will disipate 20 Watts itself.

In short, a 3.2 Ohm 20 Watt resistor is not easy to come by, you have to buy the right part, usually a physically large ceramic sandfilled resistor, or a physically large metal oxide package resistor. You also need to be mindful there is enough heat radiation/dissipation area around the resistor, or it will become hot enough to potentially cause a fire if used at maximum volume for extended periods of time. (basically, think of it as a 20 Watt light bulb, would you let it sit behind your speaker and leave it turned on continously?)

So, to be honest, I would recommend using very low value resistors with high heat dissipation ratings, that or just use a line level volume control and adjust it.
Click to expand...
 
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