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TA4F Mini XLR to 3.5mm adapter?
Old 15th October 2013
  #1
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TA4F Mini XLR to 3.5mm adapter?

I would like to plug a head microphone that terminates with a TA4F Mini XLR female socket into a wireless transmitter that accepts a 2.5mm mic male jack (easily adapted to 3.5mm).

Presumably the adapter would need to be battery-powered to supply power to the microphone. My wireless transmitter does not do this. Has anyone ever seen an adapter that would do this?
Old 15th October 2013
  #2
Mini-XLR (TA4F) to 3.5 mm adapter cables are readily available. Since your mic needs external power (usually 2 to 5 volts, or 10 volts for some) as some small electrets do for the impedance converter, you're probably on your own as far as converting or tapping into the cable appropriately. Most small powered mics have an independent power input connection. I've never seen an external power adapter for mics with a TA4F, since they are designed to plug into body-packs that have the 5 or 10 volt mic power connection on pin-2 already.

Should be trivial if you know the mic connections. For your "Pro Earhook" headset/mic, you can simply tack on a 9 volt (#522 or 2U6)

Have you checked with the specific mic manufacturer to see if they provide a power adapter?

I've attached a schematic of the connections to a TA4F wired for a Shure input (which is what your mic is designed to connect to). Modifying a cable this way will connect the 9 volt battery to the mic and the audio output to a 3.5 or 2.5 mm plug.

You must also confirm that your mini-jack mic input is DC coupled, if not, you must connect a 10k 1/4 W resistor across the input jack to provide a DC return for the impedance converter.
Attached Thumbnails
TA4F Mini XLR to 3.5mm adapter?-ta4f-connections.jpg  
Old 16th October 2013
  #3
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Wow

Very nice, thanks! I checked for an adapter, including with Shure, with no luck.

Is there an easy way to add an audio gain dial? It doesn't need to be much.

I am not familiar with the technical side of your last sentence (except the rating of the resistor). Do you mean potentially adding a resistor across the 3.5mm jack wires? How do I test for DC coupling, or is this something I can just ask the manufacturer?
Old 16th October 2013
  #4
Quote:
Originally Posted by Greg J. View Post
Very nice, thanks! I checked for an adapter, including with Shure, with no luck.

Is there an easy way to add an audio gain dial? It doesn't need to be much.

I am not familiar with the technical side of your last sentence (except the rating of the resistor). Do you mean potentially adding a resistor across the 3.5mm jack wires? How do I test for DC coupling, or is this something I can just ask the manufacturer?

If you add a gain pot (potentiometer) to the circuit, it will complete the DC return for the impedance converter in the mic so it will work with an AC (capacitively) coupled or DC coupled input. (no other resistor needed)

Since this is a moderately low-level signal, the wiring and the gain pot should be housed in a small metal (conductive) box. The metal box should be connected to the circuit ground (the connection to pin-1 on the mini XLR connector.

See attached schematic.

The output cable should be shielded wire.

The signal level control should be an "audio-taper' potentiometer.:

One that will work is: Jamco Electronics P/N 255426
(Alpha RV24AF-10-15R1-A14) or any equivalent 10k audio taper.

Obviously, this is a passive circuit. The level pot can only reduce the signal level from the mic. If the signal is not strong enough to drive your transmitter to the modulation level you need, the level pot won't help.
Attached Thumbnails
TA4F Mini XLR to 3.5mm adapter?-mic-adapter-w-level-control.jpg  
Old 6th November 2013
  #5
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I am going to build the first schematic (without the pot). This is my first project since I was about 10 years old. That's several decades. :P In my second post above, I was thinking of increasing the amplification, not just being able to attenuate it.

Can I put a "power on" LED in the first diagram easily? I'm guessing in line with the battery. Which orientation would it go in (negative connects to positive?), and what rating should it have?
Old 6th November 2013
  #6
Quote:
Originally Posted by Greg J. View Post
I am going to build the first schematic (without the pot). This is my first project since I was about 10 years old. That's several decades. :P In my second post above, I was thinking of increasing the amplification, not just being able to attenuate it.
If you want to increase the amplification you will need to install a low-noise op amp or a mic pre IC. It becomes a significantly larger project. If you're going into a mic input you should not need any additional amplification. If you're going into a line input, there are many small, battery powered mic pres that could work. Most small, inexpensive lav mics won't take much gain because of their high noise floor. Adding more gain just raises the noise.

Quote:
Originally Posted by Greg J. View Post
Can I put a "power on" LED in the first diagram easily? I'm guessing in line with the battery. Which orientation would it go in (negative connects to positive?), and what rating should it have?
A LED power-on indicator should not go in series with the power source. The FET impedance converter will not draw enough current to light the LED and you don't want to add the series impedance of the LED to the power source.

A LED can be simply added, but even if you use a high-effeciency LED, teh LED circuit will used much more power than the FET impedance converter and will shorten your battery life significantly. Even with a fairly high effeciency LED, the LED may use 5X the power of the FET.

If it's important to have a power-on indicator, it should be wired as shown in this schematic. The resistor value is determined by the LED. You can find "universal power indicator LEDs with built-in series resistors that will work from 5 v. to (usually 15 volts) but they are almost always made with low-efficiency diodes so can draw 15 to 25 mA. If you pick a high-eff. LED you can probably get a reasonable brightness at 4 or 5 mA.

With a high-eff. red LED the series resistor should be about 1400-1500 ohms. With a low-eff. green or blue LED it may be a s low as 400-500 ohms, depending on the LED color and efficiency. Whatever you use, it will shorten your battery live from months (FET alone) to a week or less of constant "on" time.

To determine the correct resistor value, first subtract the LED voltage drop (varies with LED color) from the battery voltage.

Determine how much forward current you want to drive through the LED (look at the brightness vs. current spec sheet curve.)

Calculate the resistor needed to pass that current at the voltage remaining after subtracting the LED forward drop from the battery voltage.

Example: For a med/high eff. red LED that requires 5 ma and has a forward drop of 2.2 volts:

1. Battery 9.0 v -2.2 v = 6.8 volts drop in resistor.
2. 6.8 volts drop at a target current of 5 mA = 6.8/0.005 = 1360 (ohms)
Attached Thumbnails
TA4F Mini XLR to 3.5mm adapter?-ta4f-adapter-w-led.jpg  
Old 7th November 2013
  #7
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Great Info

Thanks for all the info. I need to think about leaving the LED out now. It depends on how long I might go without noticing the device has been turned on since I last used it. I assume a low-battery indicator would draw power not dissimilar to an "on" LED. If not, that might be ideal for my purposes.

I once looked for a university-level class that had something to do with circuit design, but found nothing. At that time I thought it must be a skill acquired along the way when learning things in a variety of classes. Being wiser now, I'll just ask. How does one acquire the ability to design circuits through instruction, like you have been doing? I have been always been interested in creating electronic devices. I can learn about each type of discrete component, but in the past that hasn't triggered any insight into what I need to do to go from something I envision to a circuit.
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