Constant Voltage Speaker Measurement Circuit

Posted on Feb 5, 2014

If there are more than some 5mV, reduce this offset value by experimenting with resistors (220K a good starting point) from testpoint Z (junction of R22/R23) to +v if there is a negative voltage or to -v if there is a positive voltage until the voltage at Cal is as close to zero as you care to go (I did it this way to save a trimpot - you can opt to place a 100K pot between +v and -v

Constant Voltage Speaker Measurement Circuit
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and a 100K resistor at the slide and have a nice, easy adjustment). Place the voltmeter at the output and adjust P1 until the reading is (the value you measured for the 8. 2 ohms resistor) times 10mV (say you measured 8. 4 ohms - adjust for 84mV) Check linearity by replacing the 8. 2 ohms resistor by a 120 ohms one (measure it`s value, also). The output voltage shall be very close to 10mV times the real value, circa 1. 2V. If the voltage is grossly out, you need to adjust R6 (again, if you prefer it can be a trimpot) by lowering its value until suddenly there will be the correct value at Out. Do not forget to do the level calibration ( step 3-g ) and output ( step 3-h ) every time you start a new set of measurements and wait for the reading to stabilize before changing frequency.

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