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Thread: $8 Class A mic pre kit

  1. #1
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    Dec 2003
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    Berkeley CA
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    $8 Class A mic pre kit

    I decided to take this to a new thread since I'm interested in this:
    >> Jim Dugger wrote:
    posted May 29, 2004 12:01 PM *** ** **** ** ** **
    ------------------------------------------------------------------------

    quote:
    ------------------------------------------------------------------------
    Originally posted by Philip Perkins:
    Cool, thanks. I'm ordering some.
    Philip Perkins
    ------------------------------------------------------------------------

    I'm quickly realizing why I *BUY* this sort of gear.

    Watch the RF. There's no filter on these little circuits, and even simple Class-A amplifiers like this one have amazing bandwidth. You can pretty much expect it, not grounded correctly, to receive all AM stations *at once* and deliver that mix to the output. I'm pretty sure I heard the call sign for AM 1160 this morning when I powered it up -- hey, my little mic pre has 1Mhz+ bandwidth!

    It's been a fun little project for sure, but there's a lot of ways to mess it up. It's absolutely fascinating to adjust the bias voltage and supply current and hear what happens. There's definitely a "sweet spot" for supply voltage and current, and it's not the same on both channels. I figure the 20% tolerance (!) of the resistors that came with the kit have at least something to do with that.

    Here's what else I can tell you. By the time you get a decent rack mount case, buy the parts to build a power supply that doesn't suck, get all the panel jacks, wire, etc, to really put it together right you are going to be in the $200 neighborhood fast.

    Will this box be better than a $200 pre? I don't know until I listen, but I'll tell you right now if it were not for the learning excercise of this it would already be a huge waste of money. Unless you are into little circuit cards and batteries hanging around open in your studio, this is no way to save money on a good preamp. <<<<<

    Ah, I hate it when things turn out like that. I'm going to build mine anyhow and see if I can
    shield them well enough to use. Have you tried this thing w/ any other mics? Or are you sick of messing with it now? I don't have great pres here to compare it to, but what would be the point anyhow. Is this thing "PAIA-level" or better than that? I realize that this sort of kit is not really
    a 3DB Forum kind of thing, but I will report
    if I can get it to be useful in any way.

    thanks for the info

    Philip Perkins

  2. #2
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    >I realize that this sort of kit is not really
    a 3DB Forum kind of thing, but I will report
    if I can get it to be useful in any way.

    The finished preamp as a product may or may not be "a 3DB Forum kind of thing", but what you learn about the design of all preamps in general is useful knowledge for any of us. Please do report all your findings - I for one am very interested.

  3. #3
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    I put mine away for now. Probably for good, actually. I got pretty frustrated with the power supply portion this afternoon.

    I need to focus on the projects I'm good at: finishing up some nice wood trim for the studio and preparing a roasted beet salad with feta and watercress for tomorrow's bar-b-que.

    I think the biggest problem you are going to run into is the circuit is very much designed around a +9v supply voltage and after you split this in half to bias the transistor you are only dealing with swing voltages in the 4v-ish range. I'm sure it's less in practice, but I looked in a transistor circuit design book and wow that math left planet earth with me way behind in a hurry.

    I got it working OK, but there's no headroom. Even if you cascade the units for more gain you are going to need a line amp to do much of the rest. I barely had to talk loud into an SM58 to overload it.

    I know you could change all the values around and rebias the transistor to a higher voltage, but I'm not skillful enough to pull it off.

    And, when you DO put that line amp in there, as I did at first, and you don't shout into the mic... it's got that unmistakable Class-A sound... they do sound pretty good, I think it's just completely impractical to expect them to perform in anything close to a +4 environment.

    Still, quite a little learning excercise.
    Jim Dugger
    Poorhouse Productions

    At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?

  4. #4
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    More news on the kit front soon. And I think it's totally a "3dB" kind of thing. What else is this site about if it's not learning and discovery and experimentation and sharing what we've learned to save others from the same mistakes.

    I found a kit that costs more but you would be proud to own and use it when you're done. More soon.
    Lynn Fuston
    3D Audio

    Making beautiful music SEEM easy since 1979.

  5. #5
    Join Date
    Feb 2002
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    KC
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    50
    I've done both of the Hamptone kits and found them to be a thoroughly enjoyable experience. Both during the build and after. Especially after!

    I would love to undertake on some "3D approved" kits in the future. Could be anything, pres, comps, mics, whatever.


    Jason A.

    [ May 31, 2004: Message edited by: Jason A. ]
    Jason A.

    3D VIP

  6. #6
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    Mar 2004
    Location
    Eastern NC
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    2,639
    FWIW, I am currently in the middle of constructing a stereo LA2A compressor with all the "traits" of the original (at least, what can currently be obtained). I built a solid-state stereo unit about 15 years ago for a local radio station, and it is still in use today!

    This has been a very educational experience, so far, and can't wait to hear the results. I have the original opto module, and have also made some clones (no idea how the clones will sound yet).

    I considered many "improvements" along the way, such as a variable compression ratio control, but have basically ruled them out for now. Maybe on my next one!

    Building a preamp is next on the list. Since there are many good design philosophies on preamp design, I've yet to decide on a particular type (i.e. tube, FET, opamp, etc.).
    Quarter Note Recording
    Kinston, NC
    www.quarternotestudio.com
    QNS on Facebook

  7. #7
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    Sep 2001
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    GOODLETTSVILLE, TN.
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    One thing to consider about pre's. A good input xfmr is going to cost more than $50.00.
    So building your own pre is not an easy thing to do.

  8. #8
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    Aug 2001
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    Fort Wayne, IN, USA
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    One of the things that most of the designers on the Preamp Panel at TapeOp agreed about was that coming up with the circuit design is only about half the battle. It's the metalwork and artwork and chassis and securing parts and certification that takes the longest. So just having a great circuit on a board that sounds killer is only the beginning. Getting it into a box that is usable and reliable and safe is also a big deal.
    Lynn Fuston
    3D Audio

    Making beautiful music SEEM easy since 1979.

  9. #9
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    NYC
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    >I considered many "improvements" along the way

    I'm going to build an LA2e too! What's an "e"? Its my modification

    Seriously, the difference between an LA2, which was a limiter only, and an LA2a with the "compress switch" selected is the value of R7.

    R7 isn't in the limiter circuit, and is introduced for the compressor mode. There have been several values for R7 over the years. I intend to put a rotary switch in place of R7 so that I can change values for different curves. For fun, I think I'm going to mark that knob in years ('62, '67).

    Don't use a potentiometer here, there is some risk of inductance. Better to use a series of choosable resistors on the rotary switch.

  10. #10
    Join Date
    Dec 2003
    Location
    Berkeley CA
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    1,874
    I built the little CanaKit $8 (that's Canadian $) Class A preamp kit, 2 of 'em. They seem pretty cool to me, wide freq, response, very flat, and pretty much w/ o color. I haven't encountered any RF issues yet, but I intend to run them off a lithium 9v battery and avoid PSU hassles. I talked and shouted at them through a few decent mics, and they seemed to have quite good headroom, at least for voice. These preamps may not be worth the hassle for a studio dedicated to recording professional musicians--as was mentioned, too much work in packaging them and making them
    suitable for hard use, and not enough (like no)
    personality to recommend them for a particular instrument or singer. But what I want is something
    pretty neutral for recording people talking--telling stories, basically. With the mics I tried (Schoeps MK41 and 4, Marshall V67G, EV 257B) it sounded
    very good on voice--I liked it better than the preamps in the mixers we have here (Sound Devices). However, it did not make enough gain to be able to use it w/ my SM7 on spoken word, a bummer but to be expected I guess. Anyhow,
    for any of you 3db-ers on a strict budget, this thing
    (run on a battery anyhow) seems like a great
    deal. More after further tests and comparisons
    (other voices, other preamps to compare with).

    Philip Perkins

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