Monthly Archives: January 2017

Measuring (baselining) with my new National Distortion Meter

I like measuring. So I bought a National Distortion Meter. It is the VP-7701a.

This device is capable of measuring the distortion of an sine audio signal. And can be useful to calibrate vintage audio equipment.


To be able to make any meaningful measurements, I need to do some baselining, meaning that I want to measure the distortion when the device is not measuring anything at all. If that makes any sense to you. It does to me anyway 😀
When the base distortion levels are extremely low, I can measure devices and discard the base distortion. But when the base distortion levels are significant, then I would have to subtract these levels from the measurement each time I do a measurement. Which I do not want, because it is a hassle.

So in order to do that I need a pure sine wave for input. The device does not provide that, at least not one that is available to the user. The device measures the distortion by subtracting its own sine waveform from the one that is being measured, and the result should be zero. But only if the supplied waveform is 100% equal to the internal sine waveform. My search for a matching waveform therefor brought me to 3 sources:

  • a pure sinewave provided by my soundcard from the computer, created in audacity
  • a test CD made for this purpose which I bought 20 odd years ago
  • a test CD that I created myself with pure sine waves created by audacity and burned to a CD

This is the test CD that I once (’90-s) bought:

It has all kinds of tests on it: frequency sweep, fase checks etc and also sine wave tones across the audio spectrum.


This is the result of my base line tests:

The results show that both my soundcard and the commercial testCD are useless for my measurements. The CD that I made myself is a lot better. It performed marginally better on my Technics SL-P777than on my Philips DVD 963SA player (SACD). The result is such that I think that the base distortion is low enough to not take it into account when I start to measure real equipment. So I will be using this CD as my source for sine waves.

One side note: my CD is recorded at 0dB level, so the player outputs 2,5V which is rather high. I may have to make a second CD where the levels are lower, like -10 or -20 dB.