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How does an inline variable resistor affect sound quality of headphones/IEM's?

post #1 of 5
1/25/12 at 11:38am
Thread Starter 
I have a Sennheiser CX 485 that I use through the headphone port of my laptop. The IEM has an inline volume control module, which, as far as I can tell (from Googling), is a variable resistor.

If I turn the IEM's volume up all the way, and turn the volume down on the laptop, I can hear a slight hiss, which I assume (perhaps incorrectly) to be some sort of electrical noise. When I turn the IEM's volume control all the way down, which is still a non-zero volume level, and turn the volume up on my laptop, that hiss is gone.

Why does increasing the resistance effectively eliminate the line hiss?

What other effects, positive or negative, should increasing the resistance have on the sound quality?
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post #2 of 5
1/25/12 at 12:48pm

I have a pair of Sennheiser CX500 and I notice the same thing, I hear hissing noise when the volume control is at max. though I only notice it on the quieter parts  of the music. I'm actually thinking on removing the volume control (would this be possible/recommended?), or this would be equivalent to having them on the highest volume setting?.

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post #3 of 5
1/25/12 at 3:04pm
Quote:
Originally Posted by avkdh View Post

or this would be equivalent to having them on the highest volume setting?.


Yes.

 

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post #4 of 5
1/25/12 at 3:11pm
Quote:
Originally Posted by Jaywalk3r View Post

Why does increasing the resistance effectively eliminate the line hiss?

 

Because it makes the sound, including the noise, quieter normal_smile%20.gif Of course, you need to increase the volume on the source to compensate, but the noise will not increase proportionally. In other words, the audio output of your laptop has a better signal to noise ratio when you set it to higher volume. But using it at a very high volume setting might (but not necessarily will) result in increased distortion.

 

Quote:
Originally Posted by Jaywalk3r View Post

What other effects, positive or negative, should increasing the resistance have on the sound quality?


Other than decreasing the volume, adding serial resistance reduces the damping factor. This is generally a bad thing, although how much audible it is depends on the headphone. It can change the frequency response (especially with some IEMs), increase driver resonance, and increase distortion.

 

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post #5 of 5
1/25/12 at 5:28pm
Thread Starter 
Quote:
Originally Posted by stv014 View Post

Because it makes the sound, including the noise, quieter normal_smile%20.gif Of course, you need to increase the volume on the source to compensate, but the noise will not increase proportionally. In other words, the audio output of your laptop has a better signal to noise ratio when you set it to higher volume. But using it at a very high volume setting might (but not necessarily will) result in increased distortion.

Other than decreasing the volume, adding serial resistance reduces the damping factor. This is generally a bad thing, although how much audible it is depends on the headphone. It can change the frequency response (especially with some IEMs), increase driver resonance, and increase distortion.

Thank you for the response.

So, if I understand correctly, for the purpose of calculating the damping factor (DF = Z_load / Z_source), the cable is considered part of the source, as oppose to part of the load.

If I am going to use the volume reduction on the IEM cable to reduce line noise, should I reduce it only to the point where the noise is sufficiently reduced, instead of turning it down as much as possible? It seems that such a strategy would reduce the damping factor as little as possible.

Incidentally, as far as I can tell from Google, the impedance of my headphone port is <24 Ohms. Sennheiser lists the impedance of the CX 485 as 16 Ohms, so the damping factor of the combination is already pretty low, even without reducing the volume with the module on the cable..
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