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Audio measurement workstation
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README.md

Audio Measurement Workstation

Initial Specifications

  • Class II appliance - no ground loops incl. display
  • Balanced inputs for measuring bridged or digital amps
  • Dedicated device
  • Decent measurement quality - PCI soundcard ESI Juli@

Design Overview

Thin Client PC FS Futro S900N

  • Class II power adapter
  • PCI slot incl. +12/-12V supply voltages
  • Dual core CPU to allow measurement and VNC remote access
  • Connectors for two SATA drives to allow increasing reliability/repairability with RAID1 configuration
  • No monitor, keyboard, mouse - access via network VNC
  • Current IP address relayed via speech synthesis announcements, using internal soundcard and speaker.
  • Inexpensive from ebay

ESI Juli@

  • Both balanced and single-ended inputs and outputs, functioning simultaneously. It allows feeding an amp with single-ended signal and measure balanced speaker output at the same time.
  • Digital outputs/inputs for extra measurements/tests
  • Full support in linux
  • Priced moderately on ebay due to lack of PCI slots in modern PCs

Software

  • Linux Mint Mate (v. 18 at the time of writing)
  • Arta measurement software running under wine
  • Control - python script hooked to ACPI power button.
    • At startup - Etherner and/or wifi IP addresses are announced.
    • Pushing the power button once - IP addresses announced
    • Pushing the power button twice within two seconds - shutting down + announcement

Operation

  • Measurement mode - remotely via VNC on another PC
  • Admin mode - possible to connect monitor/keyboard/mouse at any time

Construction

All parts are 3D printed, FreeCAD sources and STL models are included in design folder.

Since the thin client supports only half-size PCI cards, room/holes must be made for full-sized one. Upper sound jacks and PS/2 slot must be removed/sawn off.

The original holder of the secondary heat sink must be removed, the heat sink fixed directly to the bottom with a new holder.

The soundcard is connected with a PCI cable adapter (https://www.aliexpress.com/wholesale?SearchText=19cm+PCI+extension+cable). It is mounted with 3D-printed brackets. The lower brackets provide only shielding, using copper tape connected to the PC enclosure. Coax digital input and output are extracted from the multiconnector socket to individual add-on cinches.

The back panel is printed in two colors.

Holes for long cinches are drilled into the front panel, covered with a two-color printed plate.

Performance

Single-ended and balanced inputs are mixed before the ADC. Therefore, for best performance of the higher-voltage balanced inputs short-circuit the more sensitive single-ended inputs with short-circuited cinches. Inexpensive at https://www.ebay.com/sch/i.html?_nkw=RCA+Protective+Caps . In balanced-balanced closed-loop measurement this improves noise RMS from -102dB to -105dBFS.

Closed loop no signal

Closed loop 1kHz signal

Closed loop two sine 13+14kHz signal

Amps are measured with a dummy load with replaceable balanced voltage dividers made from old CPU heatsinks, optionally cooled with the original heatsink fans. I will post more pictures and resistor values.

Notes

Testing Juli@ Voltage Supplies

The PC generates +12V/-12V voltages for the PCI slot. I cut these supply pins on the connector between digital and ADC/DAC parts of Juli and fed the ADC/DAC part with 7812/7912 linear PSUs. No change in noise background was detected in Arta. It means the internal regulators on Juli are capable of filtering the SMPS-generated supply voltage sufficiently and the PCI-supplied voltages can be used safely.

Shielding Juli@

I measured the closed-loop noise floor before covering the bottom holders of Juli@ with copper tape and connecting to the case. The difference between the unshielded and shielded soundcard was less than 0.2dBFS. Therefore I assume the bottom shields are not necessary and do not have to be printed/installed at all.

No Impact of SSD on Measurement Performance

Running full RAID1 sync on the secondary SSD located right beneath the soundcard had no measurable impact on the closed-loop noise floor of 105.1dBFS.

VNC Resolution

With no monitor connected the X server starts at 1024x768. This resolution seems just enough for taking measurements/pictures while keeping the CPU load for VNC transmission acceptable.

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