Tech Forum

There are many transformers, less than $15 that are available to match low impedance microphones to a high impedance input. Some can be permanently wired while others like the Audio Technica  CP8201 are adapters that simply plug in.

Google for 'microphone transformer' to see what's available. You might also raid old audio gear. Many times, depending on your needs , exact values are not so critical and there's no risk in trying them!

I am going to go against the two prior answers. I have directly wired devices with XLR connectors to ones with phone plugs. And have wired balanced to unbalanced and vice versa. I have done this in the professional environment of broadcast TV stations where any technical problems would be instantly noticed and can not be tolerated. You just have to know what you are doing.

First, the type of circuit used, balanced or unbalanced, and the impedance of the circuit do not necessarily directly correlate to each other. Nor do either of these directly correlate to the type of connector used. Although the XLR style of connectors are generally used for balanced, low impedance (600 Ohm) circuits and phone plugs are generally used for unbalanced circuits, I have seen the opposite successfully used in many cases. Connectors are connectors and do not care about impedance or balanced/unbalanced. If they have the correct number of pins and the required quality level, they will work.

OK, now your question. Your XLR terminated microphones have an equivalent circuit something like this:

It functions as an AC Voltage source in series with a resistance. The shield connection is not connected to anything in the microphone cable or in the microphone itself except the outer case. It is for shielding only. You did not say what the impedance or signal level of the microphone is so there are two main possibilities, high impedance or low impedance (Rs). Either of these can have a number of different signal levels (Vac). This is the ideal way of wiring any microphone as it can be used with any type of input, as I will show below. If it were wired as an unbalanced device, then it would be OK for an unbalanced input but not well suited for a balanced one.

The other side of your circuit is the amplifier input. The only thing you say about this is that it is a high impedance connection that uses a telephone style jack. This could be either a two pin or a three pin telephone jack. You should look inside the amplifier to determine which it is. As for the "high" input impedance, the most common value used is 10 Kohms, but some amplifiers may differ. But it really makes little difference. 

If it is a two pin (T&S - Tip and Sleeve), then it is definitely an unbalanced input and the equivalent circuit will look like this:

Your microphone and amplifier could then be connected as follows:

Notice that the ground is not connected to the XLR pin 3 at the microphone. The two resistors form a Voltage divider so the signal delivered to the input stage of the amplifier will be a simple fraction of Vac. If Rs is low, almost the full value of Vac will be delivered to the input stage. If Rs is higher, then this will be reduced, depending on the exact values. There should not be a lot of reactance in the amplifier input stage so there should not be any high frequency roll off. In any case, the connection should work with no expected problems unless the amplifier has too little gain. If gain is a problem, you will need to use another microphone or a pre-amplifier.

If the telephone jack on the amplifier is a three pin type (RTS - Ring, Tip, and Sleeve), then the circuit will most likely look like this:

I said "most likely" because I have seen microphone inputs that used the three pin telephone jacks for other purposes, like selecting between a line and a microphone input. Also, it could still be an unbalanced input, just with the extra pin. In that case, the R and S contacts would probably be connected together and you are just back to the unbalanced situation above and you still use the hookup shown below. But let's assume that your amplifier's input is strictly a balanced, microphone input and the above circuit is correct.

For a three pin telephone jack your connection should look like this:

If you look at it carefully, you will see that there is no difference in the signal loop. Only that the ground is not connected to it so both sides are floating, as opposed to one side being grounded in the unbalanced connection. This configuration is less susceptible to picking up noise from the environment, but does not operate any differently as far as the signal is concerned. So all my comments for the signal in the unbalanced connection still apply to this balanced one.

One more thing, if a three pin telephone jack is used with an unbalanced amplifier input, the above circuit is still valid except that the bottom end of Ri is connected to ground at that amplifier connector and the lower arrow to the input stage is not present.

In short you should simply use a shielded pair for your microphone cable. Connect one wire of that pair to the T (Tip) connection at the amplifier. Connect the shield to the S (Sleeve) connection and the other wire of the pair is connected to the R (Ring) terminal if it exists and to the S (Sleeve) if it does not. This covers all cases.

A transformer may help if the microphone level is too low for the amplifier input. But that will introduce inductance and it will have it's own frequency response curve which may not be very good. Good audio transformers need a lot of steel and copper and are expensive. As I said above, a far better way to do this would be with a pre-amplifier. Even an inexpensive pre-amp will have better specs. than almost any transformer, hands down. Or get higher level microphones, but that can also be expensive.

Andre, yes you can connect an XLR microphone to a 1/4" TS (or TRS) jack. I'm 67 and am a retired studio chief engineer for a major studio in Hollywood, and it works no matter what some people might tell you.

Connect the XLR pin 1 (ground) and pin 3 (low) to the 1/4" sleeve (TS jack) or ring and sleeve (TRS jack), and connect the XLR pin 2 (high) to the 1/4" tip. Those who haven't done this get confused because, theoretically, you then have "unbalanced" the microphone, but the signal still exists. The less obvious problem is that an impedance matching transformer gives you 10-20 dB of noiseless gain so your signal-to-noise ratio is a bit better (the noise includes hum, of course, plus electronic thermal noise with a floor of -131dBm).

Finally, good transformers are EXPENSIVE (I like Jensens, jensen-transformers.com). All transformers need RC compensation on the secondary; send a square wave through one if you don't believe me and look at the ringing (ask if you want more info on this). HERE'S THE BOTTOM LINE: Connect the XLR to the 1/4" jack (use a NO-transformer adapter or wire one yourself) and see if you're happy; if not, then (and only then) think about using a transformer (and don't use a cheap one, they have more distortion). Heck, even a lot of today's semipro equipment runs XLR "unbalanced" (look at the schematics). Call me at 818-951-8646 (desk line, daytime Los Angeles) if you or anybody else wants further info.

Yes Andre, you need an adapter to mate that "Unbalanced" (Hi-Z) microphone input to a "Balanced" (Low-Z) microphone. Here are three ideas:

RadioShack sells item #274-016 (you'll need cables to connect the two ends). Here's the link: www.radioshack.com/product/index.jsp?productId=2062443.  Chances are your local Shack has it in stock.

Parts Express (www.partsexpress.com) sells a similar adapter: www.parts-express.com/pyle-ppmjl15-1-4-male-to-xlr-female-mic-cable-15-ft--248-4614.

If you're handy with a soldering iron, here's a DIY idea: www.mediacollege.com/audio/connection/xlr-jack-mono.html (all necessary parts are available at RadioShack).
All three methods will do what you need — just pick the option that's easiest for you to implement.

For starters you CANNOT use a 1/4" male plug wired to a XLR jack. The 1/4" line is unbalanced meaning there is one signal line and a ground. The XLR line has two signal lines with opposite and equal signals plus the ground. Use a RadioShack Model; 274-016 (or equivalent) A3F XLR jack to 1/4" plug adapter. This adapter contains a balance-to-unbalance (balun) transformer to match the two lines properly. The balanced line and High Impedance (HiZ) are both used to reduce interference on low level signal lines like microphones.