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Re: XLR A/B switch? design help...

Posted by Jacques (A) on June 15, 2005 at 17:46:46

In Reply to: XLR A/B switch? design help... posted by joe for tone on June 14, 2005 at 13:08:28:

See stompBoxStyleSwitcher at the given URL. Some remarks: R1 & R2 avoid letting the switched out gear inputs floating. Such a 33K load won't be noticeable with modern sources and their low 10's ohms output impedances.
R3 & R4 won't let the switched out load inputs be floating. When floating, some DC can build, which stores in the cable and input capacitance, and is very quickly discharged when switched on again, and you get a click. With R3 and R4, no differential DC can build with any healthy equipment.
So, i should bet that, with things done this way, you won't get any click even without a mute. So, replace R5 and R6 by shorts and don't wire the momentary switchbutton SW1a. In case you need the mute, wire the momentary switchbutton (not to be on for long period of time, as the load impedance as seen by the source ois only 100 ohm. A few seconds is ok) and the resistors R5 and R6.
Use a metal box to encase all that stuff, don't forget to minimize the area along the current paths by using twisted wires as shown in the schematics. Note also the condenser between XLR's pins 1 and the chassis. Its purpose is to provide a RF reference point while blocking any 50/60Hz leak.
Oh, I forgot the LEDS... well, you would need a 4PDT switch, and I don't think it's available at reguilar prices. So, instead, you could use a toogle switch for the 4PDT, its axle showing the selected channel? For the toogle switch, look at APEM serie 11000, professional switches, use gold plated silver contacts
APEM, POBOX 544, Wakefield MA01880 877-246-7890
or any other professional switch. Don' use for the 4PDT a switch which has been used to switch high currents and/or voltages, since the contact surface gets cracky and can generate noise. Use a new one.You can get such a switch for around $20. The requirement for the momentary switch is less stringent, as no modulation pass though it (but when muted). Say, $1...

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