Can I use my guitar amp as a microphone pre-amp?

Discussion in 'Digital Audio' started by escabeche, Jun 29, 2007.

  1. escabeche macrumors newbie

    Joined:
    Jun 29, 2007
    #1
    Hi, recording gurus!

    I'm a recording novice who just bought a Shure SR-57 microphone to record vocals and guitar, only to learn too late that the signal needs to be amplified in order for my Macbook Pro to pick it up on the line-in.

    Two obvious options (if I understand what I read here correctly) are:

    1) buy a USB microphone that has a pre-amp inside and can go right to the line-in;

    2) buy a separate pre-amp that takes input from the mike and sends Mac-readable output to the line-in on the computer.

    I think both of these would work, but at that point I've started to sink a lot of money into something that's just a hobby at the moment. (The store I bought the mike from offers only store credit for returns, and they don't sell any USB or powered mikes, so there's no way to flip the Shure into something I can use as is.)

    I had one other idea: I do have a guitar amp (Peavey Envoy 110), and I notice that on the back of the amp there's a 1/4 inch jack marked "Preamp out." So I was wondering -- if I run the mike cord from the Shure to the amp (via a mike XLR - to 1/4 inch converter, which I have) and then run a cord from the Preamp Out on the Peavey to the line-in on my MacBook (via a 1/4 inch to 1/8 inch converter, which I also have) will it work? Of course I could just try it, but having already gotten confused about this business once, I want to make sure no one says "Good god, sending a signal through a guitar amp will void your warranty / explode your soundcard / give you herpes" or whatever.

    One drawback of this plan is that, as far as I can see, it doesn't allow me to use the Shure to record the electric guitar. Is it safe / effective to plug the guitar into the amp and run a patch from the Preamp Out on the amp to the MacBook? Or does the guitar signal really need to be not just pre-amplified but amplified?

    Thanks for all help!
     
  2. zimv20 macrumors 601

    zimv20

    Joined:
    Jul 18, 2002
    Location:
    toronto
    #2
    don't do it, it's not the same kind of preamp.

    if you haven't already, read the primer (sticky up top). then start thinking about dropping a few bucks on an interface, so you'll get both mic preamps and a better a/d converter than what's onboard your mac.
     
  3. escabeche thread starter macrumors newbie

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    Jun 29, 2007
    #3
    "don't do it" as in "it'll sound like crap," or "don't do it" as in "you're going to break your computer, ya dope?"

    (yeah, i did read the sticky primer, and i guess the most likely thing is that i'll get an audio interface as you suggest ... just want to make sure it's necessary!)
     
  4. zimv20 macrumors 601

    zimv20

    Joined:
    Jul 18, 2002
    Location:
    toronto
    #4
    i don't think you'd break the computer. from what i understand, the output of the guitar preamp and a mic pre would be about the same. however, i could be wrong about that, and maybe the out of your guitar preamp would fry some mac electronics.

    the bigger issue is the kind of impedance the input of each kind of amp expects. i.e. mic impedances and guitar impedances are different by an order of magnitude. again, from what i understand, guitar preamps don't really do much of a voltage increase, they mainly do impedance matching. but mic pre's perform a hefty voltage increase, which is why you need something specifically designed for mics.
     
  5. escabeche thread starter macrumors newbie

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    Jun 29, 2007
    #5
    Thanks so much! If it's at all possible for the wrong kind of input to fry something inside the Mac, I'm not even gonna try it. Your help is much appreciated.... and now to go start reading preamp reviews....
     
  6. CanadaRAM macrumors G5

    CanadaRAM

    Joined:
    Oct 11, 2004
    Location:
    On the Left Coast - Victoria BC Canada
    #6
    OK how about this. Yes, you could break your Mac - if you feed amp level signals into the mic or line input. Sorry to state the obvious, but since you are asking the question, we have to assume you are shakey on some of the basic concepts.

    Don't do it.

    Also, the role of a guitar preamp typically is to add compression, clipping and distortion to the signal. The role of a mic preamp is to add gain without any of those other three.

    A USB microphone is so limiting because it doesn't allow you to feed in any other mics or signals.
    A halfways decent interface will give you at least two inputs, typically switchable between Mic and Instrument inputs. If you are planning to get some better condensor mikes in the future, then also check to make sure the interface/preamp has phantom power (48V) capability. For this reason, I do NOT recommend the types that draw their power from the USB port, I prefer interfaces (and hard drives) that have their own AC power.
     
  7. zimv20 macrumors 601

    zimv20

    Joined:
    Jul 18, 2002
    Location:
    toronto
    #7
    we're talking about the output level of a guitar preamp, yes? i thought those were on the order of line level. maybe not, i just mic up the damn cabinets, so what do i know? :)
     
  8. dogbone macrumors 68020

    dogbone

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    Sep 16, 2005
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    #8
    There's a presonus firebox on ebay for $180, but if you can stretch a bit there's a new one plus an AT2020 for $300, which is what I bought some time ago and I don't regret it. You could keep or sell the Shure.

    For $300 all up it's a pretty good deal and a great mic too. At least you have a standard to compare other things to. In fact it has a 'best offer on it' Offer them $250 and see what happens.
     
  9. WinterMute Moderator emeritus

    WinterMute

    Joined:
    Jan 19, 2003
    Location:
    London, England
    #9
    Depends on the actual output type from the amp, some are line level (usually of the -10db variety) some are a lot hotter than that and are designed to feed FX loops for the high impedance pedals, some are proper DI so are actually mic level signals....!

    The only way you'll fry your Mac is if you connect speaker level signals to it, as they carry signal and power and will make a mess of your input electronics.

    However the OP made a telling comment early on it'll sound like crap for anything other than a guitar, and it'll be noisy too.

    Dedicated Mic/line pre is deffo the way to go.
     
  10. zimv20 macrumors 601

    zimv20

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    Jul 18, 2002
    Location:
    toronto
  11. escabeche thread starter macrumors newbie

    Joined:
    Jun 29, 2007
    #11
    Thanks so much, everybody, this is really enlightening.
     

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