Loud Noise due to NFB - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 14th January 2011, 04:52 PM   #1
diyAudio Member
 
Join Date: Sep 2006
Location: Malta, Europe
Default Loud Noise due to NFB

I have just built the attached 6L6-GC SE circuit by JC Morrison with EF86 driver. When I first fired it there was a loud noise, so I checked the coupling caps. They were OK. What silenced it was by removing the NFB connection from the +ve side of the OPT secondary.

It is to be noted that in the published circuit the nfb is connected to a higher impedance than 8 ohm, presumably 16 ohm. My OPT secondary is only 0-8 ohm. What value of nfb resistor do you suggest I use instead of 8.2K? Do I go up or down in resistor value?

Thanks for your help.
Joe A
Attached Images
File Type: jpg 6L6-GC SE UL JC Morrison.jpg (562.9 KB, 186 views)
  Reply With Quote
Old 14th January 2011, 05:04 PM   #2
Svein_B is offline Svein_B  Norway
diyAudio Member
 
Join Date: Oct 2005
First you have to check if your global feedback is POSITIVE or NEGATIVE.
If the amp is unstable with NFB, you may try to swap the output wires from the OPT.
When you connect the feedback the volume should become lower.

The 8 instead of 16 ohm OPT means that you have a little less feedback (about 70% of the original with 16 ohm tap. You could then reduce the feedback resistor a little for the same feedback ratio. The optimal ratio should be determined by listening.

Svein.
  Reply With Quote
Old 14th January 2011, 05:06 PM   #3
diyAudio Member
 
kavermei's Avatar
 
Join Date: Jan 2005
Location: Lokeren, Belgium
Send a message via MSN to kavermei
This might be caused by having the feedback wired in the wrong way. In this case you wind up with feed-forward instead of feedback, resulting in oscillations.

Try to swap the connections to the output transformer secondary winding.

Kenneth
__________________
Never send a human to do a machine's job. --Agent Smith
  Reply With Quote
Old 14th January 2011, 06:45 PM   #4
diyAudio Member
 
kstagger's Avatar
 
Join Date: Dec 2003
Location: Grand Rapids
I've built something very similar to this circuit (EF86 -> UL connected EL156) with no problems at all - one of my favorite amps. Definitely try swapping your transformer leads.
  Reply With Quote
Old 14th January 2011, 06:50 PM   #5
diyAudio Member
 
Join Date: Sep 2006
Location: Malta, Europe
Thanks, friends. I wired the output from the transformer exactly as specified by the manufacturer (Transcendar). I will swap the connections and see how it fares.

I have hooked the amp (without feedback) to my main (expensive) system and while the loud noise is not there, there is a noticeable hum with no program playing. I wired my grounds in star fashion and I'm not sure if this is also part of the incorrectly wired output transformer.

Anyway, we'll see how it goes with the swapped connections.

Thanks for your help.
JA
  Reply With Quote
Old 14th January 2011, 06:58 PM   #6
Svein_B is offline Svein_B  Norway
diyAudio Member
 
Join Date: Oct 2005
The hum, it is important to find out if this is 100Hz PS ripple, or 50Hz hum.

The first can be reduced with larger PS caps after the choke.

50Hz hum may be magnetic coupling between PT and OPT, or grounding issues.


Svein
  Reply With Quote
Old 15th January 2011, 04:10 PM   #7
diyAudio Member
 
Join Date: Sep 2006
Location: Malta, Europe
Thanks Svein and the other posters. For the sake of those who find themselves in the same situation I'm posting how I solved my problem/s. Well, basically it's all based on what you have said in your posts. Swapped the secondary OPT connections, increased the capacitance after the choke by 100uF and added a 100R resistor || with .22uF between the -ve PS wires and chassis ground. The amp is now very quiet with no noticeable hum. As to NFB I soldered a 39K || with the 8.2K resistor for a target 6K8. To my ears hi-freqs are a bit pronounced, but not obtrusive.

Svein wrote:
Quote:
The 8 instead of 16 ohm OPT means that you have a little less feedback (about 70% of the original with 16 ohm tap. You could then reduce the feedback resistor a little for the same feedback ratio. The optimal ratio should be determined by listening.
OK I determined the value without any mathematical reasoning. I thought that reducing it to about 6.8K I would be more or less there. What I can't understand is why we reduce the feedback resistor to increase the feedback. Can someone provide a not-too-complex explanation, possibly with a rule-of-thumb formula.

Thanks, and best regards.
Joe A
  Reply With Quote
Old 15th January 2011, 04:17 PM   #8
Cale06 is offline Cale06  United States
diyAudio Member
 
Join Date: Sep 2009
Quote:
Originally Posted by sonata149 View Post
Can someone provide a not-too-complex explanation
The feedback resistor forms a voltage divider with the unbypassed cathode resistor.
  Reply With Quote
Old 15th January 2011, 04:36 PM   #9
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
I have hooked the amp (without feedback) to my main (expensive) system and while the loud noise is not there, there is a noticeable hum with no program playing.
Quote:
The amp is now very quiet with no noticeable hum. As to NFB I soldered a 39K || with the 8.2K resistor for a target 6K8.
One and maybe most important reason that the hum disappeared is that the NFB is now "on". Asuuming the NFB is, say 18 dB, the hum level is also attenuated 18 dB compared to situation when NFB was "off".

Quote:
To my ears hi-freqs are a bit pronounced, but not obtrusive.
Try to add a 470pF...1 nF capacitor in parallel to 6800 ohms NFB-resistor.
This will attenuate the top discants.
  Reply With Quote
Old 15th January 2011, 04:43 PM   #10
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
Quote:
Originally Posted by sonata149 Click the image to open in full size.
Can someone provide a not-too-complex explanation

The feedback resistor forms a voltage divider with the unbypassed cathode resistor...
...and when the upper resistor of a voltage divider is decreased, the voltage at the junction of the cathode resistor and NFB-resistor is increased.
When this negative feedback voltage is increased at the cathode of the 1st. tube, the gain of the whole amplifier is decreased.
  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
How loud is 250uV white noise? 81bas Solid State 5 9th April 2010 05:43 PM
Carver M-1.5t loud noise on one channel hybridge Solid State 1 25th March 2010 12:09 AM
buzz noise from loud speakers vibeline Multi-Way 9 7th January 2007 06:19 PM
Carver M1.0t Loud "tong tong" noise Visigoth Solid State 9 4th January 2006 12:17 AM
Reduce NFB but keep noise/hum down? K-amps Solid State 12 17th March 2005 09:47 PM


New To Site? Need Help?

All times are GMT. The time now is 04:58 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