Balanced Signal over 500 Metres!

[Samuel Jayaraj]

A charitable organization has requested me to help them transmit a microphone signal (picking up ambient sounds) over a distance of nearly 600 metres.

The microphone is to be placed at the gate to their campus, which will then be monitored by security personnel located at that distance.

They have already purchased a condenser mic which the French manufacturer claims will transmit an unbalanced signal upto 300 metres.

My thoughts:

Use a dedicated supply for the condenser mic. Convert the signal to balanced using a circuit like this Alternative topologies for balanced to unbalanced amplifier posted by Marcel in post #4.

Use a pair of the Elektor Class A preamp shown in this thread MAT02, 03 Elektor preamp schematic

Again use Marcel's circuit above to covert back to unbalanced and feed this into a DVR already available with them.

Use one single run of Balanced Mic cable, silver-plated, drain-wire with shielding, foil wrapped, teflon insulated.

Use damping RC networks at both ends of the cable to counter reactance.

The output expected is not hi-end but listenable through the day and night.

Not an easy situation. Any foreseeable problems, precautions to be taken etc.,

Any information would be much valued and appreciated.

Thanks in advance,

[Jan Dupont]

Take a look at transmission lines......
High Current is the issue here. Use a line level amplifier with low output impedance (20-50 Ohms) and terminate the cable at the end with a resistor with the same value could be the right solution.....

[DF96]

For that distance I would be tempted to use a radio link.

[theAnonymous1]

What about something like THS4130/OPA1632 driving a pair of BUF634T with an INA134/137 on the other end?

[Samuel Jayaraj]

Thanks everyone for your inputs. A radio link seems nice but the organization's electrical department wants a line-level link.

Jan, I was also thinking of a relatively high current. That is why I thought of the Elektor Class A preamp I pointed to in my opening post. The preamp is set at an Iq of 40mA and has an output impedance of 10E.

Alternately, I am also brooding on using a pair of Mini-Alephs ( The Mini-A ); the reactive load should not matter for the mini Class A amp.

[Jan Dupont]

No need for using class A here. You can use a class A/B amplifier. It's not the Iq thats important but the Current through the cable......

[AndrewT]

Does the cable provide a phantom supply back to the microphone end?
If so then that supply can power any tiny "follower" stage. This follower stage should be capable of driving that 500m of cable capacitance. That is not a particularly difficult task.

And keep the transmission impedance balanced. You must reject noise before it contaminates the signal.

[Samuel Jayaraj]

Attached herewith are the Microphone already in possession with the concerned party. It requires only +12VDC.

Andrew, are you saying 2 follower stages, driving a balanced signal?

I was also contemplating on using 2 Mini-Alephs ( The Mini-A ) to drive a balanced signal.

It does appear that line level stages may not be as suitable as a power stage.

Small power follower, Mini-Aleph or a small Class A/AB amp (2 for balanced drive)? Thanks in advance.

[AuroraB]

Why on earth would you need a power amp for this?????
Any decent mic preamp, with a low impedance balanced line driver will do the job.
Then terminate as appropriate at the receiving end and feed to mix/amp or whatever will be used to monitor this.
Use a decent cable - properly balanced and preferrably screened, depending on your RF environment. Depending on impedance, you may loose some treble, though.....
I run party-line intercoms, converted to balanced transformer drive, at distances above 1 km.....

[sakis]

....if the sound quality is not the #1 issue and cables already exist then this is very very easy
100v technology is the no1 solution when it comes to things like that ...

also CAT5 ( probably existing ) or simple telephone cables can be used

find any preamplifier fits your needs, and any simple amplifier like TDA 2002 will do . drive 100 v small existing anywhere transformer , use another transformer on the other side which probably have a 0-4-8 R secondary with a divider of a couple of resistors and you are done with no more than 10USD ...

offers also fully differential ground so no noises of any kind will insert the signal even if the distance is 1 klm

that is common practice .....