[R-390] Subject: Re: R-390 Audio Transformer

[Dave Maples]

All: The 70.7 volt transformer is marked with various wattage taps on the
primary side, and speaker impedances on the secondary side.  Assuming the
speaker I plan to use is matched to the proper secondary tap, then if 70.7 V
RMS is delivered to the primary, the speaker will be driven with the power
marked on the primary tap.  For example, if I select the 1-watt tap on the
primary side, and connect an appropriately-matched speaker to the secondary
(e.g. 4-ohm speaker to the 4-ohm secondary connections), then if I deliver
70.7 volts RMS to the primary, one watt of audio will be delivered to the
speaker.

Now for the math.  The ^ symbol indicates raising the number before it to
the power after it.  In this case all we are going to be doing is squaring
some numbers.


Since P= E^2/R (i.e. " P equals E squared over R"), then P*R= E^2, and R=
E^2/P ("R equals E squared over P").

For this activity, we want to find the load impedance of the transformer
primary for a given wattage tap, so for the 1-watt example, we have:

	R= (70.7)^2/P, or

	R= (70.7*70.7)/1 = 5000/1 = 5000 ohms

(As an aside, now you know why the 70.7 volt standard was invented...it made
the numbers really easy to calculate.)

If I have a 10-watt transformer, and select the 10-watt tap instead of the
1-watt tap, I get:

	R= (70.7*70.7)/10 = 5000/10 = 500 ohms

If I have an 8-watt transformer and select the 8-watt tap, I get:

	R= (70.7*70.7)/8 = 5000/8 = 625 ohms

I can also use a 25-volt speaker transformer as well:

	R=(25*25)/P = 625/P

If I have a 25-volt transformer, I can use the 1-watt tap and get 625 ohms,
just like with the 70.7-volt transformer.  Either one will work pretty well
against the 500-ohm source from the R-390, and will work very well against a
600-ohm source from some other equipment.

What about a filament transformer?  Well, for a 4-ohm speaker I need an
impedance transformation of 500:4 = 125:1.  In order to know the voltage
ratio involved, I take the square root of the impedance transformation.  The
square root of 125 is 11.18, and the square root of 1 is 1.  That means I
need a voltage ratio of 11.18:1.

A 12-volt transformer will provide a ratio of 10:1, which is close.  A
10-volt transformer would be better but is non-standard.  If desired, I
could use the 12-volt transformer with a small series resistor to raise the
impedance to not overload the R-390A.  A 6.3-volt transformer would
underload the R-390 output.

It doesn't have to be exact; it just needs to be reasonably close.

All this is offered for what it's worth...

Thanks,
Dave WB4FUR