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Last Saturday I attended Skinny Living concert in Cologne, and some stage setup puzzled me. What is the purpose of taking the output of an amp with a microphone, such as on this picture?

A microphone in front of an amp

The venue is relatively small but well equipped, there were about 60-70 persons in the public and a stage of the corresponding size, if this is relevant.

Michaël Le Barbier
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3 Answers3

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This is standard procedure for electric guitars. See, a guitar amp is almost rather part of the instrument than just a loudspeaker: it critically shapes the sound. You might say the speaker cabinet is a substitute for the body of an acoustic guitar: its primary purpose may be to just get you heard, but there's a lot more than just loudness to the art of building well-sounding guitars. You want to boost a wide range of frequencies, but some more and some less†.

So, you get a very different result if you just plug the guitar into the PA, compared to the “proper” amp sound that's usually desired. Of course, if the amp is loud enough to get to all the audience then you don't need a PA at all for guitar – but such a loud amp can be quite a pain for the other performers on stage. It also isn't great for overall band sound because it bleeds in the vocal mics etc.. And for bigger venues, even a couple of Marshall stacks may not be able to get the guitar sound satisfyingly to the back rows. Modern PAs are much more effective at this.

Hence it's usually better to have only a relatively quiet guitar amp (merely loud enough so everybody on stage can hear it enough, particularly the guitarist herself) but use a mic to bring its sound to the audience too, much like you could use a mic to bring the sound of an acoustic guitar to the audience. (Just, because even a quiet amp is much louder than an acoustic guitar, it's much easier to actually get it heard well.)

What's a bit strange about this particular setup is that this seems to be an acoustic-guitar amp. Acoustic amps are different from electric-guitar amps: they are designed to not shape the frequency response much, but give the clearest possible representation of the guitar's own sound. Hence it makes IMO little sense to mic an acoustic-guitar amp. But some guitarist prefer to do it this way nevertheless.See RockinCowboy's comment below.

†Most electric-guitar cabinets are strong in the midrange (with different scoops and peaks), which is good for a powerful presence in the mix, but they cut off the high treble range rather strongly. This is particularly important for the distorted sounds typical of rock/metal guitar; those would be really annoying if you played directly into the PA.

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leftaroundabout
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  • Voted up. Very well put. There is a whole sound science around this. The OP should have googled a bit more, I think. – blusician Nov 07 '16 at 01:22
  • Thank you very much for your detailed answer! Still some amps (if not all) have an output line that could be plugged into the PA – but then the musicians would not hear the instrument – only the mix – which also speaks for the setup I took a picture of, right? – Michaël Le Barbier Nov 07 '16 at 06:41
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    @MichaelGrünewald it's certainly possible to route the signal from an amp straight on to the PA; you can still use the amp's speaker for monitoring. But the line-out on electric-guitar amps is not intended to be plugged right into a mixing console. It's intended for extra effects and/or another guitar amp. This output signal sounds really nasty over full-range speakers. An output that can be plugged into the PA will be labelled _DI out_. Acoustic-guitar amps have them and modern bass amps too, but electric-guitar amps usually don't. – leftaroundabout Nov 07 '16 at 08:59
  • Many modern guitar amps have DI outs with a speaker emulation circuit. They can't stand up fully to a well-miked amp, but if you are limited in your selection of mikes (e.g. as a visiting engineer), then you can get quite some mileage out of them. The are often also wired *before* the main volume knob which gives you a consistent level even if the guitarist turns up his amp over the course of the gig (as they invariably do). – Jörg W Mittag Nov 07 '16 at 09:59
  • Worth noting that the choice of microphone as well as its placement also has a large impact on the final sound. Many characteristic guitar "tones" comes from a combination of instrument, effects chain, amp, cabinet, microphone, and microphone placement. – J... Nov 07 '16 at 14:00
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    Great detailed answer. But as far as I know the Roland "Cube" amp pictured is an electric amp and looking at Roland web site under products, Cube amps only appear under Electric and Bass amps. Roland makes some great acoustic amps but they are not "Cubes". Some of the Cube model amps have an acoustic simulator built they all offer channels for clean or distorted electric. The newer Cube amps have iOS connectivity as well as a phone/recording out jack. – Rockin Cowboy Nov 07 '16 at 16:59
  • @RockinCowboy I have a micro cube and it does have one "model" in its model selector that is meant for clean amplification for acoustic guitars or keyboards or whatever. It doesn't sound very clean but it also doesn't sound like a guitar amp. Either way, in the pictured situation, I'd guess a model suited for electric guitars was selected. – Todd Wilcox Nov 08 '16 at 04:01
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There are many electrical advantages to "air-gapping" the PA from the backline. You avoid ground loops and other kinds of issues that arise from connecting equipment directly. Even more important, changing from one band to another is much easier when you just move the mic out of the way, let the guitarist/tech set up the back line, and then place the mic. No impedance matching, ground lifting, adapter cable finding, or other annoying tasks required.

I suppose this isn't as true as it once was, but for many amplifiers, you have no choice but to put a mic in front of them! Classic all-tube amps almost never had any kind of output that could be run to a mixer. Modern tube amps and of course digital modeling amps can often have "recording outs" or "main outs" that can be run to a mixer, but they usually sound pretty lame at best.

Also the microphone is an extension of the PA system, which is kind of like an instrument played by the FOH engineer. As an engineer, it helps a lot to get to know a set of mics really well, and then use those mics in ways such that you can control the sound coming into the mixer with great detail - like getting familiar with a certain guitar. Using the same mic for all the guitar amps that you see from different bands helps put a similar sonic stamp on the electric guitar sound, which really helps make mixing decisions quickly.

I greatly prefer a stage setup where everything is miked and nothing is on a DI. Usually keyboard players do not bring an amp, and I suppose that for a lot of gigs, a DI from the bass head is best if you can get it, but there is always that annoying part of figuring out how to plug the bass DI in.

Todd Wilcox
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    Good answer as usual, though I definitely don't agree with your preference for miking everything up. Nowadays it's fortunately getting standard to have properly matched DI outs included in bass amps, and higher-end keyboards also start employing XLR outputs. Why it hasn't always been done this evades me – a good DI connection is so much more reliable than anything involving a mic! Plugging an XLR cable in a bass amp is also significantly faster yet than properly aligning a mic to a guitar amp's speaker. Really, guitar-cabinet sound signature is the only good argument for a mic. – leftaroundabout Nov 07 '16 at 09:14
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    Good answer. So far nobody has mentioned one other advantage to miking a guitar cabinet. You can alter and shape the sound through the PA (to get the sound you want) by simply changing the placement of the mic in front of the cabinet. The distance, the angle and the part of the cabinet the mic is pointed at will all affect the sound of the output. You don't have that ability with a DI connection to the PA. – Rockin Cowboy Nov 07 '16 at 17:09
  • @RockinCowboy: yes, but that too can actually be a big disadvantage, at least live. When you're in a hurry to get the backline ready for the show, the last thing you need is lots of millimetre trial-and-error till the guitar sounds right. In practice the mic will often just be jammed somehow in front of the speaker (“that'll have to do”), and the sound only fine-adjusted through the mixer channel EQ while the band is already performing. In this regard, a digital convolution cabinet is just as flexible while eliminating the error potential. – leftaroundabout Nov 07 '16 at 18:13
  • @leftaroundabout In *my* practice I don't just plunk the mic down, but I also don't worry about millimeters. I use experience and intuition to place the mic, with a few go-to placements. Of course there is "error potential" when working live sound, but audiences can be very forgiving, especially if the instruments can sit well together. That's one area where digital modeling really shows its difference - it just doesn't sit in a mix as easily - even when miked! Also, I have found DIs from bass heads to be **less** reliable than mikes, because you don't know where that head has been! – Todd Wilcox Nov 07 '16 at 23:23
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    For a fact, many digital amp simulations are horrible. In particular, they often seem to be designed to sound good when playing just guitar alone over headphones (perhaps the problem is just that the owners set up their presets in just this way). The result, as you say, is that such a guitar will never properly sit in the mix. But it's not fair to generalise this to “_all_ digital cabs don't sit well in a mix”. My dream setup for live guitar would be: a nice small tube combo on stage, DI parallel to the speaker through convolution response (recorded from the same amp under studio conditions). – leftaroundabout Nov 08 '16 at 19:52
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This looks like an guitar amplifier. The amplifier is a key part of the sound of an electric guitar. If you want to capture that sound, you need to mic the amp, rather than using a DI.

I believe you can get amp modelling pedals, but I've mostly seen microphone setups like your picture.

A counterexample would be a keyboard amp. The amp here is usually not part of the desired sound, so we bypass it with a DI. Note that this is not true for some keyboard sounds (organ with Leslie, some electric pianos like Rhodes and Wurlitzers).

endorph
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