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Author Topic: An Easy Hookup from a Projector to a Stereo Amplifier
Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

 - posted December 07, 2015 05:26 PM      Profile for Steve Klare   Email Steve Klare   Send New Private Message       Edit/Delete Post 
2015 marks five years I’ve been sending my sound tracks through my home audio system. It’s like a lot of improvements I’ve made in my “theater”: it’s something I did figuring on occasional use, but once I tried it I liked is so much it became part of standard operations. Much the same: I hung a large screen behind the living room curtains and as a result there will probably never be a couch in front on the living room window again!

Operating through an external amplifier has a lot of benefits. There is bass response through large cabinet speakers that if you were able to produce through a projector’s 5 inch round speaker I’d put you up for a Nobel Prize: it just aint happening! It means you now have access to any features that a consumer audio system might have. Sound is muddy? Turn down the bass and crank the treble. It also means that the heavy lifting is being done by an amplifier with a lot more capacity which can be driven a lot harder without producing distortion. You can even use a set of stereo headphones and watch films without waking up the whole house! (Hobbies and Families: where diplomacy is fundamental!)

As I said: this is something I’ve been actively doing about five years now. I started trying as soon as I got into sound and ran into a technical obstacle (and an architectural one too, my Amp used to be on an entirely separate floor of the house before we renovated.) that stopped me for a while, but I’ve slain both of those dragons now and I can define how anybody can do this.

What I’m saying here applies most directly to Elmo ST series machines, but is generally applicable once you figure out how to connect up at the machine end. I’m also orienting this towards the ST’s monaural Auxiliary outputs. They are line level, which is what most amplifier inputs are expecting, and they are also controllable from the projector’s volume control. The nice thing about this is it saves you some walking when you’d really rather just watch a film. At home the stereo sits all the way at the back of the house. If I used that as the volume control I’d adjust zoom and focus at the machine but the volume 15 feet backwards from the machine and 30 feet from the screen. It’s just nice to have all the controls centralized in one place. (What if you had to sit in the back seat to turn on the windshield wipers?)

The first problem you run into here is interconnectivity. The Elmo aux is a single 1/8” female monaural connector. The inputs to most amps are two female RCA connectors. They’ll look like these and hopefully not already be busy with a CD player or other audio input (“Aux.In” is good as long as it’s still auxiliary!):


Bridging the gap between these separate worlds takes two steps. First you need one of these “Y” cables:

Stereo "Y" Cable

(These are available in many different lengths from many different sources)

-and since this is a monaural output driving a stereo input you need one of these mono to stereo adapters between the projector and the “Y” cable :

Mono to Stereo Adapter

Without the monaural/stereo adapter you will only get sound through the left speakers. Remember also: your cable is a stereo connection. If you have a twin track machine and decide to go for stereo at the speakers later on, it will only be a matter of changing what’s at the projector end of the cable. Let’s leave that for another day: The Wright Brothers left it for Lindbergh to fly to Paris after all!

Please Note: I am using Amazon as a seller because so many people have accounts with them. I do NOT own any Amazon stock and I’m letting you know this stuff is available from many other suppliers. You can even pick it up at radio Shack providing you can still find a Radio Shack!

You could make this work with a great many other machines, you just have to work out the initial connection and take it from there. What if you have (for example) a Eumig 800 series? The long cable will still work: you would need a 5 pin DIN style connector to a 1/8” stereo female. As far as I know you’d have to fabricate this yourself (I made mine), but we’ll talk more about that later on.

There’s a respectable chance the first time you hook this up you will get wicked hum out of it. The odds are this is not your projector and not your amp either: it’s really the whole system interacting with your house wiring and possibly even the world beyond.

Believe it or not, the most important voltage in the world is actually Zero volts. Voltages are always measured from one point with respect to some other so we have this idea some of us call “Ground”, others “Earth”. Theoretically if you measure the voltage between any two points directly connected to the planet anywhere, you will always get a great big Zero: this becomes the reference Zero Volts other voltages are often based upon (-kind of like an electrical sea-level…). Theoretically as well, Love is forever! (Just ask a divorce lawyer about that one!).

-So let’s face the fact that we live in an imperfect world and see how we can deal with it!

For various reasons, real world voltages between “grounds” are often more than zero, and if you measure the voltage of one “grounded” device with respect to ground on another “grounded” device, this far from theoretical difference in voltage will get mixed into your desired signal and you’ll hear more than you bargained for if you shove it through a speaker. This is called a ground loop and it’s been the bane of electronic communications for well over a century!

You’ve probably heard this happen in real life: it’s very common in Public Address systems. Even with no sound into the microphone there is this low, irritating buzz coming out of the speakers!

-You too can have this in your own home, or if you’d like I can show you what to do about it.

What you need is some device that takes the projector ground referenced voltage and shifts it to the ground level of the amplifier input. These are often called ground loop eliminators. They are common as dirt and not expensive either.

For example:

Groud Loop Eliminator

(You can also be a friend to a friend of film collectors and ask Steve Osborne about the one he sells!)

This is inserted into the connection between the ”Y” cable and the amplifier input. It’s kind of optional …until it’s not! (When you need it, you really need it!)

When I first tried making this connection years ago I had this problem big time. Only when I understood that my projectors are actually grounded to the water pipes in the house and my amplifier is grounded to our cable TV provider and one “ground” wasn’t at all close to the other “ground” could I start to unravel it. I actually built a module that combines a ground loop eliminator, a 60 Hz. active filter for projector hum and compatible connectors for the projector and Amp , but that’s a story for another day.

When you get all this hooked up, it’s good to start with the volume on everything turned down low. With a fairly typical sound track operating, turn up the volume on the projector to where it usually sits when operating on the internal speaker. Then raise up the volume on the amp until you have enough sound in the room. You can leave the Amp volume here and just operate from the projector.

Once you’ve gotten past the sheer joy of feeling your sound tracks through the floor, you may notice the regular hum you are used to hearing through the projector speaker seems a little more pronounced. You have hung a more responsive Amp on that signal and actual woofers: no miracle here. The same will go for magnetic hiss from the stripes. There are things you can do about these too (graphic equalizer for one thing), but that’s enough for today: every journey starts with a single step. Today let’s see if we can shake some stuff off the shelves, we can get picky some other time!

There are a lot of places you can take this from here. For example this setup is for only one projector: how can you do changeovers using a second machine? What if you want to add in other audio sources for intermissions? I added in a mixer panel for that and you can read about it here:

Adding a Mixer Panel

Other projectors may (or may not) act differently. The Eumig 800s for example have a 50,000 ohm resistor in series with the Aux out instead of the Elmo’s 600 Ohms. This means you will need to crank up the volume on the Amp to get the same results. The aux level is also fixed so your only volume control would be the one at the Amp. Eumig didn’t exactly have a handle on their own internal grounding so I’d be very surprised if you were able to squeak one of these through without using the ground loop eliminator.

I find my Eumig and my own sound system are a lot nicer to each other with the mixer as a go-between. At the output of the panel everything connected to the inputs becomes equal and it just makes everyone play nicely. It also puts a volume control with the projector: exactly where you need it to be.

-the fun part of these Eumig 800s is unlike many audio devices when you plug into the Aux the internal speaker stays active, so you get an extra speaker up with the machine if you want! I find having an independent audio channel competing with my external setup is downright confusing, so I usually leave this speaker turned way down.

The main idea here is for the sake of a small investment you can get much better sound and be pleased you did. Doing things like this keeps any hobby new. You will find yourself pulling films off the shelf you haven’t watched in years just to hear what they sound like.

All I ask is a wide screen and a projector to light her by...

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