Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > Audio Measurement and Testing
Speaker level pad for measurement.
David Sturzenbecher:
--- Quote from: Helge A Bentsen on May 05, 2023, 10:40:46 AM ---I forgot to specify that this isn’t for load testing, it’s for verifying DSP settings with a speaker connected :)
--- End quote ---
I have used a VIBox from LinearX in the past for this. Here is a snippet of my response to this exact question on the signal to noise discord a few days ago.
Sadly the owner of LinearX passed away a number of years ago, so the VIbox is no longer available. Physical-lab appears to make a similar unit (IMP Box), but I have no first hand experience. The VIBox was nothing more then a voltage divider circuit with precision matched components (actually 2 voltage dividers as it had a 0dB "low" side and -40dB "high" side). I smoked the low voltage side of my VIBox, and the one resistor was something like $15 to replace... with a minimum purchase quantity of 400. You can easily make your own with slightly less precise components for way cheaper. You can also use a passive DI but depending on the quality, you may see some phase deviations due to the internal transformer. This can of course be calibrated out, but something to be aware of.
Another posted posted a link to this.
http://www.atelier-der-tonkunst.de/produkte/pol-ddi
David Sturzenbecher:
I also built my own VIBox into a 1RU panel.
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David Sturzenbecher:
Here are some more links to VIBox info...
https://www.prosoundtraining.com/2013/11/15/impedance-measurement-comparison-part-2/
https://manualzz.com/download/7201779
http://www.excelsior-audio.com/Publications/Impedance_Measurement.pdf
Edit to Add (Sad the images are lost)
https://forums.prosoundweb.com/index.php?topic=139087.0
Ivan Beaver:
One thing that is very important with a lot of moderm amplifiers who do not have a speaker terminal that is NOT ground (ie floating) is to have an isolation transformer between the amplifier pad and the input to the test system.
Depending on how you wire it up, you could damage the amp or the test system if you don't physically isolate the two systems.
What I prefer is a normal pad to drop the voltage, and then go into a line level transformer (not the mic level ones used in DI boxes).
I had a bunch of old Altec plug in transformers that are quite good and could take a lot of level. Of course there are others, Triad, Jensen etc.
You could also use a "hum eliminator" type transformer as most of those are line level.
The pad I use is typically 100:1, so that I can run amps at full output and still stay around line level.
A quick description: From the speaker outputs, 1 leg goes through a series resistor (say 100K) and then to a parallel resistor (1K). One side of the transformer is hooked across the 1K resistor, and the other side of the transformer goes to pins 2 and 3 of the measurement system
NOTE: There will be a little bit of a low freq rolloff (depending on the transformer used and freq), but you can measure that before you measure the DSP in the amp so you can compensate for that.
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