Hum on projector sound playback .

[David Hardy]

I am interested to read if it's possible to totally eliminate or reduce hum significantly when playing back projector 8mm film soundtracks. Especially when playing back the soundtrack via a Hi-Fi or Home Theatre System . Has as anyone succeeded in this ? I have tried a number if simple methods over the decades but so far with no luck .

[Paul Adsett]

Hi David,
Yes hum is a bear of an issue to totally resolve. I have found that a Bluetooth connection to the A/V amplifier essentially eliminates any residual hum, but it does introduce some audio lag (about 100ms) which you may find unacceptable. Some people have suggested using an analog to digital optical converter and an optical cable connection to the A/V receiver, I have not tried this, but I suspect you would still end up with audio lag similar to Bluetooth.

[Steve Klare]

I've had pretty decent luck dealing with hum. The thing I learned along the way is it comes in two flavors and really cleaning up hum requires dealing with them both.

The first kind I'll call "projector hum". It's from things like power supply ripple on the amplifier circuitry and stray magnetic fields being picked up by the sound head. You can hear it in the internal speaker even when the projector is operating disconnected from any outside audio system. One of the ways you can tell that it's projector hum is you can make it louder and softer with the volume knob and it goes away with the audio "off".

I dealt with mine for connection to an external amplifier by building up a 60 hZ. notch filter on a vector board, but there are other ways. For example, I know that Paul uses a graphic equalizer. This happens to be a whole bunch of notch filters inside the same box and one of them will be marked "60 hZ.", since Paul lives in North America.

Dave, for you this would be 50 hZ. since that is the power-line standard in the UK.

The second source of hum is more insidious: Ground Loop Hum. This hasn't got a lot to do with the projector: it'll be there even with the machine audio turned off entirely. It is often much worse than projector hum too. Until I figured it out, the internal speakers on my machines sounded much better!

Ground loop hum comes from voltages inside house wiring: things like magnetic coupling of the power lines onto the ground conductor. It's usually only a couple of volts, but unfortunately the line level signal from the projector is pretty small and easily overwhelmed. The projector audio out is referenced to the machine chassis. The amplifier input is referenced to the amp's chassis too. If there is some stray voltage in the house wiring's ground lines and the two devices are grounded at different locations, this gets added in to the signal we actually want to amplify. (Mine have about 25 feet of cable in between the two ends: plenty of opportunity for trouble!)

Same vector board: I built up a differential amp. The long and short of what this does is allow the projector's ground to travel all the way back to the amplifier end separate from the house wiring and then shifts the level down to the ground of the amplifier.

Paul's doing the exact same thing with his Bluetooth connection: instead of a wired connection separated from ground, he's taking his audio and launching through the airwaves! I'm sure his Bluetooth receiver is referenced to the ground on his amplifier: problem solved!

There are isolation transformers that do this too. There is no electrical connection between one side and the other, but they are magnetically coupled: same theory, same results.

The shortcut to this is to have your amp grounded at the same outlet as the projector. This is the best thing to do but not always possible, so it becomes a challenge.

[Rob Young]

Yes, I'd forget the internal projector amplifier and take a line out (or as good as you can get) from the projector to an external amplifier. Paul and Steve are absolutely right; some form of graphic equaliser is required to reduce low level noise (and top end hiss for that matter).

I use this one currently;

Behringer FBQ800 V2 Compact Graphic Equalizer at Gear4music

Depending on your projector's playback capability, you will loose some degree of low level bass response (my Elmo ST!200HD seems capable of floor shaking bass; my Beaulieu 708el less so). Another benefit is that you can loose some low level bumps and rumble that could occur with later Derann paste stripe.

Also, I do use a big old isolating transformer. New ones are not cheap, but will help enormously, not only with hum but with all kinds of domestic mains interference (oven thermostats popping on and off and being amplified - I kid you not, I've had this!!) should your home be prone to any.

Here are some ideas, but I personally haven't used these;

ALLEN AVIONICS AUDIO HUM ELIMINATORS AND ISOLATORS - Analogue

Slightly off topic, but an equaliser that controls left and right channel separately I've found invaluable to "re-balance" a of stereo soundtracks. Older units seem to all do this, but the newer ones such as the Behringer alter both channels together. Fine for mono, but for stereo super 8, you'd really want two units if you wanted to have full control.

[Steve Klare]

Agreed on the top end hiss. Many films (especially old ones) were never meant to be heard full audio spectrum. When I first got my own system going there was too much hiss and sibilance going on. There is this spectrum called the "Academy Curve" which rolled off anytyhing above 10 kHz and this was a theatrical standard to get rid of these nasties for a long time. My circuit already had a series resistor on the output for short circuit protection, so I added in a small capacitor across the output, and it took care of it.

-once again: a graphic equalizer could do the same job and offer more flexibility too. (-really not a bad way to go.)

I've found since LED lighting came out, especially when used with a dimmer these can put a nasty buzz on my external sound system. We have porch lights on a motion sensor, and if somebody comes up the walk (or a cat runs through the yard, or a big leaf blows past...) it's announced on my speakers ("BZZZZZZZZZZ!!!!"), ever since I installed LEDs in the porch ceiling. (It was fine for years with incandescents)

-fortunately, the next switch over from the dimmed lighting is a regular old outside lamp on a switch without a dimmer. This is what welcomes our guests on movie nights.


There is another way to reject a ground loop: optically. Most optocouplers don't pass audio signals well since they aren't linear and will distort audio waveforms.

-but wouldn't you know? -about a month after I got my own system built up and working nicely, they discovered a ground loop where I work and also a linear optocoupler which they used to fix it!

The system I built with a diff-amp is affected by series resistance within the signal source and will only reject ground hum well at one specific value. Unfortunately with different machines there is a lot of variety (Two examples Elmo: 600 Ohms, Eumig 800 Series, 50,000 Ohms!).

An optically based system wouldn't have had this problem, but I was too lazy to scrap what I'd built and start over (-excellent way never to finish anything: starting over again and again!). I compensated my system for "Elmo" and used the Eumig on its own speaker. Eventually I added a four-channel audio mixer at the front and re-compensated my circuit to play nicely with this. I can plug any line level source I want into the mixer and they all sound just fine.

[Rob Young]

Moving on slightly from reducing hum, but equalising the sound for the room can bring huge benefits. Cinema auditoriums are supposed to be calibrated individually but many are not. Hopefully David as an ex projectionist can add insight to this.

Loud doesn't have to mean "offensive". Loud bass, for example is quite emotive, but loud treble and clarity should be the same. Unfortunately, it is often associated with screeching ear blasting. A good cinema can sound very loud indeed without being tiring or offensive to listen too, if it has been properly calibrated and adjusted.

On a simple domestic level with super 8, I've found that an equaliser can help enhance clarity, but also curtail offensive high frequencies too and having lived in quite a few different houses with a cinema set up in different rooms, I've been surprised just how differently the eq needs to be from one place to the next.

For example, my current room needs quite a lot of higher end curtailing without losing any clarity, but to reduce tiring "screech". I was a little reluctant at first as I've sometimes struggled a little to eek out higher frequency clarity from super 8, but after some trial and error I realised that those darn mid to higher frequencies were just too plain irritating in this room.

As most of us will know I'm sure, but most amps for home cinema use have an eq program which uses a microphone placed at different positions in the room to equalise the sound for different room shapes and sizes. To be honest this too can be hit and miss and I reckon there's nothing better than your own ears, although to be fair most are pretty good at ironing out problems. I worked in Hi-Fi back in the day and sound recording professionally, and one thing's for sure; people can hear things very differently!

Anyone remember in the late 80's when Dolby Spectral Recording came in? The benefits were good in a properly calibrated auditorium, but for many the experience was ear spitting treble.

[Brian Fretwell]

Yes I seem to remember the Dolby CP50 had a 27 band equliser for ech chanel. Not graphic but rotary presets designed to be set up using a micophone and meter system.