Mystery Ampex box ... what is it? What can one do with it?

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Grapejuice
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Mystery Ampex box ... what is it? What can one do with it?

Post by Grapejuice » Mon Dec 29, 2008 7:29 am

Sorry to double post, but I thought maybe after getting not too much info on the 'Gear Talk' board, the ubertechs in here might have a thing or two to say... see below.


Posted: Sun Dec 28, 2008 3:56 am Post subject: Mystery vintage Ampex passive amplifier/pre ?
Here are some pictures of an cool looking little vintage Ampex box I found kicking around my parent's house. It used to belong my Grandfather which would put its likely purchase date sometime in the 40's -50's.


http://picasaweb.google.com/grapejuice/ ... directlink


It is mono in/out and seems to be extremely lo z. There are two transformers (?) in it marked 600hm, which hopefully you can see in the pictures.

If I set up a connection this way...

Mxl sdc mic - 12ay7 mic pre - 12ay7 trs out (w/ output cranked to 10) - into ampex box 1/4 in - into m audio fast track ultra mic/instrument input at unity gain - DAW

... the box seems to pass a decent sounding not to noisy signal at about 15-20 db quieter then the same set up less the ampex box.

It also occurred to me that this box might be a headphone amp, although the output using it for this application is so low i think this is unlikely?

Anyone identify it? I'd love to know exactly what it is so that I use it correctly for it's intended purpose, and also perhaps replace the lead.

Looking forward to your answers

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RodC
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Post by RodC » Mon Dec 29, 2008 9:20 am

I thought we answered it pretty well, its an Attenuator of some sort. There is nothing active about it. Those are 600 ohm power resistors and a dual gang pot.

If you could take a more in focus pic of the connections to the pots we may be able to draw out the schematic for you. (Maybe put your camera in micro mode) It may even be a headphone tap, or a recording attenuator.

When you pass a signal through it and turn the knob does the signal go down?
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RoyMatthews
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Post by RoyMatthews » Mon Dec 29, 2008 10:00 am

I must admit I'm stumped in regards to exactly what it is. I spent an hour or so poking around and I found a parts list but that isn't listed on there.

It is some type of attenuator.

I'm curious about it's exact purpose because it's just a module. The curved end makes it seem like it wasn't a part of some other device. I almost think it was attached to the bottom of a desk.
The knob didn't look like any of the knobs on any of the Ampex devices I've seen for the era. It might be some type of random utility device.

You can always getting some info here
http://recordist.com/ampex/
They must know something.
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nclayton
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Post by nclayton » Mon Dec 29, 2008 11:29 am

I agree with RodC, I think it's a headphone box. Speaker level goes in through the wire, headphone level goes out through the 1/4 jack. The impedances look right for this kind of thing and also the unshielded pair going in looks like the kind of wire people used for speakers a lot 30-40 years ago.

Grapejuice
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Post by Grapejuice » Tue Dec 30, 2008 4:19 am

Hey, no slight intended to the folks who answered me on the Gear board, it was valuable information you gave, I just wanted to see if I could fish out some more guesses over here in mad scientist world.

Sending any level through the box does decrease it's volume greatly, yes. Because there is no markings on the front of the box it's hard to tell which way is 'up' or 'down' on the pot, but it acts as a level control as well. Basically, with the signal path I describe in the initial post, a very hot signal goes into the box and with the pot turned fully in one direction the signal is inaudible and in the other direction somewhat audible. The inputted signal I describe is decreased by about 20db when it's outputted.

I think headphone box is likely, and it would also fit with the reel to reel tape recorder I believe it belonged to.

Thanks for your answers

R

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