Film-Tech Forum ARCHIVE: Optical sound

This is topic Optical sound in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Matthew Taylor (Member # 2310) on 04-02-2005, 08:49 AM:

I have connected my stereo cell into a stereo mic preamp with a 600 Ohm resistor between the ground and signal on each channel at the cell end.

In searching through the archives I read about the need to use a capacitor on each channel to remove the DC element from the cell signal. Is this needed for the health of the preamp or does it make a difference to the quality of the sound?

Posted by Gordon McLeod (Member # 33) on 04-02-2005, 09:43 AM:

If the preamp is transformer coupled input a cap isn't needed since the transformer will block any DC component
But most direct coupled the input opamp may have an issue with DC

Posted by Matthew Taylor (Member # 2310) on 04-02-2005, 10:02 AM:

What difference would it make to the sound if there was an issue?

Posted by John Anastasio (Member # 607) on 04-02-2005, 10:48 AM:

Adding enough DC bias directly to an input could drive the signal into a non-linear portion of the device's amplification curve, resulting in some distortion.

Posted by Matthew Taylor (Member # 2310) on 04-02-2005, 10:57 AM:

yep, that's what I got. Thanks for that.

Posted by Matthew Taylor (Member # 2310) on 04-04-2005, 03:43 PM:

Thank you all for your advice - I find this forum so useful.

I put the capacitors in series with the two solar cell outputs and it has removed the distortion I used to get with the more well modulated parts of the soundtrack.

A thought occured to me. I read that it's best to use a 1:1 600 Ohm transformer for inputting the signal into a pre-amp. Is it possible to use a non-equal ratio transformer, eg. 1:50 or something, and negate the need for a preamp altogether?

Posted by John Anastasio (Member # 607) on 04-04-2005, 07:20 PM:

Sounds on the face of it like it would work, doesn't it? Sorry...a transformer isn't an amplifier. You can't get something for nothing. While the voltage will indeed be stepped up, it will be at the expense of the current. That's why they call it Ohm's law, not Ohm's suggestion. Transistors are current amplifiers, not voltage amplifiers. It's why we have to put transformers at the outputs of tube amps...to turn all that voltage change back into current change to run speakers.

Posted by Clive Carmock (Member # 2882) on 04-04-2005, 08:02 PM:

I am about to change the mono cell in my 35mm projector to a stereo cell, which came from a Westrex 7000.

The current mono cell has a single core with screen cable that connects to a 3 pin XLR plug, however the new cell has three wires, none of which are screened. Is it likely this will pick up mains hum in the machine?

I plan to connect this to the same XLR plug on the chassis. I asusm eit is OK to connect L & R to two pins and use a common ground as per the output fromt he new cell?

I would use two core + screen cable to connect to the pre-amp.

Any advice much appreciated.

Clive

Posted by Sam D. Chavez (Member # 1841) on 04-05-2005, 12:06 AM:

Solar cells need to be loaded down to something like a 50 Ohm load to get true flat frequency response. A solar cell fed into a high resistance or impedance input like the 10K Ohms load found typically found on a power amp input will cause the low frequency response to roll off substantially.

Posted by Gilbert Travin (Member # 2138) on 04-05-2005, 01:19 AM:

quote:
(Matthew)I put the capacitors in series with the two solar cell outputs and it has removed the distortion I used to get with the more well modulated parts of the soundtrack.

A thought occured to me. I read that it's best to use a 1:1 600 Ohm transformer for inputting the signal into a pre-amp. Is it possible to use a non-equal ratio transformer, eg. 1:50 or something, and negate the need for a preamp altogether?

OK Matthew !

If you put polarized capacitors (such as 1 uf, 16 V), be carefull to put + to cell (assuming preamp input is 0V) !
If you use transformers, choose shielded models, if not you will get 50 Hz inductions, thus hum !!!
[Smile]