Some questions about headphone preamp THD+N vs Power Chart

[Richarrd]

DUT sheet shows this data.
and here is my steps of measuring.
1. I use cosmos Loadboard ,switch to unbalanced and 32ohm，plugin to headphone output, and use multimeter to read the Vrms and calibrate REW generator to the actual reading.
2.I plug XLR out from loadboard to cosmos ADC input, and calibrate with this Vrms to my REW RTA too.
3.I started stepped sine on RTA and get these charts.

32ohms high gain(mW)

32ohms high gain(V)

300ohms high gain(mW)

300ohms high gain(V)

I'm not very familiar with the calculation method for the actual Watts generated under a certain impedance with V, (I'm sorry, I'm really a beginner), so it seems like there might be some missing calculations here? The actual mW conversion I obtained from REW doesn't match the official parameters, and I don't know why.

 

[staticV3]

Did you make sure to enter the load impedance value into REW as well? Otherwise, the mW numbers will be false.

 

[Richarrd]

Did you make sure to enter the load impedance value into REW as well? Otherwise, the mW numbers will be false.

oh I think that's the reason! no I never enter that and I even don't know where to enter it…

 

[DVDdoug]

I'm not very familiar with the calculation method for the actual Watts generated under a certain impedance with V,

Power (Wattage) = Voltage squared / Resistance.

i.e. With constant voltage, power is inversely proportional to resistance (or impedance). When you cut the impedance in half, power doubles.

The "basic" power formula is Power = Voltage x Current.

Another variation is Power = Current squared x Resistance.

The relationship between voltage, resistance, and current is defined by Ohm's Law (Current = Voltage / Resistance).

The 3 power formulas are derived using simultaneous equations.

Resistance (and impedance) is "The resistance to current flow".

 

[staticV3]

oh I think that's the reason! no I never enter that and I even don't know where to enter it…

On REW's latest early access build, you enter the load impedance in the RTA window under Settings->Appearance:

 

[Richarrd]

Power (Wattage) = Voltage squared / Resistance.

i.e. With constant voltage, power is inversely proportional to resistance (or impedance). When you cut the impedance in half, power doubles.

The "basic" power formula is Power = Voltage x Current.

Another variation is Power = Current squared x Resistance.

The relationship between voltage, resistance, and current is defined by Ohm's Law (Current = Voltage / Resistance).

The 3 power formulas are derived using simultaneous equations.

Resistance (and impedance) is "The resistance to current flow".

thank you! that's very detailed！

 

[Richarrd]

On REW's latest early access build, you enter the load impedance in the RTA window under Settings->Appearance:
View attachment 311093

OK that's what i missed! so if i connect to loadboard with 300ohms I simply enter 300,and 32ohms enter 32,is that right? cause I see it's for dBW and worrying about making another mistake

 

[staticV3]

so if i connect to loadboard with 300ohms I simply enter 300,and 32ohms enter 32,is that right?

Yes!

 

[Richarrd]

Yes!

thank you!!