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I have an idea for a project where I want to pass a line level signal through my electric guitar.

The idea is to have a 3.5mm jack input on my guitar, to which I can input a line level signal. Then I want this signal to go through the guitar lead and into my amp. I also need to be able to play guitar at the same time, so this signal also needs to go to the amp.

My initial concerns are that the line level signal will be far too hot for the amp, and that also the signal might interfere with the pickups, so may need to be isolated somehow (sorry, it's been many years since I dabbled in electronics).

Also, I realise that this will sound terrible - this isn't so I can jam along with my iPhone, its for some specific weird effects.

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    \$\begingroup\$ Why? It doesn't appear the guitar is doing anything to this signal other than passing it on. If so, just run it into another input of your amp, or mix after the guitar amp has turned the guitar signal into line level too. Ask about the real problem, not some strange solution to it you dreamed up. \$\endgroup\$ – Olin Lathrop Jan 15 '16 at 12:24
  • \$\begingroup\$ Yes, I just want to pass the signal on through the guitar, at an appropriate level. The idea is to install an iPad with custom software onto the guitar, so that I can trigger sound effects whilst playing. I want to run it through my effects path, so it would need to run down the same cable as the guitar signal. \$\endgroup\$ – ricky_g Jan 15 '16 at 12:41
  • \$\begingroup\$ You'll really need some kind of a power supply for the guitar in this case, since using a passive summer for the signals would really wreak havoc with the impedance of the guitar. With a PSU you can simply use an op-amp summer. This will also change the impedance, but in this case similarly to what a buffered pedal on your pedalboard would do. \$\endgroup\$ – Timo Jan 15 '16 at 12:46
  • \$\begingroup\$ If you intend to use the iPhone as an effects machine forhte guitar then what you probably want to do is something like an iRig adapter that runs your guitar pickups through the phone, then run the phone output through an attenuator and out the 1/4" connector that your guitar already has. \$\endgroup\$ – JRE Jan 15 '16 at 13:16
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Since you're modifying the guitar then you could do the following:

  • Replace the mono guitar lead socket with a stereo lead.
  • Wire pickups output to tip and sleeve as before. The guitar will work as a normal guitar.
  • Wire your 3.5 mm plug sleeve to 1/4" jack sleeve.
  • Wire your 3.5 mm plug tip to 1/4" jack ring (middle contact).
  • Make a stereo 1/4" guitar lead. Balanced microphone cable should be good for this.
  • Make a splitter box for your amplifier. 1/4" stereo in, 1/4" mono out for guitar amp, whatever connector you like for second amp or second channel on guitar amp.

The advantages with this setup:

  • Guitar still backwards compatible.
  • Independent volume and tone possible - sound effects flat - guitar distorted, flanged, etc.
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  • \$\begingroup\$ I really need to pass the 3.5mm jack signal through my guitar pedals too though, since I use a lot of effects and loop pedals. I expect I could use an active mixer just before the first pedal in my chain though and use your solution. Or possibly fit an active mixer in my guitar and use a mono cable from there... \$\endgroup\$ – ricky_g Jan 15 '16 at 14:51
  • \$\begingroup\$ Yes, either would work. \$\endgroup\$ – Transistor Jan 15 '16 at 19:01

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