Church Sound and Video Advice > Audio Tips > Connecting Musical Instruments
We need to connect musical instruments to our church sound system, how should we do this?
Cable Selection:
A musical instrument such as an electric guitar or electronic keyboard uses a High Impedance, Unbalanced output. This is designed to operate effectively up to 15 or 20 feet without picking up hum and electrical or radio interference. If you have a short distance from the instrument to your mixer, or you are operating in a very interference free room, you might be able to plug the instrument directly into a Line Level input on the mixer using an instrument cable with 1/4 inch phone connectors on each end. This cable must be a shielded instrument cable. Don't try to use an unshielded speaker cable.
Converting the Instrument Signal:
For permanent installations or greater distances to the mixer, you need to convert the instrument signal from High Impedance Unbalanced Line level to Low Impedance Balanced Microphone level. This can be accomplished in one of two ways. The most often seen method is using a Direct Interface Box (DI Box). The instrument is connected to the DI box using a standard instrument cable with 1/4 inch phone connectors on each end. The DI box is then connected to a microphone jack using a standard microphone cable. By converting the signal, you greatly reduce the possibility of interference pick up and make the instrument look like a microphone to your mixer. If you want to also feed sound to a local guitar or keyboard amplifier, some DI boxes have a second Unbalanced output which will do this.
Direct Interface Boxes:
Alectro Systems sells several models of Direct Interface boxes. A couple of popular ones are the DB25 (passive) and ADB2 (active). Both have unbalanced instrument inputs and unbalanced line level and balanced mic level outputs. Both have attenuation and ground lift switches. The ADB2, active box has a very broad frequency response from 1 Hz through 50 KHz, while the DB25 covers 50 Hz through 15 KHz. For instruments with very low or high harmonics such as organ foot pedals, the active box might be the better choice. It does however require phantom power.
The Direct Interface Cable:
The other method, which I prefer, is to use a Direct Interface Cable. This performs the same function as the DI box except it accomplishes it using one component instead of three. The DI25 Direct interface cable is a 25 foot cable with a mic connector on one end and a 1/4 inch phone connector with built in transformer on the other end. It plugs into a keyboard, guitar, VCR etc. and converts from the high impedance unbalanced signal out of the instrument to low impedance balanced mic level. The other end plugs into a standard microphone jack. This cable enables you to plug any instrument into any microphone jack without picking up hum and electrical interference.
The DI25 Cable saves you money and set up time since there is one component instead of three. It is also more immune to interference since it converts the signal right at the instrument output instead of at the box end of the instrument cable. If you want to use a local amplifier for the instrument, you can connect from the instrument to the amplifier, then from the amplifier "pre-amp" or "line" output to your microphone jack using the DI25 cable.
DI25 Direct Interface Cable
For more detailed information on this
subject, please refer to " What's
all this about High and Low impedance, Balanced and Unbalanced,
Mic and Line level? " in
the Sound Advice Section.