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Power supply ripple???

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Posted on March 20, 2017 at 16:40:11
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003



OK thought I would learn more about this scope and see if there is some nasty ac on the power supply. On the below post there were some thoughts on why the voltage drop across the choke was different than a resistor. Ac ripple was brought up so I thought I would check it.

Being a light weight when it comes to scopes I posted a picture of the screen. I seemed to be getting around 100 to 120 vpp but according to the chart on the screen it was showing 1 kz to 4kz moving all over the place. So I am wondering what the scope is picking up?

The probe and ground are right after the last cap before the interstage transformer. Could the chokes or transformers be picking up 1kz to 4 kz from some where???

Well I am open to all comments and different test procedures etc... Would like to learn how to use this thing.

Thanks Tom


PS. Kind of looks like the stock market with all those numbers on the screen.

 

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RE: Power supply ripple???, posted on March 21, 2017 at 06:18:59
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 Is the noise present with the amplifier turned off? Have you tried this with the preamp tube removed?


 

RE: Power supply ripple???, posted on March 21, 2017 at 08:08:53
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003



Pulled first tube and the voltage when drop some but what was really different was the Hz went from 1-4kz to 10-30hz.

So I turn the power supply off and the scope stayed almost the same. So I took the probe off the power supply and scope went back to normal.

OK my conclusions are the the 1-4kz stuff on the tube in picture was Coming from 1st stage of phono section. The second picture with tube out is using the 3 chokes for antennas.

So it seems we are getting higher Hz from the phono stage and low frequency from the chokes picking something from the air. I did not think low frequency was pick up thru the air.

Well I can use all the information I can get.

Thanks Tom

 

RE: Power supply ripple???, posted on March 20, 2017 at 23:41:03
6AS7_6SN7
 In order to sort it out you need FFT.
Spectral analysis will tell you the true harmonics of noise.
You have to download the time history to an external computer for FFT.
What you see in the time domain is essentially wideband white noise.

 

RE: Power supply ripple???, posted on March 21, 2017 at 07:43:36
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 My scope has the FFT function on it. Was reading through the various functions and I admit I am not following what is going on what to look for etc etc etc.

If you could give me some idea of what is going on and how to test for it that would be great. Any text that I could read would be nice also.

Thanks Tom

 

RE: Power supply ripple???, posted on March 21, 2017 at 10:05:37
6AS7_6SN7
 Great!

The FFT will show you all the line harmonics and other CW components (if any).

I personally prefer the 'flat top' window (that gives the best measurement of continuous wave components).

By the way, the standard Hanning or Hamming windows works fine as well.

Scopes with built in FFT (and time averaging in the case) are very powerful measuring instruments and as soon as you learn to use it you will see.

Hint - If you select some CH2 low voltage signal (synchronous with mains) as the trigger source (for example the filament 6.3 or 12.6 VAC) you can easily average the noise on CH1 and THEN apply the FFT (to the averaged noise signal): this allows to see a new universe of previously unseen signals that were buried in the noise.

For example, if there is an even minimal magnetic coupling of the current pulses of the HT transformer/rectifier into the HT they emerge from the noise with even moderate averaging.

And by the FFT of the averaged signal you can see the CW spectrum much more clearly and fully exploit the maximum dynamic range of the instrument.

 

RE: Power supply ripple???, posted on March 20, 2017 at 20:39:26
FenderLover
 Don't think it's PSU AC ripple. Is the PSU disconnected form the rest of the circuit? Maybe, something downstream is picking up EMF or external signal interference?

 

RE: Power supply ripple???, posted on March 20, 2017 at 20:55:19
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 It was connected to the right channel phono stage. Will Check it tomorrow with out the phono stage connected.

Do you think that picture on the scope would cause any noise regardless of where it is coming from?

Thanks Tom

 

RE: Power supply ripple???, posted on March 20, 2017 at 22:51:51
FenderLover
 Well, if the scope's image is not an artifact to the scope, itself --- it prolly does contribute some small percentage to background noise of your system.

I'll bet when you disconnect the phono input, the hash will drop significantly. There is a lot of gain going on in that section. So, it makes it susceptible to amplifying noise, from whatever the source.

 

RE: Power supply ripple???, posted on March 21, 2017 at 08:16:31
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Pulled first tube and if you go up under Triode Kingdom post there is a picture and some explanations. Do not know what hash is but if that is the 1-4 Hz ripple then you are right.

Would be interested on your thoughts to the new picture and findings above.

Thanks Tom

 

RE: Power supply ripple???, posted on March 21, 2017 at 23:35:27
FenderLover

Not sure the scale this image is at. But, sure looks like random white or pink noise. Is your scope plugged into an outlet that has some appliances running on it? Like a refrigerator or A/C-heater? Maybe a computer?

Can you run same setup through an ultra-isolator or some such isolation device? To clean up any artifacts riding on top of the AC line.

 

RE: Power supply ripple???, posted on March 22, 2017 at 09:11:18
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Hey Fender will have to look into isolation device. Why with the power supply turned off and the probe attached could you get this type of reading. When I took the probe off the screen was flat enough. So is the noise coming up the ac cord of the scope and getting grounded to the power supply by the probe?

Thanks Tom

 

RE: Power supply ripple???, posted on March 22, 2017 at 11:22:06
FenderLover
 From the AC line. Or the scope is picking up stray external signals, perhaps.

 

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