Film-Tech Forum ARCHIVE: Sound on a hobby implementation...

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Posted by Craig Huegen (Member # 7666) on 06-06-2013, 06:56 PM:

Hi all,

Many years ago I used to be the pimply-faced kid projectionist at a local drive-in, first learning carbon-arc and then to a Xenon/platter conversion. At the time, I never learned all the details about the technical stuff behind it, I just ran the show.

Fast forward 20 years, after the (different) local theater's conversion to digital projection, I was given the projection equipment; however, no processors or amps were left after the theater changed hands. It's a Simplex X-L projector with SH-1000 sound head. Exciter bulb's been replaced with a JaxLight, and the preamp is stuffed in the back of the sound head - mono cell only with original slit-lens.

Where I need help? Mono sound. I'm trying to connect that path to a consumer amp that has only unbalanced, line level inputs. I also have a vacuum-tube mic pre-amp to get the signal to a line level.

I have read that a 680-ohm resistor in parallel with the photocell will allow for it to be used without a processor, connected to a mic input / pre-amp.

So, my questions:

* Does the JaxLight change that approach at all?
* Should I still use the JaxLIght "line amp", and if so, where should the resistor go -- still on the photocell input, or the output, or shouldn't need it? Any adjustment to the value of the resistor?
* If the JaxLight line amp is used, which terminals should be used for attaching to an unbalanced amp? E/L+ or L-/L+? Do I need to do impedance matching between cold and shield, or just tie them together?
* How should the whole mess be grounded/shielded?
* Any other tips or tricks?

I briefly tried a 470 ohm resistor on the non-amped photocell and couldn't really hear a thing at full volume, which makes me think I'll need that amp in there.

I know this set of questions is out of the ordinary, but I have the film path working (including the platters), I have a replacement for the 3-phase Christie Xenon supply working reasonably, and now it's just a matter of getting the sound to work.

Posted by Ed Inman (Member # 2583) on 06-08-2013, 03:00 PM:

Some general thoughts: The Jaxlight will dramatically reduce the volume from your photo cell compared to a traditional exciter lamp--and the resistor reduces it even more. So, unless you are running a print with a cyan track I think you will be more satisfied with the results from a traditional lamp. A Jaxlight and regular exciter lamp are usually easy & quick to change out in the same soundhead. If you have a cyan track and absolutely must use a Jaxlight you might try lowering the resistance or eliminating the resistor entirely and manually adjusting the frequency response with an equalizer. The results may be so-so but still better than running cyan with white light. I'm not sure about the Jaxlight preamps--I think they were made to be used only with a dedicated processor, but there may be some way of using them with a conventional amp. I don't believe the photo cell itself should require grounding.

Posted by Craig Huegen (Member # 7666) on 06-08-2013, 10:16 PM:

Thank you, Ed.

I have another bulb bracket for this head, so I can keep the Jaxlight in one, and a white bulb in another. I have no exciter bulbs in my "spare parts" box, though, so will have to put a couple on order.

I did manage to get reasonable sound out of the projector by using an 820 ohm resistor, eliminating the jaxlight preamp, and cranking up my mic tube pre-amp up to full gain input/output as well as using the +20 dB gain switch. The power amp had to be set nearly all the way up, but you could at least hear the soundtrack reasonably well.

I confess I managed to let the magic smoke out of a couple of speakers when testing using the jaxlight's line amp in the mix, I think the signal was a bit too hot -- so I think I'll keep that off to the side for now. [Smile]

I'll still accept any more tips/tricks if anyone is familiar with interfacing a Jaxlight system to a standard amp.

Posted by Monte L Fullmer (Member # 2797) on 06-08-2013, 11:59 PM:

Thing with using a conventional exciter lamp on cyan tracks is that the sound results will have a much higher signal to noise due to the much lighter track than the old silver track of the technicolor days, or the magenta track of the acetate film usage. Plus, high frequencies will suffer using exciter lamp on cyan tracks - why the reverse LED track setup.

Sadly, the jaxlight concept was a cheap band-aid fix for users to use LED just to replace the existing exciter bulb, but they forgot that using the slit lens takes tonnage of light to illuminate the photocell or SS pickup behind the slit lens to get the proper signal to noise level.

Another problem with slit lens assemblies, esp on old sound systems, is that these lens assemblies do fill with oil from leaky projectors above also reducing their effiency and needs to be replaced anyway.

With usage of the Jaxlight, one has to crank up the preamp level just to get the bare minimum of level required for decent playback, and in do doing so, the increase of noise goes along with it.

I bet that one can do a search on used BACP LED reader assemblies coming on line from theatres that have coverted to digital and are willing to sell off these units for an affordable price and this will be a major improvement over the JAXLIGHT noise syndrome that they are well known for.

Posted by Jeffry L. Johnson (Member # 453) on 06-09-2013, 11:42 AM:

quote: Monte L Fullmer
much higher signal to noise

Just the opposite. Higher noise level and lower signal level equals lower signal to noise ratio.

Higher signal to noise ratio GOOD.

Lower signal to noise ratio BAD.

Posted by Monte L Fullmer (Member # 2797) on 06-09-2013, 05:55 PM:

Thx for the help .. I was wondering if I had those terms crossed or not.

Posted by Craig Huegen (Member # 7666) on 06-09-2013, 10:18 PM:

I knew where you were going with S:N... no worries.

I think "affordable" has to be qualified for a hobby implementation. [Smile]

For this one, I think "affordable" means "extra cheap". I'll be on the lookout, and in the meantime I'll play with a few different values and see what I can make work the best - and I'll also give a standard exciter bulb a go.

My slit-lens assembly still seems to be pretty clean and while the projector isn't fully oil-free, it seems to be holding it well. Perhaps a bit of experimentation with today's super-bright LED's might be in order.

Posted by Lindsay Morris (Member # 1430) on 06-09-2013, 11:21 PM:

I and many others have had quite good results from using the 3W Red LED's currently available.
The main trick is to get them "spot on" the optical axis of the slit lens.
On a pair of VERY old Westrex heads (206B) I utilised the original widely adjustable lamp holder to take a solid piece of brass rod the same diam as the exciter lamp base with a flat machined across the top of it to take a 3W LED so it faced the slit assembly.
The length being such as the C/L of Led was the same as the C/L of the previous lamp.
Then removed the centre pin wiring for the lamp.
Powered Led from the exciter supply via a AC/DC supply available in Australia which gave exactly the 0.8A DC power to run the Led.
A bit of careful alignment using the original lamp adjustments soon had a nice bright red slit showing on a piece of paper taped to the area where the film runs thru. The 206B heads are straight down "pull thru" heads with a flat sound gate assembly but the same can be done on a R3 rotary type drum with the paper taped around the drum and tweak the lamp adjustments to give the brightest red slit.
Finish off by running a sound test loop to polish off your adjustments and you are done.
Have used the same methods on RCA 1040 and Westrex R3 heads as well as on a Ballantyne to get close to correct and finish with a sound loop.
The solid bit of brass makes the perfect heatsink for the Led but just be aware that "some" Led's need an insulator between the case of the Led and the brass as the case is sometimes connected to one side of the Led itself...a check with a meter will show that.
The 3W Leds last for ages but if you want to be able to adjust for ageing an adjustable DC supply helps but the max current is 1A. Much more and poof the led is gone.
You can get enough illumination from a 3W Led to drive the solar cells quite well and in fact I did one setup that still used valve amps and had the original photo electric cells in the heads. The 3W Leds were able to adequately drive the photo cells to give good sound once the DC cell voltage was taken up to around 80V from the original RCA setting of 60V. The gear then all stayed "original" as the owner wanted.

Posted by Craig Huegen (Member # 7666) on 06-10-2013, 12:26 AM:

Thank you for the experience.

I think I'll be trying to scrounge up a good-old LM317 to give a 3W LED room to run.

Posted by Lindsay Morris (Member # 1430) on 06-10-2013, 12:48 AM:

Just make sure that you use a TO3 LM 317 as the TO92 types even though rated at 1.5A get VERY hut running anywhere near 1A whereas the TO3 (LM317K??) run cool as when mounted on a scrap of ali as a heatsink.

Posted by Gordon McLeod (Member # 33) on 06-10-2013, 08:42 AM:

Just to chime in
I would connect the Jax light preamp as they show it in the diagram and yes it usually works
The output connect to a BALANCED mic input this is important as there will be far more hum if it is not balanced
I would get a little shure mono mic mixer if it is a mono install and connect both outputs to seperate inputs on the mixer
I also assume that the solar cell in the projector is a stereo one
Alignemnt of the jax light to the slit is important and once that is done the output is usually acceptable

Posted by Craig Huegen (Member # 7666) on 06-10-2013, 09:40 AM:

Hi Gordon,

It's a mono one, so only the left channel had been connected. I changed my setup so that I was feeding the mic tube pre-amp with a balanced signal from the JAXlight pre-amp, then feeding the power amp with an unbalanced connection that bonded cold with shield. I could not get rid of the hum in this scenario, however, whether I connected shield or not to the JAXlight's E terminal, and couldn't get a signal that wasn't horribly distorted.

(I think the alignment is ok because if I turn up the pre-amp and power-amp to near-maximums, I hear the mono sound clearly.)

Using this method I managed to burn up two different mid-range speakers - perhaps from the hum, perhaps from it being driven just a bit too hard. *shrugs*

My power amp is unbalanced-only, although my mic pre-amp has both balanced and unbalanced outputs. Would you hook these to separate inputs (e.g., AUX and TAPE, or are you saying to use both outputs to drive L and R on the same input (AUX)?

Posted by Dave Macaulay (Member # 813) on 06-10-2013, 10:42 AM:

it's important to have a "balanced" input as Gord has said.
You should use the Jaxlight preamp, the direct cell signal is miniscule plus you run into the load resistor requirement.
Get a "ground loop isolator": Radio Shack has these, it's a stereo set of isolation transformers with RCA jacks. From the Jaxlight preamp, connect a shielded pair cable with the L and R outputs joined: just connect the "-" together for one wire and the "+" together for the other. Connect that to one side of one channel of the isolator and plug the other end into your amp. It will be very low level and a mic input is best, maybe there's enough signal for an AUX input but I don't know. Connect the shielded pair shield at the preamp end but not at the isolator end.
The Jaxlight reader was notoriously noisy, try connecting the ground on the preamp to the soundhead chassis to reduce hum (or try disconnecting it if already grounded).

Posted by Ed Inman (Member # 2583) on 06-15-2013, 05:59 PM:

This is probably a crazy idea, but for my home setup it seems to improve the sound of cyan tracks somewhat using a regular exciter lamp: http://www.flickr.com/photos/9645031@N07/9054086436/ The glass filter is a number 29 (deep red) which darkens the cyan. Obviously this is not suitable for a commercial theater--the cyan tracks are still a bit noisy. But overall frequency response seems better than running straight white light to me. (I might mention that I don't have any features with cyan tracks anyway--just a few trailers.) So, do y'all think I'm really improving anything with this ridiculous setup, or is it just my imagination?

Posted by Gordon McLeod (Member # 33) on 06-16-2013, 04:15 PM:

kinotone i believe had a reader that used a slide projector lamp or similar and a red filter i dont know if it was ever brought to market

Posted by Ed Inman (Member # 2583) on 10-18-2013, 06:42 PM:

Just to revisit this thread for a second: My signal-to-noise issue when connecting the Jaxlight through the line amp to an XLR balanced mic input is not so much hum as I would say hiss (when turned up loud enough for the signal to be audible). The hiss is coming from the line connected to the solar cell in the soundhead (i.e. if you unplug the solar cell from the line amp the hiss mostly goes away). It's not terrible, but still annoying--not as good as using a regular exciter lamp with a good silver track. Any suggestions on how to reduce the hiss? Would running the signal through a graphic equalizer and cutting off the high frequencies help?

Posted by Louis Bornwasser (Member # 3063) on 10-18-2013, 08:16 PM:

Normally not a problem since a processor such as a dolby removes the unproductive hiss as a part of the noise reduction process. The Reverse scan LED readers avoid this problem at the source.

It is possible to raise the signal with an absolutely perfect optical alignment which may well help a little or a lot. Jaxlight was not our finest hour. louis

Posted by Ed Inman (Member # 2583) on 10-18-2013, 11:08 PM:

Do you have a recommendation for a suitable Dolby processor I can look for on ebay or through theater supply dealer to do the job? I actually kind of hate cyan tracks, but once in awhile I run across even silver tracks that are noisy with white light and sound better with a red LED for one reason or another.

Posted by Ed Inman (Member # 2583) on 10-20-2013, 06:34 PM:

Found a Dolby CP-55 processor for about a hundred bucks--might as well give it a try. The price seems cheap enough.