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Adapter for Measuring Amplifier Speaker Outputs

Overview

  • Designed for soundcard balanced inputs
  • Designed to fit headless audio measurement workstation but will work with any soundcard
  • Can be switched to unbalanced mode (single-ended SE)
  • Designed to fit FS Futro >=S450 case (inexpensive on ebay)

Speaker Load

  • Load resistors - combination of 4R and 8R 100W resistors (ebay)
  • Old/legacy CPU heatsinks with fans
  • Fans powered by the measurement PC

Ranges

  • Ranges (voltages for ESI Juli):
    1. 1:1 (20Vmax balanced / 10Vmax SE) - divider 2 x 75R
    2. 1:3 (60Vmax balanced / 30Vmax SE) - divider 2 x (75R + 150R)
    3. 1:6 (120Vmax balanced - not supported/60Vmax SE) - divider 2 x (75R + 375R)
  • Divider resistors 2W: 20V at (75R + 75R) -> 1.3W per each resistor
  • Ground lift for balanced outputs

Oscilloscope Output

  • Each input line is connected to individual BNC connector for oscilloscope
  • Ground lift for scope BNCs

Soundcard Inputs Protection

  • Serial output resistors 680R
  • Soundcard hot/cold input protection by two internal voltage sources (2 x 2pcs A23 batteries)
  • Protection diodes - dual low-leakage diodes BAV199
  • Both sets of protection batteries checked by built-in voltmeter prior to measuring.

Scope screenshot of protection limiter kicking in for hot and cold lines of one output:

The adapter in action:

Construction

  • Wired, no PCB
  • SMD protection diodes soldered to small universal PCBs placed directly onto TRS sockets

Load-Resistors Switch

  • 3D-printed, designed for regular 32mm rocker switches

Position 2.7ohm - both resistors paralelly

Position 4ohm

Position 8ohm

Measurements

For comparison ESI Juli in virtual balanced inputs mode with L+R jacks shorted

Connected adapter, no input signal, virtual balanced inputs. Notice only minor increase in 50Hz noise, overall noise level unchanged.

Connected adapter, no input signal, native balanced inputs of ESI.

Git repository

  • All Fusion 360 backup files, STLs, front panel text image

Conclusion

The measurements show the adapter does not bring any increase in noise of the measurement setup.