Vintage Headphone Wiring Confusion

[Tofty]

Hi All,

I hope someone can help me with a confusing wiring issue I've come across.

The headphones are Lyric LP 200Ks, which are many decades old.
I received them faulty in a joblot and there are signs that they have been opened up previously.

Here is their wiring diagram, as I found it.

All the wires, variable resistors and drivers have been checked and are working correctly.

I'm just can't work out what's going on with the wiring. I would have assumed that either the ground wires or signal wires would be completely isolated from the volume resistor.

Does anyone have experience with this kind of headphone setup, or just any suggestions as to how it should be wired?

Cheers

 

[miketlse]

That diagram looks very odd to me.
To my thinking the signal should go through the variable resistor, as the means of changing the voltage/current at the driver.

The left hand driver certainly looks odd, because you seem to have the left signal and ground as inputs to the volume resistor, but only right signal and ground reach the driver.
Surely it should be left signal (after the resistor) and ground that reach the driver.

 

[Tofty]

Thanks miketlse,
Looks like it should have been as you suggested.

This is what I've changed and it seems to now work correctly:

I was a bit thrown by just how thoroughly wrong it was. One wire incorrectly routed is understandable, but the three joined red wires, which were insulated with epoxy made me think that it must have been an original feature.

The way the ground wires are also connected to the signal wires via the variable resistor was also confusing, but thinking about it more the variable resistor can be modelled like two separate variable resistors, that are linked to operate inverted to each other, with a near zero resistance connection between them at one end and a ~55 Ohm resister across the other end. This setup would seem to allow the volume adjustment to be twice as sensitive as it would be without the ground wires being part of that system.

 

[miketlse]

Thanks miketlse,
Looks like it should have been as you suggested.

This is what I've changed and it seems to now work correctly:



I was a bit thrown by just how thoroughly wrong it was. One wire incorrectly routed is understandable, but the three joined red wires, which were insulated with epoxy made me think that it must have been an original feature.

The way the ground wires are also connected to the signal wires via the variable resistor was also confusing, but thinking about it more the variable resistor can be modelled like two separate variable resistors, that are linked to operate inverted to each other, with a near zero resistance connection between them at one end and a ~55 Ohm resister across the other end. This setup would seem to allow the volume adjustment to be twice as sensitive as it would be without the ground wires being part of that system.

Delighted to have been able to help.