Rectifier switching / choke ringing noise - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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
Reply
 
Thread Tools Search this Thread
Old 10th October 2007, 01:03 PM   #1
diyAudio Member
 
zigzagflux's Avatar
 
Join Date: Oct 2006
Location: Charlotte, NC
Default Rectifier switching / choke ringing noise

Working on a prototype 5V regulated supply for 300B filaments, using LM1086.

Schematic

PSUD output

Testing shows 5 mV RF hash on the output. This was traced down to rectifier switching / choke ringing, exactly like Morgan Jones' describes in Valve Amplifiers. This is a look at the transformer secondary:

Click the image to open in full size.

I tried his Y-caps to ground method, to little avail. If anything, the frequency dropped, but the magnitude increased a little. Using schottky diodes with 0.01 uF bypass caps. Putting large (> 1uF) caps across the choke helps; I've considered playing around with much larger, possibly an RC network.

Any suggestions? I've read a number of other threads, but the consensus seemed to be throw snubbers in a whole lot of places. I've also read Jim Hagerman's article - I need to buy a few parts to do a little trial and error with his equations. I'm hoping someone has built a similar supply, and can ballpark me some values (this ain't the first DC supply for 300B's, though most seem to be cap input).

This RF business is a real ball buster.
  Reply With Quote
Old 10th October 2007, 04:00 PM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Are the 0.01uF caps in parallel with the diodes and if so are they ceramic types?

Also how much load current were you drawing when you made this measurement?

How about a very small cap from the rectifiers to ground. (Not a large enough value to make a valid pi filter however.)

The ringing looks like it is relatively low frequency, not quite sure what you can do to damp it.. One other thought would be a resistor in parallel with the choke - say a few hundred ohms..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 10th October 2007, 05:11 PM   #3
diyAudio Member
 
zigzagflux's Avatar
 
Join Date: Oct 2006
Location: Charlotte, NC
The caps are in parallel with the diodes, very close proximity. They are polyester film.

The load current varied from 1.2A to 1.5A, depending on what mfr of 300B I elect to use. The waveform really didn't change at all between these current levels.

I'll try the damping resistor idea, also maybe an RC.

While it looks low frequency (whatever you classify as low frequency), I see a very clear 10 MHz sine wave on the output, and at the adjust pin of the regulator. Scope is only a 20MHz model.
  Reply With Quote
Old 10th October 2007, 08:30 PM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Hi Zigzag,
Film caps don't make very good snubbers for rf ringing, ceramic types are really better for this spot, but are you certain that the regulator itself is not oscillating?

Some of these family of devices like some capacitance (>33uF) on the output particularly if they are driving an inductive load. I noted the presence of a resistor (R5) in series with the single C (C7) on the output of your regulator. This could be a recipe for RF oscillation with LT10XX series and LM317/350 series regulators. How far are the filter caps from the input of the regulator IC and what is the value and type of the cap right by the regulator input?

Kevin
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 10th October 2007, 08:58 PM   #5
diyAudio Member
 
zigzagflux's Avatar
 
Join Date: Oct 2006
Location: Charlotte, NC
Huh, always thought film caps were the way to go for snubbing. I've got some ceramics in the bin, maybe 0.1 uF. I'll give it a try.

Kind of followed the app notes from National on bypassing. Output is using a 1 ohm resistor (R5) in series with a 22 uF solid tantalum cap (C7). Out of curiosity, I shorted R5 to see if there was any difference; nothing.

Input cap C11 is a 22 uF tantalum. Bypass cap C9 is a 47 uF tantalum. BOM included for any further review. They are both mounted extremely close to the pins of the regulator. As the schematic indicates, I have three star grounding points for the three 'stages' of the power supply. The caps are grounded in this fashion.

I was assuming this ringing on the AC wave is working its way into the regulator, where it can't be attenuated, and passes through. You don't think this is the case?

Thanks for the help.
  Reply With Quote
Old 10th October 2007, 09:41 PM   #6
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
Zigzagflux,

The ringing shown in the oscillogram is at about 10 KHz - nothing to do with rf. To work on that film caps would be quite OK, and closeness of components/short leads will have insignificant influence. The 10 MHz that you observed (but not shown in the oscillogram) is a different matter.

Exactly how is your audible output affected? It is quite unlikely that the 10 MHz will have any effect in this type of amplifier (I presume you saw it on the power supply and not the amp output). If it is the 10 KHz, it is most likely to interfere through induction/radiation, although the latter possibility is also remote. (You are probably not using the 300B as a low level pre-amp stage!) Although one may want to have a clean dc out of principle, I am a little puzzled as to what audible effect this could have, that is why I ask.

Without further information I would put wiring high on my investigation list - common wire resistance, that sort of thing. But as Kevinkr said, it can also quite likely be regulator chip oscillation. That will be easily curable, as you do not need regulation response at high frequency.

Regards
  Reply With Quote
Old 10th October 2007, 11:40 PM   #7
diyAudio Member
 
zigzagflux's Avatar
 
Join Date: Oct 2006
Location: Charlotte, NC
Good questions.

My concern with the ringing is primarily that it shouldn't be there; if it's on the secondary, it also finds its way into the primary, and therefore has the opportunity to migrate into other circuitry. This looks very similar to the effect of a light dimmer, which does produce an audible effect on my sound. It is not present on the DC output, but the 10Mhz is there, drives right through the LCL filtering.

There are lesser concerns, such as insulation stress due to the high transients. Morgan Jones delves into some of this in his book, along with modulation products.

As far as the 10MHz, I certainly don't claim it to be audible; however, I have had other amplifiers that were plagued with RF, and the whole soundstage was just depressed, dry, and cloudy. No, you couldn't hear it, but you could tell something just wasn't right. Sure, it could have been something else, but repairing the RF (different topology, thanks Kevin) drastically improved the sound, especially with fatigue.

I am working towards a Karna amp in about 3-4 years, and am in the first stages of establishing some of the building blocks (can I build a quiet filament supply, do I like the sound of xfmr coupling, etc). In agreeement with those such as K&K and Lynn Olson, it ain't easy to build a quiet DC supply, and I am learning that. They continue to use AC heating for a reason (except their active RAKK DAC stage), and since I know too little to debate their philosophies, I do follow their advice.

If I can't get a quiet supply (such that my 20Mhz scope can't see it both at the AC input and the DC output), I will default to AC heating, and deal with the hum issues.

So, I want to at least try to get this supply (input and output) quiet. A good learning experience, even if it fails. I'll poke around with more solid grounding, and maybe even put in a DC source as a preregulator; maybe a car battery. That would certainly prove the issue is within the regulator.
  Reply With Quote
Old 11th October 2007, 12:02 AM   #8
diyAudio Member
 
Join Date: May 2003
Location: Colorado
Default Re: Rectifier switching / choke ringing noise

Quote:
Originally posted by zigzagflux
Testing shows 5 mV RF hash on the output. This was traced down to rectifier switching / choke ringing,(
Looks to me like the bypass caps you are using are resonating with the leakage inductance of your transformer. C10 is the worst culprit. C5 & C6 are not helping either.

Go back and re-read Jim Hagerman's articles on snubbers.

When you are reading, please realize that there are two different definitions of "snubbers" that get thrown around on these boards. While they are both series RC networks, the ones that Hagerman talks about are associated with the transformer and its leakage inductance. There are other people that use a "snubber" in parallel with an electrolytic cap as a "high-frequency bypass".

In your case, you want to focus on Hagerman's definition. Skip the "snubbers" that bypass the electrolytics.
  Reply With Quote
Old 11th October 2007, 01:22 AM   #9
diyAudio Member
 
pmillett's Avatar
 
Join Date: May 2005
Location: Dallas (but I am not a Texan!)
Try adding resistors in series with C6 and C5, and making them bigger. Maybe 0.1uF and 47 ohms or so.

The film caps by themselves don't dissipate the energy during reverse recovery; they are returning it to the circuit. So you either need resistance, or a really bad cap. That's why ceramics are sometimes used here - they are more lossy.

I agree that C10 might hurt, not help.

What is Q1?

Why the resistor ini series with C7?

Pete
  Reply With Quote
Old 11th October 2007, 02:56 AM   #10
diyAudio Member
 
zigzagflux's Avatar
 
Join Date: Oct 2006
Location: Charlotte, NC
GOT IT !!!

Whooohooooo !!!!

All very good suggestions, everyone. First, I will answer the latest questions, since you took the time to help. Then, I'll explain my solution.

C10 was added per someone else's suggestion at a different forum. In both this case, and the C5-C6 case, I was always a little confused as to why resistance was never added in series. Your suggestions confirm why, as well as Jim Hagerman's article. Removed C10, with no change. It will be left out. Fewer parts are always better.

Q1 is the LM1086, linear regulator.

The resistor in series with C7 was used to prevent the parasitic inductance of the regulator from resonating with the capacitance of C7. Being that tantalums have fairly good HF performance, a small, 1-5 ohm resistor is recommended by National and Morgan Jones. Shorting it out made no difference, anyway.

So, the solution was........(drum roll, please)...........an RC snubber. What a surprise, huh?

Tried numerous cap sizes and R sizes. In the end, a 0.47uF in series with 100 ohm worked best. Completely removed the ringing. What was interesting is that it worked best in two locations, identically. Right across the choke terminals was one method. The other method was per Kevin, from output of rectifiers to circuit common. In this position, though, cap alone did not work; it still wanted the series R of 100 ohm.

To be honest, that solution doesn't surprise me, and I kind of expected that to be the easier solution. Fixing the megahertz, tho, I expected to be very difficult. Once again, wrong was I (I'm getting used to being wrong).

Two words: Airport Express
The noise was being picked up by the flying leads used in the test setup. Keep the leads short and tight, noise goes way down. Turn off the Airport Express, silence. In actual use, I will be installing with shielded twisted pair, so I don't see this as being a big problem when actually installed in the amp chassis.

So, all is clean, and I'm ready to build three more of these for the other filaments. Not that this is a stellar design or anything, but the schematic and BOM is in this thread for anyone's use. It's been troubleshot enough that I think it can be classified as a fairly low noise design.

Thanks, everyone, for your input.
  Reply With Quote

Reply


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 filter LF noise astouffer Tubes / Valves 27 6th June 2011 10:51 PM
Channel Switching Noise with Dish 612 DVR Jon Giard Digital Line Level 0 29th March 2009 03:30 AM
Scope tricks for detecting rectifier/choke switching zigzagflux Tubes / Valves 19 12th July 2008 12:16 PM
EZ81 rectifier noise Stixx Tubes / Valves 24 22nd December 2007 07:25 PM
Rectifier ringing snubbers Prune Parts 10 26th September 2004 09:26 PM


New To Site? Need Help?

All times are GMT. The time now is 04:58 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2