Questions about measuring DC Offset and Noise with a digital scope.

[George S.]

So I have a good digital scope. The thought of using it to measure DC Offset and Noise came to me yesterday. Measuring DC Offset seems straightforward using the DC settings and voltage measurement functions.
I assume noise is AC on the output jacks? Is it permissible to use shorting plugs on the RCA inputs to see if they make a measurable difference?
Trying to learn more and gain insight to how this stuff works. Also trying to learn more about using my existing test equipment. Thanks.

 

[Gepetto]

So I have a good digital scope. The thought of using it to measure DC Offset and Noise came to me yesterday. Measuring DC Offset seems straightforward using the DC settings and voltage measurement functions.
I assume noise is AC on the output jacks? Is it permissible to use shorting plugs on the RCA inputs to see if they make a measurable difference?
Trying to learn more and gain insight to how this stuff works. Also trying to learn more about using my existing test equipment. Thanks.

Whenever you measure noise, you should have a low impedance input to the RCA inputs. A shorting plug accomplishes this. What you want to measure is the noise that the amplifier is responsible for, NOT the random EMI that is picked up and amplified when the input is open.

 

[George S.]

Got it Joe, thanks. Anyone else use a scope on a WOPLs outputs? Any tips or pointers that might help? Going to also look at noise with the amp plugged into the wall versus plugged into a isolation transformer.

 

[Gepetto]

Remember to flip the plug in the wall socket and measure/record both ways There will be a difference

 

[George S.]

Remember to flip the plug in the wall socket and measure/record both ways There will be a difference

Ah yes! And there it is, the easy way to determine best Line and Neutral wiring to the terminal strip or transformer primaries! Since I'm using a heavy line cord and IEC socket, I'll take one of those old adapter plugs and make it non indexed so I can flip it over at the wall plug for testing purposes. Brilliant! Now we know how to determine if our transformer primaries are wired for lowest line noise. Great idea!

 

[wattsabundant]

Although an oscilloscope is the first instrument I typically use to check out an amp, I wouldn't use it for noise measurement. Measurments such as noise and distortion would be an absolute value that would be difficult to measure with a scope. The HP 8903B is one instrument intended for noise and THD.

 

[George S.]

Don, I've watched YouTube videos of that HP being used. Maybe someday that'll be something I have. Some of the FM tuner alignment guys use those in how to videos, if not that model, one very similar.
I do know that I could see a marked difference in 60 Hz noise when I was using a PC soundcard based Real Time Analyzer, depending on which transformer primary that supply line and neutral was connected to. That's something I can control in a WOPL build and should optimize. Be real easy to hook up the scope, and rotate the power plug. After verifying the outlet is wired correctly! I remember you say you shorted I think a scope because the outlet was wired wrong.

 

[mlucitt]

Pay attention to the amplifier ground in reference to the scope ground. It may not be the same as the probe or test lead shield on your scope. You can try it both ways.

 

[George S.]

Pay attention to the amplifier ground in reference to the scope ground. It may not be the same as the probe or test lead shield on your scope. You can try it both ways.

Was wondering about that. Done some reading since I had the 60 Hz issues on the preamp build. One thing mentioned on other forums was grounding between the DUT and soundcard often reduced 60 Hz on the RTA. I never tried using a chassis ground to the soundcard.