Loudspeaker Tester

Posted on Jul 26, 2012

Testing loudspeakers is covered in some detail in the passive crossover article (see Design of Passive Crossovers), but it is irksome at best to have to fiddle about with clip leads and components lying all over the workbench. This simple project is intended to make life that little bit easier when you are trying to determine the optimum frequency compensation network for woofers and midrange drivers. Of all the components, the pot is most likely to cause problems. The suggested value of 30 Ohms was used in my case because I just happened to have one lying around in my junk box, but they don't seem to be easy to get.

Loudspeaker Tester
Click here to download the full size of the above Circuit.

A very basic tweeter attenuator may work OK, and the value is not that important. Even a resistance as low as 8 Ohms could be used, with a switchable 8.2 Ohm resistor in series so that the range will be from 0 to 16 Ohms (near enough). I shall leave it to the individual to determine the best way to achieve the required range, which is typically 0 to 20 Ohms. The switch SW2 and associated 100uF cap is optional, and increases the range up to 200uF - in most cases this won't be necessary though. The caps are all bipolar electrolytics, and this is perfectly OK, since they will not be used at high power, and their sonics are not important (assuming that you can hear the difference anyway). To use the box, simply connect it across the speaker - mostly you will want the caps and pot in series, so a lead between the two (or even a switch) makes this easy. From the output of suitable small amplifier, place a 100 ohm or so resistance in series with the output. Sweep the signal frequency across the speaker. Resonance will not be affected, but there will be a "magic" combination of capacitance and resistance that will make the impedance above resonance completely flat. No maths, no spreadsheets, just a quick frequency scan and twiddle a couple of knobs to get the values needed to ensure that the crossover actually will work at the design frequency. The network derived from the testing will be placed in parallel with the...

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