Running headphone signal through my wireless tx

[jdr1973]

Hello,

I'm trying to step down a signal so that I can send it through my tiny video/audio tx. The 12v dc powered headphone signal is at 8 ohm. The tx calls for HIGH IMPEDANCE. I was told 1000 ohms or 4.7k or more. I have a pack of 1k, 4.7k, & 10k resistors.

I assume I should just install the resistor on the positive wire. I don't want to fry my tx. I already ran the headphone signal straight into my tx and it still works.

Any advice on resistor size or anything else?

Thanks

James

[phil_g]

You can feed a high impedance input from a low impedance source, impedance matching is more important when maximum power transfer is required, rather than at signal levels. If the signal level is too high you can wire a potential divider using the resistors you have there. This is on the signal lead, its nothing to do with 'positive' wire as you asked.
Firstly, how do you know the signal is too high a level? Does it distort?
If not then leave everything as it is, but if it does I'd suggest a 10:1 divider as a start:


10k
(signal)---------/\/\/\/\---------+-------------------(transmitter input)
!
/
\ 1k
/
\
!
!
(ground)--------------------------+----------------(ground)

If it still distorts, add a further 10k in series with the first 10k, and a further 1k in parallel with the first 1k. That gives a 40:1 divider. If it still distorts, you need to buy some 100k's !

Cheers
Phil

[jdr1973]

I'm novice when it comes to electronics but I'll study your reply. Also manufacturer talks about 2 volt peak to peak.

I hear no sound at all even when I had no resistor. I did test the tx and it works fine though.

[jeffs555]

It all depends on what is driving the headphone signal. It is a matter of voltage and power. At 8 ohms, it would take 1/2 watt to drive to 2 volts(power is equal to the square of the voltage divided by the impedance). That would be more power than most headphone outputs need, so it may not be designed to deliver that voltage. Some 8 ohm outputs will drive high impedance and voltage just fine, some will drive the high impedance but will distort, and some just won't drive enough voltage into a high impedance source.

If you get no sound at all with the direct connection, I would think you need some form of impedance matching transformer. It could be either a physical 8ohm to 1000ohm transformer, or a low input impedance high output impedance amplifier. Radio Shack sells a small 8ohm to 1000ohm transformer for $2.99. http://www.radioshack.com/product/index.jsp?productId=2103254