Choke input filter LF noise - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 22nd November 2008, 03:47 PM   #11
diyAudio Member
SpreadSpectrum's Avatar
Join Date: Nov 2007
My reasons for thinking this is not a scope error are the following:

1. All digital scopes that I have used don't trigger at very slow time scales. They run like a strip chart. Triggering is not necessary when you can see everything as fast as the scope puts it out. The term for this mode is 'untriggered roll'. I don't know if this HP scope does this or not, but the screen captures are valid whether the scope triggered or not. It saw those voltages on its input during that time period.

2. The screen captures are quite continuous for this to be a high-frequency (>50MHz) signal aliased into this display. Does this scope have any kind of anti-aliasing filter built in? If not, how can you trust any measurement that it makes?

3. The measurements make intuitive sense. If you feed a 60Hz sine wave (+random noise) into a low-pass filter with a cutoff frequency less than 60Hz, you should get out some residual 60Hz and very low frequency noise.

Truly, you can never completely trust any measurement that you make, I just see no reason to doubt this one. As I stated, I have seen comparable levels of very low frequency noise (not oscillating at any regular frequency) on the output of my choke as well. I assume that it is just fluctuations in line voltage with time. I guess my power isn't perfect.

If one is bothered by it, there are much more complicated circuits that can get rid of it.
  Reply With Quote
Old 22nd November 2008, 09:47 PM   #12
kevinkr is offline kevinkr  United States
diyAudio Moderator
kevinkr's Avatar
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I see this sort of stuff all the time on the inputs to my supply regulators and it is definitely from small voltage variations in the AC line. It is very difficult to observe these very low frequency line voltage variations when you are looking at a 60Hz 170Vpk sinewave on your scope - you really won't see it clearly, although you will see plenty of other even more horrifying things like a clipped sine waveform, (lots of distortion) whole dropped cycles, sags and surges of a couple of cycles or more, and huge transients. (Use a very high speed scope and you will even see multi-kV transients that last up to a couple of uS - often lightning strikes many miles away..)

A few hundred mV to several volts of LF voltage variation will be typical and should not be a cause of concern unless your audio circuitry has relatively low psrr (which most do...) - still the only place this should be a concern would be in a high gain situation such as one a phono or tape stage.

In short I would not worry or obsess over it, I don't even bother to try and "fix" it any more. You can reduce it a lot, but it is almost impossible to get rid of short of active regulators.

Long ago I had an HP 54200A and it certainly has enough sampling bandwidth that aliasing would never be an issue at 60Hz and below. Somewhere beyond 10 - 20MHz (I don't remember the one shot sampling rate any more, but it was not very high) with non repetitive waveforms aliasing becomes a real concern. I will also say I found it less than ideal for low level analog work due to quantization noise, and that is why I got rid of mine.. YMMV
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
  Reply With Quote
Old 23rd November 2008, 02:26 PM   #13
diyAudio Member
Join Date: May 2005
Location: Pretoria, South Africa

By now you would have realised that your scope patterns are very normal! It would be better after R1, where I presume you take off the more sensitive stages of your amp (although your R1=50 ohm is quite small. Can that be increased?) One usually take off power stage anodes only, right after a choke input choke (I did not look up your circuit).

Circuits like the Quad II took off the anodes right after the rectifier (C-input filter), where matters looked even more ugly, without audible effect. Hook up and listen.
  Reply With Quote
Old 23rd November 2008, 03:17 PM   #14
infinia is offline infinia  United States
diyAudio Member
infinia's Avatar
Join Date: May 2005
Location: SoCal
First if you don't have a good trigger using any scope esp a digital sampling one, the measurement is garbage (big period).

Using Your Oscilloscope to Measure a Noisy Signal
Stable Triggers Required
Before you analyze your signal, you need a stable display
which can be a problem if your signal is noisy, making it
difficult to create a stable trigger. Most oscilloscopes come
with several features that assist you with this problem.

I encourage you to visit for their vast amount or technical app notes

As others have pointed out a wideband sampling scope is not the best tool in the shed for these type of measurements.
Using a digitizing sampling oscope to measure low freq small signals you just have to be a little smarter than the avg bear.
1) You are measuring a small 120 Hz signal (not 60Hz). limit your sampling rate.
2) ADC input BW to the lowest possible (diode switch noise can be an aliasing influence here) An active LPF is a good idea here.
3) Get a stable power line related trigger (build an external trigger if you have to)
4) LF random noise is not a problem here.
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 6th June 2011, 03:40 PM   #15
diyAudio Member
Rundmaus's Avatar
Join Date: Aug 2005
Hi there,

I can confirm the existence of the described LF variations in the DC output of choke-input supplies.

The setup is: 480-0-480 transformer > 5AR4 rectifier > 5H, 87R choke > 120F cap > 5H, 87R choke, 120F cap

The 100Hz ripple on the DC is about 10mV, just as Spice simulations predicted. Superimposed on the ripple are chaotic LF variations of about 200mV with frequencies of sub-Hz to a few Hz. Although the posted oscilloscope images of the OP are no longer available, this sounds very much like the behaviour the OP described.

If I do a frequency response analysis of the filter chain, there is almost no filter effect up to about 10 Hz (-0 up to -5dB max), so it sounds sensible that LF variations make it through the filter chain.

It is definitely no scope (sampling) error, I used an analog oscilloscope.

Question to the tube experts here: Are you sure that this effect is due to line voltage variations and does not indicate a problem with the choke-input supply? If yes, I will leave everything as it is and live with it. Remains to be tested if this LF garbage makes it to the output of the amp - which is not the case I hope.

Question two: Is this a special behaviour of choke-input supplies? I fired up the negative rail of my amp (180-0-180 transformer > EZ81 > 20F > 220R > 220F > 110R > 220F > 110R > 220F) yesterday and there were absolutely no LF variations visible.

But current draw is much lower in this case, and filtering capacitance much higher.

Frequency response analysis showed about -20dB in the LF area for the second filter chain.

  Reply With Quote
Old 6th June 2011, 03:54 PM   #16
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Both a choke input supply and a CLC supply are LC resonators, typically at a few Hz. Under constant load and constant mains input they will behave similarly, because the rectifier conduction will impose a short circuit at one end (almost all the time (L in) or twice per cycle (C in)) and so in effect you have an LC in either case. This may ring if the damping provided by the load is inadequate - often the case nowadays when people seem to think that bigger caps are necessarily better.

When the mains input varies they will behave differently. The choke in will simply pass through any variations below its rolloff, so you see the mains variations appearing at the output. The cap in will be temporarily disconnected from the mains during any sharp reductions in mains voltage, until the caps have discharged enough to get the rectifier conducting again or the mains has risen again. You will see mains variations looking more like a sawtooth i.e. somewhat smoothed out. This assumes that the choke in has sufficient load that the rectifier conducts for almost all of the cycle.
  Reply With Quote
Old 6th June 2011, 04:42 PM   #17
piano3 is offline piano3  United Kingdom
diyAudio Member
Join Date: Aug 2010
Location: London,UK
DF96, how would you expect this to impact,if at all,on the sound in the case of fairly good PSRR?
  Reply With Quote
Old 6th June 2011, 05:24 PM   #18
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Depends on how good is "good", and how it is done. Subsonic noise will not be directly audible (that is what subsonic means) but it could modulate the signal and so generate IM sidebands. This would be most noticeable, other things being equal, in circuits which use cancellation to get good PSRR rather than filtering etc. For example, a push-pull output will have good PSRR because of the balance but it will be just as prone to IM from the supply as a similar single-ended stage.

How would it sound? In extreme, as unwanted random tremulo!
  Reply With Quote
Old 6th June 2011, 06:24 PM   #19
piano3 is offline piano3  United Kingdom
diyAudio Member
Join Date: Aug 2010
Location: London,UK
Is sufficient resistance in series with the choke enough to eliminate the possibility of a problem?
  Reply With Quote
Old 6th June 2011, 07:09 PM   #20
diyAudio Member
Rundmaus's Avatar
Join Date: Aug 2005

But I have about 120-130mA current draw down that supply rail - with enough series resistance voltage will drop enough to make a transistor amp instead...
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
choke input power supply filter audionut Pass Labs 55 20th May 2013 08:17 PM
Choke Input Filter on Class A PS john65b Power Supplies 2 20th April 2009 09:48 AM
Choke, filter for white noise grandcanuck Analog Line Level 0 10th September 2008 04:20 PM
Choke Input Filter. G Tubes / Valves 8 11th February 2008 08:57 PM
SOZ Choke input filter? gnomus Pass Labs 48 14th May 2004 06:41 PM

New To Site? Need Help?

All times are GMT. The time now is 03:56 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2