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I'm trying to figure out the easiest way to record an electric guitar simultaneously to two tracks, one track getting the clean signal and the other getting a signal that has been processed through a Vox ToneLab ST effects unit. The problem I'm facing is that I need two line-level signals for recording, and the ToneLab ST doesn't have any additional DI output.

There are several obvious solutions, and I'm just trying to figure out the caveats they may have:

  1. Simply split the guitar signal and run it into both inputs. I didn't think the guitar signal would be strong enough to try this, so I haven't yet, but will. Should I be concerned about impedance issues? (The line input has 10 kOhms according to the manual.) If the two input impedances are very different, is this a problem, and can I fix it by just inserting a resistor?
  2. Insert a cheap dedicated guitar preamp between the guitar and the effects unit, and record its output signal. This seems like it is guaranteed to work, but will it negatively affect the live sound (which is the output of the effects unit run through a guitar amp)? I guess it just comes down to: How clean will a cheap preamp actually be?
  3. Split the guitar signal before the dedicated preamp, and use the preamp only for recording. Seems like it could solve both issues, since the guitar signal would be connected to two high-Z inputs, and everything else stays the way it currently is.
  4. Mod the effects unit to add a clean output, just behind the built-in preamp. Is this even practical? Would a short circuit (e.g. due to plugging/unplugging a cable) damage the preamp?

I hope this is the correct place to ask. Other solutions would be welcome as well, of course.

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Simple split

I didn't think the guitar signal would be strong enough to try this

Splitting in itself does not "take away" anything from the signal. It's no problem at all to use a Y-cable and feed the guitar to two guitar input (not a signal problem, at least – you will probably get ground-loop hum, however). The only relevant thing is the total parallel impedance you're feeding. We can ignore the Vox' guitar input here: as the 10k line input is so much lower impedance, it dominates. And unfortunately, guitar pickups are ancient, horrible design that can't really handle such a low impedance (it takes away all the pickup's resonance and turns it into a pretty dull low pass).

In principle, you can indeed fix this by inserting a resistor, but it needs to be really lot bigger – I'd recommend at least 200k, better 500, and with that the line-input's level gets even lower, which probably results in unacceptable SNR.

The only "correct" configuration of two parallel inputs, as you say, is if both are High-Z to begin with.

Cheap guitar preamp, to amp & rec

I wouldn't use a preamp that deliberately alters the signal in any way except perhaps gentle tube saturation. An actual guitar preamp is not a good idea, but a mic preamp (those used for voice in studio recording) with instrument input can be a good choice indeed. However, even that does rather "too much" for the amp route.

Split before cheap preamp

Again, both High-Z is crucial, so this should work. If the preamp doesn't alter the sound in a "destructive" way, this can be ok.

Mod the effects unit

Sure you can, but it's probably not worth the effort & damage risk.

What I'd recommend

Use a standard active DI unit. These have high impedance inputs (though usually not as high as optimal), zero-processing parallel outs to go to the amp/processor, and clean&stable mic/line-level outputs for recording.

  • +1 for active DI. This is a standard industry solution. – Rory Alsop Nov 4 '14 at 10:25
  • Thanks, that is really the perfect answer to my question! By "I didn't think the guitar signal would be strong enough to try this", I actually meant "to record it via a line input". I didn't realize that the impedance has physical implications as well, very interesting. Maybe I overlooked a really simple solution, though: I need to ask the guitarist whether he can insert the effects unit into the effects loop of his amp instead. Then I can hopefully just record its input and output signal. Otherwise, I will go the route you suggested. Thanks again! – Sebastian Reichelt Nov 6 '14 at 19:13

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