How to ad a headphone jack to a simple speaker?

January 10, 2007 10:06 AM

AudioFilter: how can I add a headphone jack to a simple speaker?

The channel monitor in the booth at our theater uses a very tiny, crappy speaker to monitor audio output to the various channels in the auditorium. When I watch movies from the booth, I have a hard time hearing the movie over the noise of the projector and exhaust fan. I'd like to make it possible to use headphones on the monitor, so I could hear the movies a bit better.

There's only one speaker in the monitor, and it has two wires going to it (one of which I think is a ground of some sort). What do I need to add a headphone jack, and how do I do it?

posted by bjork24 to technology (13 comments total)

Is the monitor speaker plugged into or hardwired into the channel monitor? I'm guessing the later but wanted to make sure.

Is there any way you could post pictures of the setup?
posted by JeremiahBritt at 10:17 AM on January 10, 2007

Speakers and headphones really are not so different. They are not directly interchangable though. If you are decent with a soldering iron you could probably do this.

What you're going to need to do is make what's called a Zoebel network. This is basically just a resistor and a capacitor that are intended to compensate for the frequency response characteristics of a speaker a bit. For you application this is not critical.

I couldn't find anything really simple to show you, but check out this pdf for a simple DIY amp

Check out page 4. Ignore the little circuit board except to see that there are 2 wires coming out of it. One is the "ground" and the other is the active wire.

What I would do is this: remove the current speaker. However, I'd keep it around and wire it to a 1/4" speaker plug. Then I'd take the wires that were connected to the speaker and wire them up like the diagram, going to a 1/4" mono speaker jack (to attach the speaker back if you wanted to) and the other 1/4" stereo jack is for the headphones. You may or may not need something to amplify the headphone output more, it may not be loud enough. There are headphone amps you can buy, or, as I found via a recent askme question of my own, you can make one: CMoy Headphone Amp

There is a lot of peripheral info you'll need to learn to do this, like how to solder, which wire is the ground (it does actually matter, if you want to avoid a bad hum), what terminals of the jacks are ring/tip/sleeve, etc. I think it's doable though. Feel free to ask more questions in this askme question or hit me up via email.
posted by RustyBrooks at 10:26 AM on January 10, 2007

I may make mistakes on this (so feel free to correct me), but you can remove the speaker. Then add a jack for the size of headphone you plan on using in the space of the speaker. If you know which wire is the ground, run that to the sleeve of the headphone jack (the part that would connect to the barrel) and connect the other wire to the tip connection.

Then you should be able to plug your headphones in. There might be an impedance matching problem though.
posted by drezdn at 10:29 AM on January 10, 2007

Or follow Rustybrooks advice.
posted by drezdn at 10:30 AM on January 10, 2007

The speaker is hardwired.

I don't have the monitor out right now, so I can't post pictures, but you can see the audio schematics here (PDF - page 14).
posted by bjork24 at 10:42 AM on January 10, 2007

Woops. I meant page 15, not 14.
posted by bjork24 at 10:42 AM on January 10, 2007

How many screens do you have? Truthfully, it might be worth your time to get a booth monitor with a headphone jack. For someone who's pretty comfortable with soldering, it's not that hard of a job. For someone who's not, it's another story.
Another benefit of getting a new monitor is that you *might*, depending on the model, be able to get a stereo mixdown of the 6 channels, rather than a dual-mono (doubled to each ear from the mono speaker). Not like that matters much for dialog, but still.

Granted, this all sucks if you've got 8 (or 20!) screens, but adding a headphone hack to 8 screens ain't no party, either.
posted by god hates math at 10:45 AM on January 10, 2007

Ah, shoot. I see. Your booth monitor isn't just a monitor. At my previous place of employment (oh, how I long to be in a booth again), we had passive monitors that were wired to a set of outputs from our Dolby Processor.

Also, have you thought about asking this on the Film-Tech forums? Projectionists (as I'm sure you know) are an opinionated bunch, and I'm sure somebody will be able to help.
posted by god hates math at 10:50 AM on January 10, 2007

There are missing details..

In principle, you can get away with a potentiometer, a female 3.5mm miniplug, and the opposite of whatever connector is on the end of the speaker cables. If the speaker is hard wired, you'll need to choose an appropriate connector, and splice that into the speaker wire first.

It's possible that you won't have enough volume, in which case you will need a headphone amp, but I would only think about that if the method outlined above isn't adequate.

So, is the speaker hard wired? If not, what connector does it use? If so, do you ming cutting it to add a connector in the line? Does the speaker have any sort of volume control, and where is that control located? Sounds like there might be some strange channel mixing going on, can you elaborate on how the speaker's signal is generated from the movies audio tracks (a mix, one channel at a time, who knows what)?
posted by Chuckles at 12:15 PM on January 10, 2007

should have read ahead :P, checking schematic now..
posted by Chuckles at 12:16 PM on January 10, 2007

Can't read the schematics (blurry) but what's the number on the amp (triangular piece) on page 15 that precedes the speaker? You might be able to just solder a headphone connector across the speaker if the amp can handle the load. Is there any markings on the speaker as to what impedance (ohms) it is?
posted by Ferrari328 at 12:37 PM on January 10, 2007

The amp is an LM384. There will be no problem hooking headphones up to it. So, it would be an easy mod, but it would require going inside the EX500 to do it - may or may not be an option for you, bjork24, with warranty considerations and such..

If you want to go with the mod, I (or whoever gets to it first) can mark up the schematic for you, and point you at the correct part.. Typically it would involve a switched 3.5mm mini-plug jack, so when the headphones are plugged in the speaker is disabled. It would come down to one cut wire, and about 4 new connections, as well as a hole drilled in the case for the new jack. If you don't want to disable the speaker, you can skip the cut and just make two or three connections, still need the hole though (unless there is already a hole you can make use of)..
posted by Chuckles at 3:19 PM on January 10, 2007

UPDATE: I just parallel wired a 1/4 jack onto the speaker wires. Worked like a charm. No resistor needed. The monitor volume control works for the headphones too. Thanks for all the help guys!
posted by bjork24 at 6:10 PM on January 10, 2007