FM transmitter with XTerra 705

[Goaty]

I recently tried using a wireless FM transmitter module (as per the Vallon thread post #707 ) with my Xterra 705 and a set of FM wireless headphones but the results were most disappointing with very weak and distorted audio through the headphones. I also tried a standard FM broadcast receiver and, whilst easier to tune, the audio was still poor.

It seems that the headphones socket output of the Xterra is fairly heavily attenuated and is insufficient to drive the FM transmitter.

Possible solutions may be to build a preamp for the FM module or open up the Xterra and change the attenuation.

The preamp option seems overly complicated and bulky and I would be reluctant to mod the Xterra as it is only new and would probably void the warranty, so I was wondering if anyone else had encountered this problem and has a simpler solution.

[Skippy]

That seems odd. Normally, metal detector outputs are huge, and can swamp the inputs of radio transmitters, and a dummy load is needed to bring normal behaviour.

[Davor]

Maybe there is a strong bias in headphone output. Most detectors do not respect 0V DC on audio outputs.

[F117]

It seems that the headphones socket output of the Xterra is fairly heavily attenuated and is insufficient to drive the FM transmitter.

When I connected an FM transmitter to my GoldmaxxxPower, I had to attenuate the signal from the headphone socket. If your audio signal is too strong, it will lead to a heavily distorted = clipped signal at your FM receiver.
Attenuation + AC coupling as mentioned may improve your setup.

[Skippy]

You need to work out if the audio output is between the L and R channel, or, more likely, between (L and R together) and Ground.

And then add a dummy load of 10R to 50R across the output, before sending it to your transmitter. And if that doesn't perform properly, AC coupling the input of the transmitter with 1uF - 22uF would be worth trying.

[Goaty]

Thanks to everyone for your most helpful suggestions, especially regarding possible DC bias on the audio output.
As soon as I get time I will make some measurements and see what is happening with the output.
I know the FM transmitter is working OK as I have tested it with a signal from the function generator.

[Goaty]

You need to work out if the audio output is between the L and R channel, or, more likely, between (L and R together) and Ground.

And then add a dummy load of 10R to 50R across the output

I tried this with 47ohm load resistors from each of L & R to ground.

The audio modulation was just audible on an FM radio receiver with the vol on full on both the radio and the headphone output of the Xterra. The radio signal meter showed a very strong carrier level. No distortion of the audio evident. Decreasing the headphone audio output on the Xterra from full to minimum simply dropped the audio from the radio to nothing so it does not seem to be an overload problem.

I have not had time yet to try with coupling caps or to measure for any DC bias.

The input impedance of the FM transmitter module seems to be quite high - in the megohm range.

I have also noticed that the audio level of the Xterra is not unbearably loud into 'normal' headphones even with the volume level set to maximum.

[Skippy]

So, did you work out which pins the output appears on?
That determines where you then connect your transmitter inputs.

If it's between (L & R together) and Ground then stick one load resistor from L to Ground (might be worth trying a larger resistor, say 50 - 150R) and connect the transmitter ground to detector ground, and transmitter L & R both to L output.

If the detector output is between L and R, put your load between L and R. Connect transmitter ground to detector L, and transmitter L and R inputs to detector R.

[Goaty]

So, did you work out which pins the output appears on?

Next job on the list. I had a trip to the city yesterday to have a molar removed ready for an implant so didn't feel like doing too much afterwards.

It will be interesting to see if I can detect the titanium screw when they put it in

[Skippy]

I have a titanium pin holding a cap on, my propointer can detect that OK.