How many turns in an audio transformer?

Discussion in 'Wireless & RF Design' started by WBahn, Nov 28, 2013.

  1. WBahn

    Thread Starter Moderator

    Mar 31, 2012
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    I finally ordered a bunch of parts to start playing with a simple crystal radio. The main two I needed were some germanium diodes and high impedance earphones. I figured I could wind the coil and build a variable capacitor (though I ordered a small inductor kit and also some 365pF variable caps).

    The germanium diodes came in but the earphones did not, and I would like to play with this over Thankgiving with my daughter. So I am thinking to use some earphones I have around here. I think they are 32Ω, but they might only be 16Ω. So I want to make a transformer to scale the impedance. Ideally, if I use a turns ratio of 10:1 that would get me either 3200Ω or 1600Ω, either of which is probably enough. Not looking for performance, here, just that it work.

    I have some enameled magnet wire - a few different sizes in the 28 AWG range - to work with. My question is, how do I determine how many turns to use -- or at least a minimum number of turns that is likely to work. Also, can I use an air core form, such as an empty toilet paper roll? Would a smaller diameter, such as a soda straw, work better? Would it work better if it were wound on an iron (or steel) rod. I have some large nails (3/8" diameter) I could use.

    I'll be experimenting a lot, but I'm looking for some thoughts to guide my starting point. Thanks!
     
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  2. vk6zgo

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    Jul 21, 2012
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    Are you really particularly interested in making an audio transformer for the fun of it,or is the crystal radio the main interest?

    If the latter,how about grabbing a mains transformer out of an old syle"wallwart" & using it?

    I have heard of people doing this if they can't get hold of a speaker transformer when playing with tube radios.

    Obviously they aren't optimised for audio frequencies,but people seemed to have had some success with them.
     
  3. WBahn

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    Mar 31, 2012
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    Thanks for the response.

    Yes and yes. While I am interested in winding a few coils and transformers of my own to gain some practical insight, I don't care too much about doing it as part of this particular project -- in particular, I don't what it to become the focus of this project.

    That's an idea. I don't think I have any non-working ones around here and don't know if I'm willing to sacrifice one just to get the transformer. But I may be able to scrounge through some boxes and come up with a transformer intended for long-shelved power supply project that might work.
     
  4. Tesla23

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    May 10, 2009
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    Your target impedance sounds a bit low, there is some data here
    http://www.arrl.org/files/file/Technology/tis/info/pdf/9809067.pdf

    I'd suggest that to avoid a weekend where your daughter thinks dad spent all thanksgiving winding some transformer thing, use an op-amp as an impedance buffer - you are much more likely to make a radio and impress your daughter (and you can cheat and add gain if needed), you will have good selectivity (less chance of hearing two garbled stations together), and then the magical radio (to you) that doesn't need batteries can come when the crystal earpieces arrive.

    I hope it all goes well, but as someone who even tracked down some galena to impress his kids with a crystal set made with a piece of rock, I now know that it can take a lot to impress the smartphone generation.
     
  5. WBahn

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    Thanks for the link. It's interesting and something I'll explore down the road a bit (not too far down the road, I suspect).

    I'm definitely concerned about doing a project that is a bust. She recently turned six, so she isn't going to have much of an appreciation for the significance as it is. I was talking to her last night and she didn't seem to be too impressed by the notion of making a radio at all. Definitely a child of the smartphone age.

    My plan is to do it first by myself to get it working and then, if successful, do it "for the first time" with her.

    I was encouraged by how much she got into constructing a water bottle capacitor and measuring it. She had no idea what we were doing, but she seemd to enjoy the process of doing it for it's own sake. Of course, it only took a few minutes from start to finish. I probably need to make sure that the radio project is doable in fifteen minutes or so by having almost all the pieces already to go.
     
  6. #12

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    Nov 30, 2010
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    I'm pretty sure a core is necessary. If it wasn't, Leo Fender would have found out how to do it. (Speaking from personal experience.)
     
  7. WBahn

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  8. Dr.killjoy

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    Apr 28, 2013
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    How big of a transformer are you looking? ??
     
  9. Tesla23

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    May 10, 2009
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    I think the key thing is that they talk of getting the impedance to around 100k. They talk of results with a 1K:8Ω Radio-Shack audio transformer:

    "Because I was using the phones in series, the better-performing units were actually presenting an impedance of 100 to 120Ω. When used with the Radio Shack transformer, the transformed impedance of the 120-Ω phones was about 2.8 kΩ, as compared to the 1 to 1.5-kΩ impedance presented by the lower-impedance phones. The higher-impedance units caused less detector loading and, therefore, greater audio output. The transformer loading, however, was still too great to allow proper receiver tuning."

    My take out from the article was that it is quite a specialised job to get the impedance high enough for good performance. A low risk alternative is to simply use a FET-input op-amp as a unity gain buffer. You will have a beautifully high-Q tuning circuit for selectivity and added power gain from the FET.

    Just note that to transform to 100k requires a winding reactance of the order of 100k, and at 500Hz this is 32Henries.
     
  10. WBahn

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    I've seen numerous references to headphones for crystal sets driven directly being in the 2000Ω range.

    I don't think that 100kΩ is the reactance. That kind of self-impedance is absurd (so it seems to me) in something that is often sitting an ice cube sized package. I'm guessing a lot of it is the winding resistance.
     
  11. Tesla23

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    It will all come down to how long your antenna is and how close you are to the transmitter, (and possibly how many strong signals you don't want).

    When I was a kid, I knew nothing about impedance matching and Q, but I managed to make my long wire antenna, through my crystal set, drive a loudspeaker loud enough for close personal listening. It was about a 150Ω speaker if I recall, and I didn't use an audio transformer. I did have a very long antenna, a good ground, and only one local station. With what I know now, the setup probably didn't have any significant selectivity (and was probably far from optimum).

    Re your comment on inductance, if the device is to behave as a transformer you do need the winding reactance to be of the same order as the impedance you are transforming (preferably somewhat greater), which is why high impedance audio transformers are very specialised. You will find this discussed at the bottom of p10 of the document on audio transformers that you linked.
     
  12. WBahn

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    Big in what way?

    I don't care about physical size -- within reason.

    I do care about it being something that isn't going to take hours to construct.

    I don't care about performance, as long as it will let us here a single high-power station (KOA, which is a clear-channel station whose antenna is about thirty miles from here).
     
  13. WBahn

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    I'm coming to the realization that it is the reactance as well. It is just so far out of my experience to think of tens of henries of inductance in such a small space. Then again, my experience with inductance are superconducting magnets which, by the nature of the ones we used, were air core. They ranged in inductance from 1.5H to 74H and were large critters.

    But in looking through some stuff on the net over the last little bit I'm coming to the conclusion that dozens of henries in an audio transformer is not unreasonable, after all. I admit that it's gonna take me a while to wrap my mind around it. Time to pull out Wangsness.
     
  14. MrChips

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    Oct 2, 2009
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    I have some working experience with building crystal sets.
    I have one set up right now.

    I believe the antenna is important. About 50 feet of long wire aerial as high as possible outdoors is best. I moved my antenna indoors for the winter and can still get good reception.

    For a basic LC circuit I use a tuning capacitor from an old radio. I will have to check on the maximum capacitance but I suspect it is about 500pF.
    For the coil I wound about 75 turns of 28 AWG enameled copper wire on a cardboard toilet roll.

    If you can find a point contact crystal diode that would be best. Otherwise a 1N34 works ok.

    If I remember correctly, the two pairs of headsets I used as a lad were 10kΩ and 20kΩ. These are now hard to find. The crystal set kits sold today come with crystal earphones.

    I don't recall ever seeing any one using matching transformers to match with low impedance headsets. I am not sure that that will work.
     
    Last edited: Nov 28, 2013
  15. WBahn

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    I've found numerous sites that talk about using matching transformers with speakers or low impedance headphones on crystal sets. Have no basis yet upon which to know if, or how well, any of them work.
     
  16. bertus

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  17. vk6zgo

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    Jul 21, 2012
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    Yeah.I think they are kidding themselves!:D

    My feeling is that it would look like a mismatch at the secondary,& not reflect directly to the primary,unless,as you say,there was sufficient inductance in the primary,which seems unlikely.

    Tube type speaker transformers may have packed a fair amount of inductance,as they are about the same physical size as the 14Hy HT filter chokes which were common in Australian radios.

    In Oz,those speaker transformers usually matched 5kΩ or 7kΩ to 2Ω,3.5Ω,or 15Ω loudspeakers.

    Re: the "high impedance 'phones----the old S.G Brown headphones I had as a kid were 2000Ω.
    The magazines used to warn against the use of the 600Ω Military surplus headphones which were widely (& cheaply) available at that time.
     
  18. Tesla23

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    May 10, 2009
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    You seem interested in the fundamentals of what's happening so just to give you an idea what you are working with:

    To get some estimate of the impedance at the antenna, assume a 10m vertical wire, 4mm dia. At 1MHz the input impedance over ground is about
    0.44-j2150Ω,

    (to see how this is calculated:
    http://www.ece.rutgers.edu/~orfanidi/ewa/ch22.pdf eqn 22.3.24
    this is for a dipole, so a monopole over ground has half the impedance)

    or in parallel terms 74pF || 10.5MΩ.

    Shorter wires will have higher impedance, longer lower. Slant wires will be different but still of this order.

    So this is very high, you really can't match to this, but the closer you get the better your radio will work. Unless you have a really strong signal, impedance matching will be important.

    The minimum signal levels in an AM service area in Australia (I suspect similar to US) are:
    Rural: 0.5mV/m
    Suburban: 2.5mV/m
    Urban: 10mV/m

    To determine the antenna voltage from the field strength, for an electrically short antenna like this, it is 0.5 E L, where E is the field strength and L is the monopole length. So the minimum o/c antenna voltages you would see are for a 10m antenna:

    Rural: 2.5mV
    Suburban: 12.5mV
    Urban: 50mV

    (If you want to see how the antenna voltage can be determined from the incident field see here: http://puhep1.princeton.edu/~mcdonald/examples/receiver.pdf)

    Add to this is selectivity. Assuming your tuned circuit tunes to 500kHz at 400pF, then L = 250uH. Lets analyse this at 1MHz, so at 1MHz XL = 1570Ω. Your coil probably has a Q of say 200, so to achieve a -3dB bandwidth 10kHz at 1MHz you need a loaded Q of 100, so the loading Q of your detector is 200. This means the detector load, (referred to the top of the tuned circuit if you are tapping it down), is about 300kΩ.
     
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  19. MrChips

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    I think your analysis is bang on. I measured the signal from my antenna and got 50mV.

    Here is my simplest crystal radio.

    [​IMG]

    [​IMG]

    I am using only one section the ganged tuning capacitor.

    For better selectivity I have used two coupled inductors (not shown). The second col is wound on a cardboard tube of smaller diameter (try plastic or aluminum wrap rolls) that slips inside the larger toilet roll coil. Then I can adjust the desired coupling by sliding one coil over the other. The diode is connected to the second coil. The selectivity is superb. I can easily pick up four stations at 900, 1010, 1150 and 1280kHz.
     
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  20. MrChips

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    Correction: The number of turns on the first coil is 48.
     
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