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Electronic Experts - What is this (Bridge Amplifier)?

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Electronic Experts - What is this (Bridge Amplifier)?

Old 06-10-2019, 10:24 AM
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Default Electronic Experts - What is this (Bridge Amplifier)?

I buy surplus lab equipment and sell it on eBay. I have this item that says "Bridge Amplifier" on it. But, when I Google that all I find is how to bridge amplifiers to combine channels. I suspect this is something different. Some kind of test equipment perhaps. Anybody know for sure?
https://www.ebay.com/itm/15351348394...84.m1555.l2649







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Old 06-10-2019, 03:25 PM
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Just eyeballing (along with my weak electrical engineering 101 memory) it I'd say the top dial is a high pass filter (there is a capacitor on the upper dials), and the selector determines the rolloff frequency of the high pass filter.

the lower selector determines the gain (amplification) on the remaining signal.

So the device would filter out low frequency stuff and boost the signal, for two separate channels.

Again just guessing and I'm a mechanical engineer, not electrical......
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Old 06-10-2019, 04:15 PM
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Old 06-10-2019, 04:35 PM
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Yeah, I I've worked in the broadcast arena for the past 30 years. Telephone calls are done with a box called a Hybrid that does all the processing to make crappy phone calls sound better when aired. There's a lot more to it than a high pass filter and a gain control as Ma Bell cuts certain frequencies from the audio of a phone call so it can be handled by a 2 wire, unbalanced system over long distances (more than ~100'). Boradcasters have been using telephone hybrids for 40+ years. Besides, you'd never cut frequencies from a phone call, the signal has already been significantly trimmed - that's why phone audio sounds so crappy.

With 2 channels, this thing just looks like an adjustable stereo high pass filter with gain compensation. Could have been used for anything and looking at the back of it, there's a section with a breadboard (home made circuit) in it. No telling what it was used for but a modern DSP is about 100 million times more useful and capable.

Probably a nice conversation piece but otherwise worthless.

Last edited by Back-in-Black; 06-10-2019 at 04:41 PM.
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Old 06-10-2019, 04:40 PM
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Originally Posted by Splisskin View Post

Correct.
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Old 06-10-2019, 04:45 PM
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Originally Posted by Splisskin View Post

didn't see that post - amplifies strain gauge signals which by their very nature are quite small. That makes a lot of sense.
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Old 06-10-2019, 05:45 PM
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Originally Posted by Back-in-Black View Post
didn't see that post - amplifies strain gauge signals which by their very nature are quite small. That makes a lot of sense.
Yes and "bridge" refers to a wheatstone bridge which is the type of circuit used with strain gages. For measuring a single gage there would be one strain gage and 3 fixed resistors. Load cells and similar devices have all 4 as active strain gages for both higher signal and temperature compensation.....
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Old 06-10-2019, 07:02 PM
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Originally Posted by Center Crusher View Post
Yes and "bridge" refers to a wheatstone bridge which is the type of circuit used with strain gages. For measuring a single gage there would be one strain gage and 3 fixed resistors. Load cells and similar devices have all 4 as active strain gages for both higher signal and temperature compensation.....
This.
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Old 06-11-2019, 08:24 AM
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OK. Thanks. The additional keywords may help sell it. Think $99 is too low?
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Old 06-11-2019, 12:29 PM
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I have deleted my guess as it was obviously erroneous. Kudos to those who nailed it.
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Old 06-11-2019, 03:13 PM
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They were all wrong. That piece is valuable. The Golden Gate Bridge Authority had bridge amplifiers made to amplify the brightness lights on the bridge during foggy conditions. With LEDS, they were replaced. Probably worth a good tax write off if you donate to a museum.
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