Noise in CG - 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 3rd December 2003, 07:09 AM   #1
chrish is offline chrish  Australia
diyAudio Member
 
chrish's Avatar
 
Join Date: Oct 2003
Location: Sydney
Default Noise in CG

Have just completed my first amp project, an inverted two channel LM3875 using this schematic. The only changes I have made are that I used 0.1uF caps across 1000uF, used an additional 4000uF for each power supply rail and used a 56k resistor to ground at the input instead of vaiable resistor (using as power amp). I am getting some noise at the speakers. When the amp is connected to pre-amp and volume of pre-amp is turned past 1/2 way I am getting a low level 100hz hum from the woofers that increases in volume when volume knob on pre is increased. This noise is inaudible with volume less than 1/2. When pre amp is on there is also a hiss coming from the tweeters. This hiss does not change volume with movement of the pre-amp volume control, but stops completely when pre-amp is switched off. There is NO noise at all from speakers when there is no input to amp (pre-amp turned off or RCA cables disconnected). Any help on where to start looking to solve these problems would be appreciated.

Thanks,

Chris
Attached Images
File Type: gif invertedlm3875_r2_1(2).gif (8.9 KB, 628 views)
  Reply With Quote
Old 3rd December 2003, 07:48 AM   #2
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Why do you have so high resistance values? As pot I would have chosen 10 k or so. High values creates noise. What is the purpose of the 22k?

Have you tried to shortcircuit the 18k? Is it necessary?
__________________
/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 3rd December 2003, 09:31 AM   #3
Jax is offline Jax  Sweden
diyAudio Member
 
Join Date: Nov 2003
Location: South Sweden
Try removing the 22k, it injects bias current into the signal ground. This resistor will set the DC gain to 10 which in my opinion is unnecessary. The 10k + 4.7u + source impedance will provide a gentle rolloff to DC unity gain without it.

I myself have 330k/15k + 2.2u and a 100k pot and a 1k from + to ground on my 3886's and they are totally quiet. I do have a zobel and a 10 ohm + coil on the output though, no resistor. (I still don't understand why people add a resistor to the output).

If you still want that 22k, try moving it to the power ground instead.

Have you checked your preamp if it can drive this low impedance (<10k)? Hum and noise could come from it.

Is the input connector insulated from the chassis? If not, insulate it and wire its ground to the signal ground point. I have pots, so I have wired the connector ground to the CCW lug of the pot and from there to the signal ground.

Are both the preamp and GC signal grounds connected to protective earth? If so, you have a nice ground loop. You could try letting the GC signal ground float above the chassis but do not break the protective ground
  Reply With Quote
Old 3rd December 2003, 09:42 AM   #4
chrish is offline chrish  Australia
diyAudio Member
 
chrish's Avatar
 
Join Date: Oct 2003
Location: Sydney
My knowledge of electronics is rather basic, so I have stuck to the schematic that I downloaded from one of the chip amp sites. I am happy to experiment, but would really appreciate some direction ratther than just randomly changing parts... No, I have not made any changes yet. You suggest try removing 22k to ground and trying 10k at input?

Thanks guys,

Chris
  Reply With Quote
Old 3rd December 2003, 02:24 PM   #5
GregGC is offline GregGC  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Toronto
It may be some GND loop problems. How is your gnding donne. Can you describe it please.
  Reply With Quote
Old 3rd December 2003, 03:07 PM   #6
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Quote:
Originally posted by chrish
My knowledge of electronics is rather basic, so I have stuck to the schematic that I downloaded from one of the chip amp sites. I am happy to experiment, but would really appreciate some direction ratther than just randomly changing parts...
You can get much infomation just to read the datasheet and availabe application notes which in this case is very good. If you don't understand so much you can still compare good truths and what you have found on the net.

If you want an inverting amp, start with a "clean" connectionm described in datasheets and application notes. Then if it works start changing.

Much of the LM3886 stuff is valid for LM3875.

http://www.national.com/ds.cgi/LM/LM3875.pdf
http://www.national.com/ds.cgi/LM/LM3886.pdf

http://www.national.com/an/AN/AN-1192.pdf
__________________
/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 3rd December 2003, 09:23 PM   #7
chrish is offline chrish  Australia
diyAudio Member
 
chrish's Avatar
 
Join Date: Oct 2003
Location: Sydney
Thanks for the help guys. I tried connecting the amp directly to an old Luxman cd player that I have that has a variable output. There is virtually no noise when cd on pause and volume full. Maybe it is a problem interracting with the pre amp? For info the pre amp is an old Adcom GFP555 II.

GregGC asked me to describe grounding. I have used star grounding, all signal to point on chassis next to input and power ground next to the 1000uF filter caps (behind caps in this view).
Attached Images
File Type: jpg amp2.jpg (59.0 KB, 443 views)
  Reply With Quote
Old 3rd December 2003, 09:25 PM   #8
chrish is offline chrish  Australia
diyAudio Member
 
chrish's Avatar
 
Join Date: Oct 2003
Location: Sydney
Different angle...
Attached Images
File Type: jpg amp3.jpg (39.6 KB, 430 views)
  Reply With Quote
Old 3rd December 2003, 09:27 PM   #9
chrish is offline chrish  Australia
diyAudio Member
 
chrish's Avatar
 
Join Date: Oct 2003
Location: Sydney
And full view of part completed case with both channels...
Attached Images
File Type: jpg amp4.jpg (52.3 KB, 428 views)
  Reply With Quote
Old 3rd December 2003, 11:04 PM   #10
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
At what volume does the output start to clip (distort) when connected to the pre-amp? If it is at less than about "2:00" on the volume dial, then decrease the gain of your amp by reducing the value of the feedback resistor. Try 100K instead of 220K. This will reduce noise, especially the omni-present background hiss that your pre-amp is creating.
  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
Low noise transistor and noise figure Real_Macgyver Solid State 28 27th July 2012 09:42 AM
Hum and Noise with Ono rsbay Pass Labs 0 26th February 2006 12:51 PM
NOISE - RFI and EMI jackinnj Everything Else 9 26th February 2005 01:01 PM
Noise Bukitoo Analogue Source 2 17th March 2004 02:37 PM
DSP and DAC noise nFORCE Digital Source 4 29th April 2003 03:18 PM


New To Site? Need Help?

All times are GMT. The time now is 02:52 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