[R-390] 600 to 8 ohm speaker converter

[Jim Whartenby]

KenThere should be a label on the line transformer which tells you how much power the speaker will see when using the various transformer taps.  The trick is to use the power formula to figure out the proper tap to do an impedance transformation.
Use the power formula P = I X E then substitute for I, E / R then you get  P = (E X E) / R
Solving for R the equation now reads R = (E X E) / P
Using a 70 volt line transformer:

R = (70.7 X 70.7) / 10 watts = 5000 / 10 = 500 ohms
So using an 8 ohm speaker connected to the secondary and the 10 watt tap on the transformer the speaker impedance will be transformed to 500 ohms which is close enough to the typical 600 ohm output impedance of the R-390 receiver.
If your audio line transformer is not for a 70 volt line but is for either of the other two common audio line voltages, 25v or 100v, simply use the square of that voltage divided by the various power levels to find the one which comes closest to 600 ohms.  Then use that wire color and the transformer primary common to connect to the R-390 and the two secondary wires connect to the speaker.
Don't forget to clip any bare wires from the other transformer taps so that they do not have a chance short to each other and cause other problems!
Regards,Jim



Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Robert P. Meadows <rpmeadow at bellsouth.net>
To: 'K PERALES' <kenperales at comcast.net>; R-390 at mailman.qth.net
Sent: Mon, Feb 14, 2022 1:05 pm
Subject: RE: [R-390] 600 to 8 ohm speaker converter

The R390 series of radios have a 600 ohm audio out system to feed distribution amplifiers. 
The speakers that were used then were 8 ohm for the most part, but, driven by specific amplifiers at the point of listening to the radio.
You can properly accomplish the listening deed with for the Navy installation, acquiring an AM 215 (from memory) and speaker.
Or as most do, with limited audio signal due to the radio design, use a 600 ohm to 8 ohm speaker transformer.
R

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of K PERALES
Sent: Monday, February 14, 2022 1:49 PM
To: R-390 at mailman.qth.net
Subject: [R-390] 600 to 8 ohm speaker converter

When I got my R-390 it has what I assume to be some kind of transformers to allow a modern 8ohm speaker to be hooked up.  I did not think to diagram how the wires were hooked up.  Is there any manual that tells a non-electrician how to connect these transformers to the R-390A to let me listen without headphones.

Ken