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 4th April 2008, 07:46 PM   #1
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Default Weird Input transformer hum.

Hi all,
I am reaching the end of the development of my latest amp. After a few ups and downs it seems to be panning out well.
However I have a weird hum issue. Let me describe the input circuit a little. I have input transformers with the input floating on the primaries. The secondaries are biased at -300V via a negative rail, and drive both inputs of a LTP. The problem is I have some buzzing hum. This goes away if I touch the sleeve of the input phono plugs. It doesn't respond to grounding the input sockets to the case at the case. I haven't yet attempted to ground the sockets to the main star ground. This seems to be the opposite of the usual situation where touching the screen induces hum.

Has anyone got any ideas whats going on.

Shoog
  Reply With Quote
Old 4th April 2008, 09:13 PM   #2
diyAudio Member
 
pmillett's Avatar
 
Join Date: May 2005
Location: Dallas (but I am not a Texan!)
Is the buzzing there with a source connected, or without?

I've had some similar issues using input xfmrs. With the input floating, if I connected a source (CD player) that had a 2-wire AC line cord, I got buzzing. It went away if I grounded one side of the primary. I think the source was the EMI filter on the (ungrounded) CD player inducing common-mode AC onto the floating "ground" and output of the CD player, which was then getting electrostatically coupled across the input transformer.

A different CD player, which had a real ground connection, didn't cause this type of noise.

Also had buzzing issues with nothing connected. I believe this is just that the input transformer, with an open primary, is an incredibly sensitive electromagnetic transducer. It was picking up noise from the power supply, and also anything else nearby. Terminating the primary in any reasonable source resistance eliminated it.

Pete
  Reply With Quote
Old 4th April 2008, 09:13 PM   #3
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Problem solved - the earth connection wasn't good enough. Absolutely quiet now.

The only remaining problem I have is that the gain is to high. I could do with losing about half the gain, but I already have the transformer configured as a 2:1 step down.

Shoog
  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
weird ef86 input stage in a Philips amp honinbou Tubes / Valves 4 5th August 2009 05:37 AM
Input transformer: floating input = distortion ErikdeBest Tubes / Valves 13 20th January 2009 09:36 AM
Weird Quad II mains transformer help hd38 Tubes / Valves 2 12th June 2007 02:36 PM
Identifying weird signal transformer Klimon Tubes / Valves 4 28th August 2006 12:50 PM
weird passive transformer problem sef Everything Else 0 3rd November 2003 06:05 PM


New To Site? Need Help?

All times are GMT. The time now is 10:10 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