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Guitar "wireless jack" from broken telephone headset

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TaDa

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I've still not dismantled the headset yet!!

I think I'm going to go for the preamp on the guitar end and the pot on the amp end in the hope they will assuage my fear of blowing up the important bits

Thanks for all the advice- I'll report back when it's done
 

TaDa

Member
I'm not done but I've only got 3.7v and I don't know if that significantly changes the preamp circuit

I was expecting to find a mini aerial in the mic boom but there's only two slim wires leading to the mic - so that's a bonus (I think)
 

audioguru

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I'm not done but I've only got 3.7v and I don't know if that significantly changes the preamp circuit

I was expecting to find a mini aerial in the mic boom but there's only two slim wires leading to the mic - so that's a bonus (I think)
3.7V sounds like it comes from a lithium rechargeable battery that is 3.2V when low and 4.2V when fully charged. The Jfet preamp must be powered from 9V. I do not know of a Jfet that works at only 3.2V.
Of course an electret mic has two wires, a signal wire that must be biased at a positive voltage and a ground wire. If you connect the wires backwards then the mic will not work.
 

TaDa

Member
Thanks audioguru

Yes, the battery is a 3.7v Li_ion one - nastily bulging - I think it might need replacing :)

So I guess I have to go with the non-preamp solution then - heh ho - hopefully the impedance required by mic will be similar to that required for the guitar.

I'm going to completely remove the mic - I was just pleased to see there wasn't a third wire in there being used as the aerial - I've found the aerial now its rather integral to the case and making it difficult to take it all apart!
 

audioguru

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A bulging Lithium battery is dangerous and almost useless. It is old, or has been discharged to a voltage too low, or has been fully charged for too long. Get rid of it at a battery recycling bin before it catches on fire.

An electret mic is powered by a positive voltage that will upset a guitar magnetic pickup. An electret mic produces tens of mV but a guitar pickup produces hundreds or thousands of mV. An electret mic feeds a preamp with an impedance of about 10k to 50k ohms but a guitar pickup feeds a preamp with about 3M ohms.

Did you try the simple volume control and capacitor that I recommended in post #18?
 

TaDa

Member
Sorry, no, I'm going very slowly today I finally opened the headset up fully (and broke the aerial which was glued in place hidden behind some other plastic bah!)

I see where the mic is wired and I'm not sure how I'm going to wire it up but that's for tomorrow now

Thanks
 

TaDa

Member
I've tried to think about this but I don't see how it works...

The mic input to the transmitter is two wires - one is at 3.7v and the other 0v (ground, what you will) the signal from the mic comes back along the these wires somehow - I'm guessing as some sort of 'wave' over the 3.7v line. If I play the guitar input through a capacitor into the 3.7v line how does the guitar's puny signal (maybe 1v and very little current) ever impose its 'wave' over the 3.7v DC?
If I attenuate that signal even more by running it through a 100k pot surely that just reduces the voltage competing with the 3.7v?

You also mentioned that the guitar would normally feed a preamp of a megohm or more but if I run it over a 100k pot surely I'll be drawing more current than the guitar can provide - will this damage the guitar or distort the signal? should I not be using a megohm or 10megohm pot?

Its late for me - time to go to bed!
 

audioguru

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The output of an electret mic is not connected directly to a power supply positive output, instead it is fed a small current from a series resistor fed from the positive voltage. The resistor is the load for the Jfet inside the mic. The signal is developed across this resistor. The DC voltage must be blocked from entering the volume control with the capacitor I showed.

If a 100k volume control causes the guitar to sound muffled then try a 1M volume control. 100k will not overload a guitar pickup.
 

dr pepper

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A common impedance for instrument amps is 47k

Your guitar will impose its waveform on the mic line with the same ease the mic did!, there will be a resistor from + battery to the mic connection, your multimeter input resistance is so high the resistor will have no effect and you'll read battery voltage.
You just need the capacitor to prevent dc from reaching the guitar, the pickup in the guitar allthough it'd probably survive would sound totally different without it probably really bad as its core would saturate with the dc.
 

audioguru

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A common impedance for instrument amps is 47k
No. 47k is the input resistance of a moving magnet phono pickup.
Simply look in Google Images for Fender Guitar Amplifier Circuit and you will see that their "Princeton" amp had a 10M input resistance and their "deluxe" amp had 5M input. Their "Champ-amp" had only 1M input.
Most Marshall guitar amps had 1M inputs.
 
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