Valve HT power supply common-mode 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 6th July 2014, 11:15 AM   #1
diyAudio Member
 
Join Date: Dec 2011
Default Valve HT power supply common-mode noise

Hi guys, I am having trouble deciding whether or not to use a toroidal or an IE transformer for my HT supply.

Do you think that noises(also known as common mode noise) that slip through from the AC main to the HT supply via the interwinding capacitance of the power transformers would induced unwanted distortions(i.e. hums)?

I do know that due to the capacitive coupling between the heater filament and the cathode, the heater power transformers must have a low interwinding capacitance + shield + bobbin or IE transformers to stop main AC noises from getting through. However, for the HT(plate/anode supply) in a pull-push configuration, I believed that common mode signals are canceled, thus says a toroid power transformer could be used. Even though toroid would let more AC main noise through because of their higher interwinding capacitance.

Last edited by pha0001; 6th July 2014 at 11:17 AM.
  Reply With Quote
Old 6th July 2014, 01:37 PM   #2
disco is offline disco  Netherlands
diyAudio Member
 
disco's Avatar
 
Join Date: Apr 2006
Location: Holland
My 2ct is that some builders go to extremes for that ultra low noise "black background" experience, a concept that can't be had without inclusion of all parts. If you are satisfied with "a reasonable result without much hassle" you might get away without attention to CM noise because there are bigger issues involved in a power stage.

The regulation of the power transformer is an overlooked property. On the other hand a bulky 200VA unit is overkill for a pre amplifier, where the smaller radiation distance of a toroid in combination with CM filtering might be beneficial.

I never obtained an answer to the question I posed whether or not it will be harmful for AF signal integrity when CM noise passes or oscillation occurs, way above AF.
__________________
jaap
  Reply With Quote
Old 6th July 2014, 02:29 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by pha0001
Do you think that noises(also known as common mode noise) that slip through from the AC main to the HT supply via the interwinding capacitance of the power transformers would induced unwanted distortions(i.e. hums)?
I'm not sure if you are asking about noise, hum or distortion: three different phenomena.

My understanding is that toroids can let through more differential-mode noise due to better HF magnetic coupling between windings.
  Reply With Quote
Old 6th July 2014, 03:10 PM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
They generally have lower leakage inductance than EI types. In this case the EI has an advantage, however this can be addressed quite effectively with a combined CM/DIFF mode choke or good quality line filter on the input side of the transformer.

The other thing to bear in mind with the toroid is you quickly loose the benefit of its good magnetic containment if you allow it to saturate during any part of the rectification cycle. Over sizing here is good.

I use a mix of both. Recently Antec has started offering transformers with electro-static shields which helps with HF noise.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 6th July 2014, 07:21 PM   #5
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
There is little point wringing your hands over common-mode noise unless the rest of the design is already exceptionally well optimised. Is you amplifier exceptionally well optimised?
  Reply With Quote
Old 6th July 2014, 11:03 PM   #6
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
This of course is an excellent point. In practice it has caused me little or no trouble. Unintended loops have always been the bane of my my existence.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 7th July 2014, 03:35 AM   #7
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
Quote:
On the other hand a bulky 200VA unit is overkill for a pre amplifier
oh but i have seen a local build with such sized traffo and a 5U4 tube rect, feeding a circuit that only required a few mA at just about 120volts plate B+....

laws of physics have been casually ignored....
  Reply With Quote
Old 7th July 2014, 08:09 AM   #8
diyAudio Member
 
Join Date: Dec 2011
Seem like no-one here have much of an issue with common-mode noise getting through.

But wouldn't everyone here agreed that common-mode noise is canceled in a pull-push configuration? The reason is because common mode signal is of the same magnitude, polarity and common to both valves. Thus it get subtract from one another in the output pull-push transformer?

If what I stated is true, then wouldn't everyone agreed that a toroidal power transformer is a perfectly acceptable(even though it let more CM noise through) replacement for an EI core transformer in the HT supply?

The reason i asked this question is because of the popularity of EI core power transformer in valve amp, while toroidal is some what rare.

Last edited by pha0001; 7th July 2014 at 08:16 AM.
  Reply With Quote
Old 7th July 2014, 09:13 AM   #9
disco is offline disco  Netherlands
diyAudio Member
 
disco's Avatar
 
Join Date: Apr 2006
Location: Holland
Not quite, it has meaning if everything else is top notch.

CM signal is cancelled in a differential amplifier: it amplifies differences in grid signals and ignores signal common to both inputs. A push pull amplifier does not have this property.
__________________
jaap
  Reply With Quote
Old 7th July 2014, 09:47 AM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by disco
A push pull amplifier does not have this property.
Yes it does. A perfect PP output stage will cancel any supply rail noise, although there would still be supply rail intermodulation. Big snag: no PP output is perfect.

In my early days of learning electronics I would often obsess over unimportant details while completely ignoring much more important circuit problems. Part of the art of electronics is knowing which things to worry about, and which things to ignore (perhaps after doing a back-of-envelope estimate of their likely magnitude).
  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
Common mode feedback dirkwright Tubes / Valves 28 9th March 2012 12:26 AM
Differential amplifiers ...noise, common mode etc. peskywinnets Analogue Source 17 24th February 2011 11:45 AM
differential design, common mode noise rejection Sebastiaan Solid State 0 17th April 2006 08:16 PM
Power Supply Common Wire... EchoWars Solid State 4 4th December 2003 12:01 PM
Using common mode chokes in filament supply. G Tubes / Valves 8 9th December 2002 03:10 PM


New To Site? Need Help?

All times are GMT. The time now is 07:13 PM.


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