Film-Tech Forum ARCHIVE: Using Proj 2 input for a 16mm machine

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Posted by Lindsay Morris (Member # 1430) on 03-28-2017, 07:57 PM:

Just tussling with trying to integrate a restored Australian 16mm machine so that it appears as Proj 2 in my home cinema.
Originally the machine had a valve amp with a valve preamp built into the projector.
That has been replaced with a solar cell strip excited by a 3W LED.
Naturally enough the output is quite low as compared to a 35mm solar cell & when fed into the proj 2 input on 2 types of cinema processors to get any sound the master fader has to be at 10 with the preamp card pot wound flat out. Certainly not useable like that.

I tried a preamp module that when fed with the 16mm signal could deliver around 250mV which was enough to directly drive a power amp OK. Not exactly what I wanted though.
But when hooked to the processors, the input impedance of the solar cell inputs is so low that it drags the preamp module output way down so there is more hum than sound [Mad]

Has anyone tackled this sort of issue in the past??? I am now sort of thinking that an emitter follower type interface might be the better way to get around the vastly differing impedance mismatches between the very low input Z and the 50K ohm output Z of the preamp module.
Have tried a couple of low Z to high Z Mic transformers but they appear to be rather unstable & create high frequency oscillation around 20khz & at a very HIGH level.

One processor is the Panastereo CSP1200 & the other is the Ultra-Stereo JSX-1000A.

Posted by Leo Enticknap (Member # 534) on 03-28-2017, 09:07 PM:

Is there any reason why you can't just put the valve preamp back again?

We have an Eastman 20 with the original valve preamp still in it. The output from that goes to the mic input on an AP20 (set to line level, obviously), and the resulting audio is, to my ears, as good as 16mm optical is ever going to get.

We have several spare valves and at least 10 spare exciters, and given the infrequency with which the machine is used (probably about two hours a month, on average), I'll be six feet under by the time we run out of either. However, if you have a supply problem for exciters and/or valves, I can see that the solid state retrofit would make sense.

Posted by Gordon McLeod (Member # 33) on 03-28-2017, 10:40 PM:

What make of processor are you using?
All solar cells are very low impedance and like to feed virtual ground type inputs
because you are using I assume is a red LED the solor cells output will be very low so it needs to be boosted ( The same way USL did with the Jax light and also on the reverse scan 35mm soundhead a small high gain preamp is located close to the cell to boost it up to the ~-20db that the processors optical preamp

Posted by Lindsay Morris (Member # 1430) on 03-29-2017, 04:20 AM:

Well the valves had to go as much of the wiring was rubber covered that was no longer so....much of which was totally bare.

It seemed easier to move up a notch & give the machine a decent workover & get it to interface with modern gear & make integration with the current 35mm machine & other bits of sound gear much easier. As part of the rebuild I hooked out the old 115V 1000W filament lamp & replaced that with a 200W car headlight xenon & the screen light is vastly improved & coupled with a scaled down maltese cross intermittent makes this machine VERY gentle on 16mm stuff that is now quite old & brittle.

It does work OK on the Mic input but I thought just toggling the 35mm inputs over as needed would be more workable & logical when running the gear.

I thought I was OK once I built up the preamp module & it drove a 100W power amp with its output quite well.

Not so of course as I found out feeding either the Panastereo CSP1200 or the Ultra-Stereo JSX-1000A because as Gordon says the inputs for the solar cells are virtual earth inputs so thus VERY LOW input Z.

Am using a white 3W LED as no point going Red as no 16mm stuff has Cyan tracks so level is a tad higher than if using a Red LED.

I wondered about the Reverse scan setups & how they got on feeding into the normal solar cell inputs & I am guessing that they have a Emitter follower output stage in that built in preamp.
Maybe someone who has had experience with the build of the reverse scan setups can shed some light on what they did to overcome the impedance mis-match.

I need about 20dB gain which the preamp does easily.. Have dug up a simple Emitter follower circuit that I think will suit so one transistor & and handful of components later it just might work OK... a bit of vero board hung on the back of the processor & fed from the 15V +ve rail might work OK. [thumbsup]

Posted by Sam D. Chavez (Member # 1841) on 03-29-2017, 12:46 PM:

It's the IR portion of the light that gets modulated by the film soundtrack. How much IR is produced by your white LED might be the issue. Exciter lamps had plenty of output at 660 Nanometers which is is the band area you want.

Posted by Lindsay Morris (Member # 1430) on 03-30-2017, 08:43 PM:

Thanks Sam. IF I get nowhere with this emitter follower circuit I might pull the exciter assembly out and try a 3W Red LED & see what sort of signal I get with that.
The output with the White LED feeding straight into the solar cell I/P IS well down. So much so that the sound tracks are barely audible with the Master fader Full on and preamps on both types of processors would fully up.

More tinkering to come...what else is there to do once retired eh?? [Big Grin]

Posted by Emiel De Jong (Member # 4225) on 03-31-2017, 04:18 AM:

Using the projector2 input on the CP65 for 16mm is exactly what I do in my home cinema, I am using an exciter lamp with the solar cell though: no problem with the input level here, and excellent sound. It's a Bauer P5 portable, slightly modified and what the previous owner did in solving the problem of getting rare exciter lamps was making an adapter for using a very regular lamp; for a professional environment unacceptable since a quick change is not guaranteed, for home cinema not such a disaster:

Posted by Steve Kraus (Member # 476) on 03-31-2017, 08:18 AM:

Perhaps this might be a solution.

This is the pre-pre-amp board for a basement reader, in this case Kelmar's though I think this might actually have been made or at least designed by Dolby since they were the one giving instructions on raising gain by cutting the jumpers. The actual pickup device is still a solar cell I believe.