Using graphic equaliser to suppress hum

[Martin Davey]

Thanks all for the very interesting responses. There's certainly a lot to get through and refer to on this thread.
I've been exposed to HiFis all my life, back to the 1970's, with my father being a Hi Fi enthusiast. Hum in sound was not generally heard on home hifi, but cine seemed to be the poor relation.
The hum is something that appears mainly in silent sections, with louder passages masking over the hum, but I'm conscious that its there. Recently I purchased an Eumig mark S, using a tube amp and I was struck how pleasant the sound was, in this case using an external speaker, and not much hum at all. Even though its not a HiFi source the sound did not seem so crude as it had less background noise to compete with.
Often my projectors are plugged in either a P.A amp (which has no bass/ treble controls) or and AV amp, on which the bass/ treble controls are hidden away in the software, rather than a physical knob. The AV amp is set to neutral settings for content off Bluray etc and sounds perfectly good.
It just makes sense to have the equaliser just to remove the hum from the various film projectors output, to tidy up the general sound, before it enters one of these external amps.
I sometime use headphones for listening too.
The 'cleanest' audio I get is from 2 B&H TQIII's, and is very nice. My Elfs follow a peg down (they have audio outs added). The 3 Elmo st1200s again below, with the Eumig 710s at the bottom. The Chinon 7800h is also quite hummy. I listen to the audio from other formats, old and new and enjoy hum free sound, and want to get something better out of the cine equipment.
I thing I would go for an equaliser when one becomes available.

[John Taylor]

I had a couple of interesting email exchanges last evening regarding this, one was from a collector of nearly 40 years who said other than the Eumigs, most projectors should really only become noisy with or hiss when the volume control is turned to half or more with Elmos being among the very best performers for sound.
An average film with decent sound should not normally need to be up this high, and if a good branded standard analouge amp is used, hiss or hum shouldnt be heard at all, unless a print has a poor or faulty sound.

[Steve Klare]

Hum is a changeable problem. For example with the little round speaker in the projector, it's kind of thin: you can hear it, but the small speaker is not that friendly to low frequency tones, including hum. Then comes the day you do what comes naturally and add a big external speaker, and those bass tones (including the hum) come up. It sounds "better", but not "right". Then you do the ultimate: add in an external amplifier. Now you are messing around with ground loop hum. This is actually the worst hum of all: loud and buzzy. It has nothing to do with the projector (-even happens when it's turned "off") and actually starts out in the house wiring. There are ways around this like leaving the amplifier ungrounded (they usually have two prong cords for just this reason) or at least grounding it at the same outlet as the projector. In my case, the amp was grounded through my home entertainment system, which was itself grounded through TV coax. 75 feet away physically and more than 2 Volts RMS electrically.

I had no choice: it was very real, and very loud. I had to fix.

(-actually that part of it was kind of fun! 🙂)