Audio transformer for crystal set help

[ecollen]

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

 

[Bluejets]

Seems easy enough to find ...Ebay

http://www.ebay.com.au/itm/EI-14-Ce...457422?hash=item2359dc2e8e:g:kt8AAOSwuTxWCM0G

 

[hevans1944]

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.

 

[ecollen]

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

 

[ecollen]

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.

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

 

[duke37]

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.

 

[Colin Mitchell]

See talkingelectronics.com website for a very large article on CRYSTAL SETS and how to create an amplifier with and without a transformer.

 

[ecollen]

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.

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

 

[ecollen]

See talkingelectronics.com website for a very large article on CRYSTAL SETS and how to create an amplifier with and without a transformer.

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

 

[CDRIVE]

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

 

[ecollen]

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

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

 

[CDRIVE]

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

 

[ecollen]

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

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

 

[CDRIVE]

Hi Chris

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

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.

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.

Cheers,
Chris

 

[ecollen]

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.
Cheers,
Chris

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

 

[BobK]

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

Bob

 

[ecollen]

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

Bob

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

 

[CDRIVE]

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

Cheers,
Chris

 

[duke37]

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.

The diode in the supply is to protect the circuit from reverse polarity.

 

[CDRIVE]

Oh a Dummy Fail Safe!

One old fart to another.. Salute!
Chris