1/4" to XLR A/B Box? Am I an idiot?

[guitar_santa]

Hi all, I searched for anyone info for this kind of project but didn't find anything. I'm sorry if this is a repeat.

I have a Scarlett 2i2 interface, into which I run stereo microphones. I use that rig for teaching GED classes as well as guitar lesson online.

I've been toying with the idea of running my guitar rig DI into my Scarlett and doing a little streaming or whatever, and so what I've been doing is unplugging my mics and plugging in my guitar cable from the pedal board. I'm more or less satisfied with how it sounds, but I'm lazy and would like to build myself a little box that would let me switch between the guitar rig and the microphones like an A/B thing.

Is this a profoundly stupid idea? Is there some impedance or other such thing I'm overlooking?

[FiveseveN]

Welcome to the forum!

Is there some impedance or other such thing I'm overlooking?

Yes. You can't plug in a guitar into the XLR mic input, it has 3kΩ impedance. While the jack input in INSTrument mode is 1.5MΩ and also has some more headroom. And of course, you can't use both unless you hack together an unholy chimeric connector, which would be more involved than the actual switching part.
If the extra convenience is worth the signal degradation (which might be slight but you'll have to test) you can buffer the guitar before switching. I would just get an interface with more inputs.

[guitar_santa]

I figured there was something like that.

So, 3kΩ  is too low, right? Because of the low output impedance of the last pedal in the chain (a GE-7)?

Adding a fat, juicy resistor or something wouldn't help I assume? I'm probably better off getting a Solo or somesuch and plugging it into a different usb or upgrading to a different interface?

I'm very dumb at this so I appreciate your help!

[FiveseveN]

No, it's the other way around: you want the lowest source impedance driving the highest load impedance, so a resistor can only make things worse. However, you said

plugging in my guitar cable from the pedal board

and I assumed there was a guitar at the end of that cable. The GE-7 does buffer the signal, though it's not particularly low impedance: BOSS says 1K and "Recommended Load Impedance 10 k ohm or greater". I'd say get a jack to XLR adapter/cable and try it out for starters.

[guitar_santa]

No, it's the other way around: you want the lowest source impedance driving the highest load impedance, so a resistor can only make things worse.

Okay, so the interface would be the load? So we want a high input impedance on the interface and a low output impedance on the pedalboard?

The GE-7 does buffer the signal, though it's not particularly low impedance: BOSS says 1K and "Recommended Load Impedance 10 k ohm or greater". I'd say get a jack to XLR adapter/cable and try it out for starters.

Interesting. So (if you don't mind my relentless questioning, feel free to tell me to STFU) how do buffers affect impedance? Or do they not really?

What is a good frame of reference for impedance - like what's a large and small amount of impedance? Am I correct that amps typically have like 1M or so input impedance?

[FiveseveN]

Okay, so the interface would be the load? So we want a high input impedance on the interface and a low output impedance on the pedalboard?

Right.
A buffer presents a light load and a stiff source (i.e. one that can drive heavier loads), it's a transparent impedance transformer. Helps fragile signals get where they need to be.

Am I correct that amps typically have like 1M or so input impedance?

Yes, 1M is conventional for instrument inputs in both amps and pedals.
The thing about impedance is it's complex, described by a real and an imaginary number, not just an amount. 1M is a nominal value for the frequency span we're interested in. A pickup may be 15K at DC (0 Hz) but it will rise consistently due to its high inductance until its resonant peak (5—10 KHz), then drops down again as the capacitance becomes more significant than the inductance. This is why you lose treble first as you load down pickups.
So it's a fairly deep rabbit hole if you want to go down it. The basics have been described in many posts here using various analogies, just look around.

[guitar_santa]

I'll have a look around. Thanks for your help!

[PRR]

......I assumed there was a guitar at the end of that cable. ....... I'd say get a jack to XLR adapter/cable and try it out for starters.

There's another trick missed here. This particular "interface" (like many now) has COMBO XLR & 1/4" jacks. These are really two separate inputs. The XLR connection goes to a high-gain 3kΩ microphone preamp. The 1/4" connection goes to a lower-gain 1.5MΩ input. https://focusrite.com/en/usb-audio-interface/scarlett/scarlett-2i2

> unplugging my mics and plugging in my guitar cable from the pedal board.

Maybe because I spent decades plugging and unpluging audio, but....

That seems to me an ideal method. You'd have to be switching-over hundreds of times a day to justify the extended labor to build both a mike preamp and a guitar preamp and a switch. Before that you could look in BanjoWorld and see how much for a four-input interface with both types of inputs.

[ashcat_lt]

If the mics are dynamic (don't need phantom power), you could probably do it the other way.  XLR>TRS from the mics into the instrument input should work fine.  Unless you've already got the gain all the way up, you won't miss that 3db or so difference. 

If the mics are condensers (need phantom power), well you wouldn't want to send that to your guitar pedal anyway.