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 wkang598 14th June 2008 09:14 PM

basic question on volume control

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I am trying to build a little passive volume control unit, stereo in, stereo out, to be hooked up between my no-adjustable-volume CD-player and my headphones. But I am encountering a problem that the headphone impedance is much smaller than the pot resistance value and hence altering the volume.

I am using a 100k dual-gang audio taper radio shack pot. Taking the left channel for example, the two ends of the pot are connected to the left signal and the shield (ground). The center wiper is connected to the output. And the shield for the output is connected to the input shield. (as shown in the image, sorry for the crappy drawing)

Without headphone attached, everything looks fine. Half-way on the knob outputs a voltage roughly equal to 1/7 of the input voltage. BUT when a headphone with 40 ohm impedance is attached, the voltage across the headphone is almost zero since the 40ohm is connected now in parallel. I could only hear something when the knob is turned to almost full scale.

Is there any way to solve this problem? Thanks!

 theAnonymous1 14th June 2008 09:21 PM

You need a headphone amp circuit or at least a buffer between the output of the pot and the headphones. You can't use a high impedance pot as a voltage divider for low impedance headphones.

 valleyman 14th June 2008 10:59 PM

As mentioned, a headphone amp or buffer is by far the best solution but if you need to keep things simple you could just put a suitable variable resistor in series with the headphones

 wkang598 15th June 2008 01:52 AM

Thanks for the advice. I guess that a buffer normally means an op-amp or some kind of circuitry that requires power? Am I right?

Quote:
 Originally posted by theAnonymous1 You need a headphone amp circuit or at least a buffer between the output of the pot and the headphones. You can't use a high impedance pot as a voltage divider for low impedance headphones.
That's a good idea! Since the resistance of my portable headphones are roughly on the same scale.

Quote:
 Originally posted by valleyman As mentioned, a headphone amp or buffer is by far the best solution but if you need to keep things simple you could just put a suitable variable resistor in series with the headphones

 unclejed613 15th June 2008 04:49 AM

you need to use a 50 ohm pot instead of 100k. a 100 ohm might work as well. once you get the 100k pot turned down to 90%, you have 1000 ohms in series with the headphones. that reduces your signal to (assuming 120 ohm headphones) 12% of it's original level (much lower if that's a log taper pot).

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