Wrongly wired extension/volume control cable.

2012-07-05 07:51:55

I just bought an inline volume control (1m mini-stereo extension cable with in-line pot) and the pot seems to be wired up in the reverse of what I expected.

It appears that the input to the pot (output from the player) is connected to the slider, while the output side (to the phones) is connected to the other side of the pot. (btw First side of the pot still common/ground of course.)

Is this what I'm thinking, someone just wired up a bunch of these backwards and hence why they're flogging them for $1 on ebay, or is there an actual reason that someone would do it this way.

Wrongly wired extension/volume control cable.

Reply #1 – 2012-07-05 08:01:48

Ok, playing around with these now and while not the traditional "voltage divider" configuration, for basic volume control they do actually work.

As the volume control is lowered it places an ever smaller resistance in parallel to the player output, and an ever larger resistance in series with the phones. So there is no doubt that this will control the volume, just seems weird that someone would choose to do it this way, particularly as it basically shorts out the player output at low volume settings.

Wrongly wired extension/volume control cable.

Reply #2 – 2012-07-05 10:31:18

That is the normal way I always see in schematics and have used in several audio hardware home built projects.

Wrongly wired extension/volume control cable.

Reply #3 – 2012-07-05 14:27:02

No device's output will be happy playing into a lower and lower resistive load as you turn the volume down.  With this volume control set at zero, the unit's output plays into a dead short.

Wrongly wired extension/volume control cable.

Reply #4 – 2012-07-05 14:41:14

I don't believe uart said that at minimum volume there is a dead short across the amplifier, just that the load is smaller and smaller.

This is actually a common way of providing non-linear volume vs. pot position with a linear pot. There is a small series resistor from the amplifier to the slider. At low volume settings this acts as a divider from the amplifier, with a large series resistance to the headphones to further attenuate the signal. At high volume settings the resistor divider aspect gives very little variation due to the series resistor being much smaller than the pot's resistance. Instead you have smaller and smaller series resistance to the headphones.

This is not a particularly good way of implementing volume control, both because it presents variable load to the amplifier, and variable source resistance to the headphones. The only way for it to work at all is to have a large disparity between the source impedance of the amplifier and the load impedance of the headphones.

Wrongly wired extension/volume control cable.

Reply #5 – 2012-07-05 15:08:17

Thanks for the info guys. Yes I was worried about this device presenting a short circuit to the driving device, but it may indeed have some minimum resistance in the slider circuit. I did a rough tests with an ohmmeter it didn't seem to go below about 8 ohms, so perhaps there is something there. (It actually goes to some minimum but then open circuits right at the end, presumably to completely disconnect the phones at the zero volume point).

Unfortunately I wanted this device specifically to increase the driving point impedance for a cheap mp3 player that has waaaay to smaller coupling capacitors to drive anything under a few hundred ohms. Anyway, I popped it apart and re-soldered it in about 5 minutes and now it works like a charm (just swapped around input and output wires btw). I can pretty easy get the driving point impedance up over 350 ohms and still get enough volume, so it's basically "fixed" an otherwise unusable mp3 player for me.