Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

The 120Hz Blues...
The 120Hz Blues...
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
Thread Tools Search this Thread
Old 24th February 2008, 02:25 AM   #1
kff322 is offline kff322  United States
diyAudio Member
kff322's Avatar
Join Date: Mar 2006
Location: Berkeley Heights, NJ
Send a message via AIM to kff322
Question The 120Hz Blues...

Hey guys! Ive been having a bit of trouble with a Headphone Amp Ive been building mostly an evil 120Hz Hum.
But First A little Background

- The headphone amp is based on the "Optimized" Morgan Jones Amplifier Schematics outlined here. >> http://www.headwize.com/projects/cmoy5_prj.htm

- The actual amplifier works without the Hum when tested on a high voltage bench power supply.
The Hum started when I added my power Supply. (Schematics also on that site)
Xformer = Hammond 250-0-250
(B+) Xformer >Full Wave> 220uf > 3K 10w >220uf >2H >100uf
(Filament) Xformer > Bridge > 4700uf > Tubes

Things Ive Tried to fix this
- Moving Wires, particularly making sure the inputs arent near the main Xformer.
- Running it from my Variac or just Mains makes no difference.
- Checked Grounding which uses a central point isolated from the chassis.
- Earth-grounding the chassis leads to small hum reduction.
- Moved B+ to Bench Powersupply, no difference - I had a similar problem like this on a previous project, was fixed when I grounded the Center tap from my Filament Xformer though it wasent nearly as bad. Tried Grounding the Negative of the Filament bridge rectifier to Main Ground, no difference.

Things I will try tomorrow.
- See if changing the filament supply to unfiltered AC makes a difference.
Other then this, I am out of ideas that would make this a simple fix.

Thanks for any help!
  Reply With Quote
Old 24th February 2008, 06:04 AM   #2
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
SHiFTY's Avatar
Join Date: Jul 2002
Location: New Zealand
Is your filament supply grounded? The best way to do this is two 47 ohm resistors in series across the tube filament, then ground the junction of the two resistors.

Your power supply should generally have the largest capacitance last to avoid oscillation. Try full wave (diodes)->100uF->2H->220uF->3k->220uF, that seems to be the usual way to have it.

Also you could use a second stage in the filament supply- like transformer->bridge->X ohms->1000uF->Y ohms->4700uF

Where X is a small resistance to limit current surge and Y is slightly bigger, but not big enough to drop the voltage by too much. Trial and error required...
  Reply With Quote
Old 24th February 2008, 06:26 PM   #3
kff322 is offline kff322  United States
diyAudio Member
kff322's Avatar
Join Date: Mar 2006
Location: Berkeley Heights, NJ
Send a message via AIM to kff322
Yeah it was the filament supply, It runs dead quiet on AC which is good enough for me.


DC is used to make things quiet. More trouble then its worth I quess. Or I just need a filament Transformer with a ground-able center tap...

  Reply With Quote


The 120Hz Blues...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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
ReVox G36 15ips mod (100/120Hz) Steerpike Analogue Source 0 1st August 2009 01:38 PM
5dB dip at 120Hz in my speakers? cjv998 Full Range 12 21st September 2007 11:10 AM
Is it reasonable to have a sub go from 20 to 120hz? bluegti Subwoofers 5 18th September 2006 01:13 AM
powering up old amp-120hz at output? portlandlay Solid State 4 11th December 2005 04:36 AM

New To Site? Need Help?

All times are GMT. The time now is 05:52 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio