Buzzing changes with volume pot position... - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

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 August 2004, 07:54 PM   #1
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
Default Buzzing changes with volume pot position...

Well, I built a tube amp and everything is absolutely perfect with it, except that there is a a buzzing sound when the volume pot is anywhere but max volume, at max volume there is no buzz (or hardly any at all) but as I move the volume lower the buzz steadily increases, and it is very lound at minimum volume. I'm gathering that htis is a grounding problem, I've got a star ground, and I'm not sure what else to try. The fact that it changes with the volume control (which is right at the input to the circuit) baffles me as to why this is happening, any help would GREATLY be apreciated.
  Reply With Quote
Old 10th August 2004, 01:16 AM   #2
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
I'd seperate the PSU ground and signal grounds, but whatever...

Assuming you use a normal voltage divider volume control, clearly there must be two things: 1. there is a signal (with respect to the first (input stage) cathode) at the pot GND; and 2. there is another source loading the pot, so that AC voltage drops to zero at the *open* end. If you have a signal connected, it could well be a sign of ground loop (try ungrounding some connected equipment). If it does this without anything connected, it could be a sign that the control is reversed and picking up stray noise in your case (is it near an HV line?).

The absolute best way to eliminate internal ground loop hum, BTW, is to 1. isolate PSU from amp circuits, and 2. connect input ground to 1st stage, and 2nd stage/output if possible, grounds. The quad 6L6 amp I made had nasty problems for a long time until I finally ran the 12AX7 cathodes to the input jack. (Everything was running through the chassis otherwise; damned if I know where the loops came from in the first place, but this nipped it in the bud. Bit of a coverup rather than fix-it-for-good situation but what are ya gonna do.)

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 10th August 2004, 04:17 AM   #3
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
Oh, thanks Sch3mat1c, I didn't even think of seperating the signal and power grounds, I'll try that first. But what did you mean by "connect input ground to 1st stage, and 2nd stage/output if possible, grounds"? I think I just didn't understand the wording. And again, thank you alot, seperating psu and signal ground had never even crossed my mind!
  Reply With Quote
Old 10th August 2004, 06:26 AM   #4
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Click the image to open in full size.

See the black wire that snakes around the left and top? That's the power switch wire. Well immediately below the switch (top center) is the potentiometer (it's an uh... 100k dual gang Alpha from Mouser). Above it is the input jack, which by its nature happens to be connected to the chassis. It's not easy to see on this small pic. Well anyway, the ground tab on this jack is connected to the pot's ground, and the first stage (triode; 6U8 on the right) is also grounded to this point. The other stages are grounded locally, including the 6L6s which are grounded at their sockets. They don't matter because the 50mV of hum pales in comparison to the 50V required drive; however, 50mV at the input of such a high gain amplifier will drive you NUTS.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 10th August 2004, 05:25 PM   #5
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
ted the signal and power grounds and that helped, how it helped was decrease the buzz at lower volume levels. Now, at no volume, the buzzing is almost gone, but as Ibring it up the buzzing increases until it's at mid volume, then it decreases again and at max volume the buzzing is almost gone again. I'm using a linear pot so the loudest buzzing is when the resistance is equal going to ground and from the source, and it's loud. A max and min there's barely any, but in the middle there's alot.

Thanks for the tip, got some of it away that way.
  Reply With Quote
Old 10th August 2004, 05:44 PM   #6
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Alright, now that implies that there is a high-impedance hum fed to the GRID and it is being shunted by the pot at min, and max depending on what's connected. I can't get my hands on your amp so all I can say is think what could be giving it a signal.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 10th August 2004, 07:20 PM   #7
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
Ah, so a high impedance signal fed into the grid would produce an even louder hum when no inputs are connected, right? Because I just tried that and indeed that's what happened. Although, I have no idea what could be causing that. Well, at least I know what to look for, thank you so much for the help and time btw.
  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
Is setting an alps volume pot to the full position the same as bypassing? boidos Solid State 3 12th February 2009 07:57 PM
25-position switched attenuator using a pair of 5-position switches? ray_moth Tubes / Valves 1 4th May 2008 08:02 AM
Open end position in a TL teodorom Multi-Way 4 12th May 2004 11:33 PM


New To Site? Need Help?

All times are GMT. The time now is 11:19 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