[R-390] 600 to 8 ohm speaker converter

Dave and Sharon Maples dsmaples at comcast.net
Mon Feb 14 14:47:41 EST 2022


All: For a 25.2-volt speaker transformer, a 1-watt tap presents a load of
635 ohms.  For the 70.7-volt speaker transformer, an 8-watt tap  presents a
load of  625 ohms, and a 10-watt tap presents a load of 500 ohms.  

The general formula is R = (E*E)/P, where
	R = transformer impedance presented to receiver
	E = either 25.2 or 70.7 volts, depending on the transformer
	P = transformer tap power rating (in watts)

The secondary should match the characteristic impedance of the speaker.
Recognize that the speaker impedance is neither high-precision nor steady
across all frequencies, so worrying about the mismatch between the 600-ohm
output on the receiver and the impedance of the speaker / transformer combo
may not be worth doing.  Something in the 500-600 ohm range should be
adequate (closer is better, but you use what you have).

Thanks,
Dave Maples 

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
Behalf Of Barry
Sent: Monday, February 14, 2022 14:17
To: K PERALES <kenperales at comcast.net>
Cc: R-390 Mailing List <R-390 at mailman.qth.net>
Subject: Re: [R-390] 600 to 8 ohm speaker converter

If I remember correctly, a 70 volt line speaker transformer can be used and
I think that's what you're showing on the left.  An 8-ohm speaker goes to
the 8-ohm output lines and the line output goes to one of the wattage taps.
I can't see the labeling on the back side of the transformer in you picture
so not sure which wires those are but typically there would be one common
input with multiple taps for various wattage (or volts).  As I recall, there
is a recommended tap to use but I think I tried a few of them to see which
one gave the best output.

HTH,

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "K PERALES" <kenperales at comcast.net>
> To: "R-390 Mailing List" <R-390 at mailman.qth.net>
> Sent: Monday, February 14, 2022 12:49:17 PM
> 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
> 
> 
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