Noise & attenuation - 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 30th March 2010, 02:41 PM   #1
diyAudio Member
 
Alfetta87's Avatar
 
Join Date: May 2003
Location: Alsace
Default Noise & attenuation

Hi there,

I bought some time ago a preamp kit from asia using the 6N3 tube, with a valve PSU.
I'm really happy with the sound, although I have some issues:
- I have a 50Hz hum noise, whatever I do, central grounding, separate PSU for heater and HV, and even with the simplest wiring (no input selector) I can hear it, especially when the volume pot (100k log. mounted before the preamp.) is in the middle position.
- The output level is way to high; at 9 o'clock, I have already full power, and the potentiometer is not so accurate at that low position.

My thoughts: the hum is not extremely high, so when I'd attenuate the output level by let's say 10dB, or even more, both problems would be resolved I guess.

At the output I'm driving a Lundahl LL1547 to connect an XLR output socket.

Questions:
Where should I place the attenuator: just after the output stage, and before the transformer?
.. or after the transformer?
What would be the right values for the serial/ parallel attenuator to preserve the output impedance (a friend of mine calculated about 600 ohms)? I'm running it with an UcD400 amplifier.

Thanks,
- dan
__________________
It is worth to keep your mouth shut and let people think you’re stupid; rather than to open it and thus eliminate any doubts...
  Reply With Quote
Old 30th March 2010, 03:01 PM   #2
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
I would insert a resistor in series with the potentiometer.

A 220K resistor should get you approx 10dB attenuation - though both this value and the pot value seem high - a 47K resistor/47K pot should work better.
  Reply With Quote
Old 30th March 2010, 04:12 PM   #3
diyAudio Member
 
Alfetta87's Avatar
 
Join Date: May 2003
Location: Alsace
OK thanks, but my potentimeter is BEFORE the stage - thus I can attenuate the signal but not the hum.
__________________
It is worth to keep your mouth shut and let people think you’re stupid; rather than to open it and thus eliminate any doubts...
  Reply With Quote
Old 30th March 2010, 04:35 PM   #4
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
Ouch.

Is the filament supply AC or DC?
  Reply With Quote
Old 30th March 2010, 10:29 PM   #5
diyAudio Member
 
Alfetta87's Avatar
 
Join Date: May 2003
Location: Alsace
DC, regulated (LM317).
__________________
It is worth to keep your mouth shut and let people think you’re stupid; rather than to open it and thus eliminate any doubts...
  Reply With Quote
Old 30th March 2010, 10:36 PM   #6
jrenkin is offline jrenkin  United States
diyAudio Member
 
Join Date: Nov 2007
Location: Denver, Colorado
I am no expert, but it seems that the hum must be either a grounding problem or coming in from your source. Where would 50Hz come from? Not your power supply (should be 100Hz, right?), not your regulated DC heaters (100Hz and minimal). I would recheck the grounding scheme, especially the mains grounding and power trafo grounding and then try a different source and finally look at the transformer orientations. Just some thoughts.
  Reply With Quote
Old 31st March 2010, 05:21 AM   #7
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
On either side of the transformer, the output impedance will be affected by the L-pad, and a receiver will affect it once again. Plus this way, you are essentially throwing away gain, which is kind of suboptimal.

Where is the cold terminal connected to and where is the ground? Do you also get the hum with no source connected and the pot set to zero? If the hum varies with pot position and the pot is before the stage as per your first two replies, I would think of eliminating the stage as the source of the hum.

It does sound like a ground loop to me if we can rule out the tube supplies. High input impedance + high pot values + XLR wiring can all lead to hum issues.
  Reply With Quote
Old 31st March 2010, 06:37 AM   #8
diyAudio Member
 
Alfetta87's Avatar
 
Join Date: May 2003
Location: Alsace
Tried everything, transformer orientation, ground loop (any ground connected to a single point), even w/o wiring the XLR socket or even the output trannies.
Hum is minimal when the pot is in zero or max position, and increases when middle position.

But the hum is not of high level, thus my thoughts to attenuate the output... no way to do this without increasing the output impedance?
__________________
It is worth to keep your mouth shut and let people think you’re stupid; rather than to open it and thus eliminate any doubts...
  Reply With Quote
Old 31st March 2010, 07:03 AM   #9
danzup is offline danzup  Romania
diyAudio Member
 
danzup's Avatar
 
Join Date: Oct 2007
Location: Bucuresti
did you ever think that hum come from not enough anodic filtration ?
Just add condenser on +B voltage and see if thing are improving .
__________________
There are not better tube than 6P3S and 6N2P for my ears !
  Reply With Quote
Old 31st March 2010, 12:33 PM   #10
diyAudio Member
 
Alfetta87's Avatar
 
Join Date: May 2003
Location: Alsace
Quote:
Originally Posted by danzup View Post
did you ever think that hum come from not enough anodic filtration ?
Just add condenser on +B voltage and see if thing are improving .
I tried this,but no improvement.
__________________
It is worth to keep your mouth shut and let people think you’re stupid; rather than to open it and thus eliminate any doubts...
  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
Input attenuation on Leak amp patrickamory Tubes / Valves 15 16th July 2007 06:52 PM
Adding extra attenuation to S&B TX-102 mlvp Everything Else 10 13th July 2007 01:32 PM
headphone amp attenuation elementx Headphone Systems 1 31st January 2006 05:34 PM
Attenuation Box for Guitar Amp Keeso Instruments and Amps 7 3rd August 2005 11:39 PM
How to put fixed attenuation between preamp and amp. Lpad or just resistor in series? PUCK Parts 1 19th November 2003 04:59 PM


New To Site? Need Help?

All times are GMT. The time now is 09:18 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