8mm Forum: What causes that hum?

This is topic What causes that hum? in forum 8mm Forum at 8mm Forum.

To visit this topic, use this URL:
https://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=010615

Posted by Stuart Reid (Member # 1460) on April 06, 2016, 03:48 PM:

One thing I had forgotten about in my sojourn away from Super 8 is that damn hum that sound projectors seem to exhibit through the amplifier path as soon as they're switched on. My question is: what causes it, and of course, can it be rectified, reduced or removed?

Posted by Steve Klare (Member # 12) on April 06, 2016, 03:57 PM:

Hi Stuart,

If you are talking about a projector standing alone, the two biggest factors are power supply ripple and pickup of stray magnetic fields from the power transformer by the sound head.

If you are connecting to an external amplifier you can take all the stuff above and add in a layer of hum due to ground loops. What this amounts to is all the signal voltages are referenced to the chassis of whichever device is either sending or receiving them, but any voltage between the two chassis gets mixed in too.

In a perfect world ground is ground (or "earth" if you like) and there is no voltage in between, but the world is a long way from perfect.

Ground loop hum is often much worse than regular old projector hum, and until I figured out how to kill it I stayed on the internal speaker.

What's your settup like?

Posted by Andrew Woodcock (Member # 3260) on April 06, 2016, 04:21 PM:

Some machines certainly score worse than others regarding ground hum from the Super 8mm range.

Findings from the recent ones I've used or witnessed are;
1/ Eumig S938 - hardly any
2/ ST 1200 is not bad at all on this front, especially the HD.
3/ Beaulieu Stereo - hardly any at all until very high volume level is reached.
4/ Bauer models - awful for ground hum until the heads load. Then they are fine (ish), especially the later 525 / 610.
5/ Agfa Sonnector, not great either for ground hum
6/ Elmo GS 1200- some but not overwhelming by any means.

To me its more of an offense, the circuit's that don't ground the loop hum from head and transformer when they are idle like the Bauer Studio range. It's very annoying having to quickly lower the volume slider as a film reaches its end. A bad design here in that one respect.

As Steve says, ways to solve include extra grounding wherever possible especially on input / output connections to separate equalizers and amplifiers and the inclusion of devices like ground loop isolators for placing in circuit,in between such devices.

Also much of this issue here can be purely down to some of the ridiculously low recording levels that come with our films originally. If you are having to really crank that volume pot to gain anything like reasonable listening levels of audio, then it's only to be expected, that of course some hum and hiss will follow from a magnetic track of any kind.

If levels are high, touching 0db for peaks or thereabouts, then even the worse offenders here sound very very good comparatively.

Posted by Stuart Reid (Member # 1460) on April 06, 2016, 04:37 PM:

Hmm, actually I guess it's more of a hiss than a ground hum. I expect some of these projector amps could do with having the capacitors replaced as they're now very old and probably leaky?

Posted by Andrew Woodcock (Member # 3260) on April 06, 2016, 04:39 PM:

It certainly helps Stuart, that's for sure!
Not found too many leaking though yet thankfully, but it certainly helps to change these out when the electrolyte becomes ineffective as they age and dry out.

Posted by Steve Klare (Member # 12) on April 06, 2016, 05:24 PM:

I've tried going the capacitor route, and given the age of these machines it should have helped, but it didn't as much as I hoped it would.

Hiss is best dealt with by rolling off the frequencies up in the teens to 20 KHz. In our soundtracks there's really not any sound up that high so those frequencies are good for nothing but noise.

A lot of people do this with graphic equalizers.

Posted by Stuart Reid (Member # 1460) on April 07, 2016, 02:25 AM:

Thanks Steve, good advice. I'm going to try and EQ it out.

Posted by Steve Klare (Member # 12) on April 07, 2016, 08:05 AM:

-then again before I got the mixer/equalizer and the audio interface and the amplifier and the speakers and all the cabling I could usually take the edge off the hiss with the tone control.

These days I just leave it turned all the way up and clean up the sound downstream of the projectors.

What would be nice about an equalizer is not only could you stifle the hiss, you could also put a notch at 50 Hz. and do the same to the hum.

Posted by Gary Crawford (Member # 67) on April 07, 2016, 09:15 AM:

Having been in the audio world for my job for almost 50 years, I'd say hiss is the hardest darn thing to handle. You want ALL the high crispness of the soundtrack. You don't want the hiss. Most times the two are unfortunately co-mingled. Some of that hiss is right in the middle of the frequencies you want to hear in the track. Eq or filter too much and you almost always take out some of that good frequency range in the original sound. It's almost always, especially with consumer equipment, a compromise between the sound you WANT to hear and the his you DON'T want to hear.
Generally in well recorded soundtracks the rolling off of highs is not needed. The sound obliterates it. That's why I don't recommend trying to eq the hiss or hum from a source like a projector with no film running. Sure, you can filter the heck out of the amp noise, but when you go to run a film, you end up with half the highs on the track gone and some of the lower frequencies gone, too.

it's a tough job.
The older Eumig's pre HQS 800 models had so much hum, it was distracting, even when a film was playing. Since then the 824's made a big improvement. The Elmo st1200HD series have some amp noise that seems to dissolve once you have film running to cover it up. The GS is better. Both have more noise coming out of the power amp, the one that feeds the speakers, than coming from the aux out or monitor lineout outputs.

Great, careful recordings using machines with good sound heads, properly aligned and clean, recorded at good levels, properly biased for the type of sound stripe....all these things really can make a world of difference in how Super 8 soundtracks can sound.

Posted by Steve Klare (Member # 12) on April 07, 2016, 09:30 AM:

'Morning Gary!,

Hiss is definitely a situational thing. That's why I keep a flashlight back by my machines. Unfortunately the folks that built my mixer panel never expected it to be used in the dark!

I always run with the 60Hz notch, though. It doesn't kill the projector hum entirely, but it so thoroughly injures it that any normal soundtrack overwhelms it. (-even just the other ambient sounds in the room...)

There is still plenty of bass left.

The day somebody in a musical hits the second "B" up from the bottom of the keyboard, that note will get hurt, but so far I've gotten away with it!

I think there is a fair amount of psychology going on with this stuff too. If you turn on modern consumer electronics, you have to crank the volume up to disastrous levels to hear any kind of hum at all.

-then we use a 40 year old movie projector and listen to it with that same set of ears.

I bet a 40 year old TV or stereo would seem hummy to us too!

I didn't think my Elmo STs were hummy when I was operating on the internal speaker, but these days every time I operate unplugged from the external setup they sound absolutely awful! (especially at first...)

-just expectations!

Posted by Stuart Reid (Member # 1460) on April 07, 2016, 11:35 AM:

I wonder if there would be a way of getting a balanced XLR out from the projector pre-amp? Probably way beyond my competency.

Posted by David Fouracre (Member # 3883) on April 07, 2016, 12:18 PM:

I may be covering a previous response regarding hum, but here goes anyway....
Eumig and other machines often employ small coils on 8mm formers in close proximity to the replay heads. Very often on twisted wires and `floating`! these are hum balance coils and a very light touch with a non-metalic tool can reduce or remove hum by cancellation of field pick-up from the transformer.
On single track machines, turn the volume mid way on a running machine (without film) and gently move the visible hum-balance coil to best position (lowest hum) and repeat for other track for twin track or stereo units.Only a small rotation or twist is usually required.
On certain machines, these unsuported hum-cancelling coils are just `dangling`on the end of a twisted pair and thus regular movement of the projector over time can displace them from their most effective location.
May help your hum problem if not caused by earth loop!

Posted by Stuart Reid (Member # 1460) on April 07, 2016, 01:18 PM:

Interesting, David. Would you know if the Elmo ST1200 employs this method?

Posted by Andrew Woodcock (Member # 3260) on April 07, 2016, 06:14 PM:

No Stuart, it doesn't unfortunately. Very much a Eumig thing these but David is spot on and Lee Mannering has covered this tweek on here previously

Posted by Paul Adsett (Member # 25) on April 07, 2016, 07:08 PM:

My Elmo GS1200'S have hum cancelling coils right next to the sound head, and they can be positioned for minimal hum.
Not sure if the ST1200 is the same.

Posted by Andrew Woodcock (Member # 3260) on April 07, 2016, 07:32 PM:

I've never seen any Paul on my ST 1200s. Not that I've ever had to do any work of any kind on their amplifiers, I've always found the electronics on these to be highly reliable.

But from having these apart for so many other mechanical reasons, I can't say I've ever seen any of these noise cancelling coils which are very prevalent on the Eumigs.

Posted by Steve Klare (Member # 12) on April 08, 2016, 08:07 AM:

ST-1200 doesn't have hum bucking coils.

Mine does make more hiss than my other machines, but not a lot, and nothing that either the machine's tone control or the treble control on my mixer can't handle.

What I'm finding with mine is the head is pretty forgiving of some films that don't play well on my other machines. I have a couple of films that the level pulsates up and down a couple of times a second on other machines that on the ST-1200 the level is rock steady.

I don't know why this is happening, maybe the stripe is actually zig-zagging side to side a fraction of a millimeter, maybe something else, but the ST-1200HD isn't bothered by whatever it is at all.

(What a pain when you have to match films to machines!)

Posted by Andrew Woodcock (Member # 3260) on April 08, 2016, 08:15 AM:

The heads are great on the Elmo's Steve. Extremely durable and their stripe contact is a little wider also than on other models which means they allow for greater mistrack between recording head and playback head, when different.

The only downside to having a slightly wider head contact, especially on the balance track side of things, is there is a far greater risk of putting lines down the film frame edges from doing so.

This is something that the later Beaulieu Studio models redressed by making their heads even narrower than the originals, however with this, comes the trade off of course that the balance stripe may in some cases, sound low or muffled in it's recording.

When the same film was replayed through a ST 1200HD M/O that I had not so long back, the balance track would then sometimes, play clearer and louder dependent on the original recording head tracking used by the distributors.

UBB.classic^TM 6.3.1.2