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Thread: transistor microphone preamp

  1. #1
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    transistor microphone preamp

    Hi friends!

    Im working on a project. It is basicaly a ballanced rack version of the Akai 4000DS MKII microphone preamp. I have some thoughts about the circuit and some modifications, so my intent here is to discuss a little bit how the circuit works, what components I may dont need, and what is the best way to balance the input and output of the circuit.
    We have one of these tape machines in the studio, and we like very much that preamp, it works really nice with mikes, and if you plug a guitar in the mic input it becomes a superb fuzz effect.

    Im planning to add a THAT1606/1646 to balance the circuit, Im just not shure about the impedance diferences interfacing this chips with the circuit.

    THAT1606 and 1646 have 5K of input impedance and 50ohm of output impedance. The akai preamp is rated 30k of input impedance. The way it is, this preamp, is a very sensitive device, you have to keep the gain very low to keep the signal round. So I have two main questions:

    Is 5k a good input impedance for a microphone?

    The 50ohm output impedance of the 1606 may be two low for the 30k preamp input? will this make it further sensitive?


    Thank you and sorry for my english errors

    akai-preamp.png

  2. #2
    Senior Member Enzo's Avatar
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    Um... do you have any information about these ICs and the circuit you intend to use them in? That you could share here? HAve you considered simple transformers?


    You will want to decide which setting you prefer on the TAPE SPEED swiching circuit, then hard wire in your preference.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
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    Quote Originally Posted by Enzo View Post
    Um... do you have any information about these ICs and the circuit you intend to use them in? That you could share here? HAve you considered simple transformers?


    You will want to decide which setting you prefer on the TAPE SPEED swiching circuit, then hard wire in your preference.
    The only info I have about the ICs are the datasheets, and the complete schematics of the Akai preamp.
    I omitted completely the last transistor stage (TR4) to keep the things simple, and Im taking the output right from C13. Before I made my prototype I did some experiences with the real circuit of the tape machine. With a signal tracer I took the output from C13 and I came to the conclusion that I dont need nothing more that these 3 transistor stages.
    I also considered using transformers, but I prefered to do it electronic if I can get to a clean and good result, due to some desing constraints like costs, space and alike.

    AKAI SCH
    http://sportsbil.com/other/Akai/AKAI...0MKII%20SM.pdf

    THAT1606
    http://www.thatcorp.com/datashts/THA..._Datasheet.pdf

  4. #4
    Old Timer J M Fahey's Avatar
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    Well, you donīt actually *need* balanced inputs there, specially since you will be using that preamp as some kind of "effects box", to impart its flavour to sound.

    Since you already tested (and liked) it, you must have unbalanced the microphone , unless you added an input transformer.

    Personally Iīd build it as shown, only adding an XLR Mic In connector, with pins 1&3 shorted and grounded, and pin 2 "hot" (European convention) , or 1&2 shorted and 3 Hot (Japanese convention). Sound is exactly he same, of course.

    Those Mic inputs were not meant for XLR balanced Mics but for cheap "Home Recording" type ones: dynamic (600 ohms), unbalanced, with standard 1/4" plugs , or same thing but 1/8" unbalancd plugs in cassette recorders.

    If you go away from original design, you might as well build (or buy) a modern Mic preamp
    Juan Manuel Fahey

  5. #5
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    Quote Originally Posted by J M Fahey View Post
    Well, you donīt actually *need* balanced inputs there, specially since you will be using that preamp as some kind of "effects box", to impart its flavour to sound.

    Since you already tested (and liked) it, you must have unbalanced the microphone , unless you added an input transformer.

    Personally Iīd build it as shown, only adding an XLR Mic In connector, with pins 1&3 shorted and grounded, and pin 2 "hot" (European convention) , or 1&2 shorted and 3 Hot (Japanese convention). Sound is exactly he same, of course.

    Those Mic inputs were not meant for XLR balanced Mics but for cheap "Home Recording" type ones: dynamic (600 ohms), unbalanced, with standard 1/4" plugs , or same thing but 1/8" unbalancd plugs in cassette recorders.

    If you go away from original design, you might as well build (or buy) a modern Mic preamp
    Well, the thing is, I might be wrong, but I think this is not a bad preamp at all. For example I can't see too much differences between this circuit and the BA283 on the Neve 1073 preamp, of course the akai circuit doesn't have a 2n3055 driving a transformer at the output, neither high quality components but I was quite impressed with the prototype I did.
    The idea to make it balanced Its because I think the preamp deserves it, and I'm planing to make it a 500 series module.
    I'm more convinced now that the input transformer it's the best way to do it, but Anyway, Think will be a good idea to lower the input impedance a little bit, may be add some shunt to ground from the base of Q1. I'm not to worrried about the output, I'm pretty shure that the THAT1646 will do the job nicely.

  6. #6
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    Quote Originally Posted by Enzo View Post
    HAve you considered simple transformers?
    Now im considering using a transformer at the input but Im not shure about the ratio and impedance...

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