An idea about DMM tuning

[timma100 10+ year member]

Here is my idea, when an amp clips, it doesn't produce more AC voltage, it just produces a distorted wave. So, theoretically, if you had a high quality true rms DMM on the speaker outputs of the amp like you normally would for tuning. Couldn't you turn up the gain, (very slowly mind you) until the voltage stopped increasing? then back it down a hair, like .1V AC from the max.. Wouldn't that achieve the maximum output with out clipping?

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[KyleBechtold 5,000+ posts]

Here is my idea, when an amp clips, it doesn't produce more AC voltage, it just produces a distorted wave. So, theoretically, if you had a high quality true rms DMM on the speaker outputs of the amp like you normally would for tuning. Couldn't you turn up the gain, (very slowly mind you) until the voltage stopped increasing? then back it down a hair, like .1V AC from the max.. Wouldn't that achieve the maximum output with out clipping?

try it.. you would need an accurate dmm.. I am buying an oscope so I will compare once it arrives..

 

[timma100 10+ year member]

try it.. you would need an accurate dmm.. I am buying an oscope so I will compare once it arrives..

Thus is the issue :| lost my dmm in my last move, no cash for a proper one (~$100-150).. The issue is going to be people with cheap DMMs, I have a feeling that a clipped signal will appear to be a lower/different frequency to a DMM (sharper drop offs and rises) , and if its cheap your going to get different voltage readings just from the "apparent" frequency change... But I am excited to find out the results, the only unknown left would be HU clipping... hmm

 

[jockhater2 10+ year member]

hmmm. I should try that. I have a $600 DMM. although it is about 10 years old

 

[timma100 10+ year member]

hmmm. I should try that. I have a $600 DMM. although it is about 10 years old

I would still expect it be a boss of a dmm //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif What brand?

 

[maylar 10+ year member]

You would be hard pressed to discern when the waveform shape stops increasing amplitude with a DMM. Be aware that "clipping" doesn't always flat top like the classic pictures we've seen. Sometimes you get a dip in the peak or a nasty change in waveshape before the peaks go flat. Only an oscilloscope will tell you that.

 

[jockhater2 10+ year member]

I would still expect it be a boss of a dmm //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif What brand?

Ummm. Snap-on. 596A I think

 

[AStovall 10+ year member]

Sort answer ..No

Long answer

The waveform dose not suddenly go square wave, Even when well into clipping you can still get AC content.

This image is when i clamping my MB ref1.800 note the RMS reading on the top of the meter, its AC coupled. Note the 5volt difference.

A DMM will only show you part of the picture. Im not a fan of DMM tuning ,becasue if you don't fully grasp whats going on you can do more harm than good.

if you are going to use a DMM you need a precision high power resistor, or a clamp on current meter,when using the sub as a load. The sub will not be stable impedance when playing,its impedance will shift due to frequency,box rise,and heat making that silly formula every one like to throw around useless for this task.So you need a clamp on meter to read current ,so that you can calculate the impedance of the driver at your test frequency then you can calcualt weather or not you are at the amps rated output...but even then you can still be clipping but you will at lest be in the ballpark of you maxamum gain setting...at that frequency . You can use the formula only if you are using a known load such as a precision resistor...And even then you have to be very careful becasue as the resistor gets hotter resistance will drop or become a short if you cook the resistor which is a great way to explode fets and capacitors inside the amp.

 

[maylar 10+ year member]

^^ I agree.. except for...

...And even then you have to be very careful becasue as the resistor gets hotter resistance will drop or become a short if you cook the resistor...

Wirewound power resistors have a positive temperature coefficient. They increase with heat.

... picky picky, I know

I'm not a fan of DMM gain setting either.

 

[AStovall 10+ year member]

^^ I agree.. except for...


Wirewound power resistors have a positive temperature coefficient. They increase with heat.

... picky picky, I know

I'm not a fan of DMM gain setting either.

good catch. lol