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Topic: Sound output from Eumig

Steve Klare
Film Guy

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

posted January 13, 2016 10:22 AM

The problem really wouldn't be so much the projector all by itself, but the combination of the projector and the amp.

If your amp input is high enough in impedance a lot of the signal voltage will get through and you'll have plenty of power in the speakers at a reasonable volume setting.

-but let's say the amp input impedance is low enough that it cuts the signal voltage in half.

So you'll only get half the volume?

No, power is proportional to voltage squared: you'll wind up with 1/4 the volume. (-and so on down to silence...)

A problem you'll also run into is when the signal gets really weak and you really crank the volume to get it loud enough, the hum and other nasties get boosted up too.

My Eumig is a train wreck directly through my amp: it takes the mixer in between to fix it.

-though it does sound good this way!

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

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Lee Mannering
Film God

Posts: 3216
From: The Projection Box
Registered: Nov 2006

posted January 14, 2016 04:19 AM

Now that is a top tip Paul and Maurizio [Smile]

Using my own Eumig 800 range sound projectors for silent films I typically disengage the sound head pressure by falling short of the second main control click. By slowly turning it just to just switch the lamp on its possible to stop short of the final click as it were. Have been doing this for decades to no ill effect, think they call that a soak test. [Big Grin]

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John Armer
Expert Film Handler

Posts: 139
From: Lancaster, UK
Registered: Jan 2015

posted January 25, 2016 01:59 PM

Steve, you mentioned:

Here's a thought: You can make your own Aux. Out with a DIN speaker plug, a simple resistor network and an RCA cable. You plug into the external speaker jack and tell Eumig to keep their constipated Aux.!

Are you up for a little soldering?

Yes I can get this done at work. What value resistor would you recommend?

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Steve Klare
Film Guy

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

posted January 25, 2016 03:00 PM

Something like this:

(Sorry for the scribble: I'm known more for prose than penmanship!)

Basically what this is doing is fooling the machine into thinking it's driving an external speaker, then dividing that voltage down and putting a low enough fraction of it into a line level amplifier input.

This is for driving a monaural amp. A stereo amp would need the same except it would be two 47K resistors and two RCA connectors branching off the top and bottom of that 10 Ohm load resistor.

This would be good to build inside a small plastic project box with cables coming out either end.

This will get signal into the amp. It in of itself it does absolutely squat about hum either from the machine or from an external ground loop.

In this case the ground loop is best dealt with by plugging the projector and amp into the same outlet and not having the amp connected to anything else except speakers. When my amp is also connected to my video system I get a wicked ground loop!

The projector hum is usually dealt with on Eumigs by manipulating hum buck coils. (I wish you better luck than I had!)

My first crack at externally amplified sound was something like this: it was the beginning of the journey, not the destination. Getting decent sound out of Super-8 is very doable, but it usually isn't plug and play!

If you do it, start out with all of the volume controls (projector and amp) set as low as possible and incrementally adjust them all up until you have enough volume without distortion.

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

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John Armer
Expert Film Handler

Posts: 139
From: Lancaster, UK
Registered: Jan 2015

posted January 25, 2016 03:51 PM

Steve, thanks very much... that's perfectly clear. I should be able to get that made tomorrow... and will let you know!

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John Armer
Expert Film Handler

Posts: 139
From: Lancaster, UK
Registered: Jan 2015

posted January 26, 2016 02:28 PM

No joy I'm afraid. The amp I'm connecting to behaved very strangely with the circuit in place. The sound was initially great, but after about a minute it went very quiet. I turned the volume up on the Eumig and then it was too loud! I turned it down so it was at an acceptable level and it was fine for a little until the whole cycle repeated itself.

Back to the drawing board. I think I may experiment with some sort of buffer to get around the impedance issue as you did with your mixer, Steve. It was definitely worth the experiment though!

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Steve Klare
Film Guy

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

posted January 26, 2016 02:55 PM

Interesting!

It worked for me, although I eventually abandoned it because of the hum. (I think it's still in my junk bin...)

Something you might try as an experiment is connecting a speaker into your external speaker jack just to make sure there is nothing funky with your projector. (This should not be complicated enough to cause such mis-behavior!)

If you are of a circuit-oriented mind, you might try building an instrumentation amplifier:

Instrumentation Amplfier

Your two inputs would be both legs of the line out signal.

What this does:

1) It busts the ground loop
2) It is very high input impedance so you get the full signal voltage despite the 50K internal resistance.

(If I did this I'd put in something like a transorb between each leg and ground to protect those op-amp inputs from static discharges and other misadventures.)

I built a diff-amp as my own anti-ground loop circuit:

Diff-Amp Schematic

It works very well but it's very picky about the value of the resistance inside the projector. I added in a pot in the feedback to fine tune it for the Elmo Aux (600 Ohms) but it completely goes off the rails with the Eumig's 50K. It works really well with my mixer because I built a kludge compensation circuit to go in between.

I know how I'd sidestep all this now, but I'm not motivated enough to build it all over again. (-really great ground isolation circuit I found out about like 3 months too late!)

What I have was supposed to be just an experiment, but it's close enough. I have enough other projects I'm not looking to do the same thing twice for such a small improvement!

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

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