Headphone Testing Help

[wardenclyffe]

A friend of mine asked me to repair some headphones the other day. The only problem was general wear where the wires come out from the end of the jack. I cut the jack off and went to add a new one but I couldn't figure out which wires were the left and right channel. I tried to pull the rubber coating off the the old jack to figure out how they were soldered on but goofed it up so badly it was useless.

I know very little about audio signals so my question is how I can best test the headphones to figure out which side is left and right. I am assuming I can feed in a sine wave in the audio range and listen for a tone in headphones but I don't want to damage them in the process. Is there an amplitude limit I should stick to? Some other places have suggested putting a 1.5V battery across the terminals and listening for a pop in the headphones, but I am skeptical of that approach. Any suggestions would be much appreciated.

[JohnS_AZ]

I've done the 1.5V battery thing many, many times.

[Zero999]

The 1.5V battery will work but don't do it with the phone in your ear - it's really loud.

[Simon]

you can sometimes use you meter in low ohms mode

[Zero999]

That won't tell you which is left and which is right though.

[Simon]

it will if you listen for the crackle ! not sure if it works on digital meters though

[pmrlondon]

There's probably no surer way than the ones mentioned. Sometimes the core wire colour is a hint but it is by no means standard. Last pair I had to change the plug on had enamelled wire - that was a bit of a pain, but I think the colours helped there.

[ziq8tsi]

it will if you listen for the crackle ! not sure if it works on digital meters though

It does, but you will probably need to be wearing the cans to hear the low test currents from a DMM.

One of the non-intuitive features of unamplified analog meters was that on the lowest ohm range they put a significant fraction of an amp through the device under test, even at only 1.5V.  Enough to make an incandescent torch bulb glow dimly for instance, and certainly make loud clicks on a headphone.

Whereas on the highest ohm ranges the open-circuit voltage could be 9V, but the short circuit current would be only microamps.

[Simon]

you have just elaborated on my point

[Mechatrommer]

you repair earphone and you dont have anything to play from? radio? cd player? pc with sound card? very odd. use a battery then, scratching the wire on it, u'll hear something.

[Ronnie]

Connect a capacitor in series with the 1.5 V battery to avoid shorting it continuously by the speaker coil. During transient speaker coils like inductors are open circuit but will be short circuit after several µs or ms (depending on the resistance of the circuit).

[vk6zgo]

Do the headphones have markings to show which is physically L or R?
If so, just keep trying pairs of wires  using the method posted by others above,until you find ,say, the L one.
The other pair are the R.

But,wait--there's more! The above finds the two sides,but it doesn't necessarily get them phased correctly.

The only easy way to do this is if you can somehow see the diaphragms.
You then put the two pairs in parallel,& do the ohmmeter test as before.
If both diaphragms move in the same direction,they are phased correctly.If not,reverse one connection.

The other(harder) way is to use a tone or mono music,again with the earpieces parallelled,& see if you can hear the difference.
The wrong way round should subtract,the right way,add,but I think you may need "Golden ears" to notice.

VK6ZGO

[Simon]

But,wait--there's more! The above finds the two sides,but it doesn't necessarily get them phased correctly.

The only easy way to do this is if you can somehow see the diaphragms.
You then put the two pairs in parallel,& do the ohmmeter test as before.
If both diaphragms move in the same direction,they are phased correctly.If not,reverse one connection.


VK6ZGO

The common earth is usually quite obvious on g
headphones

[vk6zgo]

You are right,that is the normal configuration.
I was led astray by the fact that fact the stereo phones that I set up for mono use for Ham radio are odd ones which use two separate pairs.
My first HF receiver was quite happy with the mismatch of two earpieces in parallel,but the transceiver I bought later,wasn't,so I connected them in series.
I couldn't have done that if it had one wire always commoned.

VK6ZGO

PS:- Come to think of it,of course I could have connected them in series, just by ignoring the fixed common!
It isn't how I wired them,though as they did have separate pairs.

[pmrlondon]

There's probably no surer way than the ones mentioned. Sometimes the core wire colour is a hint but it is by no means standard. Last pair I had to change the plug on had enamelled wire - that was a bit of a pain, but I think the colours helped there.

Self-followup time:

I've forgotten how I stripped the enamel - and the same thing has just cropped up again. Any hints on how to get (soldering-proof) enamel off wires?

[ciccio]

I've forgotten how I stripped the enamel - and the same thing has just cropped up again. Any hints on how to get (soldering-proof) enamel off wires?

Enamelled wires are hard to solder...
sometimes I had success with a VERY short immersion in a VERY hot solder pot,  sometimes I was capable of burning the enamel with the flame  of a gas lighter,  sometimes the VERY hot tip of the soldering iron was enough.

[HLA-27b]

Rubbing some fine sandpaper helps a great deal. A lot less messy than burning with flame.

[Psi]

I normally just scrape them carefully with a sharp craft knife or scalpel rotating the wire as i go.
You can normally see where you've scraped off the enamel off because the color changes.

However if the wire is really thin, (eg lots of strands all enameled separately), then things get tricky.
A flame is probably the easiest way.

[ejeffrey]

[quote author=pmrlondon link=topic=3452.msg59188#msg59188
I've forgotten how I stripped the enamel - and the same thing has just cropped up again. Any hints on how to get (soldering-proof) enamel off wires?
[/quote]

At home I use a scalpel.  At work when I need to strip a long length, or very delicate wires with very tough coatings I use formic acid.  Watch out for fumes... it isn't particularly dangerous as chemicals go, but just a whiff is like snorting vinegar.