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-   -   DMM gain setting (https://www.hdforums.com/forum/audio-systems/1114818-dmm-gain-setting.html)

slyedog 05-08-2016 04:17 PM

The sticky is flooded and no since clouding it up. Im not trying to get in a pissing match nor talk anyone down here. There are some methods how testing with dmm being left out here and since nutz isn't around as much I'm throwing this out there how I was taught to do so. Using this as a USEFUL discussion only!!! I am trying to grasp the ac voltage dc current concept. Lets use a JL xd400/4v2 as an example amp. 75x4 @4 ohms. I will connect my dmm leads to channel 1 +/-. At those terminals with 4 ohm load I should see 17.32v. Now I can go check the bridged output of channels 1&2 . It will still show the 4 ohm output if I am doing this correctly of 28.2 . You are running the 2 channels in parallel so the voltage will change. That's how the power is double from the 2 ohm rating of the amplifier. This chart JL uses is kind of a handy reference. I personally have never used a DMM to test as ear was always adequate until the high end setup I now have. I used the scope and DMM to PROTECT my investment. Not squeeze every drop of unclipped power to it. I have been applying DC theory to A/C which is different. In dc parallel current increases but voltage does not. I apologise if this is hard to understand. I am trying to grasp how some figures are given. Example my amp bridged displayed 32.4 v at near clipping, In bridged mode. But others are showing higher per channel than my voltage per speaker. Now is that peak voltage being measured? Then if so I am right on point with my thought process. So the single channel voltage will be .614 of the bridged voltage.

AAWAV 05-09-2016 09:21 AM

when bridging, you do not take the channels parallel but in series.
it stacks the 2 AC signals (1000Hz test sine wave) "on top of each other", as 1 of the channel is 180 degrees reversed.
Scope is simply the "best" way to dial in amp (theoretically spoken right).

Not sure what you are referring to regarding DC. I do not use DC measurement on amps (been a while since I was in college; there is a DC component to the whole amp thing and it should not be a high value).
most if not all amps will NOT double the amp RMS in 2 ohm (1 channel) compared to 4 ohm. there will always be a "loss".
you will see a 2 ohm load draw more Current (A).

hope this helps

slyedog 05-09-2016 09:45 AM


Originally Posted by AAWAV (Post 15127012)
when bridging, you do not take the channels parallel but in series.
it stacks the 2 AC signals (1000Hz test sine wave) "on top of each other", as 1 of the channel is 180 degrees reversed.
Scope is simply the "best" way to dial in amp (theoretically spoken right).

Not sure what you are referring to regarding DC. I do not use DC measurement on amps (been a while since I was in college; there is a DC component to the whole amp thing and it should not be a high value).
most if not all amps will NOT double the amp RMS in 2 ohm (1 channel) compared to 4 ohm. there will always be a "loss".
you will see a 2 ohm load draw more Current (A).

hope this helps

It does. I was attempting to say the 4 ohm bridged load will always be 2x the 2 ohm rated output. No matter what amp, bridged will always be twice the 2 ohm rated output . The whole step up DC input to AC voltage/current does get confusing. I thought the amp used AC voltage and DC current somehow on the outpit. It's been almost 20 years since I went to tech school and I switched to HVAC after few terms. Trying to regain some knowledge.


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