Need help with old mic amp

[simon7000]

What the noise reduction does is not have a projected light "Bias." In the even older version zero signal (I.E. half of the final sound track area at 50% modulation) was Clear final film. That would create noise on the projection playback from any dirt or scratches on the film. By using Black as the no signal the playback would have less noise.

[toptip]

Originally Posted by simon7000

What the noise reduction does is not have a projected light "Bias." In the even older version zero signal (I.E. half of the final sound track area at 50% modulation) was Clear final film. That would create noise on the projection playback from any dirt or scratches on the film. By using Black as the no signal the playback would have less noise.

So I thought, from everything I read, that Auricon used "variable density," not variable area, looks like I was wrong. Now I understand it even less: do you mean, rather than 50-50, with no signal, the entire track is black? Why would the system not be just designed that way, rather than this "NR" system? A photocell will not read a -tive values anyways, so an all black track opening with increasing volume? At the loudest bits, with the track wide clear, any scratches would probably be masked.

There is a wonderful Encyclopedia Brittanica film on Youtube about this. There were three systems used; var density, one sided var area and symmetrical var area. The same projector played back all three.

[toptip]

Originally Posted by costis_n

i can't believe it! There was one such 16mm camera in the closet in my film school. This camera could record audio right on the negative!! Pretty unique feature, normal workflow is to have a separate tape recorder. I wonder if this contraption was for that camera or a standalone one.

I think this was the complete Auricon sound set-up. The device on the lower right hand transfers sound to film. My amp is the one right above that.

[simon7000]

Any system that modulates the amount of light reaching the projectors photocell will have the same results. The better the system the less noise... of course. With a variable density system the noise level is higher than a variable area system that uses all black as zero signal.

The unit you have is intended to drive a voice coil actuator with a permanent magnet for variable area recording. The lamp intensity is controlled by a rheostat fed by 12 volts.

With no signal it is all black, as the signal increases this system moves the 0 value to enough light to allow just full scale excursion, no more. So if the peak level of the signal is only 10% of full scale then it will only modulate from black to 10% open area. As the level rises to 100% modulation, it will eventually use all of the area.

Open or clear area is noisier due to dirt, scratches and the nature of the photocells used. Without any light they were quieter than when exposed to light. So these guys came up with a system of noise reduction that varies the area in use to match the peak signal level.

[trobbins]

The mic transformer can be easily measured for step-up ratio, and nominal bandwidth response using matched resistive loading. If it is in a mu-metal screened can then it may have good use for phono.