Using Mechanical Relays - Part 2 - Signal Switching
For audio switching, the DPDT relay is most useful. It allows you to select how the input signal will be redirected, as well as where the output signal originates from. We will examine a simple, yet common use for audio signal switching.
There will be 2 modes of operation - they are the Pass-Thru Mode and the Signal Processor Mode. Many Guitar Stomp Box effects operate in this way - when you turn them on you get the effect enabled, when you turn it off, there is no effect enabled. We are going to provide the same function using a DPDT relay.
In the Pass-Thru Mode you want to have the audio signal be unaltered - This will be the default operating mode, and as such, you want the failure mode of the relay to be the Pass-Thru Mode - This means that you will be using the NC (Normally Closed) and C (Common) connections of the relay switch for this function.
In the Signal Processor Mode, the input signal will be routed thru the NO (Normally Open) and C (Common) connections of the switch. This means that the Signal Processor will only be activated when the relay is energized. If, for any reason, the relay is no longer energized, then the relay will revert to the Pass-Thru Mode.
NOTE: You will notice that this design is for single sided unbalenced signals. If you wanted to utilize balenced signals (such as Low Impedance XLR microphone connections), you would need a 4PDT (4 Pole, Double Throw) relay to handle the 2 additional signal lines.
Signal Ground is shown here as well. A component of all analog audio signals is Signal Ground. While the relay contacts won't be switching Signal Ground and really don't need it for anything, you can rest assured that the Signal Processor has to have it, as does any mixer/power amp stage down the line. The problem with Signal Ground is that if wired incorrectly, it can be a source of impossible to locate hum somewhere in the wiring. The solution to this is pretty simple, but it may require you to make up special cables to prevent ground loops. You want a single Signal Ground connection at the input, a single Signal Ground at the output, and only 1 Signal Ground connection going to each Signal Processor that is being switched.
NOTE: You may hear a 'pop' as you switch to the Signal Processor - this is often caused because there is no ground reference for then Signal Processor input while operating in Pass-Thru Mode, and when you switch to the Signal Processors input, its likely to be at some other voltage level than Signal Ground. If you have this problem, add a 1 Meg resistor between the Input and Signal Ground on the Signal Processor - this will maintain the ground reference when there is no actual input to the Signal Processor.
You will need a power source for the coils on the relays, as well as On/Off switches to cause the relays to energize and cause the connections to change to Signal Processor Mode. You may want to also know the status of the On/Off switch that controls the relay.
I prefer 12 volt relays, mostly because they are used in automobiles and as a result, there us a large selection of different switch configuration relays available from any sources. In the USA, the local Radio Shack often has many to choose from. Auto Parts stores also sell them (but they may be marked differently than I show here - you may need to use a VOM to figure out which connections go to which switch contacts. Many electronics parts catalogs also sell mechanical relays (some examples are: Mouser Electronics, Parts Express and MECI Surplus Parts). I also prefer DC (Direct Current) Relays, mostly because I don't want a 50 Hz or 60 Hz power source (in the relay coil) that close to my low level audio signals - Anything that avoids additional signal interferance is a good thing.
To control the relays, a set of On/Off switches might be arranged like this:
As you can see, the control of the magnetic coils that operate the switches have no direct connection with the signals being switched - that means that any signal type can be switched without knowing much about the signal processor, or having to match up its internal voltages in any way (which is often the case if you are attempting to use solid state switches).
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