Hum in active speaker caused by floating flyback SMPS - 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
Reply
 
Thread Tools Search this Thread
Old 20th February 2016, 05:00 PM   #1
diyAudio Member
 
Join Date: Oct 2010
Location: Denmark
Default Hum in active speaker caused by floating flyback SMPS

Hi there,

for an active speaker I build a 150W flyback converter delivering 30V to an power amp and +-18V to linear regulators for some analogue OPamp circuits.

The power supply is floating, having no protective earth connection, to save space and this is where the trouble starts. The primary and scondary grounds are connected with a Y-cap 2n2.

The input circuit following a neutrik XLR connector is a symmetrical opamp buffer stage. An highpass before that with 10kOhm to GND keeps the impedance low. When the whole PCB is connected to the ground of my measurement system or with a cable to protective earth the speaker is silent. But when disconnecting the GND from the board, a big hum comes out of the speaker. Measuring that, you can see 50Hz/60Hz with harmonics till some kHz. Even without any audio cable at the input, the hum is huge. When putting in more Y-cap this gets worse. Without the Y-cap it gets a little better, but still huge. When I get closer with my hand to the XLR or the PCB the hum gets louder.

Does anybody know that behavior? And what is the connection between having no earth connection and this big hum?

This is the first time that I design a SMPS without protective earth. With an earth connection, there are normally Y-caps to filter for CM-noise. How can I do that when I have no earth?

Best wishes and a great weekend
  Reply With Quote
Old 20th February 2016, 07:08 PM   #2
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: The great city of Turnhout, BE
Blog Entries: 7
Hmmm. 50Hz with higher harmonics normally come from a rectification of the 50Hz mains. Do you have that anywhere in your system?

Can you provide a schematic of the wiring for clarification?

Jan
__________________
Music is dither to the brain; lets me think below the usual chaos - me
Linear Audio Vol 12 is out! Check out my Autoranger and SilentSwitcher on Kickstarter!
  Reply With Quote
Old 20th February 2016, 09:44 PM   #3
diyAudio Member
 
Join Date: Jan 2010
Yes, this is a well known effect and the reason why wall warts generally introduce much noise into audio systems. The Y-cap conducts 100Hz-noise generated by the primary rectifier into the secondary circuit. All in all this is common mode noise current growing with Y-capacitance carrying many harmonics caused by the primary rectifier.
As you mentioned secondary grounding helps.

I would recommend to connect the Y-cap with primary PE instead of secondary gnd. This should work even with secondary floating.

Last edited by voltwide; 20th February 2016 at 09:50 PM.
  Reply With Quote
Old 20th February 2016, 11:55 PM   #4
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 142
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Quote:
Originally Posted by Hannes1 View Post
Does anybody know that behavior? And what is the connection between having no earth connection and this big hum?
Its common-mode noise, as you mention in your next paragraph. And introducing the earth provides a lower impedance path for those CM noise currents to flow, so they don't go via your input circuitry back to the source component (or measuring device).

Quote:
This is the first time that I design a SMPS without protective earth. With an earth connection, there are normally Y-caps to filter for CM-noise. How can I do that when I have no earth?
You'll need a common-mode choke. Try one from Murata. But beware it probably won't eliminate the hum, just reduce it. You'll need to re-think the input circuitry most likely.
__________________
The heart ... first dictates the conclusion, then commands the head to provide the reasoning that will defend it. Anthony de Mello
  Reply With Quote
Old 21st February 2016, 08:46 AM   #5
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
Quote:
Originally Posted by Hannes1 View Post
Hi there,

for an active speaker I build a 150W flyback converter delivering 30V to an power amp and +-18V to linear regulators for some analogue OPamp circuits.

The power supply is floating, having no protective earth connection, to save space and this is where the trouble starts. The primary and scondary grounds are connected with a Y-cap 2n2.

The input circuit following a neutrik XLR connector is a symmetrical opamp buffer stage.
Surely symmetrical? Instrumental amplifier?

Quote:
An highpass before that with 10kOhm to GND keeps the impedance low. When the whole PCB is connected to the ground of my measurement system or with a cable to protective earth the speaker is silent. But when disconnecting the GND from the board, a big hum comes out of the speaker. Measuring that, you can see 50Hz/60Hz with harmonics till some kHz. Even without any audio cable at the input, the hum is huge. When putting in more Y-cap this gets worse. Without the Y-cap it gets a little better, but still huge. When I get closer with my hand to the XLR or the PCB the hum gets louder.

Does anybody know that behavior? And what is the connection between having no earth connection and this big hum?
Y capacitor sets your reference potential ("signal GND") to a power line potential, and the stray capacitance to the environment injects noise currents into inputs. Probably input impedances are not equal, therefore noise currents don't cancel each-other and your shielding system is not complete, therefore stray capacintances "connect" to a very different potential. High harmonic content is because of the (stray) capacitive coupling of noise.

Quote:
This is the first time that I design a SMPS without protective earth. With an earth connection, there are normally Y-caps to filter for CM-noise.
Y caps are for switching frequency and above. (Common mode choke also, so it will not help with this problem.)

Quote:
How can I do that when I have no earth?
Tricky question, no simple answer.

But as starting: shielding is a must, and symmetrical inputs must have exactly the same impedances.
  Reply With Quote
Old 21st February 2016, 09:12 AM   #6
diyAudio Member
 
Join Date: Oct 2007
the converter's flyback transformer needs a tight coupling , hence a lot of common mode noise injected into the audio gnds. if you have a chance use a flyback transformer with dual shield windings between prim and sec. or redesign to a LLC resonant design with intended weaker coupling between prim and sec. this way you can really minimize the noise injection..
  Reply With Quote
Old 21st February 2016, 11:50 AM   #7
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
Quote:
Originally Posted by basreflex View Post
the converter's flyback transformer needs a tight coupling , hence a lot of common mode noise injected into the audio gnds. if you have a chance use a flyback transformer with dual shield windings between prim and sec. or redesign to a LLC resonant design with intended weaker coupling between prim and sec. this way you can really minimize the noise injection..
These are very good advices again sw freq noise!

However this doesn't solve the problem of floating and not properly shielded system.
  Reply With Quote
Old 21st February 2016, 03:44 PM   #8
diyAudio Member
 
Join Date: Jan 2010
no, the solution is in posting #3, obviously ignored by the experts here.
  Reply With Quote
Old 22nd February 2016, 07:53 AM   #9
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
Quote:
Originally Posted by voltwide View Post
no, the solution is in posting #3, obviously ignored by the experts here.
1: Y capacitor is needed to short capacitive coupled high freq noise. You will get 100 kHz (+ modulation + harmonics) noise on output GND instead of 50 Hz. Not a great deal.
2: Even if there were no discrete capacitor from mains voltage, stray capacitances would still be there. As basreflex pointed well: "flyback transformer needs a tight coupling". This would still couple 50 Hz as well.
  Reply With Quote
Old 22nd February 2016, 07:57 AM   #10
diyAudio Member
 
Join Date: Oct 2007
i'm working on a dcdc usb isolator using the llc principle. only 2pf coupling between prim and sec.. (toroid sectional winding)
  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
Quad 303 hum caused by HDMI connection Alnasir Solid State 6 6th October 2014 12:11 PM
A 65W Flyback SMPS tylerjbrooks Power Supplies 19 3rd November 2012 12:13 AM
Active clamp flyback SMPS eem2am Power Supplies 1 23rd December 2011 08:37 AM
self oscillating flyback smps blackmamba Power Supplies 15 19th October 2011 07:51 AM
my 80W flyback smps rikvancoolwijk Power Supplies 13 24th March 2011 11:52 AM


New To Site? Need Help?

All times are GMT. The time now is 08:26 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki