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Old 14th October 2011, 04:48 PM   #1
toptip is offline toptip  Turkey
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Default Need help with old mic amp

I have two of these Auricon NR25 amps. They are very attractively made. The original purpose was to convert a microphone (and a line) input into a strong enough signal to drive a "galvo" (a coil driven mirror) to "write" an optical sound track on film, in the "variable density format." This last, I do not have a use for.

But it has a headphone output also and I want to use them as mic amps only. As an audio circuit it has some funky features. In particular the first and second (L to R) 3V4 pentodes are for "noise reduction" (the "NR" in the name). Those apparently make sure that unused portions of the optical track are "blacked out" to prevent light leakage. As you will notice they form a separate circuit, with its own batteries. The amp will work without them.

Now this amp has certain frequency "corrections" as the galvo peaks around 5kHz and drops thereafter. I would like to eliminate them as they affect the headphone output also. My question is, can someone tell me what is eq and what just feedback loop? For instance the L/C network around the "K-1909" coil seems like a hi-cut. Can I bypass it?

Any help would be endlessly appreciated.
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File Type: jpg Auricon NR-25.jpg (164.0 KB, 79 views)
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Old 14th October 2011, 05:54 PM   #2
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What a delightful device. The obsolete tube types, the use of "B" and "A" batteries for field use, all take one back to a time just after camaras were cranked by hand. It is interesting that there were suppliers other than RCA and Western Electric, the two large corporations usually credited on screen. One of my neighbors bought a 1935 van truck once painted up for Western Electric. Great Hulking thing, probably longer and taller than a current Ford E350 15 passenger van, but the windows were only in the back.
Due to the extensive use of transformers as stage coupling and output devices, I doubt this will ever be a 20-20000 hz device. However you can fiddle with changing the coupling capacitors and feedback capacitors. Modern capacitors are much cheaper than they were in 1930. AC resistance of a series RC pair is proportional to 1/sqrt(2pi*F*R*C) where C is in farads, R is in ohms and F is in cycles per second. .01 microfarad=10e-8 farad.
I would expect limited upper frequency range is important when using a galvanometer as the transducer.
It also might be difficult to find an earphone at 2500 ohms, though these used to be common.
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Last edited by indianajo; 14th October 2011 at 06:00 PM.
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Old 14th October 2011, 06:09 PM   #3
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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.
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Old 14th October 2011, 06:35 PM   #4
sbrads is offline sbrads  United Kingdom
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That K-1909 circuit could be a high cut but also could be giving a boost at 5kHz depending on Q and LC values. You could try shorting the inductor but you may need to also lift the cap after it that goes to ground or put a somewhat smaller cap in its place or risk losing treble. It's difficult to read the values in this part of the circuit.
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Old 14th October 2011, 06:52 PM   #5
toptip is offline toptip  Turkey
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It is probably for the camera, among others. It comes in a case with its batteries, etc., but in old pictures people carry this and camera together. There is also a separate gadget to burn the final (optical) soundtrack to film, which also uses this amp.
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Old 14th October 2011, 07:11 PM   #6
toptip is offline toptip  Turkey
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The whole amp has only six capacitors. This (the 0.0004mF) seems to be the only one that is part of an identifiable LC network. There are two others after the second tube/valve but those are for a "tone control" which I want to keep.

Thanks for your input. I should just bypass the coil and disconnect the 0.0004mF and see what happens, unlikely to cause much harm.

If you see anything else, I will appreciate. I am attaching a picture of the device (as I received it) and of its microphone (post-restoration, really a badge engineered E-V 635).
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File Type: jpg Auricon 7.jpg (400.5 KB, 57 views)
File Type: jpg Auricon NR-25 pic.jpg (26.8 KB, 55 views)
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Old 14th October 2011, 07:28 PM   #7
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I really don't think that this would be a good candidate for a high quality microphone preamp.
The microphone input transformer was designed for a 50 ohm microphone.
The coupling capacitors won't pass low frequencies.

I believe that this recording amplifier was designed to record dialog on an optical 16mm film soundtrack. As such, it purposely has a limited frequency response. Probably 100Hz to 5kHz.

The first two 3V4 tubes drive a volume level indicator. The third 3V4 drives the 2.5k headphone output. I don't see anything in the schematic that would reduce the noise.
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Old 14th October 2011, 07:39 PM   #8
toptip is offline toptip  Turkey
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Quote:
Originally Posted by indianajo View Post
Due to the extensive use of transformers as stage coupling and output devices, I doubt this will ever be a 20-20000 Hz device. However you can fiddle with changing the coupling capacitors and feedback capacitors.
It also might be difficult to find an earphone at 2500 ohms, though these used to be common.
I have no expectations of "high quality" -- just want to remove any deliberate limitations put in for the original purpose. The transformers are mostly potted devices, not sure if this indicates any better quality. Actually the headphone transformer is the only one that is not, and I thought I might replace that with something of lower output impedance. The literature says, "it is optimized for the 'latest' stethoscope-style earphones!"
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Old 14th October 2011, 07:57 PM   #9
toptip is offline toptip  Turkey
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Quote:
Originally Posted by Frank Berry View Post
The first two 3V4 tubes drive a volume level indicator. The third 3V4 drives the 2.5k headphone output. I don't see anything in the schematic that would reduce the noise.
It is not "noise reduction" in the sense we know it now. See attached from the manual. I am yet to figure out what this means or how it works.
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File Type: jpg Auricon NR25 Manual_Page_10.JPG (211.8 KB, 52 views)
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Old 14th October 2011, 08:02 PM   #10
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I don't have a clue about that either.
Thanks for the information.
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