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Audio transformer for crystal set help

Discussion in 'Audio' started by ecollen, Oct 19, 2015.

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  1. ecollen

    ecollen

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    Jul 31, 2015
    Hi,

    I am building (this is elementary level electronics) a simple two-transistor amplifier for a crystal set that will allow me to listen through a small 8 ohm speaker rather than through an earphone. The circuit requires an 8 ohms to 1 k centre tapped audio matching transformer (it feeds the signal from the second transformer to the speaker), but I am having difficulty trying to find this transformer.

    I was wondering whether anyone could perhaps suggest how I can get around this problem, please. Where could I find such a transformer, or could a different one be used/adapted? Is there any other way around the problem?

    In my electronics junk I have also found three transformers (still on boards) that I thought might work here, and that I’m thinking of trying out. But can anyone perhaps tell just from this picture (if I manage to get it loaded!) whether any of the three might perhaps work (I can also post slightly more detailed pictures of the individual transformers - and also of the circuit if it’s relevant), or would it be a waste of my time desoldering them and trying them out?
    All assistance would be much appreciated.
    Errol
     

    Attached Files:

  2. Bluejets

    Bluejets

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    Oct 5, 2014

    Attached Files:

  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    IMHO it's never a waste of time to remove transformers and try to discover their specifications through electrical testing. You will learn a lot about transformers that way. You might even get adventuresome and remove the insulated copper windings to salvage the cores to make your own custom transformers.:D
     
  4. ecollen

    ecollen

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    Jul 31, 2015
    Hi Bluejets
    Thanks for your trouble. This will work, will it? I'll try ordering one anyway. I will however keep looking for other solutions in the mean time - because with our postal system nowadays there's only about a 50/50 chance of it ever reaching me. Maybe I should order two, separately, so I've got a better chance of getting one :)
    Thanks.
    Errol
     
  5. ecollen

    ecollen

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    Jul 31, 2015
    Hi Hevans
    Many thanks for your reply. Yes, I think I should try fishing one out for a start and try it out. I can't think of any simple way to test it though - probably just putting it into the circuit and seeing if it works will be best. I don't know if these are power transformers or not - I expect they are - but I think the name just comes from what they're used for, so if it's got the right turns ratio it should work for me, regardless of what it was used for formerly. I will give it a go. I don't think I'm quite ready to be that (!) adventuresome yet to unroll the wire and try making my own. It's an idea to save up for later though.
    Thanks,
    Cheers,
    Errol
     
  6. duke37

    duke37

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    Early transistorised radios used transformers and you could take the transformers out of these or use the audio amplifier complete.
    Later radios used transformerless circuits.
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    See talkingelectronics.com website for a very large article on CRYSTAL SETS and how to create an amplifier with and without a transformer.
     
  8. ecollen

    ecollen

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    Jul 31, 2015
    Hi Duke
    Many thanks for the information. I didn't know that. I've got a few old radios lying around. I'll take a look at one or two of them and see if I can identify the parts.
    Thanks,
    Errol
     
  9. ecollen

    ecollen

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    Jul 31, 2015
    Hi Colin
    Many thanks, There's a massive amount of information in that article. I'll look at in more detail and decide on which circuit/s to try.
    Cheers,
    Errol
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Good luck with your project but from a purist perspective "Crystal Sets" can't have active components. At this point in its life your project is now a Transistor Radio.

    Chris
     
  11. ecollen

    ecollen

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    Jul 31, 2015
    Thanks, Chris. I'll go on with it anyway. I've ordered the transformer from Ebay as mentioned by Bluejets above. Now it's just a matter of waiting to see if it gets here, from Hong Kong. One nice thing though is that there's no postage. Pretty neat.
    Cheers,
    Errol
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Errol, I buy lots of stuff from China on ebay. I guess about 1 out of every 20 orders don't make it here but the seller or ebay always credits my paypal account. You can't be in a hurry though. Typical 20-30 day delivery time.

    Good luck,
    Chris
     
  13. ecollen

    ecollen

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    Jul 31, 2015
    Hi Chris

    Thanks for letting me know. I've got a bit more hope now. There's one big difference though - I have to rely on the South African postal system! But, well,we'll keep our fingers crossed, which always helps :)

    In the mean time I've desoldered the components that I started talking about in the beginning and have discovered that only one of them is a transformer - the one near the middle of the picture that is obviously a transformer. The other two that I thought might be possibles - the one in the foreground and the other one in the upper left - turned out to be chokes. Anyway, now I've learned something. Whether the transformer will work or not remains to be seen. It's not centre tapped - I don't know if that makes a difference.

    Cheers,

    Errol
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I looked through this thread but I didn't see the schematic of your circuit but since it specifies a center tapped xfmr it's probably configured as a Class B Push - Pull output.

    What Duke says here (he's an old fart) is quite true. At that time it was not an easy task for engineers or techs to transition from vacuum tubes to bipolar transistors. So output transformers that were absolutely required with vacuum tube technology just carried over to solid state design but it didn't last long. When the industry began to realize the astronomical cost and space saving of solid state it wasn't long before wet feet engineers developed transformer-less audio outputs.

    Cheers,
    Chris
     
  15. ecollen

    ecollen

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    Jul 31, 2015
    Hi Chris

    No, I didn't put the diagram on - only the photo - but here it is. I doubt if it's that transformer. Don't forget, this is rock-bottom elementary stuff. Yes, interesting about the transformers and transistors. I must definitely have a peer into some of old radios sometime to see if I can become any the wiser.
    Cheers,
    Errol
     

    Attached Files:

  16. BobK

    BobK

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    Clearly it does not need a center tap since the center tap is left unconnected!

    Bob
     
  17. ecollen

    ecollen

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    Jul 31, 2015
    Hi Bob

    Oh, yes. I'm wondering - perhaps they specified a centre tap because one of the other circuits in the book needs it. (It's just a little thing where the list of all the needed components is given at the beginning of the book, and then several circuits are given.) In any case I'll try it when I get there.
    Cheers,
    Errol
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, as Bob said... The CT isn't used in that circuit. What's the diode for in the supply rail?

    Cheers,
    Chris
     
  19. duke37

    duke37

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    Chris said "What Duke says here (he's an old fart) is quite true."
    There is one old fart here and lots of new ones.:p

    The diode in the supply is to protect the circuit from reverse polarity.
     
    Martaine2005 and ecollen like this.
  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Oh a Dummy Fail Safe! :D

    One old fart to another.. Salute!
    Chris
     
    Martaine2005 likes this.
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