Setting amp gains with DMM for bridged load - diyAudio
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Old 29th September 2015, 11:33 PM   #1
diyAudio Member
Join Date: Sep 2015
Default Setting amp gains with DMM for bridged load

Hi everyone,

Hoping I can get a bit of help with a little problem I'm having.

I'm trying to set the gains for my subwoofer amplifier using the DMM method. I've done this before with my front stage speakers without any issues.

The amplifier is a 2 channel Class A/B Clarion XR2220 capable (on paper) of producing 190 watts RMS @ 4 ohms when bridged. The subwoofer presents a 4 ohm load, and my head unit is a Kenwood KDC-U556BT with 2.5v preouts. All EQ is flat, no loudness/bass boost/bass extend etc and the subwoofer volume on the HU is at max. I am using a 50Hz sine wave at 0dBfs to try and set the gains and have calculated I'm looking for a voltage of 27.56v.

When I connect the DMM to the amp as usual to set the gains, I attach the probes to the correct terminals for bridged mode and adjust the gain so I see the desired voltage, with the HU volume at 75%. However, with the gains set like this the sub is almost inaudible... like you wouldn't even know the car had a sub in it! The cone barely even vibrates visibly in the boot.

Out of interest, I tried setting the gains with the bass boost on at the amp (I don't want to use it but just wanted to see if it made any difference) and with the gain turned all the way down it reads over 40v.

This doesn't sound right at all, I'm wondering if there is a problem with the multimeter? It was set to 200v AC mode as required.

Right now I have the sub amp gain set by ear with no audible clipping (I am a recording/mix engineer professionally so I do have a good ear for it!) but I would prefer to set it with a DMM to be more accurate.

I bought this subwoofer to replace a no-name cheap underseat 8" woofer rated at 55 watts RMS and, if I go by the DMM reading for gain setting, the old cheap one was about 10 times as powerful as the new one!

Am I missing something here? Any help would be much appreciated!
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Old 30th September 2015, 12:59 AM   #2
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
My opinion... for what it's worth.

Set it so that you have use of approximately 75% of the volume control's range and so that it sounds like you want it to sound. Then, when driving it hard, listen for distortion or stress (drivers bottoming...) and back off of the volume. Music is too variable to have one 'perfect' setting.
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