Custom A/B Switcher - Schematic
|This is a
design to built your own a/b switch with one input and two outputs (or the
other way around), it needs a latching footswitch to change the
signal routing and DC Volts, the amount depens on the relay. There are
many a/b stompboxes available, but they are useless for people who use 19"
racks. So this design is especially for those who want to use an a/b box
which can be controlled by a normal on/off footswitch.
This is a quality design that should give you noiseless switching. It switches the output to the ground when it's not used. This design is going to be used by me as a mute switch for my preamp and the latching footswitch connection allows me to control it with a Midi Controller such as the Nobels MS-8. So this is real handy for all you 19" rack freaks. If you use some quality components and good cable then this a/b box shouldn't alter your sound.
The relay switches when the tip of the footswitch is connected to the ground. The unused output goes through a 50 Ohm resistor to the ground, which makes sure that the poweramp stays quiet and noiseless.
If a popping noise is audible when switching, then connect a 1M Ohm resistor between the tip and ground of each in- and output (not the footswitch jack). I've done this to my a/b switcher out of precaution and the switcher is in my setup dead quiet.
If you use it as a muter and there is still some (switching) noise, then add a 10K resistor to pin 9 and connect it to the ground. This is ofcourse not necessary when you use it as an A/B box. But you could also just built it as a A/B box and place a jack connector with a 10K ohm resistor between the tip and the sleeve and leave it in the output B. Then you can always use it as a A/B box if you want to. This 10K resistor keeps the output of your preamp "loaded", and not open. I've tried this in my setup, but it wasn't needed
The bridge-rectifier is there to enable the use of most common DC adapters, no matter it's polarisation. If you place a 470uF elco between the + and - of the bridge-rectifier you can also use an AC adapter. The voltage for a DC adapter has to be a minimum of 9V and maximum 15V (or 12V AC). For other coilvoltages (for the relay) you should change your supply-voltage!
Pay attention to the -(negative pole) of the bridge-rectifier, it should not be connected to the ground of the signal, this could cause a hum. The switching-circuit of the relay is completely seperated from the signals!. For the best result it is advised to use a shielded box (the once with conductive coatings on the inside are real handy or for maximum stability use a aluminium box) to prevent noise. The shielding of the box is then connected to the signalground.
Keep in mind that with this design the possibility of groundloophum is still there when you use it as an A/B switch to switch signals between multiple devices that all have ground. Using the same outlet for the equipment might help prevent this. If you want to prevent groundloophums caused by the grounds of the devices, I advice to use isolated connectors, then the connector doesn't make contact with the box automaticly and can decide which outputs you want to ground. You could simply add an on/off switch to lift the ground of each connector and the box if you prefer that. This should keep those groundloophums away, although I only advice to do this when a groundloophum does occur. You could also add another DPDT relay to switch the ground from
the input to outputs.
Resistors: 10 kOhm and 50 Ohm
Diode: 1N400x x = anything, these are all 1A DC
Connectors: isolated connectors are prefered, adapter connector
Case: shielded box
If you are not known with these kind of projects, take a look at Aron Nelsons' webpage. It has a good FAQ.