Audio Measurements

To get more information contact me at: webmaster@audiodesignguide.com

Audio Measurements
Aug 2018 in progress

INTRODUCTION

I have started the measurements on my projects in the year 2003 with thee Clio system 6 by Audiomatica, this was a PC PCI audio card (16bit 44KHz) with an external unit and a software working only on Windows XP
After some years this system has been upgraded to the version 8 adding the possibility to use the M-Audio Transit USB as audio card instead of PCI card.
The Clio system was designed for loudspeaker measurements but it was possible to use 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 and to leave this old system on Windws XP I have designed the EMU input project able to measure 1000Vrms like the anode of a 845 tube.
This project used the M-Audio 0404 USB audio interface and the Arta software but also if the M-Audio 0404 USB have 192Khz A/D converters the limit is 20KHz for the measurements.
For the loudspeaker measurements I have used the good condenser microphone Behringer ECM8000, this requires phantom power (+15 to +48 V)
The ECM8000’s ruler-flat frequency response from 15 Hz to 20 kHz, and its omnidirectional pickup pattern give you a clear, accurate reading but it have no calibration file.

It is obvious that to have a true measurement system and overcome the various limits encountered, it is necessary to have for each type of measure a specialized systems.

Measure of distortion
it is necessary a very low distortion generator in the audio band also with fixed frequency and a good ADC with a minimun resolution of 24dB.
Measure of frequency response
In this case we need an sytem able to generate and measure waveforms in a wide band at least 1Hz to 1MHz but also 12dB are enought.
Measure of loudspeakers
Here we need a good microphone with calibrated file and an adc in the audio band 20Hz to 20KHz, 16dB are enought.
If the microphone integrate the adc is possible to eliminate many problems of long analog cables.
Measure impedance and Thiele/Small parameters of 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

Full galvanic isolation between USB digital part and ADC part.

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:

Two general-purpose analog inputs:
Differential, ±25 V, 1 MΩ || 30 pF, 12-bit, 100 MSPS ADC with 25 MHz bandwid
Two general-purpose analog outputs:•Single-ended, ±5 V, 50 Ω, 12-bit, 150 MSPS DAC with 30 MHz bandwidth
Two variable power supplies•0 V to 5 V, –5 V to 0 V, 50 mA
16 digital input or output pins•3.3 V, 1.8 V, 100 MSPS, 5 V tolerant
Two digital triggers•3.3 V or 1.8 V, 100 MSPS, 5 V tolerant

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:

Scopy - panel Network Analyzer (suggested)
Alice - panel Bode plot for frequency reponse

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)

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.

All tools can be detached by dragging them out of the left Tool Launcher onto the application or by double clicking the instruments if that setting is set in Preferences - https://wiki.analog.com/university/tools/m2k/scopy?s[]=detach#user_guides

VRMS is available in measurements -https://wiki.analog.com/university/tools/m2k/scopy/oscilloscope?s[]=measurements&s[]=scopy#using_built-in_signal_measurement_features

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:

Windows/Mac/Linux software
Special features such as calibrated SPL measurement enabled under free Room EQ Wizard (REW) software
Frequency response: 20 Hz - 20kHz +/-1dB with calibration loaded
Resolution & Sample rate 24bit ADC @ 48kHz
Max SPL for 1% THD @ 1kHz 133dB SPL @ 0dB analog gain setting
USB Audio: USB Audio class 1.0 Driverless interface for Windows, Mac & Linux
Power USB Powered (5V) -Blue LED indicates unit under Power
All UMIK-1 are calibrated with a unique calibration file

To have a fast integration with the calibration file I suggest to use the REW software.

REW is free room acoustics analysis software for measuring and analysing room and loudspeaker responses working on Windows, MacOS and Linux.

REW uses a logarithmically swept sine signal for its measurements instead of MLS so the room affects the measurement result if it is not produced in the near field.

Here follows the frequency responce of the Pioneer PM16A1 with the 2nd and 3rd harmonic.

MEASURE IMPEDANCE AND THIELE/SMALL PARAMETERS OF LOUDSPEAKERS

I am using the DATS V3 Computer Based Audio Component Test System.

Here the measurement of the impedance of the Monitor 4 with the two LCR cells for the compensation.