Original rack mount speakers for the Collins R-390A are getting scarce and once found are very expensive. I decided to build my own speaker that would compliment the audio of the R-390A.

  I researched speakers and enclosures and soon discovered this can be a complicated subject. I will skip all of the experimentation and give details of the final product. The speaker described sounds great for AM, music and response is very good for CW.

  I had a blank 19x7in. rack panel and decided one speaker would be sufficient. I was lucky and located a NOS 1960 Jensen Concert Series speaker for $10.00. The speaker uses an alnico magnet and measures 5 1/2in. diagonally between mounting holes. The enclosure is 6 1/2in. tall, 7 3/4in. across and 7in. deep. A Collins 600 to 8 ohm audio transformer is used to match the speaker.

  Since I used the local audio to feed the speaker a switch is needed to turn off the speaker when headphones are used. The rack handles are aluminum and can be found at home improvement stores. These are sold as drawer pulls.

  Much experimentation can be done with speakers and enclosures. The speaker I have described has been in constant use for 4 years and is still a pleasure to use.

Part II:   Audio Impedance Matching Transformer

  This is an audio impedance matching transformer I use with the Collins R-390A. The receiver uses 600 ohm output for the speaker and headphones. This transformer allows you to use modern 8 ohm headphones. The transformer could be mounted in the receiver, but I prefer to keep the receiver original. An added bonus is that it can be used with other older tube type receivers.

   

  73, James, KC5LDO

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