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Old 2nd January 2007, 03:42 PM   #1
toolkit is offline toolkit  United Kingdom
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Default Wiring a rotary selector switch

I am reboxing and doing the standard mods on an SI T-amp but want to include a simple selector switch so I can switch inputs between CD and Phono preamp.

The question I have after sitting here scratching my head and getting confused about poles/throws etc is what kind of switch I need to get, i.e. how many pole - 3 (L+R+Grnd) or 4 with both grnds - do I combine the grounds on input or should I switch them separately? Does that make sense?

If anyone could even suggest an available part it might help me see through the fog. I am currently thinking the 3 or 4 pole break-before-make switch here: http://www.maplin.co.uk/Module.aspx?...o=2417&DOY=2m1
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Old 2nd January 2007, 03:51 PM   #2
EC8010 is offline EC8010  United Kingdom
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It's unusual to switch grounds as well as signal, but if you're going to do it then you might as well go the whole hog and switch them separately. A four-pole switch...
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Old 2nd January 2007, 04:33 PM   #3
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If youre happy with just a couple of inputs then a rocker switch also works (double pole) All signal earths are then tied together at the phono inputs.

The switch you've identifed is acceptable, anything with more than 2 poles, you can always ignore the ones you dont need...

Andy
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Old 2nd January 2007, 06:57 PM   #4
Nordic is offline Nordic  South Africa
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I have succesfully used the method andrew described.

All grounds connected and 2pole way switch allows me to choose between 4 sources.

You have to try and position the switch close to the connectors (keep wires short as possible to prevent crosstalk), and maybe use a long shaft to reach it from the knob.
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Old 2nd January 2007, 11:12 PM   #5
Pano is offline Pano  United States
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I use the switch you linked to. Works well. I use the 2 pole 6 position version.

Just try your grounds together were they come in via the RCAs. Only the signal hot wire (RCA center) needs to go thru the switch. Bring left out of one of the tabs in the center, right out of the other.Then go to the volume pot - then to the amp.

You'll need some way to tell which of the outside tabs is connected to the center tabs, and at which position. Just use a continuity tester of some kind to map out the connections.

I have also used the 4 pole 3 way version. Either way, I like to tie the unused tabs to ground.
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Old 2nd January 2007, 11:20 PM   #6
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What they said...

With two inputs, there is not much reason to use a rotary... and not much reason to switch the grounds. In this case, go buy a rocker or toggle switch... you will be looking for a DPDT On-On configuration. That will have two positions, either source 1 or source 2, no OFF. Only the L & R wires get connected to the outside pins. The center pins go to the inputs on the board. Try it, you will like it. You might actually be able to get these at a hardware store.

I have an intuition that the toggle will be longer lived than a rotary. Much simpler construction, fewer parts, etc. Did I mention cheaper?

EDIT: The closer the inputs to the switch, the better. The long leads can be the "outs" from he switch. You can shield those if you think it's necessary.
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Old 3rd January 2007, 10:12 AM   #7
EC8010 is offline EC8010  United Kingdom
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The reason for switching grounds as well as signal is to reduce ground noise.

The specified rotary switch is more suitable for signal switching than a randomly chosen rocker/toggle switch that may have been designed for switching power.
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Old 3rd January 2007, 11:40 AM   #8
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Quote:
Originally posted by EC8010
The reason for switching grounds as well as signal is to reduce ground noise.

You mean that you are connecting/disconnecting the grounds that come from the sources before they reach the amp chassis?

Never tried that. But I haven't been having ground noise problems.
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Old 3rd January 2007, 11:53 AM   #9
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by panomaniac
You mean that you are connecting/disconnecting the grounds that come from the sources before they reach the amp chassis?
That's right. If two sources were bonded to mains earth, switching grounds would eliminate hum loops. Video recorders, in particular tend to use switching supplies with a pair of capacitors from line and neutral to chassis, effectively sitting the chassis at half mains voltage - that will certainly inject noise.

Although I've not yet tried it myself, I think switching grounds makes a lot of sense.
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Old 3rd January 2007, 08:39 PM   #10
moamps is offline moamps  Croatia
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I've designed and built a few preamplifiers where only the ground of the active source was connected to the ground of the input circuitry of a preamp. Switching the ground is not as easy as it may seem because the ground must be connected before the signal is connected so as to avoid an unwanted impulse (spike) at the output of the preamp. The impulse may occur if the signal connection is made before the ground connection. It is therefore adviseable to use make-before-the-break switches for input switching, which means that a simple toggle switch may not be such a good idea. The best idea yet may be to use some specially designed (a la carte) multiple-deck switches with different switching sequences.
Generally speaking, I think that ground switching is an excellent idea for minimizing the noise caused by local ground loops (as someone has already pointed out).

Regards,
Milan
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