PRO AUDIO
The Unique Geek

The Unique Geek
November 16, 2011

What’s a preamp? And why do I need one?

When I used to think of preamps, I used to think of them as a lifeless utility device. I suppose they are, ultimately – I’ve never had a conversation with one and they do serve a function. But, preamps also have their own personalities. Let’s go back to how I first really knew they existed.

That’s a preamp too?

Way back in time (a land before the laptop computer) I owned an acoustic guitar and bought a pickup to put in the guitar because I was too broke to own both an acoustic and an electric. But, recording the guitar was always awful because I captured a ton of noise and it was always so darned quiet. I don’t even want to tell you what I was recording on, it’s too dark, I can’t go back there – just know that there were about 10 people recording audio on the computer back then and I wasn’t one of them. I went to the local music store to buy a fun pedal that would make my guitar sound cooler. I mentioned to the guy behind the counter (while I eyed the cool noisemaker pedals) about my problems with that stupid, quiet guitar. “Oh, you need a preamp”, he said, “that’ll make it louder.” This, too, was in a pedal form. When I got home and plugged it in, I was shocked to learn that it did make my guitar audible. The only down side was it didn’t echo, shimmer or sound like I was playing in a canyon. It was one of many “recording ‘duh’ moments” I’ve had over the years.

So, what was it I was suffering from with that weak guitar signal? Just an “instrument level” signal is all. It turns out mics, guitars and basses are all about 20 to 70 decibels below “line level.” The terms “instrument level”, “mic level” and “line level” simply denote the level at which these products put out voltage. Instrument pickups’ signal strengths are closer to mics (some actually are microphones) and line level signal strengths are commonly found in CD players, MP3 players, amplifier outputs and such. At the end of the road, or near it, for a microphone or instrument pickup, the signal needs to get to line level. Then, that signal is strong enough to amplify (guitar amp, PA, what have you) or record. So, it is utilitarian in the sense that you have to get the signal strength to a good level to reproduce the sound. Then you have to consider the myriad of choices you have in what kind of preamp you use.

The $50 mic pre

The $4500 mic pre

Like anything in life, there are many companies out there that are willing to take your money under the premise of offering you a solution to your problem – the weak signal. If you aren’t sure how important the preamp is, just consider that you can get a $50 two-channel preamp all the way up to $4500 for two channels. Most of the preamps we use are just there and we don’t really think about them; the mic input on a mixer, the mic or instrument input on an audio interface and even the input on a guitar amplifier. At that connection – when you see the gain knob – that is the preamp you are going into. So, consider this: if there is such a thing as a $5000 mic preamp, how good is that input you are putting the mic or instrument into?

Every single component in the signal chain colors your sound. Think about the signal chain for a moment. For just a microphone with a simple path, there is a lot of opportunity for something to influence the original sound you captured in the mic – be that a vocal or whatever. The signal chain for the mic would be: the mic itself; to cable; to preamp; to analog to digital convertor; to recording software (which has its own sound believe it or not); to digital to analog convertor; to cable; to speakers. The preamp is an important step – you are converting its signal. The same can be said for the A/D (analog to digital convertor); it’s converting an analog signal to a digital signal – and this is another important task.

Look Ma! It’s comes with preamps

What many people don’t realize when they choose a recording interface, for example, is that the preamps in the box have their own sound characteristics and so does the A/D section of the box. How do these preamps sound? How does the A/D sound? If you choose a preamp separate from the audio interface – will it sound better? That depends on what you choose. But, suffice it to say; if you have a preamp that you like the sound of (let’s say it is an old tube preamp that you think makes everything sound more juiced up), then you need only select a good I/O with good A/D & D/A.

So what makes a good preamp? Well, for one thing – how does it sound? One of the things you can get from a great preamp is no discernable character at all. Imagine that. I went to a friend’s studio ages ago, and he had just bought a $2500 two-channel preamp. I thought, before plugging a mic into it that I knew very well, “man this is going to sound awesome.” And what I heard floored me. It bored me. It had no sound character at all. I said, “this sounds like nothing – why is it $2500?” He told me, “that’s just it – it has no sound, no color.” It took a day or two for me to realize that he spent that much on the preamps because it had no flavor at all, not even vanilla. That means, if you have an instrument and you like the way it sounds, or a mic or a voice, that you won’t have to consider what the preamp will do to the sound. It will simply boost the signal and move it on down the chain. You have to spend an inordinate amount of money to get the preamp to be “transparent” – meaning, that it has no characteristics that are audible.

Now, having “no sound” isn’t necessarily all we hope to achieve. It is great to think, “what would I like on here – a tube sound, neutral sound, warm sound”, etc. It is fun to think about what flavors we wish to add to the sound we mean to amplify or record. Here are some traits to listen for in a preamp – does it sound brittle and harsh on the highs, is it dark and muddy or does it sound exactly like what your own ears heard in the acoustic or analog world?

Red guitar and sweater optional – natural finish preferred

You’ll also notice on cheap preamps that overtones and harmonics sometimes don’t exist on the other end of their electronics. This is why you have to A/B two preamps. You really have to compare preamps to know the personality of one versus the other. But, let’s narrow down easy ways to test a preamp. Put a mic right in front of the sound hole on an acoustic guitar. If you record that guitar and you don’t hear a lot of the boomy-ness you heard while recording it, but you hear a lot of brightness from the strings, this means the preamp didn’t translate all of the content (sound waves) coming out of the sound hole. A mic in front of the sound hole (straight on) of an acoustic guitar should be unusually boomy and require lots of eq and compression to get this sound under control. Even if the acoustic guitar is known for being a thinner sounding one, there is an inordinate amount of low end coming out of an acoustic guitar at the sound hole. Same goes for a mic too close to a male vocalist. Have them speak into a mic and intentionally put it up close to get that proximity effect (lots of chest oomph) and listen to what happens with different preamps.

Choose your preamps for what they do to the sound. Choose your preamps for what they don’t do to the sound. Regardless, actually choose your preamps and don’t just take what is given to you.

R = Resistor, C = Capacitor, T= Transistors – Feel free to make your own…if you’re a nerd

Comments

  1. Bryan Bowman says:

    awesome, very well written and informative…