Misc: How to connect microphones to your keyboard


You are trying to connect a microphone directly to the MIC input jacks on the back of your keyboard, typically to drive vocoder effects. You're not hearing the mic at all, or perhaps you hear it but the signal is really weak or noisy. You've done all the troubleshooting with the GLOBAL settings of your keyboard to ensure that the input routing is correct and you're sure you've set the MIC gain level on the input jacks correctly.

Facts about microphones and how they relate to your keyboard's input jacks

First off, nearly every keyboard that has MIC inputs at all expects to receive an unbalanced signal via a quarter inch mono jack input. Keep this in mind as you read the following.

Most studio and stage microphones are designed with a 3-pin XLR male connector at the base. This is a 3-wire "balanced circuit" which means that the signal from the microphone is driven in-phase between the pair of pins referred to as L/"live" and X/"shield" and in antiphase between the pair of pins referred to as R/"return" and X/"shield".

The lead from the microphone to the mixing desk will have a 3-pin XLR female connector at the microphone end, and either a 3-pin XLR male connector or a quarter inch TRS (Tip Ring and Shield) jack plug at the mixer desk end. The TRS plug is an identical plug to quarter inch "stereo audio" plugs, but the wiring is different and it carries a Mono balanced signal using three wires to do so.

NOTE: in the context of XLR Mic connections, L and R does not stand for Left and Right in the context of stereo signals, it stands for Live and Return in the context of mono transmissions using a balanced line driver.

Two types of microphones

  1. Dynamic (unpowered) - the balanced line signal is achieved by wiring the microphone's coil like a transformer with a center tap going to X/"shield" and the phase and antiphase winding outputs going to the L and R pins.
  2. Condenser (powered) - the balanced line signal is achieved using a small preamplifier within the Mic. Some require batteries and others require the application of 48v DC (phantom power). If phantom power is required then some mixer desks and mic preamplifiers can provide it and it is applied between the L/"live" pin and the X/"shield" pin, noting that most cable will use the Red wire as the "Live", the Black wire will be the "Return" and the braided shield will be, well, the "Shield".

Why all this complexity?

The MIC signal is a very low level signal that is easily swamped with intererence picked up on the cable. Here's the neat trick. The same "interference" is normally picked up on the L/"in phase" circuit as on the R/"out of phase" circuit. Of course the "in phase" signal is the same as the inverse of the "out of phase" signal. So the receiver circuit in the mixer desk subtracts the "out of phase" signal from the "in phase" signal, and in so doing the "interference" signal is completely cancelled out. Brilliant!

Options for connecting Mics to your keyboard

  1. High quality MIC connection - use a balanced XLR to XLR lead or an XLR to TRS lead to plug into a mixer desk with balanced inputs (either XLR or TRS), or else use a balanced MIC preamplifier to convert between balanced and unbalanced. Then use a short output patch lead (mono tip and shield quarter inch jack) to connect an unbalanced output into the keyboard.
  2. Cheap dynamic microphone, use the sort normally supplied for use with home karaoke sets. These mics normally have a single unbalanced coil wired between the "tip" and "shield" of a quarter inch jack plug. You can plug this straight in to your keyboard.
  3. "Cobbled together" balanced to unbalanced cable, this will only work with unpowered "XLR" microphones. Connect the Red (live) and braided shield wires from the XLR lead to the Tip and Shield of a quarter inch jack plug, and connect the Black (Return) wire via a 600 ohm resistor to the Shield pin of the quarter inch jack. This has the disadvantage of transmitting only half the signal from the MIC, and the signal to noise ratio is half that compared with a cheap karaoke mic. The purpose of the 600 ohm resistor is an attempt to avoid seriously unbalancing the other half-coil of the Microphone.

NOTE: With either the cheap MIC or the cobbled together "unbalanced" cable, you will be limited to lead lengths of no more than 3 meters. Beyond this length the interference pickup becomes noticeable.

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