That's a good idea... so, run the 1Vrms test with your attenuator in place to get the precise attenuation ratio at the SC input. Then use this to scale up your attenuated noise to it's actual value. Do I have that right?
BTW - I was careful. I used my DMM to verify the amp output was in fact at zero (DC and AC) before plugging into the SC. But I do understand there's always a risk when connecting directly.
BTW - I was careful. I used my DMM to verify the amp output was in fact at zero (DC and AC) before plugging into the SC. But I do understand there's always a risk when connecting directly.
I have a 22:1 attenuator on my rig. To calibrate REW for this diminished signal, you Calibrate the signal in the Generator in REW. Put a TRMS meter, or 'scope, across the speaker terminals after connecting your dummy load. Open the 'scope tab and the Gen tab. Instead of setting the signal using dbFS, click the Volts button, and enter 1v. The measurement at the speaker terminals will be the true output of the DUT, but the signal at the audio interface will be lower.
Right next to where you enter the voltage value, there's a box labeled Calibrate. Click, enter the value of the TRMS meter and OK. REW should be calibrated now for your PS output level. Confirm accuracy at a higher voltage point and adjust your calibration if necessary.
This video has good info on this subject.
Right next to where you enter the voltage value, there's a box labeled Calibrate. Click, enter the value of the TRMS meter and OK. REW should be calibrated now for your PS output level. Confirm accuracy at a higher voltage point and adjust your calibration if necessary.
This video has good info on this subject.
Hi @dubadub, I use that process for testing with a signal. But testing for noise with the input shorted took me a few tries to figure out.
Also, realizing that my old Tektronix 2246 scope gives a different voltage reading than my Amprobe AM-510 DMM. I decided my newer Amprobe is likely more accurate than my scope which is at least 35 years old at this point.
Also, realizing that my old Tektronix 2246 scope gives a different voltage reading than my Amprobe AM-510 DMM. I decided my newer Amprobe is likely more accurate than my scope which is at least 35 years old at this point.
There's no mention of TRMS on the webpage for the Amprobe AM-510 DMM. Not all multi meters do the calc to display TRMS.
@RickRay, It is peak-to-peak. I converted to RMS by dividing by 2.828 then compared to my DMM AC reading. The DMM read a bit higher.
@dubadub, Its not a true RMS meter. However, my understanding is that for a pure sine wave, its AC measurement of RMS voltage should be fairly accurate. Maybe I'm wrong? Or maybe this explains the slight difference between the two.
I attempted to validate what I measured against the specs of my Focusrite. The datasheet says its Maximum Output Level is +15.5 dBu for balanced. This should be 4.614 Vrms or 2.31 Vrms unbalanced. I entered this into the Full Scale calibration in REW and set the signal generate to Volts. I measured the signal generator's output on my scope and DMM. My DMM was closer to the expected value than the Tektronix scope.
However... they where both pretty close to each other. I don't think it makes that big of a difference when my only objective is to determine approximately what the output power is for quoting measurements.
@dubadub, Its not a true RMS meter. However, my understanding is that for a pure sine wave, its AC measurement of RMS voltage should be fairly accurate. Maybe I'm wrong? Or maybe this explains the slight difference between the two.
I attempted to validate what I measured against the specs of my Focusrite. The datasheet says its Maximum Output Level is +15.5 dBu for balanced. This should be 4.614 Vrms or 2.31 Vrms unbalanced. I entered this into the Full Scale calibration in REW and set the signal generate to Volts. I measured the signal generator's output on my scope and DMM. My DMM was closer to the expected value than the Tektronix scope.
However... they where both pretty close to each other. I don't think it makes that big of a difference when my only objective is to determine approximately what the output power is for quoting measurements.