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Topic: Simple resistive (potentiometer) phones volume ctrl. (Read 4143 times) previous topic - next topic
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Simple resistive (potentiometer) phones volume ctrl.

Some earphones come with inline volume control. From what I can make out the ones I've seen are just a potentiometer (variable resistor), typically 0 to 1 k-ohm. You can also get these as something like a 1 meter extension with inline volume if your headphones/earphones don't already included it.

Recently I've noticed that on a really bad portable player the sound can actually be improved by using one of these, by simple virtue of the increase in input resistance. (This is on a rubbish cheap player that has very noticeable low freq roll off when driving low impedance phones). The basic idea is that as you turn the volume down on the inline control then you increase the input resistance. For example, with a 1k pot I can increase my resistance to 300 - 500 ohms, completely masking any low frequency coupling-cap limitations, and still be able to compensate for the volume loss by cranking up the player.

Assuming that available volume level isn't an issue, I was just wondering is there's any other audio degradation that the use of these is likely to cause. Does such a high resistance, like 500 ohms for example, typically cause any high frequency roll off or any other artifacts?

Simple resistive (potentiometer) phones volume ctrl.

Reply #1 –
Depends on how flat the earphones' impedance response is. Dynamic in-ears tend to be quite uncritical, while some open dynamic headphones are fairly picky already, and a number of balanced armature IEMs are very fussy and quickly show altered frequency response.

Simple resistive (potentiometer) phones volume ctrl.

Reply #2 –
Quote from: stephan_g on
Depends on how flat the earphones' impedance response is. Dynamic in-ears tend to be quite uncritical, while some open dynamic headphones are fairly picky already, and a number of balanced armature IEMs are very fussy and quickly show altered frequency response.

Thanks Stephan. What type of frequency response artifacts are you most likely to get? I've been told that if the resistance is too high then you can get low frequency coil resonance (that would otherwise have been electrically damped by a low impedance source). Is that the main artifact, or can you also get high frequency limitations as well?

Simple resistive (potentiometer) phones volume ctrl.

Reply #3 –
Quote
I've been told that if the resistance is too high then you can get low frequency coil resonance (that would otherwise have been electrically damped by a low impedance source). Is that the main artifact, or can you also get high frequency limitations as well?
You'd need the impedance vs. frequency curve for the particular headphones/earphones.    Usually, we don't have such a curve available.        With headphones, I believe the biggest resonance-frequency is usually in the lower frequencies, but not at the lowest frequencies.  But, there may also be an impedance increase at the highest frequencies if the driver starts behaving like an inductor...  I don't know...

Frequencies where the load impedance is highest will be boosted, and frequencies with the lowest impedance will be cut.  The effect is minimized with low source impedance.

Quote
From what I can make out the ones I've seen are just a potentiometer (variable resistor), typically 0 to 1 k-ohm.
Since one terminal of the pot is connected to ground, you don't simply have a resistance in series with the headphone.  You have a series-parallel circuit with the highest source impedance at "mid" position, and the highest load impedance at minimum volume.  (Volume controls have an approximately logarithimic taper, so the electrical mid-position is not the mechanical mid-position.)

It gets rather complex when the amp capacitance/impedance is a factor...  The pot presents a variable load to the amp as well as a variable source to the headphones.  The effects work in opposite directions at different frequencies....    Higher load impedance tends to result in flatter response, but higher source impedance tends to result in worse frequency response.

So...  it's unpredictable...  Just use your ears.    If you feel that adding a resistor or pot improves the overall sound, go ahead and use it.    (Just be aware that your earphones/headphones are not performing exactly "as specified", or as tested by the manufacturer.)    Or, get a "better" PMP, or if you don't need portability you can get a headphone amp.

Simple resistive (potentiometer) phones volume ctrl.

Reply #4 –
Quote from: uart on
Assuming that available volume level isn't an issue, I was just wondering is there's any other audio degradation that the use of these is likely to cause. Does such a high resistance, like 500 ohms for example, typically cause any high frequency roll off or any other artifacts?


You'll have a really low dampening factor, so probably more distortion.  On the upside, as you've noticed, putting the resistor effectively shifts the cutoff frequency of the coupling caps, therefore flattening the frequency response (at least for a capacitively coupled device).

This paper has some interesting test results measuring distortion with series resistors:

http://www.benchmarkmedia.com/discuss/site...ance-Part-2.pdf

It would seem to be much the situation you are in. 

Quote from: uart on
I've been told that if the resistance is too high then you can get low frequency coil resonance (that would otherwise have been electrically damped by a low impedance source). Is that the main artifact, or can you also get high frequency limitations as well?


Without electronic damping you'll see the mechanical resonance of the system, which is usually pretty low.  I don't think you'll see much high frequency effects though.

Simple resistive (potentiometer) phones volume ctrl.

Reply #5 –
Quote from: uart on
Some earphones come with inline volume control. From what I can make out the ones I've seen are just a potentiometer (variable resistor), typically 0 to 1 k-ohm. You can also get these as something like a 1 meter extension with inline volume if your headphones/earphones don't already included it.

Recently I've noticed that on a really bad portable player the sound can actually be improved by using one of these, by simple virtue of the increase in input resistance. (This is on a rubbish cheap player that has very noticeable low freq roll off when driving low impedance phones). The basic idea is that as you turn the volume down on the inline control then you increase the input resistance. For example, with a 1k pot I can increase my resistance to 300 - 500 ohms, completely masking any low frequency coupling-cap limitations, and still be able to compensate for the volume loss by cranking up the player.

Assuming that available volume level isn't an issue, I was just wondering is there's any other audio degradation that the use of these is likely to cause. Does such a high resistance, like 500 ohms for example, typically cause any high frequency roll off or any other artifacts?



This depends on the impedance curve of the headphones or earphones. High impedance phones are likely to be affected to a lesser degree than low impedance phones.
 

Simple resistive (potentiometer) phones volume ctrl.

Reply #6 –
Quote from: DVDdoug on
You'd need the impedance vs. frequency curve for the particular headphones/earphones.    Usually, we don't have such a curve available.

Actually, for quite a number of models we do.

See e.g. Headroom: Build a Graph, InnerFidelity: Headphone Data Sheets, Golden Ears' measurements.

Multi-driver BA IEMs can be extremely picky (look for an impedance plot of UE super.fi 5 Pro oder Triple.fi 10 Pro - you get a rollercoaster between about 6 and 60 ohms). Some open dynamic cans are not particularly uncritical either (e.g. HD595/598: 50..60 ohms min, >220 ohms max). Their closed cousins tend to show less of that (compare e.g. AKG's K24x to K271x). And then there are a number of dynamic IEMs whose impedance response is rather flat indeed, plus orthos which also tend to pass for an Ohmic resistor.