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At North Ridge Community Church in our Worship Center we have a mono system. Mono meaning that there is no left or right, there is just the one speaker. Our room doesn’t have a good shape for everyone to get a good stereo feed, so mono it is.
Almost every CD player, computer, MP3 player or media player is in stereo. On a sound board, to get these media devices connected, you often have to use a stereo channel or two mono channels (one for left and one for right). With a mono audio system you combine the left and right channels inside the sound board to send to your speakers.
Most churches require a lot of media devices to play from. At NRCC we have a computer, iPhone/MP3 player, CD Player and DVD player. If you add that up that is 8 different connections into the board for 4 devices! If your church has a smaller format audio console, those add up quick and take up your channels!
One way to fix this is by using a stereo to mono summing circuit. This isolates the left and right signals and then joins them together to form one output that you can send to the sound board.
As we can see in Figure 1, we have our left and right channels. The easiest way to think of this is, think of an RCA cable. So you have your red and your white cables. The pin that sticks out of the RCA is the positive, the part on the outside is the ground or negative. This circuit takes the positives of both the left and right, puts them through a 1,000 ohm resistor, joins them together and then puts it into one single RCA jack. This circuit works well, you may loose a bit of volume using this, but it’s nothing a gain adjustment can’t fix!
One way to improve on this is to add an audio isolation transformer to isolate the audio ground. By isolating the audio ground you remove any chance of ground loops (humming/buzz).
As we see in Figure 2, there is a transformer added into the mix. This transformer, in a simple way of thinking about it, copies what is on the left side and pastes it on the right side while keeping both sides electrically isolated from each other.
In the photograph above is a old broken “Live Wire Solutions Direct Box SPDI” in which I removed the transformer. I separated the two 1/4 inch jacks and connected them together via 1k resistors, this goes to the -20dB pad switch, then goes into a 1:1 audio isolation transformer and then gets connected into the XLR connector. Pin 3 and Pin 1 are connected together on the XLR connector. Positive is connected to Pin 2. This box is now converting my stereo feed into a mono feed.
By making cables or small converter boxes you can “sum” the stereo to mono before your mixer and save yourself some channels. If you have any questions feel free to post below!