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    Rookie making an audio step-up transformer

    Hello everybody!

    I've recently (re-)started my journey in electronics. Last time I tried was very long time ago though. I started with books but not everything I read I understand.

    So I started playing with an amp for a dynamic mic and then I read I could use a loudspeaker as a mic which is interesting to play with (as I have a pair of very old ones).

    The circuit uses 8 Ohm loudspeaker and it's connected to a primary coil (Rp = 8 Ohm). The secondary coil "sees" TL082 (I haven't read much on op amps yet...) and it's set Rs=1kOhm for the secondary.

    And here's where I'm confused. I to make this transformer myself, so I read everything it's available to me on this topic. The transformer is an audio step-up transformer. In this case, if this is an impedance matching then the turns ratio would be 11.

    1) Correct me if I'm wrong on impedance matching. Or is this just for voltage (signal) step-up?

    2) With turns ratio = 11 I can make 5/55 turns or I can make 50/550 turns. My understanding is if the inductance depends on L and dI/dt then I have to know an approximate dI/dt (current change rate). But how can I find this value? To measure from the loudspeaker?
    If that's the case then I need to know the inductance and start from there - it's pretty easy to calculate number of turns then (considering material and core).

    3) I want to make this transformer with a toroidal core. And I found the following core from Epcos with the following characteristics:

    T38


    T36


    The optimum frequency range of T36 better suits the audio as far as I understand.
    Overall, will such a toroidal transformer work in this case? Would it work with T38 (as I already have one)?

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    Re: Rookie making an audio step-up transformer

    Audio transformer needs to care for main inductance which forms a high pass with the load impedance. If you want e.g. < 30 Hz lower cut-off frequency, the primary main inductance Lh1 must be > 8 /(2*pi*30) = 42 mH. T38 core has AL value of 4410 nH/nē, respectively you need at least sqrt(42 mH/ 4410 nH) = 98 primary turns, about 1100 secondary turns. Not very comfortable for a toroid core.

    Microphone transformer, if used at all, is regularly designed for minimal noise rather than impedance matching. Instead of using TL082 with a transformer, you better use a low-noise OP optimzed for low source impedance, e.g. OP27.

    If you still prefer a microphone transformer, you should use a laminated iron core.


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    Re: Rookie making an audio step-up transformer

    Winding you own transformer is a difficult task.
    Why not just buy a small audio transformer, such as one of these?
    The small ones are only a few dollars.
    Zapper
    Curmudgeon Elektroniker


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    Re: Rookie making an audio step-up transformer

    Quote Originally Posted by FvM View Post
    Audio transformer needs to care for main inductance which forms a high pass with the load impedance. If you want e.g. < 30 Hz lower cut-off frequency, the primary main inductance Lh1 must be > 8 /(2*pi*30) = 42 mH. T38 core has AL value of 4410 nH/nē, respectively you need at least sqrt(42 mH/ 4410 nH) = 98 primary turns, about 1100 secondary turns. Not very comfortable for a toroid core.

    Microphone transformer, if used at all, is regularly designed for minimal noise rather than impedance matching. Instead of using TL082 with a transformer, you better use a low-noise OP optimzed for low source impedance, e.g. OP27.

    If you still prefer a microphone transformer, you should use a laminated iron core.
    Can you please tell me where are these came from? Or a book/link to the theory. I mean I'd like to know where is this formula coming from. Thank you!



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    Re: Rookie making an audio step-up transformer

    Quote Originally Posted by crutschow View Post
    Winding you own transformer is a difficult task.
    Why not just buy a small audio transformer, such as one of these?
    The small ones are only a few dollars.
    I had a hope that would be easy :)
    Well, I didn't know they exist! Thank you for the link!



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    Re: Rookie making an audio step-up transformer

    A loudspeaker makes a poor microphone because of its strong low frequency resonance. It also does not need a transformer. The cheap old poor-sounding intercom at a gas station uses its speaker as its microphone.
    Today, an inexpensive but high performance electret microphone is used with an audio opamp like an OPA134 as its preamp.


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    Re: Rookie making an audio step-up transformer

    Quote Originally Posted by Audioguru View Post
    A loudspeaker makes a poor microphone because of its strong low frequency resonance. It also does not need a transformer. The cheap old poor-sounding intercom at a gas station uses its speaker as its microphone.
    Today, an inexpensive but high performance electret microphone is used with an audio opamp like an OPA134 as its preamp.
    This is interesting. Can you please tell me what is "high performance electret microphone"? I mean what params should I look for?

    And then I'll try to find a circuit... when I'm done reading about op amps.



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    Re: Rookie making an audio step-up transformer

    There are many good quality electret microphones stocked at Digikey. The first one listed costs $.77US and its datasheet shows its fairly wide frequency response and its low noise level. It does not list its distortion but I expect it to be very low.


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