Ground loops, speaker hum and buz - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 24th October 2008, 11:29 AM   #1
steevo is offline steevo  United Kingdom
diyAudio Member
 
Join Date: Sep 2005
Default Ground loops, speaker hum and buz

I have built a giovanni Stochino Mosfet Power Amp and have been testing the amps, they work fine apart from what seems to be ground loops *** soon as I connect them either or both input to my Audiolab surround sound processor output.

I am confused as to what is going with the ground loops.
I am running both amps off 1 +/-50 V supply for the ouput
stage and 1 +/-55V regualted supply for the other stages.
The contruction notes specifiy using 2 sperate supplies for the amp.

I have connected the power ground for each amp to a star point
which is run well away from the center tapping of the power capacitors.
The Ov for the +/- 55V regualted supply is also connected to the star
point.

1.) When powered without the inputs connected there is a definited low level hum from the speaker, I am not sure why this is, When I conect the mains earth to both input signal grounds on the phono connectors for each amp the hum disappers and there is virutally no buz or hum from either amp. Not sure what is going on here why the mains earth should erradicate this.

When I connect my Audiolab to the either or both amp inputs then
buz and hum can be clearly heard from the speaker. And I cant think why this is, If its eliminated when I connect the mains earth to the signal ground then why does hum and buz come back when the audiolab is connected. If I then disonnect the mains earth from my amps signal ground so that only the audiolab is still connected to the the mains earth then the hum and buzz is still there.

I am well confused as to what is going on.

Can anyone help
Thanks
Steevo
  Reply With Quote
Old 24th October 2008, 11:33 AM   #2
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Have a read at this -- all of it
3 stage LIN topology - NFB tappings?
  Reply With Quote
Old 25th October 2008, 10:40 AM   #3
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Because you are using one xformer for supply, which means a common GND node for both channels at star point and another one in your surround processor unit, it is caused a nice ground loop. Make a sketch on the paper and you will find the loop.
Now the only solution it is the seperation of the ground of the input RCA sockets with 2 resistors of 10 from the shield of signal cables. Maybe this can resolve a little your problem.

Regs
Fotios
  Reply With Quote
Old 25th October 2008, 09:31 PM   #4
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
If its not a ground loop then it could be hum on the power supplies. Some designs are very prone to amplifying hum on the power rails.
I usually put a 39R in series with power supplies to the driver section and then use 1000uF caps on the driver supplies.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 26th October 2008, 09:43 AM   #5
diyAudio Member
 
Nico Ras's Avatar
 
Join Date: Nov 2005
Location: East Coast of South Africa
Although some advocate ground loops as being the cause of hum, it is not necessarily the case as it needs an understanding of what creates the hum.

Assume for a moment that your +-50V supply is actually +50.1V and -49.9V, then the 0V is actually not 0V. Now assume that the other piece of equipment has the same problem and you find the 100 Hz ac component superimposed on the "ground".

The amplifier will do its job in amplifying this and you will hear it as hum on the speakers. Hence nothing connected no hum and something connected produces hum.

Regulators and filtering does not get rid of this problem.
  Reply With Quote
Old 26th October 2008, 10:11 AM   #6
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
The mains ground is a safety ground. It should be connected to the chassis or enclosure but not to any part of the circuit.
The star ground(s) of the amp should really be star grounds. I don't know what you mean by saying that the start grounds are 'well away' from the cap ground. The star ground should be the midpoint of the supply, the return of the supply caps and the speaker return. If you have a single supply for two amps, the star ground should be common for both amps, both speaker returns going to this star point. Then you have the signal ground. The input RCA's should of course be isolated from the chassis, and the screen continued to the circuit board input entry. There should normally not be a connection between that input ground and any heavy current points. For reference, you can connect the screen of one input only to the star ground or the chassis through a small (1-10) ohms resistor.
If your samp hums without any input then there is a problem with the grounding of the amp ground and ref points. Try to solve that rather then trying to cover it up with other stuff. Including other pieces of equipment before solving this only takes you further away. Divide and conquer!

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W S Maugham
Check out Linear Audio!
  Reply With Quote
Old 27th October 2008, 11:26 PM   #7
steevo is offline steevo  United Kingdom
diyAudio Member
 
Join Date: Sep 2005
Default Giovanni Amp Hum Problem

I think my problem is more complicated than just a ground loop, with both amps running from the same supply, and 1 channel only connected to my audiolab processor preamp I still get hum/buz, without any inputs connected the amps are as quiet as a mouse, so with 1 input connected only there is no ground loop between the amp beacuse they are only coonected together at 1 single star point, I think each amp pollutes the ov's return to the star point when an input is connected , and to eliminate it I need to re-route the feedback returns, and coupling returns from each amp to the single star point, in other words redesign the grounding on amp . I would need to hack and cut the large ground tracks on each borad to do this so I aint gonna bother. The grounding arrangement was not designed for a common supply I think, the original article does recomend seperate supplys to avoid ground loops, so I'm gona stick with seperate supplys. I tried to save money but in the end not worth it, Then I can be confident of getting the performance specificed by Giovanni. But i have learnt alot about grounding in the process.

steevo
  Reply With Quote
Old 28th October 2008, 07:19 AM   #8
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Bearing in mind all the usual safety warnings, is your preamp connected to mains earth. Is the power amp ? Try disconnecting in the plug one or both grounds temporarily.
Anything like that would certainly create aground loop. Is the OV line of the power amp connected to mains earth ?
Do bear in mind all the safety implications, what I have mentioned above is just to try as a test, not to leave like that
  Reply With Quote
Old 28th October 2008, 11:33 AM   #9
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Also, you can keep the cost low by ordering the next time one simple transformer - of the same power - but with individual secondary windings per channel. The cost will be the same.

Fotios
  Reply With Quote
Old 28th October 2008, 04:49 PM   #10
diyAudio Member
 
Nico Ras's Avatar
 
Join Date: Nov 2005
Location: East Coast of South Africa
Draw your power supply section, and we have a look see. Your problem is rooted there.
  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
Ground loops and containment jneutron Parts 49 18th February 2011 02:28 PM
UK laws - dealing with ground loops? johnm Power Supplies 9 4th August 2008 03:05 PM
Safety question concerning ground loops derMichi Chip Amps 5 9th June 2006 08:31 AM
P3A and ground loops biff584 Solid State 5 14th March 2003 02:21 PM
Earth/Ground Loops JohnClark Tubes / Valves 2 14th November 2001 12:49 AM


New To Site? Need Help?

All times are GMT. The time now is 12:09 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