[R-390] Official specs

[Larry Haney]

Jim,  I agree with this posting of yours *except* for the 1st and last
statements.

1.  First you said: 'What has been overlooked is that there is an impedance
transformation from 50 to 125 ohms.'  We are all very aware of this fact.

2.  Lastly you said: 'To convert the* SG voltage output* into the voltage
actually seen by the R-390, multiply the SG reading by *0.1235* or divide
the SG reading by 8.097, either way works.'  That is *not right* at all.
You just went through a nice step by step explanation about how to
determine the power loss, then you use that power loss ratio (0.1235) to
determine the voltage seen by the 390.  *Wrong, wrong, wrong.*  The last 3
steps in your procedure are: 1. dB = 10 Log ^ (.00247watts / 0.02 watts),
2. dB = 10 Log ^ 0.1235, 3. dB = -9.083.  *No real disagreement there*.
The input watts to the da-121 = 0.02 watts, the output watts from the
da-121 = .00247 watts, that's a 12.35% loss of *power* in watts, not
voltage.  You *can not* use the 0.1235 *power loss* relationship to
directly calculate the *voltage loss* relationship of the da-121 as you are
doing in your last statement.

One way to correctly calculate the voltage coming out of the da-121 (Vout),
would be to use the formula:

Vout = Sqr rt (Pout (watts) x impedance (ohms))

Where Pout is the power coming out of the da-121 (in this case, 0.00247
watts) and impedance is the da-121 load impedance provided by the 390, 125
ohms.

Vout = Sqr rt (.00247 x 125) = 0.5556 Volts

     .00247 x 125 = 0.30875
     Sqr rt  0.30875 = 0.5556
     Vout = 0.5556 volts

Vout is what's going into the 390 (in this scenario).

Regards, Larry


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On Wed, Oct 23, 2024 at 9:35 AM Jim Whartenby <old_radio at aol.com> wrote:

> What has been overlooked is that there is an impedance transformation from
> 50 to 125 ohms.  Any calculation that ignores this transformation is in
> error.  The only solution that accounts for different impedances is by
> looking at the respective powers at both input and output.
>
>
> 1 volt into the DA-121 gives 0.556 volts out.  Looking at the power-in
> verses power-out using the respective impedances:
>
>
> Power = voltage squared / resistance
>
> Pin = 1 volt ^2 / 50 ohms = 0.02 watts
>
> Pout = 0.556 volt ^2 / 125 ohms = .00247 watts
>
> dB = 10 Log ^ (Pout / Pin)
>
> dB = 10 Log ^ (.00247watts / 0.02 watts)
>
> dB = 10 Log ^ 0.1235
>
> dB = -9.083
>
>
> To convert the SG voltage output into the voltage actually seen by the
> R-390, multiply the SG reading by 0.1235 or divide the SG reading by 8.097,
> either way works.
>
>
> Regards, Jim
>
> Logic: Method used to arrive at the wrong conclusion, with confidence.
> Murphy
>