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Author
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Topic: Sound output from Eumig
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Steve Klare
Film Guy
Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003
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posted January 10, 2016 03:50 PM
There may be something more going on here than just the signal strength. My 800 series Eumig has an internal 50,000 ohm resistor in series with the output. The typical amp input is 10,000 Ohms. When you plug this output into that input it drops that signal to 1/6 what it started out as.
It would be interesting to see what the specs are on your 'machine.
(BTW: S938 is spec'ed at 47,000 Ohms. It could be a family trait...)
50K is a huge number: Auxes on Elmo machines have 600 Ohms in series!. With these you get 10,000/10,600 of the full signal strength: a pretty respectable fraction.
I plug my Eumig into a mixer: the 20,000 Ohm input gives me a fighting chance (2/7>1/6): besides, it's a pre-amp.
[ January 10, 2016, 04:53 PM: Message edited by: Steve Klare ]
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All I ask is a wide screen and a projector to light her by...
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Steve Klare
Film Guy
Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003
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posted January 11, 2016 08:46 AM
I can't say much about the grounding on the 900+ series, but the 800s can be pretty bad. This can produce some respectable hum if you connect to an external amp.
When I decided to build a real external sound system, I knew about ground loops, so I decided to see how big a problem they might be first. I measured the voltage between the chassis on a couple of machines and the third prong of the outlets they were plugged in. For my Elmos I got something like .01 Volts AC, but my 800 series Eumig I got about .50 Volts, and it tended to drift around a lot too. I took the thing apart and found out the bracket the transformer was mounted on had the green wire running to it, and the bracket is bolted to the chassis, but there is no direct ground to chassis.
I griped this and heard back that someone had this so bad he actually got a shock off his Eumig! What I did is find a couple of star washers of the right size and I put them on the mounting bolts to dig into the metal on the bracket and the chassis and improve the connection. It dropped the ground voltage down a lot and it stayed constant. It wasn't perfect, but much better.
Now there's a potential safety issue here for sure, but what about audio? Well, the "hot" side of the line level signal comes from the output of some kind of amplifier circuitry, but the return is connected to chassis. This means when you connected this signal to some other chassis (example: amplifier) this voltage winds up mixed into the signal and you can get hum you'd be downright proud of if hum was what you were after all along! (A friend of mine actually "fixed" this problem by deciding to like the hum!)
My Eumig still hasn't lost all its ability to cause a ground loop. What I do about that is I operate it without its chassis directly connected to the chassis of my Amp. My mixer is capable of taking inputs in unbalanced (signal, shield) and balanced (signal(+), signal(-), shield) connections. I usually connect one Elmo as unbalanced to ground the whole projector end of the hookup. All the other projectors, especially the Eumig, get hooked up like this:
Projector audio hot: Signal(+)
Projector chassis: Signal(-)
Mixer ground: Shield
What this does is only amplify the voltage between projector hot and chassis and it takes the stray voltage between Eumig chassis and house ground entirely out of the equation. (ground loop: busted...)
This reduces the hum down just to the internal hum (the one the hum bucking coils are intended to reduce. They can't fix a ground loop.)
I've been running my 800 series a lot the last two weeks and connected this way the audio is pretty nice! The hum is still there, but it's way in the background and if this was as bad as it ever got I'd have left it alone.
It means if I'm using my sound system the Eumig can't ever be the only projector on the table**, and the mixer has to be there too, but at least it can join the party. It has a nice lens and I've found it's very good set of for 'scope.
** I could build some sort of grounding box, but I don't need another project!
[ January 11, 2016, 09:49 AM: Message edited by: Steve Klare ]
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All I ask is a wide screen and a projector to light her by...
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