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

[rbethman]

Group,

The math is great. There is a 'but' in this all.

The audio output on the rear apron/back panel is, (according to 
TM11-856A), is a *whopping* 500 mW.

This also needs to be put into the equation.

It hasn't.  How much audio is lost by power in the windings?

Curiosity makes myself want to know.

Bob - N0DGN

On 4/22/2012 11:18 AM, Dave Maples wrote:
> 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
>