Source Selector for Car Audio (a - b switch) wiring help - diyAudio
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Old 20th February 2013, 12:26 AM   #1
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Default Source Selector for Car Audio (a - b switch) wiring help

I have a Volvo that uses fiber optics from the HU to the amp and is not expandable.

I would like to add a standalone iPhone -> Pre-Amp -> Amp and then have a "junction box" with a switch that will switch the speakers from the factory amp to the second standalone amp.

I have four channels of audio to switch (LR, RR, LF, RF). Is there a switch that's appropriate for this? I think I could use two DPDT switches (one for front speakers, one for rear), but I'm getting confused about how to wire that and if it is ok to use a common ground.

Any suggestions appreciated.

Thanks!
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Old 20th February 2013, 03:32 AM   #2
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You need 1 DPDT relay for each pair of wires going to the speakers. The speakers will connect to the movable contacts of the relay. The two amplifiers will connect to the stationary contacts of the relay.

Be VERY careful when doing this. If you make a mistake and damage the OEM amplifier, it may be VERY expensive to repair.

You can NOT use common grounds for the speakers.

If you don't understand relays, read the 'relays' page of the car audio site (link in sig line below). It's #36 in the directory.
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Old 20th February 2013, 02:37 PM   #3
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Thanks Perry. Will this work? I didn't draw the connectors for the rear yet, but it would be the same...

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Old 20th February 2013, 02:57 PM   #4
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No. You cannot have any wires of the two amps directly connected. You have to switch ALL speaker wires.
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Old 20th February 2013, 04:12 PM   #5
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Thanks - would a set of relay's like this be appropriate?

12VDC/3A DPDT Plug-In Relay & Socket : Plug-In Relays | RadioShack.com
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Old 20th February 2013, 04:32 PM   #6
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The current rating is marginal and only suitable for low power amplifiers.

If you use radio shack relays, you most definitely want to use sockets. When I used them (years ago), they had a high failure rate. There are better relays but they're significantly more expensive. The higher current relays at radio shack would likely be more reliable due to higher contact pressure.
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Old 20th February 2013, 04:53 PM   #7
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Are these a better option?

12VDC Coil DPDT Miniature PC Relay : Miniature PC Relays | RadioShack.com

I'm only using an Alpine 4 X 45W RMS @ 4 ohm amp so not trying to push monster power through them.

Thanks for all the help.
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Old 20th February 2013, 05:24 PM   #8
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RS#: 275-218 plus the socket may be a better choice.

The 3 amp relay you listed will work but you should expect to replace relays from time to time.
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Old 20th February 2013, 06:51 PM   #9
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So am I on the right track now? (again missing some wiring, but should give the idea). I'm not sure if I should wire the relays in parallel or in series?

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Old 20th February 2013, 06:59 PM   #10
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It looks OK but there is too much blue on blue to tell definitively.

Don't forget an appropriate fuse for the wire you're using where you tap for 12v.
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