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Audio Measurements
Aug 2018 in progress


INTRODUCTION

I have started the measurements on my projects in the year 2003 with the Clio system 6 by Audiomatica, this was a PC PCI audio card (16bit 44KHz) with an external unit.
After some year these has been upgraded to the version 8 using the M-Audio Transit USB as audio card (24bit 96KHz). 
The Clio system was designed for loudspeaker measurements but it can be used also to test amplifiers with some limits.
I have destroyed 2 times the input circuit of the Clio system during tube amplifier measurements.
To overcome these problems I have designed the EMU input project able to measure 1000Vrms like the anode of a 845 tube.
This project use the M-Audio 0404 USB audio and the Arta software.
Also if the M-Audio 0404 USB have 192Khz A/D converters the limit is 20KHz for the measurements.
To increase the frequency band and the distortion resolution are necessary 2 different systems for THD and frequency spectrum.


MEASURE OF DISTORTION

It is necessary an ultra low distortion sine generator like the Victor 1kHz board with an harmonics typically about -150dB.
Symmetrized FET in AGC is the "heart" of this generator (see schematic).
It is necessary an external power supply DC voltage/current 35V / 25mA regulated with a LM317 or LM317HV.
In order to purchase this sine generator contact Victor Mickiewicz by email and ask for the required frequency 1KHz, 10KHz, ....


Measure the distortion with very good resolution is not possible with normal audio card so the my choice is the Olivine 2 an USB Analog To Digital Converter by Altor Audio.
In order to purchase this USB ADC contact Alexander Torres by contact page.

     
 

  • 2 channels, 96/24 digitizing
  • Better then -140 dBFS Noise Floor
  • Better then -110 dBFS 2nd/3rd harmonics @0 dBFS signal.
  • Better then -135 dBFS 2nd/3rd harmonics @-20 dBFS signal.
  • UAC1 operation (not need driver  for Windows) with firmware upgrade.
  • No external power supply need.
  • Input level control potentiometer added.
  • 0.5Vrms Full Scale (max., with potentiometer at right position)
  • Power and Overload LED indication.
  • Small 120x75x30mm case (Depth-Width-Height).

  • Follows the measurement of the Victor ultra low distortion sine generator with Olivine 2 unsing Arta software, the result is very good 0.0003%.

     

    MEASURE OF FREQUENCY RESPONSE

    To measure frequency spectrum I suggest an USB oscilloscope with minimun 12bit ADC and at least 1MHz band like the ISDS2062B by Instrustar.

          

     

    This USB device have 2 input channels with an impedance 1Mohm 25pF and a variable output.

    Here the frequency of the Gainclone amplifier using LM3886 chip.

    Open the panel Sweeper of the interface and connect the amplifier using this layout.

    At this time I have problem with the original 10x probe because it give a bad output result.