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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 3 different systems for distortion, frequency spectrum and loudspeakers.

 


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, ....
It is not easy regulate the output level with the on-board potentiometer because this is integrated in the sinusoid generation circuit,
so every time the position is changed, it takes a few seconds to reach stability again.


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.
  • input impedance about 10Kohm
  • 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%.

    To power the Victor 1kHz board I have used a LM317-LM337 module and a 18V-0-18V r-core transformer.

     

    MEASURE OF FREQUENCY RESPONSE

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

    ADALM2000

    It s a portable lab with 12bit ADC and 12bit DAC designed for university students

    Features:



    The BNC adapter module for Analog Discovery could be used also for the ADALM2000 but I don't suggest to use it because it does not have differential analog (scope) inputs.

    One input channel (1+) will be used as reference and output is measured on the second channel (2+).

    There are 2 software to use with this product:

    In the webpage https://wiki.analog.com/university/tools/m2k/common/firmware you can find the procedure to upgrade the firmware to the last release.

    In the webpage https://wiki.analog.com/university/tools/m2k/scopy you will find 3 download links for Scopy, one for the latest release, and two for the latest nightly builds (one for 32-bit systems and one for 64-bit systems)

    image

    Select the second or third installer depending on your system, you should use the Scopy v1.1.1.

    Here the frequency response of a pair of RC filter.

    Here the frequency of the Gainclone amplifier using LM3875 chip using Scopy Network Analyzer with automatic voltage gain.

    Here the frequency of the my PF2019 amplifier  high voltage gain on 8ohm, output cap. 4700uF and driven by 6072 pre-ampl.

    In order to create a final product to use I have put the ADALM2000 inside a chassie with standard connectors.

     

     

    LOUDSPEAKERS MEASUREMENT

    This is miniDSP UMIK-1 Omni-Directional USB Measurement Calibrated Microphone

     

    Features:


     

     

     

     

     

     

     

     

    Power Follower 2019

    2V

     

    1V

     

    500mV

     

    200mV