simple DIY mic pre

Discussion in 'Recording/Live Sound' started by Shiny_Beast, Feb 21, 2012.

  1. Shiny_Beast

    Shiny_Beast Member

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    Is there some reason why I wouldn't want to build a simple mic pre with a jfet or similar boosting the signal a modest 12db or so?

    I track my 57 straight into the converter, and the signal is at least reasonable with the boost maxed. My ribbon mic on the other hand picks up some white noise. It's not too bad, but I've thought about a really cheap booster.

    Why would my $15 2n5457 based mic pre be a whole lot worse than any of the $500 offerings out there? Do they use magic transistors? Is the input transformer that importand? I'm not gonna worry about phantom power or a level knob, but that doesn't make up the other $475. I figure 1m input imp, probably sub 10k output imp...

    what's wrong with this picture?
     
  2. Scott Whigham

    Scott Whigham Member

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    What do you mean when you say "I track my 57 straight into the converter"? I'm not aware of any converter that would allow that. I know of lots of interfaces that allow that - but those are a combination pre amp and AD/DA converter.

    I wish I knew enough about the internal electronics to help answer this question. I'm hoping some others reply so I can soak up some knowledge!
     
  3. mrface2112

    mrface2112 Member

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    A lot, really. :rotflmao

    Mic preamps are absolutely necessary. Yes, you can probably get away with a simple fixed 12dB boost, but what happens when you put that sm57 in front of a Marshall 4x12? Or a cranked tweed deluxe? You still need to amplify the mic's signal, but you might need less than 12dB. Or maybe a whole lot more with a ribbon mic in front of a classical guitar. If you ever use a condenser mic, you'll have to have phantom power to make it work.

    Maybe 12dB works. It'll do the job and make the mic louder. But how does it sound? Some jfets sound like crap, some don't. And a lot of it depends on the circuit around the jfet. Personally i prefer a transformer-coupled preamp. Overgeneralizing, transformer-based preamps sound more musical. It's a lot like the difference between a tube amp and a solid state amp.

    Part of what you're paying for with a commercial mic preamp is R&D. Part of what you're paying for is the raw components, including a power supply that is UL Approved (or ROHS approved in the UK). People don't realize how much that particular part costs. Part of what you're paying for is the time it takes to assemble the preamp. And in some cases, you're paying for brand or vintage.

    And in all cases, you're paying for a specific sound. Sometimes you want the sound of preamp _N_ on vocals, preamp _A_ on snare and kick, and maybe a pair of preamp _T_ on the guitars.

    So....can you DIY a kickass preamp? Absolutely. Take the DIY Classic API 500-series preamp, for instance. They are, for lack of a better word, clones of the classic API 312 preamps. They're available in kit form for the API 500 series rack/form factor. They cost in the neighborhood of $250 per channel once you factor in the opamp kit required, assuming you build them yourself. I'd wish you good luck in sourcing the parts to make one of those separately.

    Likewise, there are all kinds of great things over at GroupDIY. I would suggest checking that place out.

    So yes, your simple $15 jfet preamp would probably, most definitely "get the job done" in terms of boosting the signal of the mic. But in the end, it's "how does it sound" and "which one would I rather make an album with" that matters to me.

    YMMV

    cheers,
    wade
     
  4. Shiny_Beast

    Shiny_Beast Member

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    My converter takes it fine, I'm not sure what you mean "allow". I jsut plug the mic into the oconverter. With the strong signal from my amp up reasonably loud and a 57 the noise floor isn't much of an issue for my "home" projects, and talk about a transparent pre...
     
  5. mrface2112

    mrface2112 Member

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    What interface/converter are you using?
     
  6. Shiny_Beast

    Shiny_Beast Member

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    Ya I get all that, I have a dmp for when I need it, a stand alone phantom power unit etc...this would be single purpose for micing my amp. I might include a gain knob, dunno. I'm not really looking for something to get me certain sounds from high end pres, just get up off the noise floor without ruining the signal. I'll check out thqt link, thanks.


    I found a reference to this article by Scott Hampton about a simple jfet based pre, just what I was looking for. A little more complicated than one tranny, but still quite simple. It does however have an input tranny for "impedance matching". I don't really get this, most modern mics have a pretty low oputput iompedance, so why the 1:10 input transformer?
     
  7. Shiny_Beast

    Shiny_Beast Member

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    A lavry blue, I don't see what difference that makes though.
     
  8. mrface2112

    mrface2112 Member

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    Why an input tranny? Partly b/c it sounds good. :p

    Not all mics respond well to low loading. Some want a higher load. Depending on the mic being used, adjustable impedance on the input can have a dramatic effect on how the mic sounds....and in some cases, it doesn't matter at all. I'm not familiar with the circuit that you're referencing, so I can't really comment to why. But knowing Scott, "tone" (or "how does it sound") will play a big role.

    As for inquiring about your converters, like Scott, I too wanted to know what kind of converter was strong enough (had enough gain) to do something useful with the relatively low output level from an sm57. Your Lavry Blue doesn't have the optional mic preamp option in it, does it? :p

    I'm not saying this can't be done and I totally encourage you to do it.

    But as someone who obviously understands the need for world class, $1000-per channel A/D conversion that's present in the Lavry Blue, I guess I just don't understand the reluctance to picking up a pair of Neves or APIs to put in front of it. You've spent that $$$$$ on the converters, I'd throw another $1000-1500 at it and call it a day. Or better, get a lunchbox and load it with 500 series modules and have a smorgasbord of flavors at your disposal.

    cheers,
    wade
     
  9. Shiny_Beast

    Shiny_Beast Member

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    When I stopped recording all the time I was getting ready to buy a GR inv1 (whatever it's called). I don't use pres cause it sounds better straight in than through my dmp3.

    I'll probably pick up a kit along the lines of the ones you've mentioned once I decide I want a veratile real world pre. I'm more or less done with the cheap variety.

    The blue has 12+db of internal gain IIRC, which doesn't really matter to the noise floor since it's digital from there to the computer, no white noise on the way to the computer, but I assume (hope lol) the gain is pre conversion, so this get's it up where it's at least using some bits once it's gone to digital.

    Going from memory here, once into the computer it's usually about 12-20db down from something usable. I just normalize the tracks.

    Micing my cab with the 57, if I could really turn it up, wouldn't need much boost at all. At the volumes I track the noise floor is there, but it's livable. With the Ribbon, which is somewhere around 10db or so down from the 57, it's starting to get noticable.

    Both mics track fine quality wise, I haven't noticed anything missing from going straight in. I checked the specs once on my mics and the blue and the signal is "bridged" as I understand it, that is, the input imp on the lavry is always higher than the output imp of the mics. It's an ac signal :dunno I hear no sound degredation, except the noise floor. Really that's my only use for mic pres right now, is to get non balanced signals strong enough to transport through 20 or so feet of cable. Having $10 worth of "colour" lying around would be cool :), but first things first. btw, I just use an outboard phantom uint for my condensors.

    The blue has no pres. As for the mission impossible conveter, I wanted the cheaper Mytek, but couldn't find a local distributor so pulled the trigger on the blue. I'm a digiphobe, and I figure everything will benefit from this one piece of equipemt etc...
     
  10. Shiny_Beast

    Shiny_Beast Member

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    So part of it is the same old, the right kind of distortion is good, thing going on with mic pres. Which is great. I have to say though, you read about that 2002 stereo mic pre from whatever their name is, that costs over a grand a channel, and how they went to great lengths to make it perfectly transparent...

    Most of us are sitting in rooms with modern mics that put out a stong signal, through less than 30 feet of cable, sometimes balanced, that doesn't belong to a snake full of 30 other mics and electrical signals. Most of the pres we use sit on top of the desk, so by the time the cable hit's the pre the damage is done, all the pre is doing is saving the signal from the noise of travelling 6 feet across the desk into the converter. It's then transformed into digital using 50 cent converters and then hacked to pieces with a bunch of digital processing. I fail to see the need for a mic pre in most of these situations. If you have good ones that sound good, great. The cheap ones they pedal to the hobbyest are a waste of time IMO unless you jsut have to get the signal up to keep it clean, like certain mics or situations etc... If your converter is 24 bits, even a weak signal is probably better off with the limited bit range than being coloured by a cheap pre.

    IMO of course, and I'm not an SE
     
  11. TAVD

    TAVD Guitar Player Gold Supporting Member

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    The main issue I see is the jfets should see a high impedance and the mic a low impedance. That's why the transformer is important. Hamptone makes a great fet mic pre, available as a kit. It's not the only way to build a mic pre though. Download the data sheet for any one of the mic pre chips (ina217, THAT 1510, SSM 2019) and all the info needed is right there. They're designed for a low impedance input so don't require a transformer, they sound good and don't cost much. I like the ssm2019 best but that's really splitting hairs. A lot of commercial products are exactly the circuit shown in the data sheet plus a power supply. For that, use the one from Five Fish or JLM. They will easily power 8 of these preamps.
     
    Last edited: Feb 22, 2012
  12. Chrome Dinette

    Chrome Dinette Member

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    Another place to look for preamp schematics suitable for diy'ers is the Jensen Transformer site. Of course, those circuits want to be used with transformers.
     
  13. Shiny_Beast

    Shiny_Beast Member

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    I think that backwards, you want a low source and high input, which could be a problem with some other types of transistors, but I thought most jfets had a very high input impedance, like tubes. I've also read that modern mics have a lower output impedance then they used to, so the high output imp of some old mics used to be an issue, but not so much any more.

    Thanks for the suggestions, I've stumbled across a few simple schematics, one that uses a bc108 even, I'm gonna throw something together, I'm convinced it's worth trying.
     
  14. TAVD

    TAVD Guitar Player Gold Supporting Member

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    I think maybe you are confusing source (as in the microphone output) with source (as in the part of the fet that the current flows from). Modern mics do have lower output impedance compared to a classic ribbon mic, but you're talking about approx 150 ohms vs 1500 ohms. Fets have input impedance similar to tubes, and are more appropriate for things like instrument preamps, fx pedals, DI boxes.

    Would be better than the jfet, for sure. Cheap mic preamps use BJT to lower the impedance enough to allow for inexpensive opamps in subsequent stage. Just look at any older console, any cheap mic pre. Most use a similar topology. Do you want to waste time copying a ART Tube MP minus the tube? Use a 1:2 Jensen transformer coupled to a 990 opamp or THAT 1510 IC if you're on a budget. Google $5 mic preamp or look at the mic pre circuit in the INA217 data sheet. If you're really on a budget, replace the dc servo with a 470 uf capacitor.
     
  15. GregoryL

    GregoryL Supporting Member

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    Sounds like you clearly want to try the DIY route you've outlined - I say 'go for it if you want' ... you're not going to break anything.

    If it gets your signal up to or closer to line level, then your boost will be doing the fundamental job of a mic pre.
     
  16. Shiny_Beast

    Shiny_Beast Member

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    You've oost me, I was saying I understand that a low putput on the mic into a high imput on the jfet is bridged and should transmit the signal well.

    This sounds to me like you are saying the mic should have a higher output impedance than the input impedance of the jfet. Gues I don't know what you mean here.

    Here a clip just for the heck of it, a couple years old, 57 on the grill of my skylark sytaight into the converter, and push pull 6aq5 amp probabaly less than 10 watts. It's nomarlized.

    http://s273.photobucket.com/albums/...d/?action=view&current=prelessguitartrack.mp4
     
  17. TAVD

    TAVD Guitar Player Gold Supporting Member

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    Nope, sorry if that's what you took from my previous post.

    It will sound like it's been through a HPF.
     
  18. Shiny_Beast

    Shiny_Beast Member

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    I thought it's just suppose to "bridge" the connection, maybe with a loss in power transfer.

    I've been reading that a lower imp pre on a 57 mellows it out.
     
  19. jmoose

    jmoose Member

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    The gain adjustment on the Lavry (like most converters) is purely a trim to "fine tune" the operating level to your environment. You're basically plugging the mics right into a line level device and expecting it to do something it was never ever intended to.

    At the very least if your not willing to dish out for an actual microphone preamp, you should at least rent in an API or Daking and hear what you've been missing this whole time. After taking the plunge on a Lavry converter you sorta owe it to yourself...
     
  20. Shiny_Beast

    Shiny_Beast Member

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    Take an electrical signal and turn it into digital? The Lavry has a input imp of 5k, my dmp3 as an example, 3k. Is that really gonna make a world of difference to some mic with an outupt impedance of 300 ohms? The real difference is the stength of the signal and the resulting noise floor, which is really what mic pres were for in the first place, to get the signal up where it can survive a cable run.

    Sure the lavry is expecting a stronger signal, but I expect it to be able to convert something 16db down from normal just fine.

    Is there really anything else to consider?

    Listen to the recording, sounds pretty good to me. First thing I noticed was how much better it sounded than running through my DMP3.
     

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