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astouffer 20th November 2008 03:30 AM

Choke input filter LF noise
So tonight I decided to start wiring up the power supply for my new PP 429A amp. The schematic shows 1N4007 diodes but they're UF4007 in real life. The output seems to have some random LF noise and I'm not sure about how to clear it up. Here is the schematic

Now here are the screencaps from my scope

Adding a small film cap like .047uF or .1uF across the first filter cap had no result. Maybe snubbers for the diodes?

Boris_The_Blade 20th November 2008 03:48 AM

Is the load attached in those scope pics? What kind of load (dummy or tubes if so)?

SY 20th November 2008 03:53 AM

Have you looked at your power line?

infinia 20th November 2008 04:10 AM

What kind of scope are u using?
trigg error?
aliasing error?

Use a 1% resistive V divider to look at some known signal. For example the output of rectifier diodes would be interesting.

ashok 20th November 2008 04:13 AM

When you look at the dc power supply noise you will get random fluctuations , sometimes quite large , due to fluctuations in the mains supply. Anything switched in or out in the house or in the vicinity is supposed to show up on your rectified line. LF garbage being quite hard to filer out fully.

Someone suggested that if you find this on the dc supply and you want to look closely at the supply noise , build a sharp filter at about 30 Hz to block LF garbage.
If I use an ac meter to look at a tube dc supply ( in the 10 mV range ) , the noise fluctuates at random with ocassional large peaks. Haven't yet tried a filter to cut out LF garbage.

astouffer 20th November 2008 05:17 AM

Boris_The_Blade: The load is a large 3k resistor drawing 90mA.

SY: No I have not looked at the power line yet. The transformer does go through a normal IEC filter. What is the best way to look at the incoming AC? Float the ground of the scope or just measure the unloaded secondary of a lower voltage winding?

infinia: The scope is an old HP 54200A. Can you elaborate on what you mean by trigg or aliasing error?

Tomorrow I'll take a closer look at the AC line and post my findings. Thanks everyone.

infinia 20th November 2008 10:30 AM

Your description sounds like a scope that is not triggered properly esp on measurements related to the power line frequency. The trigger usually can be switched to line and then re-adjusted?

Aliasing errors due to the variable sampling on a wide band non-filtered input can give amplitude and time based errrors. The sampling rate has to match the expected timebase of the TBD signal to display properly.

SY 20th November 2008 12:34 PM

Andy, no I wouldn't float the scope at 120V. I'd either just look at one leg of the line or (better) use an isolation transformer with one side of the secondary connected to scope ground.

SpreadSpectrum 21st November 2008 05:36 AM

That's nothing a few hundred Henries won't fix :D.

It makes sense that the only things coming out of your low pass filter are very low frequencies. I don't think there are any scope errors here.

I get a small amount of random very low frequency noise out of my choke too. It bothered me enough that I am building a Maida regulator.

infinia 22nd November 2008 06:05 AM


I don't think there are any scope errors here.


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