hum problem

yaniv · 25th March 2007

Hi,

I have a PM-1000 strip.
I connected its output unbalanced to my fireface400 and I have hum :-(
but when I connected it directly to phones I don`t have any hum.

I also look at the output with scope , and I saw that the hum occures only when I connected it to the fireface ... when it connected to phones the scope shows "quiet" line..

all the power supplies are from the same source ....

Can someone help me ?!

Matt Syson · 25th March 2007

hi
It is a 'standard' earth (ground) loop problem. Search the many threads about this and you will find suggestions. You sat a '1000 strip, have you made this yourself? While the power supply metalwork MUST be earthed, the analogue ground used by the module does not have to be tied directly, so you could use a pair of 'back to back' diodes and a small capacitor or a 10 ohm resistor (3W wirewound) to link the analogue ground to mains earth on the strip. If you don't understand this, find someone who does.

Matt S

yaniv · 27th March 2007

I attched a draw of what you describe ... did i understand you right ?

also, about the small capacitor , how small should it be ? and if I understand it shouldn`t be electrolitic because of its polarity ?

thanks A lot !

Matt Syson · 28th March 2007

Hi
Sorry no it is incorrect. You have drawn all in SERIES. They should all be in PARALLEL. The capacitor should be around 0.1 to 1uF polyester. As it is in parallel with the dides the maximum voltage across the capacitor would be about 0.7 Volts.
You could add a resistor in parallel as well say 22 ohms but leave this out unless you have problems.
What this arangement does is tie the audio ground to safety ground within 0.6 volts for safety, and at HF due to the capacitor but at low frequencies there can be up to 0.6 volts difference so preventing ground circulation currents.
Diodes 1N4001 type or 1N5401 (anything power type).
Matt S

25th March 2007	#1
yaniv Gear interested Joined: Dec 2006 Posts: 21 Thread Starter	hum problem Hi, I have a PM-1000 strip. I connected its output unbalanced to my fireface400 and I have hum :-( but when I connected it directly to phones I don`t have any hum. I also look at the output with scope , and I saw that the hum occures only when I connected it to the fireface ... when it connected to phones the scope shows "quiet" line.. all the power supplies are from the same source .... Can someone help me ?!

25th March 2007	#2
Matt Syson Lives for gear Joined: Sep 2004 Location: UK Posts: 4,822	hi It is a 'standard' earth (ground) loop problem. Search the many threads about this and you will find suggestions. You sat a '1000 strip, have you made this yourself? While the power supply metalwork MUST be earthed, the analogue ground used by the module does not have to be tied directly, so you could use a pair of 'back to back' diodes and a small capacitor or a 10 ohm resistor (3W wirewound) to link the analogue ground to mains earth on the strip. If you don't understand this, find someone who does. Matt S __________________ Matt S www.mseaudio.co.uk

27th March 2007	#3
yaniv Gear interested Joined: Dec 2006 Posts: 21 Thread Starter	Thanks I attched a draw of what you describe ... did i understand you right ? also, about the small capacitor , how small should it be ? and if I understand it shouldn`t be electrolitic because of its polarity ? thanks A lot ! Attached Thumbnails

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread starter	Forum	Replies	Last Post
Frustrating hum problem, please help!	scottrbishop	Geekslutz forum	11	6th March 2007 10:42 PM
Distressor Hum Problem	JB872	So much gear, so little time!	10	12th July 2006 05:35 PM
Patch Bay Hum Problem	Caldo71	So much gear, so little time!	5	23rd February 2005 05:21 AM
hum problem - help needed	spherop	So much gear, so little time!	12	26th November 2003 08:30 PM
sm57 hum problem	I.G.M.	The Moan Zone	0	3rd August 2003 12:24 AM

28th March 2007	#4
Matt Syson Lives for gear Joined: Sep 2004 Location: UK Posts: 4,822	Hi Sorry no it is incorrect. You have drawn all in SERIES. They should all be in PARALLEL. The capacitor should be around 0.1 to 1uF polyester. As it is in parallel with the dides the maximum voltage across the capacitor would be about 0.7 Volts. You could add a resistor in parallel as well say 22 ohms but leave this out unless you have problems. What this arangement does is tie the audio ground to safety ground within 0.6 volts for safety, and at HF due to the capacitor but at low frequencies there can be up to 0.6 volts difference so preventing ground circulation currents. Diodes 1N4001 type or 1N5401 (anything power type). Matt S