Boosting signal of a dynamic microphone

Question

Recently I've bought a Shure sm57 mic and I want to use it for vocals and acoustic guitar that I will record into my DAW on computer. When I bought it I didn't know this mic needs a lot of gain (around 50db).

My first thought after doing some research was to buy an audio interface, but a lot of people claimed that the pre amp in audio interface like focusrite scarlett 2i2 does not provide enough gain for this mic. This is why I decided to buy a separate pre amp.

I know that if you set pre amp's gain to more than 70-80% it starts adding a lot of noise. Because of that I thought that maybe it is a good idea to boost signal from my microphone through separate pre amp by 20/30db (around 50% of its overall gain potential) and then boost it again by 15/20db with a help of audio interface.

The problem is that this mic needs a lot of gain and I don't want to set the gain knob to 100% on my interface or pre amp. I thought that it will work fine if I enchance the signal from both pre amps (standalone one and audio interface's built in) and won't get as much noise as I would while turning knob on one device to 100%. What do you think about that?

I'm new to this stuff. If I say something stupid, I'm sorry. I'm just a newbie :)

Thanks!

Answer 1

You should ignore the incorrect claims by others about interface mic preamps and your understandable but not really accurate concerns about noise and go ahead with a decent but affordable audio interface that includes mic preamps, such as the Focusrite Scarlett series.

The SM-57 is one of the most popular microphones ever made, if not the most popular. It's not quite right to say that it needs "a lot of gain" in that it doesn't need an unusual amount of gain compared to other microphones. In fact, I would say that if you asked 100 audio engineers what's a reasonable amount of mic gain, they would name a figure close to what is needed for a typical signal into an SM-57.

You can't say that a mic needs "50 dB of gain", because the gain needed for a specific situation depends much more on the source, and less (but still a bit) on the type of microphone. In other words, for a very loud source, you won't want to add 50 dB of gain to the signal from an SM-57 close to that source. Some sources are loud enough that you need negative gain on the mic.

The point is, you probably can't buy a mic preamp that won't work well with an SM-57. It's so standard it's probably the first mic people reach for to test their mic preamp designs. And the mic preamps built into audio interfaces are generally pretty good these days.

Splitting up the gain between two different preamps will not, in any way, give you a better signal to noise ratio. It's more likely to make it worse. It will also just make your life harder, and it will cost more. Don't bother.

Answer 2

A dedicated mic preamp will indeed generally give you a better SNR than the ones built into a budget audio interfaces, but most of those are ok as well nowadays. I haven't used the Scarlett 2i2 myself, but heard it's pretty ok.

The main reason for many people to have an external preamp is to get a tube model, which colours the sound in a way that is often desirable (especially for vocals, but also many instruments). This can also be pretty well done in mixing though, with digital plugins.

The real bottleneck is the mic itself. Granted, the SM57 is a good all-rounder especially for live, mainly because it can take a lot of a beating and still keep working. It can also be used for recording (in fact the SM originally meant “studio microphone”). But condenser microphones are superior in terms of both signal level and clarity. So for recording, I'd rather save up for another good mic, before investing in a preamp. For the time being you'll probably be ok with the SM57 and built-in interface-preamps – for close-miking, the SM57 and 58 actually give pretty decent signal level on vocals and guitar.

What you propose about gain-staging isn't really relevant. Yes, on the electronic level, high-gain preamplification is best implemented with multiple stages of moderate gain, but in recording practice it's basically only the noise level of the very first input stage that dominates the noise level or the recorded track. So an interface with really good built-in preamps, e.g. an RME, can be used just fine without external preamps, and if the signal is so weak that it's not enough then this should be fixed at the mic rather than preamp stage.