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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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Old 17th August 2012, 11:17 AM   #11
AndrewT is offline AndrewT  Scotland
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Two transformers.
Any chance that what you are describing is balanced impedance connection?
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Old 17th August 2012, 11:30 AM   #12
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in a way yes but the trick is that if you are looking for line level trafos that will be hard to find and expensive

on the other side speaker level transformers exist anywhere cost nothing and do the job with no problems except lower quality due to poor frequency response of the trafos .

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sakis
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Old 17th August 2012, 05:30 PM   #13
AuroraB is offline AuroraB  Norway
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IF you're not expecting "studio" signal quality , line transformers are available for 20-30$ each.... I use quite a number of them for voice++ (10kHz or so) quality distribution.
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Old 17th August 2012, 06:00 PM   #14
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5W speaker level transformer audio grade will not cost more than 2-3 euros or usd in any electronics shop and yes it will not be studio quality ...just almost hifi expect a bandwidth from 100-120 HZ to almost 8.5K ....more than enough for such an intercom application
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Old 18th August 2012, 09:53 AM   #15
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Thanks everyone for your responses. Absolute studio quality is not necessary.

A brief description of the application is as follows: The charitable organization is spread over a large area. The main gate is unmanned and the security personnel are stationed at the next gate which is a little more than 1/2 km away. The two points are connected by a winding road, so they are not in a straight, direct line of sight. They want to have a microphone pick up all ambient sound, synchronize this with their CCTV and make sense of what is going on in the vicinity of the main gate. The gates are shut at night and remain open during the day.

In such a situation, covering a wide band will convey more information; hence, restricting it to speech bandwidth may not be the best situation.

HDP pipe has been laid but no cable has been run as yet for this purpose.

Thanks again, for any further inputs.
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Old 18th August 2012, 10:37 AM   #16
tvrgeek is offline tvrgeek  United States
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If you have power at the gate, then I too would be thinking RF. Running 600M of cable is not cheap, and prone to damage. ( big expense if pipe was laid before working out these details.) Could you modulate the audio and piggy-back on the CCTV cable?
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Old 18th August 2012, 10:54 AM   #17
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this is crazy. you can even pick up power line hum (magnetically induced50/60 Hz) with such lengths. my advice is to place an amplifier right at the microphone site, remotely power fed with a three wire cable and have at least one more repeater.
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Old 18th August 2012, 08:11 PM   #18
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better yet. use a two wire twisted pair and treat it like a telephone system. send dc power over the wires for remote amplifier and superimpose the audio with a balanced system just like in telephony. since it works for several kilometres in telephony it will definitely work in your case.
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Old 18th August 2012, 09:06 PM   #19
jcx is offline jcx  United States
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no mention yet of phantom powered mics?

shielded twisted pair XLR cables, 48 Vdc power appled between the pair and the shield is a pro standard

active buffered phantom powered mics can have <100 Ohm output Z, will drive 1000 m of <100 pF/m cable with ~ 16 kHz -3 dB point
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Old 18th August 2012, 09:06 PM   #20
DUG is offline DUG  Canada
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The 50 ohm feed and coaxial cable is the right idea as suggested by Jan Dupont.

Cable TV uses DC and RF over the same line...30VDC and lots of TV channels.

If you need phantom power then transformer or inductor isolation of the AF and DC is another option. (see jpg's for ideas)

Make sure the Microphone side power requirements are less than 18 mA due to max 144H unbalanced current limitations.

Transformer-less dc coupled circuits can also be used with 50 ohm coax.

Try to amplify as much as possible at the microphone end.
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