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Loudspeaker Measurement System [130453-I]

Status: Finished
January 8, 2014
in Elektor magazine | November 2015 | Find it here

MEASSY- a 'measurement assistant' for the loudspeaker DIY enthousiast. Everyone developing loudspeakers knows about the hassle when it comes down on measurements. Ton's of cables, adapters, amplifiers, the soundcard and notebook are involved occupying the desk in a chaotic manner. There is seldom clear work organization or the possibility to retrieve a session done weeks ago and the gear is generally not portable.

Once the hobbyist investigates in what is really needed in terms of  a performance spec, he or she will find out that no high-end equipment is needed and a simple solution should be at hand. As a consequence, MEASSY was developed. MEASSY combines:

  1. A symmetrical microphone amplifier with up to 46dB gain
  2. A phantom power unit supplying 48 Volts (up to 8mA)
  3. A peak programme meter
  4. A 30W-amplifier with clip-detect and novel SC-protection
  5. A shunt resistor with switch-box for easy impedance measurement setup

MEASSY was specifically developed to operate in conjunction with ARTA (Dr. Ivo Mateljan, Split/Croatia), but should work with any loudspeaker development software. The guideline was: Reduce to the Max. As good as necessary, as simple (and cheap) as possible!

The design is based upon an INA163, a TDA7293 and standard components with a focus on 'value reuse' to minimize the BOM. Enclosure is a TEKO AUS12.

A first prototype is fully working assisting in measurements with the ARTA software-suite. The amplifier delivers 36W,max at 0.006% THD. The microphone  preamp has 80dB SNR and 0.01% THD at 36dB gain and -10dBV,ref (considerably better for lower gain, up to 103dB & 0.003%)

Update (20.03.14): A new version is available that indicates the operating status by a bi-color led. GREEN means everything OK; YELLOW indicates an output clip; RED means detection of short circuit- the amplifier is kept muted until power off/on!

Update (05.06.14): The peak program meter now directly indicates in dBu, so 0dB equals 0.775Vrms. This is a good match to soundcards containing 5V-supplied codecs which have a max I/O-level of 900mVrms typically.

Update (30.06.14): Dr. Ivo Mateljan and Dr. Heinrich Weber tested MEASSY thoroughly and came by confirmation of my measurements to a very good result with two exceptions. First the device shows some 50Hz-hum caused by a low-quality transformer (15 years old and cheap) and a local ground loop. Second they claimed bad EMI suppression.- There are clearly no ferrite filters at the mains and Mic-input that need to be added for low susceptibility. Also invoking protection earth helps. Moreover they also suggested to get rid of the crapy 3,5mm stereo-jacks and to replace them by solid RCA-connectors (3x). All measures call for a slightly larger case- I am considering building another prototype with a BOPLA  Ultramas52-case.


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Front view of Prototype
Rear view of Prototype
Main board, top view
Main board, bottom view
ELEKTOR- New PCB version available with SC-protection
Front Panel (Frontplatten Designer @ Schaeffer-AG)
Peak Program Meter & Gainswitch board

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