Film-Tech Forum ARCHIVE: MPU-1 Could it be used for solar cells?

This is topic MPU-1 Could it be used for solar cells? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Steven Cutting (Member # 5803) on 06-06-2012, 09:42 AM:

I have one of these and wondered if i could boost the signal of the cells to my cp50? Anyone tried it?

Posted by Sam D. Chavez (Member # 1841) on 06-06-2012, 10:29 AM:

The CP50 has a preamp intended to amplify the solar cell so why the need for boosting the signal?

I'm thinking you have 2 35mm projectors using the preamp and you want to amplify a 16mm projector optical?

The MPU can be used but it has way too much gain and is the wrong way to do it. It will work by reducing the level by putting a resistor across the input to the MPU on the solar cell wires.

Around 500 Ohms if I recall.

Posted by Tony Bandiera Jr (Member # 2365) on 06-06-2012, 12:20 PM:

In addition to what Sam said I also ask why?

1: The EQ structure of the mag cards is not right for solar cells;
2: The gain as Sam mentioned is wrong;
3: If you can't get enough gain from the optical preamp card then you have a serious problem at the projectors' soundhead. Check the exciter lamp or LED alignment first.
4: Even for 16mm a CAT 108 has enough gain to work. For 16mm (or a mono cell) jumper the left and right (+) terminals to each other, do the same with the left and right (-) terminals. Then set the gain and HF pots to match.

If for some strange reason you need extra projector inputs it is really easy to get a connector and wire up an outboard CAT 108.

Posted by Louis Bornwasser (Member # 3063) on 06-06-2012, 04:29 PM:

Are you running 16mm?

If so, try a used Ashly BP-41 bass guitar preamp. (The best 16mm preamp ever made.) I sold 11 of them in Los Angeles for Academy screenings years ago. Dolby L.A. installed them in the DGA and other places. Louis

Posted by Steven Cutting (Member # 5803) on 06-07-2012, 05:40 AM:

Thankyou for your input gentleman. I have two 35mm raycophone rear scan projectors from I would say 1950's. They have nice soundheads with a prism that directs the light from a 10v 5amp lamp that I am running at full power. I have made do with a red filter for later type tracks. alignment is spot on and I have the cp50 set to high. I have an Sra5 Sr adaptor. All works well however I'm only just able to reach the dolby level with a test film, but the volume is lower with cyan soundtracks. If i crank the amps up it sounds poor. Looking for a cheaper alternative to red led readers. Thanks Regards Steve

Posted by Robert Minichino (Member # 3495) on 06-07-2012, 07:24 AM:

What are you using for a red filter? Do you know what its spectral transmission curve looks like? Incandescent exciters put out quite a lot of IR, and solar cells are quite sensitive to that, so if your filter isn't blocking the IR your signal/noise ratio on cyan isn't going to be very good (silver-applicated tracks block the IR).

Louis's suggestion of the bass guitar preamp is a good one. I was going to suggest a mic-type preamp, but Louis's suggestion is even better.

Posted by Louis Bornwasser (Member # 3063) on 06-07-2012, 09:17 AM:

My thought is for 16mm MONO. A red herring!

Louis

Posted by Sam D. Chavez (Member # 1841) on 06-07-2012, 12:15 PM:

Steve says.

"If i crank the amps up it sounds poor. Looking for a cheaper alternative to red led readers".

It will sound a lot worse if you use an MPU for a preamp booster, and then what? You hook it up to the solar cell input of the CP50?

The MPU preamp is mighty hairball. There are many better ways to do this; the easiest is the Jaxlight which is a Red LED version of an exciter bulb. It can have its own set of issues
but at least the red light produced is at the correct wavelength the cyan track wants to see.

There are other threads here over the years describing some homemade RED LEDs replacing the exciter. Check them out.