Filter Chokes wired for CMR - 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 17th September 2014, 01:26 AM   #1
diyAudio Member
 
Join Date: Jun 2008
Default Filter Chokes wired for CMR

Starting to build a power supply for first attempt at Rod's shunt cascode RIAA circuit and the two filter chokes I have on hand for an LCLC filter are both dual coil (Lundahl).
The data sheets suggests using one coil in B+ leg and the other in ground return for CM rejection. You can also use one of them for choke input. This got me to wondering - If there is an inductance between the first filter capacitor's neg. and the rectifier neg. why wouldn't we consider that an unwanted impedance in the ground return?
Common mode rejection would be a nice addition to a phono stage power supply but what would be optimum? First L in HV string only and second L in the CMR configuration? Both L's one way or the other?

Any thoughts?
  Reply With Quote
Old 17th September 2014, 05:38 AM   #2
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
been using that for 3 years now in many of my preamp and amp builds....the psu ground then is taken off the cap at the output of the common mode choke...
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 17th September 2014, 04:02 PM   #3
diyAudio Member
 
Join Date: Jun 2008
Do you mean like in the data sheet (without the input capacitor)?
Attached Images
File Type: jpg Screen Shot 2014-09-17 at 8.55.48 AM.jpg (45.4 KB, 167 views)
  Reply With Quote
Old 17th September 2014, 04:26 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Common mode choke/inductor is more commonly used on the mains cable before it gets inside your chassis.

The can around a mains filter makes metal to metal contact around the hole that the can fits through. This can effectively makes the common mode filter OUTSIDE the Chassis. That is the best place to prevent extra interference getting to the circuits inside the Chassis.
__________________
regards Andrew T.
  Reply With Quote
Old 17th September 2014, 04:48 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by Hearinspace View Post
This got me to wondering - If there is an inductance between the first filter capacitor's neg. and the rectifier neg. why wouldn't we consider that an unwanted impedance in the ground return?
Fortunately, that's not the case- the impedance is inserted after the first capacitor. If you have the same impedance in both legs, it's essentially balanced and that means common mode reduction. IME, it's most useful for heater supplies taken from a heater winding on the main HV transformer.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 17th September 2014, 06:55 PM   #6
diyAudio Member
 
Join Date: Jun 2008
Quote:
Originally Posted by SY View Post
Fortunately, that's not the case- the impedance is inserted after the first capacitor.
Yes, in the data sheet drawing . . .

. . . but let's say an unsuspecting and entirely innocent user (no one by that description around here!) looked at data sheet and thought, "OK, it says here that this choke is good for choke input duty so I'll use the common mode reduction hookup in a choke input filter.", then the common practice of returning signal or star grounds to the negative terminal of the first filter capacitor comes under question because there's an impedance between it and the rectifier. That's what I'm asking about.
  Reply With Quote
Old 17th September 2014, 07:43 PM   #7
diyAudio Member
 
Vinylsavor's Avatar
 
Join Date: Feb 2002
Location: Lindau
Hi!

You can use these chokes in CMR connection for both choke input and cap input filters. The terminal 6 of the choke becomes your signal ground. It doesn't matter if there is a inductive connection from there to the rectifier minus terminal or transformer center tap. What is important is that everything that is supposed to be on signal ground has a low impedance connection


Quote:
Originally Posted by Hearinspace View Post
the common practice of returning signal or star grounds to the negative terminal of the first filter capacitor
This isn't really common practice this is actually bad practice.

Best regards

Thomas
__________________
http://vinylsavor.blogspot.com/
  Reply With Quote
Old 17th September 2014, 07:59 PM   #8
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
I don't think a common mode choke has much usefulness in tube circuits; in the 50's schematic diagrams, you would never see one (unless I am mistaken).

Their main purpose today is to allow a piece of equipment to pass EMC tests without too many issues (ie, block the internally generated crud from contaminating other nearby equipments).

If you really want to suppress common mode, a differential choke like the one you show is a very expensive and ineffective option: using current compensation, a true CMC can be much more effective, smaller and cheaper than a differential choke used in this role. But the real question is: do you actually need CM suppression? What benefits do you think it will bring?
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 17th September 2014, 09:54 PM   #9
diyAudio Member
 
Join Date: Jun 2008
Quote:
Originally Posted by Vinylsavor View Post
This isn't really common practice this is actually bad practice.
I'm not sure why you are saying that. Could you say more? Whenever you have a series of RC or LC filters, all the capacitor Neg. terminals in the string are connected. What I meant was making the first filter capacitor neg a star point.

Quote:
Originally Posted by Elvee View Post
If you really want to suppress common mode, a differential choke like the one you show is a very expensive and ineffective option: using current compensation, a true CMC can be much more effective, smaller and cheaper than a differential choke used in this role. But the real question is: do you actually need CM suppression? What benefits do you think it will bring?
I don't know that it's needed, but in a phono stage if noise reduction without a catch is being offerred for free (I already have the Lundahl) then it makes sense to use it. So that was my question - is there a reason not to.

I don't know anything about current compensation. Used more in sand circuits than tube (and those are more sophisticated than I have done), I have no experience. Would you give an example of how you might do it?
  Reply With Quote
Old 17th September 2014, 10:08 PM   #10
RTF671 is offline RTF671  United States
diyAudio Member
 
Join Date: May 2008
Location: North Carolina, USA
It'll bring your noise down a bit. Also since you already have them, go ahead and wire them up for CME. It offers no drawbacks versus using it in series not CME. Just follow the suggested wiring as per the datasheet.

Your other option is to wire LCLC filter using the two halves of one of the chokes. Or both! Then see which you like.
  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
FS: 4x 10H 215mA WIDEBAND CHOKES (layered chokes) DIYspaceW Swap Meet 5 13th March 2013 05:10 PM
Filter Chokes Geek Swap Meet 2 3rd February 2008 07:51 AM
Plate chokes & Power chokes JC951t Tubes / Valves 3 19th October 2006 03:18 PM
Filter Chokes Leslie Horn Parts 18 11th May 2005 09:09 PM
Wanted->Discrete balanced to unbalanced CMR converter Brian Guralnick Solid State 20 21st September 2003 03:32 AM


New To Site? Need Help?

All times are GMT. The time now is 06:14 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