Setting gains via DMM

[gunz4me2 10+ year member]

I wanted to post this demonstration hoping that some could avoid the pitfalls associated with setting their gains via DMM.

Here is a Zapco Z100S2 with nothing but a DMM and an oscilloscope connected to it. These voltages are just prior to clipping:

At first, you would look at that and say “ZOMG, my 50 watt x 2 amplifier is underrated and produces 83.44 watts RMS per channel.”

Now, let’s repeat this test with some 4 ohm dummy loads connected:

Ruh ro roggy, we are making less power at a lower volume level. Also, my meter was bouncing between 14.55 and 14.56. My photo of the volume level caught that.

Now, let’s see what happens when we crank the head unit to volume 30, the same as the maximum, unclipped output determined when the amplifier wasn’t loaded down:

Is that clipping I am starting to see? Yes it is!

So, as a warning, unless one knows the true output of their amplifier, setting gains via DMM alone may not be the right thing to do!

Click to read more...

 

[CHEMMINS 10+ year member]

O scope has always been the way to go for setting gains.....or now the Dummy Meter that SMD makes.

 

[bubbagumper6 5,000+ posts]

Actually, had you set the gain with the DMM properly you would have set the output voltage at 14.14 (sqrt(50x4)=14.14) which is well within the non-clipped range //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif

It's actually very close to what you claim is the maximum non-clipped output...thus making the dmm method actually pretty good since a dmm only costs $5

 

[CHEMMINS 10+ year member]

Actually, had you set the gain with the DMM properly you would have set the output voltage at 14.14 (sqrt(50x4)=14.14) which is well within the non-clipped range //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif
It's actually very close to what you claim is the maximum non-clipped output...thus making the dmm method actually pretty good since a dmm only costs $5

But what do you do with under rated amps? Guess off the efficiency and fuse size?

 

[bubbagumper6 5,000+ posts]

But what do you do with under rated amps? Guess off the efficiency and fuse size?

Set them according to their rated power. If you want to get into squeezing out every bit of power from an amplifier you can, you need an o-scope, plain and simple. But that's an advanced topic, setting the gain with a dmm is the cheapest/easiest way to insure noobies don't blow their shit up.

 

[CHEMMINS 10+ year member]

No point in leaving power on the table.....spend the $100 more and get an 0-scope

 

[bubbagumper6 5,000+ posts]

No point in leaving power on the table.....spend the $100 more and get an 0-scope

Are you being intentionally thick? First of all, the difference between rated power and actual power in the majority of "overrated" amplifiers would be inaudible. Second, I already said trying to get every bit of power you can is an advanced technique. Do you think every joe schmo that wanders into this forum wondering how to set up his amplifier is going to want to go out and drop another $100 on a tool he'll probably use once? Not everyone lives and breathes car audio. The DMM method is a perfectly acceptable (and cheap) way to set up equipment for novices.

 

[gunz4me2 10+ year member]

Actually, had you set the gain with the DMM properly you would have set the output voltage at 14.14 (sqrt(50x4)=14.14) which is well within the non-clipped range //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif
It's actually very close to what you claim is the maximum non-clipped output...thus making the dmm method actually pretty good since a dmm only costs $5

BUT, what if their amps are overrated and they don't know this? After all, there are plenty of 12v amplifiers out there that will NOT make rated power without some form of clipping!

 

[bubbagumper6 5,000+ posts]

BUT, what if their amps are overrated and they don't know this? After all, there are plenty of 12v amplifiers out there that will NOT make rated power without some form of clipping!

This is true. Unfortunately in this set of circumstances there's little we can do to help them. Think about it, if they were cheap enough to buy an amplifier that doesn't even do rated, there's no chance in hell they'll drop the cash on an o-scope.

 

[CHEMMINS 10+ year member]

BUT, what if their amps are overrated and they don't know this? After all, there are plenty of 12v amplifiers out there that will NOT make rated power without some form of clipping!

This is true, especially with lower end companies. People will use max power ratings as well. So people should become educated before ever touching a gain knob.

 

[bubbagumper6 5,000+ posts]

This is true, especially with lower end companies. People will use max power ratings as well. So people should become educated before ever touching a gain knob.

I most certainly agree, and I try to do that every time I see on of those threads...

 

[justafifteen 10+ year member]

Kind of on-topic, are there any places that would have an O-scope that would be down with letting you take it to the parking lot for five minutes?

My local NAPA let me borrow their DMM before I bought my own, I had to use it on like three separate occasions.

 

[bubbagumper6 5,000+ posts]

Kind of on-topic, are there any places that would have an O-scope that would be down with letting you take it to the parking lot for five minutes?
My local NAPA let me borrow their DMM before I bought my own, I had to use it on like three separate occasions.

You could try your local audio shops, any decent one will have a scope and they're always looking to make money so I'm sure they'll let you "rent" it. Other than that just about the only place that might let you use one is an electronics repair shop.

 

[CHEMMINS 10+ year member]

Not many places carry them...

If you do have a shop you deal with for your gear, they may let ya use it.....but doubt it.

 

[smd4life 5,000+ posts]

i have a vlx 25....it's rated at 12.5 watts x2 @ 4ohms...i have yet to bench it hehe