Need help with my LME 48930 amp - 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 13th October 2013, 08:40 PM   #1
fiscal is offline fiscal  France
diyAudio Member
 
Join Date: Nov 2008
Default Need help with my LME 48930 amp

Hi

I have a hum with my new LME 48930 amp. It is a 2 x 100 W (8ohms), with two pairs of 2SK1058/2SJ 102 mosfets. There is a single power supply +/50 V, ClC one, with 2 larges 22 mH capacitors, and 1,8mh self on each rail.

I don't understand why I have this hum, with the IN+ connected to ground.

Here's a capture of th e mesure with cold amp (5mv/2ms)
Click the image to open in full size.

and the same (each chanel) 15 mn later (hot amp)

Click the image to open in full size.

I try lots of thing: add big electrolytics caps, change value of bias cap, change the bias courant (130 mA on each mosfet), and no effect.

Here's the schema and the PCB. Can you help me?

Click the image to open in full size.

(there is a offset control)
Click the image to open in full size.

Thanks!

Georges (sorry for my bad English)
  Reply With Quote
Old 14th October 2013, 12:03 AM   #2
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: 98
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
You could be seeing the cap charging currents imposed on a non-zero ground impedance. To see if this is so, please show the wiring of the main 22mF rail caps and how these connect to your PCB?

The only thing I can see potentially wrong in your grounding on the PCB is that you're using a noisy AGND to feed the mute pin - you could try moving this to a quiet ground (from the star). Oh and C14 also should go to the star, not to a noisy AGND. And R5 needs its own quiet ground, since the one it currently has is polluted by the noise current through C11.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler

Last edited by abraxalito; 14th October 2013 at 12:23 AM.
  Reply With Quote
Old 14th October 2013, 08:52 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by abraxalito View Post
You could be seeing the cap charging currents imposed on a non-zero ground impedance. .
This is very likely.
Bad Grounding and particularly sharing a charging circuit with an audio circuit trace or wire.
__________________
regards Andrew T.
  Reply With Quote
Old 14th October 2013, 08:51 PM   #4
fiscal is offline fiscal  France
diyAudio Member
 
Join Date: Nov 2008
Thank you for your answer

There is the schema, the PCB and two photos of the power supply with the amplifiers:

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

There is an flip flop and a soft start on the power supply you can't see on the schema.

The star ground is the five pins near the two bridges rectifier: two pins for amplifier's ground,, two for the HP's an one for the in signal.
The AGND is connected to the earth by a 100 R/100 nF.

For the mute, I try to change with a 5,1 V zener diode and the good value resistor to have 160 A: no effect. I try an another power supply: it's worse!

May be a problem with the inductor just above the ground wire on PCB?

Georges

Last edited by fiscal; 14th October 2013 at 08:54 PM.
  Reply With Quote
Old 14th October 2013, 10:02 PM   #5
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: 98
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Thanks for providing the PCB layout for your PSU. Its clear from this that your 'star ground' isn't one in reality - that's because you have very large capacitor charging currents flowing through it (left to right) and so the tastons to the left will be at a different potential to those to the right.

A star ground really does mean a literal star - all connections to it must meet at an infinitesmally-sized point so that there are no ground impedances in common.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 15th October 2013, 08:28 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I don't know where you got that PSU PCB from, but in my opinion it belongs in the bin.

You would get better performance hardwiring instead and omitting many of the components that you have included.
__________________
regards Andrew T.
  Reply With Quote
Old 15th October 2013, 09:04 PM   #7
fiscal is offline fiscal  France
diyAudio Member
 
Join Date: Nov 2008
This PSU come from TI Application note AN-1849.

I try something like that to connect the amplifiers ground (with a weding wire): thers's no effect . May be I don't understand something with the "real star ground"

Do you think I have to remove the self (just above the signal ground?)

Click the image to open in full size.

Thank you!
  Reply With Quote
Old 15th October 2013, 10:28 PM   #8
WSJ is offline WSJ  United States
diyAudio Member
 
WSJ's Avatar
 
Join Date: May 2009
Location: GILROY, CA
The amplifier open loop gain is 120 dB (1,000,000) minimum. Any signal on the traces connected to pin 5 (- input) will be amplified 1,000,000 times. The long trace on pin 5 is an antenna. Both gain resistors should be connected to pin 5 with the shortest possible trace. This could be accomplished with a trace length of 0.25 or less.
  Reply With Quote
Old 15th October 2013, 11:37 PM   #9
WSJ is offline WSJ  United States
diyAudio Member
 
WSJ's Avatar
 
Join Date: May 2009
Location: GILROY, CA
Both gain resistors should be connected to - input with the shortest possible trace. This is also the case with discrete amps. Parallel traces next to or under the - input trace will capacitive couple unwanted noise or signals. You can verify the sensitivity of the - input by touching it with a test lead or your finger.

Last edited by WSJ; 15th October 2013 at 11:51 PM.
  Reply With Quote
Old 16th October 2013, 01:34 AM   #10
Johno is offline Johno  Australia
diyAudio Member
 
Johno's Avatar
 
Join Date: Oct 2010
Location: Up in the alps - motorcycle heaven
Fiscal,
The common mode choke (the inductor) is one of the main causes of your problems.

It is very strongly coupling the +ve rail current to the -ve rail and vice-verse - remember that in class AB the rail currents are more or less equal but opposite. Short it out and you will see improvement.

If you add inductors to the DC rails they must not couple.
  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
Passively Battery supplied LME 49710 RIAA amp for MM cartridge eaglebear Analogue Source 1 5th August 2013 12:52 PM
LME 49810 Kit thor61 Chip Amps 4 22nd February 2013 06:21 PM
LME based amp vs Rod Elliott amp benproiii Solid State 5 6th May 2011 01:59 AM
LME 49713 + LME 49600 pre amp klewis Chip Amps 16 4th February 2011 01:15 PM
National LME 49811 satanx Chip Amps 2 8th May 2008 10:29 PM


New To Site? Need Help?

All times are GMT. The time now is 06:10 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