Film-Tech Forum ARCHIVE: Opt. Pre-amp with HF Eq.

This is topic Opt. Pre-amp with HF Eq. in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=006032

Posted by Ravindra Desai (Member # 2337) on 04-06-2004, 05:48 AM:

I have sucessfully designed and tested my own stereo opt pre-amp for Solar cells as well as rev.scan. It also has a power supply for reverse scan with variable current limit.
I would like to know what excatly is HF eq. in the branded pre-amps? Is it just a treble boost ckt or is it much more? Can I have access to any schematic of the same?

Ravindra.

Posted by Marin Zorica (Member # 1716) on 04-06-2004, 09:09 AM:

I'm glad to hear that someone working what I'm working too, i don't know for eq, but i think it's linear. Can you put schematic here or email it somehow I'm wonder how it work, also what quality is sound like?

Posted by Sam D. Chavez (Member # 1841) on 04-06-2004, 10:07 AM:

The slit loss compensation is a Sin x/x function, and not linear.

The reverse scan readers from BACP, Component, Kelmar and others have a solar cell slit height narrow enough to not need compensation as they are flat when in focus to 14 kHZ.

Posted by Marin Zorica (Member # 1716) on 04-06-2004, 01:34 PM:

Can somebody describe that function more detail, and what is a frequency response for analog sound, are they any differences in response between revers scan readers, analog readers with led, and exciter lamp??

Posted by Sam D. Chavez (Member # 1841) on 04-06-2004, 05:55 PM:

The frequency response of an optical soundtrack is totally dependent on the slit height of the lens in a front scan system and the effective slit height of the reverse scan solar cell.

The roll off begins when the wavelenghth of the soundtrack begins to approach the wavelength of the slit.

For front scan systems it begins at about 2KHZ. With proper slit loss EQ they can be brought to flat up to 10kHZ. Reverse scan readers start to roll off somewhat higher and are good to 14kHZ.

(You might ask why the slits are not made smaller to be good to 20kHZ. Well, the smaller you make the slit, the less light gets on the solar cell and the higher the signal/noise ratio).

The roll off function is the same for either method. The CP55 and 65 manuals have a section describing this function along with a drawing of the shape of the roll off.

Posted by Luciano Brigite (Member # 1104) on 04-06-2004, 08:05 PM:

there are some pre-amp circuits that does not have the slit loss eq at all, a few have something that seems to be a fixed eq for it and then others that will have a variable eq. some of the manuals available here on FT shows these 3 examples I mentioned and I think one should take a look on them if is really interested in it.
I made a few pre amps in the past,when all I had was a pro-logic processor (home theatre processor), the first being without any loss eq and hte second with a rahter crappy eq stage, since it was for private use,it served me well.Later when I started servicing booths here or there ,I found out that many cheapo theatres adopted the same arrangement I did and from the 7 theatres I visited that used it, only one had a preamp board that had the loss eq, all the others had a pair of dual op-amps,one for the cell and other to raise the signal of the first to the line level required by the "processor" .
As said here before, the slit has a great influence in the sound as I had the time to perform a few tests with an old slit sens I had by altering the slit's aperture ( I do not recomend it to be done as certainly one will ruin the slit lens assembly if doesn't know what's being done. as I said, I performed tests for my own knowledge using an *old* lens)small slits will indeed get noisier because the low light output from it and the high gain that'll be required to set the preamp ( with that even using low noise IC's,there will be that "hiss" in the signal)

Posted by Ravindra Desai (Member # 2337) on 04-07-2004, 09:29 AM:

Kindly take a look at DDTS-100 decoder on creative.com
It HAS everything needede. Even split surround (Pro Logig II) for analog i/p.
Give the o/p of an optical pre-amp to it, and you are done. It is a total DIY project but you do take TOTAL control of the cabin sound don't you?

Ravindra.

Posted by Marin Zorica (Member # 1716) on 04-07-2004, 02:09 PM:

Yes, DDTS-100 decoder is really nice stuff for this kind of aplication, but we forgetting Noise Reduction System.

Ok, what about we on output of pre amp connect only NRM Cat. No. 22 (of course two of them for stereo), would be sound be better??.

and does NRM like CAT. No. 22 had any influence to eq in sound??

Posted by Ravindra Desai (Member # 2337) on 04-07-2004, 07:54 PM:

Go ahead right away and do not worry about NR or SR.
About the pre-amp, I will upload my design with HF Eq. soon.

Ravindra.

Posted by Steve Kraus (Member # 476) on 04-07-2004, 10:00 PM:

Why not copy an existing design?

Posted by Marin Zorica (Member # 1716) on 04-08-2004, 03:17 AM:

Ok Ravindra, i hope you will upload that design soon.

Posted by Ravindra Desai (Member # 2337) on 04-08-2004, 11:33 AM:

My pre- amp design is very simple, uses low noise components and since it is my design, it gives me that much additional confidence.
For example, change the Rev. Scan Pre amp IC TLO72 to NE5532 (they are pin compatible) and notice the enhancement in HF and reduction in noise (HISS) So you see, we people can better the existing designs.

As an engineer, copying is the lsat option for me.

Ravindra.

Posted by Gordon McLeod (Member # 33) on 04-08-2004, 12:02 PM:

Reverse scan preamps usually use NE5532 op amps in many designs
There is very little new and different out there but the proper preamp should be a current source design