Testing for common grounded amps

ro2207
ro2207 Posts: 4
edited March 2012 in 2 Channel Audio
Hi everyone,

I just read a thread from some years ago where someone explains how to check for continuity between the negative leads of an amp in order to determine if it's common grounded.

So I'm using my ohm meter to conduct the testing..

Should the amp be powered ON or completely OFF (disconnected) while testing?

The reason I ask is because I get different readings:

With the amp OFF, I get a high ohm reading (that suggest no continuity)

With the amp ON, the ohm meter shows a resistance of almost 0 ohms (less than 0.5ohms, therefore common grounded).

Any info on this matter will be greatly appreciated.

Cheers
Post edited by ro2207 on
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  • ro2207
    ro2207 Posts: 4
    edited March 2012
    No sure if it matters, but I'm actually testing a car head unit's internal amp... With the key ON (head unit on either standby or on mode) I get 0 ohms; with the key at the acc OFF position I get almost no reading at all.
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  • bmbguy
    bmbguy Posts: 416
    edited March 2012
    Resistance should be measured with power off. In order to measure resistance, the meter applies its own test voltage to the circuit. Any voltage generated by a circuit that is powered up will interfere with the meter's reading -- and high voltages could possibly harm the meter.
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  • ro2207
    ro2207 Posts: 4
    edited March 2012
    bmbguy wrote: »
    Resistance should be measured with power off. In order to measure resistance, the meter applies its own test voltage to the circuit. Any voltage generated by a circuit that is powered up will interfere with the meter's reading -- and high voltages could possibly harm the meter.

    Thanks for confirming. I wanted to try pseudo sorround for the rears using the "hafler circuit" (connecting both rear spk's negative leads together to provide an L-R signal). Apparently you can safely do that only when using common grounded amps.
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  • bmbguy
    bmbguy Posts: 416
    edited March 2012
    Just to confirm that the meter is working properly (battery voltage, etc), you can set it to measure resistance and touch the leads together to see if it registers zero ohms (or very close to it!).
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  • ro2207
    ro2207 Posts: 4
    edited March 2012
    bmbguy wrote: »
    Just to confirm that the meter is working properly (battery voltage, etc), you can set it to measure resistance and touch the leads together to see if it registers zero ohms (or very close to it!).

    I've tried with both analogue and digital meters. Unfortunately they don't show anything near 0. The digital meter also has a continuity setting that makes the device beep when it perceives a resistance of less than 50 ohms. I can only get it to beep if the car key is at the ACC position. Thereby, it is safe to assume that my Kenwood HU is not common grounded (so no simple way of achieving rear ambient sound :( ).
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