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 Post subject: R390A speaker match DIY
PostPosted: Jun Sun 16, 2019 11:34 pm 
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Joined: May Fri 25, 2012 3:31 am
Posts: 505
Location: Clayton, Ind.
I am pondering an audio transformer for an R390A, which puts out about half a watt at 600 ohms.

I have read many options to match to an 8 ohm speaker, which would require a turns ratio of about 8.67:1. With only half a watt, and just listening to voice, it is practical to just put the two windings over an open core and not worry about ferrite cores and such? Just wind say 100 primary turns, and about 12 secondary over that?

Note:. I have much recapping etc. before I even turn this on so I am not yet at the try it and see stage - strictly theory at this point.

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TDane WA9YI

Radio is a divine gift; God made us able to do one thing while listening to another.


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Mon 17, 2019 12:52 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2975
Location: Lexington, KY USA
Unless that voice is kinda high-pitched, it ain't gonna work.

If the iron were not necessary, they would have left it out of all those transformers.

A small line to voice coil transformer, as used in PA work will do what you need.

https://www.parts-express.com/search.aspx?N=4294967118+4294966104&Nrs=collection()%2frecord%5bendeca%3amatches(.%2c%22P_PortalID%22%2c%221%22)+and+endeca%3amatches(.%2c%22P_Searchable%22%2c%221%22)%5d&Ntt=70V*+transformer*&PortalID=1

Ted


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Mon 17, 2019 12:55 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2284
Location: Saskatoon
No. You'll need an iron or ferrite core. Otherwise, the inductance of the windings will be too low to work. Just find a small transformer with the desired turns ratio. Even a small power transformer would work fine for voice.


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Mon 17, 2019 1:03 am 
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Joined: May Fri 25, 2012 3:31 am
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Location: Clayton, Ind.
I believe you and will pursue the proper item, I see several options and may have the right power transformer here. However, what exactly does the ferrite do - feel free to point me to learned texts etc.

Or.... What kind of toroid might work, if any? Thx.

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TDane WA9YI

Radio is a divine gift; God made us able to do one thing while listening to another.


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Mon 17, 2019 1:56 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Saskatoon
The reactance of the primary side of the transformer, when the secondary is disconnected, needs to be about 4 times the impedance that the amplifier expects to see. In your case this impedance is 600 ohms, and the needed reactance would be 2400 ohms (minimum) at the lowest frequency to be reproduced. Otherwise the amplifier output stage just sees what is basically a short circuit. Since you're only interested in voice frequencies, the cutoff could be 300 Hz. A reactance of 2400 ohms as 300 Hz equate to 1.26 Henries. In order to get that kind of inductance without a ferrous core, you'd need a bazillion turns of wire. When you have a ferrous core, the inductance increases by a factor of several thousand, which significantly reduces the number of turns required on the primary and secondary. An additional benefit is that the ferrous core also improves coupling from the primary to the secondary, thus improving efficiency.


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Mon 17, 2019 4:31 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2975
Location: Lexington, KY USA
Ferrite is never used for audio transformer cores, because the core would have to be much larger than one made from transformer steel.

Ferrite becomes the choice at higher frequencies, where eddy current losses in a steel core would be too large.

Yes, a small power transformer might work. Something salvaged from a wall wart, say. Won't be "Hi-Fi", but good enough sound for this radio.

The impedance ratio does not have to be exact for the result to be satisfactory. This is a case where you can try different transformers from your junk box, and find one that works OK.

Ted


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Tue 18, 2019 11:22 pm 
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Joined: May Fri 25, 2012 3:31 am
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Location: Clayton, Ind.
Thanks for the informative replies.

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TDane WA9YI

Radio is a divine gift; God made us able to do one thing while listening to another.


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 Post subject: Re: R390A speaker match DIY
PostPosted: Jun Fri 21, 2019 8:17 pm 
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Joined: Feb Mon 21, 2011 4:19 pm
Posts: 206
Location: San Marcos, CA 92069
Back when I had an R390A, I used a salvaged transistor radio output transformer to match an 8ohm speaker. Worked great and sounded surprisingly good.


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