DMM Function Difference: mVAC & VAC

Thread Starter

Loudpipe

Joined Feb 2, 2020
7
Hello! New to the forum and seeking help on something:

I'm taking a reading off the output terminals of a home bass amplifier (Monoprice SMA-400) that has a consistent/stable squeal right around 1800 Hz and finding something I didn't expect and don't understand on my DMM readout. When on VAC function, I get 0.1794 VAC, and when on mVAC function, I get 416.38 mVAC. I expected these to be consistent; i.e. either 179.4 mVAC if VAC is the basis, or 0.4163 VAC if mVAC is the basis. What am I missing?

Separately, my DMM (a Fluke 289) has the capability to measure AC frequency, and when I select that function, I get a 0.0000 Hz reading. Perhaps the voltage is too low for it to measure? The speaker is making low but very audible noise... I would have expected to see some sort of signal frequency.

Thanks in advance for any guidance.
 

AlbertHall

Joined Jun 4, 2014
9,310
Do you have (acces to) an oscilloscope?
There may some signal(s) present at a frequency which you can't hear which are confusing the meter.
I looked at the manual for the meter and I cvan't see anything there which could cause a problem for a simple 1800Hz signal.
 

Thread Starter

Loudpipe

Joined Feb 2, 2020
7
Do you have (acces to) an oscilloscope?
There may some signal(s) present at a frequency which you can't hear which are confusing the meter.
I looked at the manual for the meter and I cvan't see anything there which could cause a problem for a simple 1800Hz signal.
Unfortunately, no. I may end up getting one at some point, but I find myself doing this kind of troubleshooting/analysis only a few times each year, and it hasn't merited purchasing one (yet).

Any thoughts on my mVAC/VAC question? I thought I knew how to use a multimeter, but surely I'm missing something obvious. I do get some noise on my mVAC function, with or without leads connected, but it's on the order of 4-6 mVAC. On the VAC setting, it's rock solid at 0.0000.
 

AlbertHall

Joined Jun 4, 2014
9,310
Any thoughts on my mVAC/VAC question? I thought I knew how to use a multimeter, but surely I'm missing something obvious.
See line 2, post #4.
Hencs the need for an oscilloscope. I often call them a 'lie detector' as you can see what is really happening.
 

Thread Starter

Loudpipe

Joined Feb 2, 2020
7
One thing I will add that I find particularly odd is that this is the second amplifier (of same model) to do this. Surely I haven't received two separate amplifiers from Monoprice with issues.
 

Thread Starter

Loudpipe

Joined Feb 2, 2020
7
Is it strange that my DMM would be giving me a reading in mVAC even without the leads connected to anything? It's 4-6 mVAC with no leads plugged into the meter, and shows somewhere around 14mVAC with leads plugged in but not connected to anything. Both values float a bit, but are fairly consistent.
 

Thread Starter

Loudpipe

Joined Feb 2, 2020
7
The manual is available on www. Search it for 'residual reading'.
I have and have read the manual. Is it your experience that meters will display 'residual readings' in an AC mode? I see the note about residual readings related to the 'AC Floor' when the leads are shorted together, but I haven't shorted the leads together, so I'm not exactly sure how to interpret that guidance.
 

AlbertHall

Joined Jun 4, 2014
9,310
You will probably get a greater reading with the inputs not connected and greater again with leads plugged in but not connected to anything else.
Noise + electrical pickup account for this. Take your meter into the middle of a field a very long way from any AC power lines and you will likely get a lower reading.
 
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