I want to measure a signal noise ratio (SNR) in an power amplifier. SNR is calculated by 20Log (Vs/Vn) when Vs is nominal output voltage in volt RMS when generate a nominal power and Vn is the noise in volt RMS (weightning A). Alternative metod is: nominal voltage output in dB minus noise (weightning A) in dB. I using the Focusrite Scarled 2i2 sound card with REW software.
An examples in my litle diy power amp.
Using attenuator from power amp to sound card.
1) set input knob to minimum in sound card
2) connect sound card output to the power amplifier (TS -> RCA)
3) connect LINE input of sound card to amp out via attenuator (-20dB)
4) gently ingrease output level untile the THR reach 1% (this is max power)
5) run SPL meter and set weightning Z and save the value displayed (101.3dB)
If you want using multimeter to measure maximum Vout of the amp, in my example is 16.5Vrms and calculate the nominal power Pnom= Vrms^2 / 8 = 34W (8ohm load)
6) calculate dBsignal: dBsignal = dBdisplayed + dBattenuator = 101.3 + 20 = 121.3dB
7) stop generator and set to zero monitor knob and read dBspl weightning A (21.2dB)
8) calculate SNR = dBsignal - dBnoise = 121.3 - 21.2 = 100.1dB
Isn't necessary calibrate level on sound card.
This methods are valid if your sound card has a high enough SNR. To check this simply disconnect the sound card input from the amplifier and short-circuit the input, the SPL value must be at least 10dB lower than the value read with the amplifier connected.
An examples in my litle diy power amp.
Using attenuator from power amp to sound card.
1) set input knob to minimum in sound card
2) connect sound card output to the power amplifier (TS -> RCA)
3) connect LINE input of sound card to amp out via attenuator (-20dB)
4) gently ingrease output level untile the THR reach 1% (this is max power)
5) run SPL meter and set weightning Z and save the value displayed (101.3dB)
If you want using multimeter to measure maximum Vout of the amp, in my example is 16.5Vrms and calculate the nominal power Pnom= Vrms^2 / 8 = 34W (8ohm load)
6) calculate dBsignal: dBsignal = dBdisplayed + dBattenuator = 101.3 + 20 = 121.3dB
7) stop generator and set to zero monitor knob and read dBspl weightning A (21.2dB)
8) calculate SNR = dBsignal - dBnoise = 121.3 - 21.2 = 100.1dB
Isn't necessary calibrate level on sound card.
This methods are valid if your sound card has a high enough SNR. To check this simply disconnect the sound card input from the amplifier and short-circuit the input, the SPL value must be at least 10dB lower than the value read with the amplifier connected.
