Using graphic equaliser to suppress hum

[Martin Davey]

Dear All,
I wanted to ask about the use of graphic equalisers, in particular to removing ground hum from projector audio, while using an external amp. I do of course realise that by doing so I'm removing all audio, not just the hum at a particular point!
Here in the UK we have 50hz power so my questions relate to that. Audio wise, is the (hum) sound dead on the nose at 50hz, or is it covering a spectrum more like 40 - 60hz? Would an equaliser with a slider at 50 hz remove the audio/ hum cleanly at that point? Only a few equalisers I've seen cover 50hz, so would this mean that any thing else (for instance sliders at 41Hz, 100Hz as examples) would fail to hit the target correctly? Do equalisers only lower/ enhance whats in each slider spectrum rather than remove it?
I've tried those cheap ground loop isolators with no acceptable results, and have the projector and amp in the same power socket. I just feel this must be the only way forward now. I'm interested to hear how others have got on please.
I would say my B & H TQ3s have the most pleasing, almost hum free sound, whereas at the other end I would say that the Eumigs are the worst offender.

[Paul Adsett]

Hi Martin.
I use a Yamaha EQ-70 10 band stereo equalizer for all my super 8 projections. It does a good job of attenuating low frequency hum from the projector. Mine has sliders at the following frequencies on left and right channels, 30,60,120,240,480,1k,2k,4k,8k,16k. Each slider bar can amplify or attenuate the signal at that frequency by +/- 12db, which means if you set all the sliders at max +12 db levels and slide just the 60hz slider all the way down to the -12db levels you have essentially attenuated the relative hum level at 60hz by 24db , which is essentially complete removal as far as hearing goes. When you attenuate the signal at 60hz then this also progressively attenuates the signal from the adjacent bands of 30Hz to 60Hz and 60hz to 120HZ , in other words you have a 'valley' between 30Hz and 120Hz with the bottom of the valley at 60Hz, so you do not get a notch attenuation at a single frequency but a smooth curve response between the levels of each slider.
Some projectors produce most of their hum, not at the mains frequency (60Hz in the US) but at the second harmonic (120Hz in the US) so you have to play around a bit with the slider levels to get the most pleasing response.
I also eliminate high frequency hiss by attenuating everything above 16k.
Yes the Eumigs are notable for their relatively high hum levels, but this is just the 800 series as far as I know. I think that Shane has reported that the earlier 700 series , which have valve amplifiers, have low hum levels, and I have a couple of 900 series Eumigs and the hum is no worse than my Elmo GS1200''s.

[Steve Klare]

A ground loop eliminator can't do a thing if the hum starts out within the projector itself. Ground loop hum would appear on both signal and return as common mode noise and it's easy to get rid of it by subtracting the return voltage from the signal voltage. When the hum is on one line but not the other, it squirts right through ground-loop rejection.

Many years ago I decided to design a filter to get rid of both the ground loop and what I'll call "projector hum" here. I thought that it was kind of silly to try to reject something without knowing what it is first, and all of a sudden this opportunity presented itself. I was using an oscilloscope at my new job a lot, and my boss told me that I should keep it in my office to keep it from being...shared. (it was like a ten acre building: it was very easy for things to get lost! Even without real theft something might turn up months later!)

So, during lunch one day....
.

This is right across the internal speaker on at Elmo ST-800. There is 60 hz, 120 Hz. and 180 Hz in there and the ratios of 10:4:6. (With 50 Hz., this should translate to 50, 100 and 150, of course.)

I thought maybe I'd build up a cascade of three notch filters: 60,120,180 Hz. This sounded like a lot of complexity, so I thought I'd just build up 60 Hz. and see where that took me. (I also worried that 120 and 180 are well up in the musical range and the three together might really clobber a music track!)

(I never buy the case these things are mounted in early on: sometimes a circuit needs to grow after you try it out!)

I learned about this weird thing that happens to audio that is a fundamental frequency with harmonics. After I knocked out the Fundamental the harmonics that remained behind combined to produce a signal that has the same frequency as the Fundamental I'd worked so hard so wipe out! The good news is that missing 60 Hz. represented about 2/3 of the power in the hum and killing it alone really took the edge off the projector hum. With an Elmo ST, the hum is low enough that the volume control needs to be at levels that would definitely cause complaints from family members when playing real sound tracks before it's audible at all.

The 800 series Eumigs really are hummier than average. I found running my Eumig through this results in hum still present, but pretty tame.

I like the approach of using an equalizer: my little interface box is one size fits all where the spectrum of the sound track is concerned. It's there to get rid of the nastiest of the nasties and does a pretty decent job of that. What the equalizer gives is the flexibility to optimize the frequency response to optimize the signal and get the nicest results in the speaker. I may go that way myself some day.

(Maybe I just felt like building something!)

[Brian Fretwell]

Yes harmonics of mains frequencies cause problems too. As well as the 800 series the Eumig 710d had hum problems, it isn't much different from an 800 series machine.

[Tim Gillett]

The hum problem with these projectors is basically due to their compactness, meaning hum producing components like a hefty power transformer which powers a 100 watt lamp plus the motor, are situated close to a sensitive magnetic head. Internal hum shields can only do so much.

My first sound projector, a Eumig 710, was even worse, using a hefty unshielded AC motor. Eumig had to design in no less than two internal "hum bucking coils" to try and address the hum problem but it was only partially successful.
I recorded sound tracks onto the mag stripe using the 710 but it wasnt until decades later when I'd completed my modified Revox tape machine to replay my old striped sound films, that I discovered much hum had been RECORDED onto the magnetic stripes. The Eumig had had so much hum in playback that it had masked the hum I'd accidentally recorded. Goodness, that was a noisy projector.

Later I acquired a Chinon SP330 projector which had less hum than the Eumig 710 but still not great. Once in desperation I completely removed the large power transformer from the projector, and added extension leads so the transformer was now far enough away from the projector not to induce the hum. Messy and inconvenient but effective.

A graphic EQ is only useful if its fader frequencies happen to coincide with the hum frequencies and harmonics, and as said, they also remove those same bands in the wanted audio. Better is a multiband "parametric" equaliser whose frequencies can be manually tuned to the hum fundamental and harmonics.

These days the projector's audio could be diverted through a software Dehum tool. In terms of hum reduction after the fact, these are probably as good as it gets, as they are infinitely tunable with very high Q filters so only the spot hum frequencies are targeted. The downside will be a slight delay in the sound re the picture but these days with modern computers this delay can be pretty minimal.

But really for best results, hum needs to addressed at source rather than trying to filter it out after the fact.

[Steve Klare]

I've always felt it is at least partially modern expectations. Back when these machines were built, everything with a power cord, an amplifier and speaker had some audible hum. Now here we go dragging these old machines forward in time to the point where our other devices seem hum-less and notice the difference. (-with 40+ year old filter capacitors, no less!)

It's like driving a mid-1960s car and saying "These brakes are terrible!". -back when the brakes were all equally terrible nobody thought they were!

[Tim Gillett]

Steve, maybe a better comparison is the cassette decks of the day which I'm pretty sure didnt produce nearly the hum of the average 8mm sound projector in playback. I'm not blaming the projector manufacturers. To reduce hum to the level of a good cassette deck of the day was probably a difficult design exercise and achieving it may have put the projectors out of the reach of most people and reduced projector portability. Obviously tired capacitors can greatly increase hum but that applies to all audio gear, then or now.

[John Taylor]

All I use is a decent make analogue hifi stereo amplifier. A single twist know control for bass, treble , balance and volume plus a loundness button if needed which it never is and two decent speakers and that does the job perfect for my 16 and 8mm films, it knocks spots off most other showings ive been to. No hum no hiss.
much of the background noise will be dependant on the projector, eumigs in particular are known for annoying louder hum through the speakers, others have a very slight hiss but only with the volume up full. If the films soundtrack is that poor that the volume has to be up more than half way, i'd consider that film to be either faulty or just poor quality.
All these number of equaliser outputs and everything else is just over the top, your ears are all you need to know how good or bad the sound is.

[Steve Klare]

Tim,

You're right: they probably could have produced a low hum projector, but it would have cost dearly! The operating requirements of a portable movie projector presented some real problems where hum was concerned.

A cassette deck has a couple of major advantages over a sound movie projector where hum is concerned: the big one is the amount of power involved. For example the movie projector has a projection lamp which can easily add 100W to the equation. Then again you look at the transport mechanism: that big gear train and all those belts and bearings: that has to add some Watts. The average home cassette deck has a pre-amp and leaves the heavy lifting to an external amp, but any portable movie projector I've ever seen has a power amp too.

For examples here, the ratings plate on my pretty middle-of-the-road Elmo ST-800 says 200W, but the plate on my Technics dual cassette deck says 19W. This difference means a much smaller power supply with things like a smaller transformer with less stray fields to find the soundhead and easier shielding. Less power means it's also easier to avoid problems like internal ground loops.

The joy too of the external power Amp is that big transformer it needs can easily be many feet away in another chassis. In an 8mm projector the power supply and the sound head are pretty close neighbors living in the same building. It's interesting too that the closest transformer and soundhead pair I know of live inside the Eumig 800 series.

I wonder too if the mechanical noise of a movie projector didn't give the designers of these machines a certain mount of cover for allowing the machines to have noisier audio: to save a little cost. A perfectly quiescent speaker doesn't mean nearly as much once you run the transport.

One of the things that got me going cleaning up my own audio was I had a back porch through sliding doors right in back of where my machines stood. I believed I'd been granted a free projection booth! I backed a machine up 10 feet and ran an external speaker up to the screen. The machines became quieter, and the sound became more local...and without the projector sound it sounded awful! That and having to walk 30 feet out and then back every time I wanted to tweak the focus or the volume made this a short lived experiment.

[Paul Adsett]

Hi Steve, a few thoughts of mine on your post! It seems that the greatest current in movie projectors is in the lamp circuit, typically 8 to 10 amps of AC and its associated EM field. I recently started running my GS from an external power supply to enable use of the 250w ELC lamp instead of the 200w ESC or 200w ELJ. The power supply puts out 24V DC, so no alternating electromagnetic fields are now generated in the lamp circuit. The transformer in the projector is still generating 24v AC of course but the lamp is no longer connected to it, so there is no current and it is no longer generating any EMI. So there should be no additional hum when the lamp is powered up. And there is'nt. So maybe an external DC lamp power supply would be beneficial for the Eumig"s. Is HID a DC supply voltage? If so have Eumig HID owners benefitted from a reduction in hum?

[Steve Klare]

Hi Paul,

In my imagination of what the 2023 Super-8 sound machine would look like, the thingy where the line cord plugs in is a switching power supply and all the voltages and currents after that are DC! Yeah, there is a little ripple and noise running around in there, but a lot less opportunities for mischief than the regular-old AC we have now.

(Let's not even mention the USB ports for automated changeovers and second-system digital sound!)

[Tim Gillett]

I suspect that the standard transformer radiates a large EM field regardless of the load, simply because of its capacity. The same for the (as I recall) shaded pole AC motor in the Eumigs. Both would difficult to encase in a shielded box, and of course overheating can then become an issue. Yes a SMPS would potentially work for the lamp but wouldnt solve the problem of the magnetically noisy AC motor. A DC armature type motor with a permanent magnet is easier to encase in a mumetal shield. I suspect most later sound and silent 8mm projectors used them.

[Carl Ingram]

I thought i might learn something here, but just gobble de gook, i wonder if martin has regretted asking ..........

[John Taylor]

I'm with you on that Carl 🤣🤣
Its great having lots of hot looking gizmo's plugged into your projector, it certainly impresses some those who visit your home for a film show, but all you need to get great sound is, Number one, a quality projector that doesn't throw out a loud hum as standard, and two, the best enhancement, a good quality stereo hifi amp. In an average home that is all you need and it will blow the roof off with a high quality sound.
A graphic equaliser will of course give you a lot more adjustment within the sound to tweak things further but for the average collector it would be over the top. It's all very nice having pictures of oscilloscopes and throwing out numbers of Hz's but to most collectors this is so unnecessary, however, for many I understand this is another aspect tot he hobby.
Carl, my advise, use a good projector and extension speaker, if you really want to enhance with twin mono or stereo, (if you have a stereo machine), add an analogue stereo amp and two decent speakers.

Martin, what super 8 machine are you using, or are you only 16mm? I think if the hum is that loud that you are actively looking to get it reduced then something may be wrong.

[Steve Klare]

That 'scope trace may actually be more interesting than it was ever supposed to be.

It was maybe the beginning of October when I sneaked a projector into my office and got that screen shot. In January I started getting a buzz through the speaker and I figured out the motor brushes needed changing.

Later on I saw that picture again and I wondered if all that hash around the trace was the worn brushes starting to arc.