Passive I/V, poweramp, distorsion & SpectraLab

[A 8]

Until recently I have not been able to measure anything but ohms and voltage but in a recent thread I found a reference to SpectraLab and their FFT spectral analyses software and could not resist putting it on one of my laptops (IBM ThinkPad T21)

Now, I have always preferred passive I/V for completly subjective sonic reasons inspite my understanding that the overall THD will be much higher then a traditional opamp I/V.

However looking at the SpectraLab plot which is actually taken at the poweramp speaker terminals I seem to be doing pretty good considering.

I am a bit confused though, taking the THD reading of 0,01244% and converting to dB gives me -78dB THD which does not align with the graph, the graph shows a max around -95dB with most of the crap at around -100dB...?

Anyone care to comment and help me make sense out of it?

/Michael

 

[Andypairo]

The -9x dB level on the graph is a power spectral density (and it depends on the frequency resolution aka bin width IIRC), while the SNR is a ratio between two power levels.. that's why they are different.

Integrating the PSD over the 20-20kHz band should give you the right result.

Cheers

Andrea

 

[IpsilonSound]

I'd also like to get a copy of the SpectraLab, could you pm me a link pls?

 

[A 8]

Thanks Andrea, makes perfect sense. I tried to hit 0dB but my volume setting pushed the input stage on the laptop into clipping.

I am still a bit confused as to why it looks as good as it does considering my use of the laptops original soundcard and the passive I/V?

The link...http://www.pmgrp.com/prod01.htm#SpectraLAB