noise from the potentiometer - 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 31st August 2005, 11:05 AM   #1
m0nty is offline m0nty  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: near Venice
Send a message via MSN to m0nty
Default noise from the potentiometer

I have finished building a headphone amplifier:
http://headwize.com/projects/showfil...szeke1_prj.htm
replacing r4 with an lm317 current source, as suggested in the project addendum

now, it has a good sound, but there's noise coming from the volume potentiometer: that is very low or nothing when it is full on or full off, but quite loud when the pot is about half
i tried to wire the case of the pot to ground, but it made it even worse... now I removed it, but it didn't change much
by the way, the noise is present on both channels at equal level

what should I do? i'm really thinking about removing it ...

i wired the first pin to ground, the middle goes to the amplifier circuit, and the last one is from the rca input
it's "reverse" wired, that is you turn the volume up counter-clockwise (but I don't think that's the problem)

any help?
thanks
Nicola
  Reply With Quote
Old 31st August 2005, 11:15 AM   #2
m0nty is offline m0nty  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: near Venice
Send a message via MSN to m0nty
here is a photo:
Attached Images
File Type: jpg img_0198.jpg (33.2 KB, 332 views)
  Reply With Quote
Old 31st August 2005, 11:45 AM   #3
MikeB is offline MikeB  Germany
diyAudio Member
 
MikeB's Avatar
 
Join Date: Apr 2004
Location: GŁtersloh
You might consider oscillations, i often had that if i had no inputfilter,
the amp oscillated when turning the inputpot to half volume.
If this is the problem, it goes aways with ~33pf-~100pf cap to gnd
after R1. If it does not change with this cap, the problem is somewhere else...

EDIT: Another possibility: The resistance of the inputpot might be too high,
did you use 100k ? If yes, try a 10k.

Mike
  Reply With Quote
Old 31st August 2005, 02:48 PM   #4
diyAudio Member
 
Join Date: Dec 2004
Location: Mt Pleasant Sc
Default Amp

Hi:
DC thru the pot will cause noise. Is there any?
Why did you use the little PC board on the pot? It is not necessary and the wiring would be neater without it.
What kind of pot is it? An ohmite pot is a better design.
Hope this helps
Ed
  Reply With Quote
Old 31st August 2005, 02:51 PM   #5
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
What kind of noise is it ? Is it hum ?

Regards

Charles
  Reply With Quote
Old 31st August 2005, 03:27 PM   #6
m0nty is offline m0nty  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: near Venice
Send a message via MSN to m0nty
i tried to put a 68 pf capacitor as eds65gto said, but it was better as it was before
also, i tried bypassing the potentiometer and going directly to the amp circuit: no noise at all! sadly the volume is WAY too high, and I can't turn it down from the cdplayer ...

so, i restored the conncection as they were before, and I noticed that when i move my hands near the circuit, the noise gets somewhat louder

i don't think it comes from the power supply (it is a 15V 1.5A switching wall wart), anyway I added a 1mF cap in addition to the 0.1uF (C3) from the power supply.

any other ideas?
meanwhile, i'll try to change the psu to see if something changes

thanks for the help
Nicola
  Reply With Quote
Old 31st August 2005, 03:35 PM   #7
m0nty is offline m0nty  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: near Venice
Send a message via MSN to m0nty
i forgot, it is hum...

i just tried to run it with another "heavier" 12v power supply.
noise is somewhat lower, but still present

... i think i'll have to build a power supply too
  Reply With Quote
Old 31st August 2005, 03:35 PM   #8
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
When the wiper is in the middle position the pot represents a source with higher impedance (0.5 times its nominal value) than the stage driving it. So the middle positions of pots are prone to capacitively coupled noise (additionally to the aforementioned oscillation possibilities).
How long are your connections ? Are they properly shielded ? What is the input impedance of the stage "driven" by the pot ?

Regards

Charles
  Reply With Quote
Old 31st August 2005, 03:48 PM   #9
diyAudio Member
 
Black Heart's Avatar
 
Join Date: Aug 2005
The problem is most likely that you don't have a very good ground return on the input of the circuit since you're apparently relying on the pot and it's wiring. I'd recommend an input termination resistor be installed in the circuit from R1 to the circuit ground to assure a good solid termination without any leads involved. Something around 50K is a standard value, but it would depend on what value you're using for the pot and what kind of load impedance you were trying for. I'm kind of surprised that was left off the schematic because it's fairly standard practice. You do have C1 in the circuit so that the input impedance doesn't effect the bias, right?

Dave
  Reply With Quote
Old 31st August 2005, 04:29 PM   #10
m0nty is offline m0nty  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: near Venice
Send a message via MSN to m0nty
Phase_accurate:
i made the connections as short as I could, but using standard red/black wires ...
about the impedance of the amp stage, i don't know, however it's a mosfet source follower, so input impedance should be quite high

Black Heart
yes C1 is present
i'll try with a resistor from r1 to ground

thank you both for the replies
Nicola
  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
Which value of a potentiometer Alfetta87 Tubes / Valves 13 23rd April 2014 08:34 PM
Need some help with potentiometer. mkapszewicz Tubes / Valves 6 29th November 2006 04:55 AM
LED potentiometer ralf Pass Labs 20 19th October 2006 05:37 PM
Looking for potentiometer paulc Class D 6 3rd February 2006 11:57 PM
Best Value of Potentiometer for T-Amp? HolyGhostFire20 Class D 0 3rd January 2006 10:39 PM


New To Site? Need Help?

All times are GMT. The time now is 08:27 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