Practical considerations building LM3886 amp - Page 4 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 5th February 2013, 12:17 AM   #31
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
Earth to Neutral is ideally zero. I wouldn't get too concerned about a few volts...but you have 103 more Volts than a few!
  Reply With Quote
Old 5th February 2013, 12:34 AM   #32
diyAudio Member
 
Join Date: Jan 2013
Location: the town where nothing ever happens
Send a message via Yahoo to Bloodmist
Thanx for confirming this. My reckless neighbors and their renovations probably ruined it. It used to be 0 with respect to neutral. Is there a way to fake the earth for the amp?
  Reply With Quote
Old 5th February 2013, 05:27 AM   #33
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
What happens if you pull the plug with your ear against the speaker. Does the hum stop instantly or does it continue for a short time?
  Reply With Quote
Old 5th February 2013, 05:43 AM   #34
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
I think your "earth" isn't so any more.
Beyond mere Audio problems, check and repair it at once for your own safety.
Get a registered Electrician to do it or *you* will be liable for any future problem, not kidding.

Now, how come you have 127V in Romania??????
  Reply With Quote
Old 5th February 2013, 07:43 AM   #35
diyAudio Member
 
Join Date: Jan 2013
Location: the town where nothing ever happens
Send a message via Yahoo to Bloodmist
gootee if i pull the plug the hum stops instantly, there remains just the very small hiss that is normal, until the capacitors in the power supply discharge. i noticed also when connecting both transformers the hum is modified. if the primary windings are in phase the hum is still 50Hz just louder than with only the main transformer. if the windings are in anti-phase the hum is not so loud but becomes 100Hz (by "ear-meter").
JMFahey, no, actually it should be 220V. in reality it's 230 on a good day when there is a lot of load. usually it sits at 247V (possibly 250V at night). that's why this is odd. if the earth was still earth, there should be 200-and-something volts between it and the power line. instead it sits about in the middle.
  Reply With Quote
Old 5th February 2013, 03:22 PM   #36
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
Originally Posted by Bloodmist View Post
JMFahey, no, actually it should be 220V. in reality it's 230 on a good day when there is a lot of load. usually it sits at 247V (possibly 250V at night). that's why this is odd. if the earth was still earth, there should be 200-and-something volts between it and the power line. instead it sits about in the middle.
OK, now that clears things up.
You do NOT have "220V/neutral" available on your (formerly 220V round pin ) wall socket but you inherited an *old* home power distribution system, once somewhat popular: 2 x 110V out of phase lines, none of which is ground.
What's grounded is the center tap of the 110V+110V distribution transformer which powers a few blocks in your neighbourhood.
I *guess* it was chosen for safety: appliances "see" 220V, but no line is more than 110V away from Earth.
Best of both Worlds ... except it's murder on hum sensitive Audio stuff.
I used to live in a house wired like that
Had to get my own ground using a 3 Meter long Aluminum bar, making a hole in a patio until I found real, black humid Earth.
Oh well.

What you measure is quite reasonable: both voltages add up to about 230V, the new European "de facto" midrange between 220 and 240V standards, and are not symmetrical because of uneven load.
And rising to 250V when unloaded at some times of the day is reasonable too, old overworked transformers and lines have poor regulation.
So, given that you have to live with it, simply remember no power pin is "neutral".
Unless you make a *big* 220V/220V transformer, primary across both "live" lines, secondary ground and hot 220V.

I was about to do the same at my old home, but then moved to a "normal" wired house, so it was not necessary.
Still have the huge transformer core and the heavy duty 5 position switch (since I was getting into it anyway), I was going to add 5 taps to fine tune wall voltage at home.
  Reply With Quote
Old 5th February 2013, 08:15 PM   #37
diyAudio Member
 
Join Date: Jan 2013
Location: the town where nothing ever happens
Send a message via Yahoo to Bloodmist
The main problem is that IT is not the problem... therefor i'm at a loss what might be. I found on the hallway an outlet with normal 220V-0V output and grounding but nothing changed. Since this apartment building is awkwardly wired, at the midpoint between 2 transformers, some outlets are wired to the old transformer, some to the newer one, that was needed only in recent years with more and more appliances drawing power.
There is no noise from the preamp. I've powered it up and it's dead silent regardless of potentiometer position. There's just the everpresent hum from the output stage. I could probably live with it as it's inaudible when say the computer is running (where it gets it's audio anyway) but it's just a mystery to me what could be happening.
  Reply With Quote
Old 5th February 2013, 09:06 PM   #38
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Well, the scope is your friend
Use it to trace hum or whatever bothers you, step by step.
  Reply With Quote
Old 6th February 2013, 05:11 AM   #39
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
So then your problem is probably something covered in post 28.

Your signal input and signal ground reference conductors (or paths, or partial paths) might have space between them, allowing them to be an antenna for hum.

Otherwise, maybe your signal reference ground at amplifier input is bouncing. Possibly you connected it too close to rectifier in PSU, instead of after the smoothing caps?

Otherwise, maybe your smoothing caps are not doing a good-enough job and your amp's PSRR is terrible? Do you not have any global negative feedback?

Otherwise, maybe your speaker wires have space between them, and are acting as an antenna.

A picture of the amplifier circuits would be helpful.

Last edited by gootee; 6th February 2013 at 05:13 AM.
  Reply With Quote
Old 6th February 2013, 06:49 PM   #40
diyAudio Member
 
Join Date: Jan 2013
Location: the town where nothing ever happens
Send a message via Yahoo to Bloodmist
while i'm gonna look for a decently priced scope online, i'll try moving the earth point to different places in the ground path. perhaps it would become quiet if i put it on the signal ground instead.
when building it i considered the following layout i found on the net (seems very revealing):
Click the image to open in full size.
Otherwise the schematic remains pretty much as previously posted by me, just the LM3886 now uses split power supply.
there could be a potential problem indeed that cables to the output connector pass close to the main transformer. i can imagine how signal induced in the output cable would go to the LM's input through the feedback resistor. if all else fails i'll dislodge the connector and place it elsewhere.
  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
Measuring speakers in-room. Practical considerations. chuyler1 Markaudio 38 2nd September 2011 09:37 PM
Considerations in enclosure-building for full range drivers Taterworks Full Range 2 23rd April 2008 07:00 PM
Considerations for building a TWW 2-Way System plamalice Multi-Way 10 8th December 2006 07:31 PM
LM3886 / LM3875 supply voltage considerations breez Chip Amps 7 22nd January 2005 06:22 PM


New To Site? Need Help?

All times are GMT. The time now is 09:51 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