Heathkit AD-1307 Power Meters

[Walter R]

Just received a HK AD-1307 power meter from the e-place that I bought for $59. It works! But previous posts indicate that the meters are calibrated for 8 ohm speakers, and to multiply by 2 for my 4 ohm speakers. But I notice in the schematic that the unit has internal calibration pots for each channel. Has anyone tried to adjust the pots for 4 ohm speakers, or am I just asking for trouble?

 

[WaynerN]

I have built several VU meters (similar to power meters), and the adjustments on my driver board are for calibration and to balance the 2 channels (via a mono source signal). The range required to adjust your meters from 8 to 4 ohms may not be enough to make the offset.

 

[Walter R]

Would there be a problem connecting the meter to the 8 ohm terminals of the power amps to get a true reading, even though the speakers are connected to 4 ohm? Since the meters probably draw very little current, I would guess not.

 

[Walter R]

I tried adjusting the internal pots and it worked! I put a signal generator into the preamp and adjusted the pots on each channel until they meter readings were about double. I’m not sure how accurate I left it, but then I’m not sure how accurate the Heathkit builder left it.

Connecting the meter to the amp 8 ohm outputs wouldn’t have worked. I have two MC275’s running in bridged mono mode, and the two 8 ohm outputs are bridged together to drive the 4 ohm speaker. So there is no 8 ohm output as configured.