problem with LM's and power supply - 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 9th March 2005, 08:40 PM   #1
diyAudio Member
 
Join Date: Sep 2003
Location: Athens
Default problem with LM's and power supply

Hello,
i have one transformer 2x24V and i want to supply one LM3875 chip and one LM1875 the same time.
because a 2x24V trafo gives us around +-35V and lm1875 can go max +-30V i am using regulator LM350..

When i power up each channel alone everything is ok.. but when i power them up together there is a terrible huge very loud noise comes out from both channels..

i suspect some ground loop because input grounds are connected together but power grounds one goes inside regulators the other is from the rectifiers directly from transformer..

i have draw the schematic i am using trying to explain my connections...

what am i doing wrong? any suggestions?

thank you very much!
Attached Images
File Type: gif ampprob.gif (62.4 KB, 530 views)
  Reply With Quote
Old 9th March 2005, 09:42 PM   #2
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
I see the power grounds connected to the input ground.

/Hugo
  Reply With Quote
Old 9th March 2005, 10:13 PM   #3
diyAudio Member
 
Join Date: Sep 2003
Location: Athens
if you talk about the green line its just to notice that the preamp connects the input grounds together....

on the amps pcb there is a star ground of input ground with power ground... but they are also connected inside the preamp.

i think that there is something to do with the regulators..

any clue?
  Reply With Quote
Old 9th March 2005, 11:17 PM   #4
moamps is offline moamps  Croatia
diyAudio Member
 
Join Date: Oct 2002
Location: Croatia
Hi Kostas,

I see two mistakes in your schematic. First, you can't mix secondaries and their respective bridges. In other words, if you use one secondary to get positive voltage for one amplifier, the same secondary must be used to get positive voltage for the other amp (not negative). Second, in your configuration, you can't use a positive regulator (LM338) to get negative regulating voltage but a regulator that's normally used for getting negative voltage (with common pin connected to ground).

Regards,
Milan
  Reply With Quote
Old 10th March 2005, 10:18 AM   #5
diyAudio Member
 
Join Date: Sep 2003
Location: Athens
i have put the positive with the positives the negatives with the negatives.... the same problem..


i have done another test.. i powered both amps .. and i connect only the one input at a time and it is working... the problem starts when both inputs are connected.

i remind you that the preamp connects grounds together from inside. so i can think that this is the problem..
the common input ground of the preamp..

what can i do to fix it?

the regulator with LM350 works fine my friend.. this is a schematic i found here in the forum from another great guy.

what can i do :-( im so sad :-(
  Reply With Quote
Old 10th March 2005, 10:37 AM   #6
shusha is offline shusha  Croatia
diyAudio Member
 
shusha's Avatar
 
Join Date: Sep 2004
Location: Velika Gorica
Send a message via ICQ to shusha
Quote:
Originally posted by moamps
Second, in your configuration, you can't use a positive regulator (LM338) to get negative regulating voltage but a regulator that's normally used for getting negative voltage (with common pin connected to ground).
Why not zz, he can use positive regulators for both sides as long as he have two secondaries (if this two secs. are from same transformer as you mentioned). I am sure you know that. His schematic with 338 looks like Pedja's. I have built several regulated PSU's by it's schematics.

http://users.verat.net/~pedjarogic/audio/index.html
  Reply With Quote
Old 10th March 2005, 10:49 AM   #7
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
Look at your power supply schematic very carefully. On the -35 volt supply, the cathode (+) end of the bridge is going directly to common ground. On the -25 volt supply, the cathode (+) end of the bridge is going to the input of the regulator. This will not work because you are shorting out the regulator.

You must use a negative regulator if you expect the two amplifiers to work from common windings.
__________________
Frank
  Reply With Quote
Old 10th March 2005, 12:02 PM   #8
moamps is offline moamps  Croatia
diyAudio Member
 
Join Date: Oct 2002
Location: Croatia
Quote:
Originally posted by shusha
...Why not zz, he can use positive regulators for both sides as long as he have two secondaries......[/B]
Bok, Hi,

Frank je dao odgovor.
Look at Frank's post.

Pozdrav, Regards,
Milan
  Reply With Quote
Old 10th March 2005, 12:07 PM   #9
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
... so if you want to use only positive regulators you must have at least 3 totally separate windings and if you fix a true negative regulator you can get away with only two secondary windings.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 10th March 2005, 12:19 PM   #10
moamps is offline moamps  Croatia
diyAudio Member
 
Join Date: Oct 2002
Location: Croatia
....or (wierd solution) keep amps grounds separated and use ground-lifted line transformers in inputs..
  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
Power supply problem Niila Tubes / Valves 6 12th April 2008 01:39 AM
Power Supply problem Landoctor Chip Amps 3 8th September 2007 07:50 PM
Power supply problem krzanik Tubes / Valves 19 1st May 2007 03:56 PM
Power supply problem PassFan Tubes / Valves 6 24th October 2003 10:03 PM
Power supply problem (or something) ir Chip Amps 4 23rd August 2003 08:44 AM


New To Site? Need Help?

All times are GMT. The time now is 09:19 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