Checking for clipping with DMM

[TurboTBird]

Hey,

I get how the method works to ensure your not over power your subs.

ie Use the max rms power rating and impedance to determine the the maximum voltage by applying the V^2=Prms*Z

But how would this check for clipping? Because isn't Vrms just the average area under the curve which would be a sign wave ideally but it would work out the same even if it was an alternating square wave.

Could I use the current setting of the meter tho to determine the signal quality? Like if I have the voltage set for 34.64V and the load is 2 ohm the tottal current shouldn't be more than 17.32amp right?... oh shit I guess I'd need a clamp meter for that.....

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[FoxPro5 5,000+ posts]

I get how the method works to ensure your not over power your subs.

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I think people should start checking for actual voltage coming from their brains with their DMMs.

 

[cj407]

in order to find out about clipping you would need an O-scope...dont know what that is then keep sending clipped signal....

 

[gunz4me 10+ year member]

To sum it up, the DMM gain setting method is useless because your amplifier will respond differently once a load is connected to it. Likewise, using an oscilloscope to determine clipping is useless without a dummy load because once you hook your speakers up to that amp, you have changed the rules of the game. Lastly, it is not recommended to use your speakers as the "dummy load" for a sine wave test unless you really know what you are doing!

 

[TurboTBird]

To sum it up, the DMM gain setting method is useless because your amplifier will respond differently once a load is connected to it. Likewise, using an oscilloscope to determine clipping is useless without a dummy load because once you hook your speakers up to that amp, you have changed the rules of the game. Lastly, it is not recommended to use your speakers as the "dummy load" for a sine wave test unless you really know what you are doing!

Thank you for posting something that is meant to be helpful.

So how can one set the amp gains correctly if the DMM method and the oscillioscope method doesn't work?

 

[gunz4me 10+ year member]

Thank you for posting something that is meant to be helpful.
So how can one set the amp gains correctly if the DMM method and the oscillioscope method doesn't work?

Using the DMM method to set your gains is one of those "get you in the ball park" techniques, even though FoxPro5 will say it merely gets you into outer space. Regardless, you still have to use your ear for the final settings. Only after you have used your ear would I recommend sticking the oscilloscope on your equipment, unless you have access to dummy loads.

One of the major pitfalls to using the DMM method involves the power output of the particular amplifier you are trying to set the gain for. Are you sure it makes rated power on your vehicle's voltage? Are you REALLY sure of this? What is going to happen when you load it down with your subwoofers or other speakers, depending on the amplifier in question?

Don't get me wrong, a DMM is a good way to establish somewhat of a base line for you to use your ear from that point forward, but it is not a means to an end. The one exception to this is when you are down tuning an amplifier to produce less output than what you need. For example, your amplifier is capable of producing 1,000 watts RMS and you only need 500 to 600 watts RMS of output, you can somewhat safely use the DMM to set your gains lower than the desired output level.

 

[TurboTBird]

Okay, thanks!

That helps a lot. I am infact trying to tune a 700Wrms amp to my subs which are 600Wrms total.

I was a little worried that my head unit was clipping tho and i was thinking that might carry through the amplifcation.

Maybe I'm just not understanding what the real problem with clipping is. I was thinking it was because for the clipped part of the signal the signal would appear to become DC.

An inductor in a DC circuit will become a short circuit in time, if the time constant is short enough I was thinking I would see a spike of current on each clipped signal.

In other words

E= L(di/dt) .. if (di/dt) the rate of change in current stops I have no opposing voltage.

 

[thch 10+ year member]

clipping can be estimated with a HU that uses a discrete based volume control, eg setting 10 of 35, vs a knob you just turn.

in this case, you can turn the gain down, and then measure the normal volume setting, and 3 taps downs. the idea is that the 3 taps down will always be the same percentage down. eg, 50% down (if each tap is -2dB).

so as you adjust the gain, you'll be able to plot how close the ratio is. eg, at 10V output (on the full output case), the ratio might be 1/2. set the HU back to normal, and set the gains for 20V, try this test again, and you might still get around 1/2. repeat this. maybe at 30V, the ratio is still the same. then at 40V the ratio is a bit less. then at 45V the ratio is 1/1.

this gives a fairly low resolution way to determine that 40V is likely too high, and 45V is definately too high. But the DMM generally only has a couple of digits output, and may have other issues.

a similar method can be used to estimate clipping on the HU, but only if the taps are the same. lower taps may be -2dB while upper taps are -1dB, or maybe all taps are -2dB or -1dB. but there are other volume affecting controls. the idea is to get a fixed and precise change, and make sure that the percentage change isn't affected by other settings.

again, the estimate will be affected by any limitations of the DMM, and is fairly lengthy.

there are other ways to estimate clipping using a DMM and creativity. A twin-t filter can be used to reject the main signal, in which case only distortion gets through. you can actually get pretty good results this way, but it does still need a bit of tuning, as your filter tends to be imperfect.