High Power Calibration
For The Woofer Tester Pro
Overview
High power test port calibration begins by connecting the line outputs to your test amplifier and returning one of the amplifiers outputs to the LOAD side of the high power test port. Be sure to connect the ground-side of the amplifier to the black 5-way binding post.

The next step configures the tester for sine test signal mode HiZP operation. This can be done from the options pull down menu, or from the Setup Control Window. In both cases selecting the HiZP mode also automatically sets the Measurement and Reference sources.

CAUTION

A ground-referenced amplifier, with the ground connected to the black terminals, must be used for proper operation. Voltage at either black terminal (ends of the sensing resistor) should not exceed 4 Vrms

If the leads are swapped, or if the amplifier ground lead is resistive, a large common mode signal will be created at the differential sensing inputs (between the black terminals). Placing the sensing resistor in the ground side of the amplifier improves the high gain sense amplifier’s Common Mode Rejection Ratio (CMRR). This increases the tester’s usable dynamic range


Connecting the HiZP Port
The tester line outputs should be connected using the same configuration that is used for acoustic testing as shown below.  This is the normal configuration for almost all tests.  Y-jack's are used to allow external line level devices to be connected.  For example, an external microphone pre-amp could be connected as a measurement input (or reference).  Or, an external pre-amp could be connected in a 'loop through' to measure its response characteristics.

hizp_cal_connect1.jpg

hizp_cal_connect2.jpg
                                          (click to enlarge)
                                          hizp_connect_small.gif


Select the hiZP port and Sine Mode Test Signal
The next step configures the tester for sine test signal mode HiZP operation. This can be done from the options pull down
menu, or from the Setup Control Window. In both cases selecting the HiZP mode also automatically sets the Measurement Reference sources.

 
            hizp_cal_signals.gif


Initial Levels

At this point the current port conditions are given before making any more connections.  If for example the tester output was 100% and the amplifier level set very high, the CalR could be damaged.  Special non-inductive 10.0 ohm and 0.5 ohm resistors are used for calibration and current sensing.   These resistors can dissipate ~30W for a short period of time.  After this, the heat sinks will warm up, decreasing the safe dissipation level to ~10W.

                    hizp_cal_1.gif

Steps 1  - Open Circuit Low Frequency Gain
In this step the unloaded amplifier gain is established using a 60 Hz frequency.   In the unloaded  case there is no voltage drop across the 0.5 ohm current sensing resistor, cable resistance or amplifier output resistance.  60 Hz is used because some volt meters are intentionally AC coupled (to block DC) or bandwidth limited to avoid measuring line 'hash' on power main circuits.

Adjust the gain of your amplifier to get about a 2Vrms reading.  If you have an external DVM connected, the reading in this dialog should match reasonably close.  

Step 2 - Open Circuit High Frequency Gain
In this step the amplifiers (and tester) high frequency roll-off is found.  At this point a bandwidth limited DVM might read differently.

Note: In both step 1 and step 2, the load side CalR is still unconnected so there no danger of excessive power being dissipated.
 
  hizp_cal_2.gif         hizp_cal_3.gif


Step 3 - Loaded Circuit Low Frequency DC Resistance
By connecting the calibration resistor (the sense resistor is already connected), the amplifier and cable resistance can be calculated. A 60Hz signal is used because this should be higher than any internal DC blocking in the amplifier, but low enough that cable inductance is minimal.

Step 4 - loded Circuit High Frequency Cable Inductance
The amplifier side indcutance is calculated at 15 Khz.  At this point it is possible to acurately measure impedance values at the tester terminals (IE speaker and cable).  To measure at the end of a long test lead, the test lead must be accurately measured using the LoZP full calibration routine.  If the load side test cable is calibrated, its resistive and inductive effects can be nulled by measuring at the end of the test cable.

           hizp_cal_4.gif              hizp_cal_5.gif

Results

Simplified calibration results and a reminder message about using test cables is given, followed by a more detailed readout of both the LoZP and HiZP calibration.  At this point the HiZP port should accurately measure impedances.
  .
       hizp_cal_6.gif     hizp_cal_7.gif


IMPORTANT!
LOADS ARE DIRECTLY CONNECTED TO YOUR AMPLIFIER!
DO NOT SHORT THIS TEST PORT

  • A 10A fuse is internally connected between the red terminals
  • The 0.5 ohm current sensing resistor is between the black terminals
  • Set the tester to 100% drive and set your amplifier to a maximum safe level
  • Note: It is very easy to accidentally set the gain back to 100%
  • Most high power tests start with 100% drive level that is then decreased
Improving Operational Results
  • The 0.5 ohm sense resistor and tester circuitry will give ~1% error when the drive level is decreased (using drive level) to ~1 mA.  At this point the signal across the sense resistor is very small.  Use the WT2 LoZP port if lower drive levels are required.
  • Best accuracy is achieved at modest 0.5-10w (2-10V)  drive levels




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