Speaker hum problem

wuzzo

668- the neighbour of the beast
Hello,
I have two PCs for recording, one with an E-mu 1820m sound card linked to a pair of Fostex active 5" speakers and another one with a Roland Quad-Capture sound card linked to a pair of Fostex active 4" speakers. Both setups work perfectly well. However, I recently tried to link the Quad-Capture to the Fostex 5" speakers and they hum like crazy. What could be causing this ?
 
Are they plugged into same electric circuit with grounded plugs? Hum usually comes from a ground loop problem.
 
In addition to power ground problems I find that connecting an unbalanced output to a balanced input can cause pretty bad hum.
 
Are they plugged into same electric circuit with grounded plugs? Hum usually comes from a ground loop problem.


Well, the Quad-Capture is USB powered- although its PC is powered by the same circuit as the 5 " speakers. However, the Quad-Capture's previous machine was plugged into the same circuit as the 4 " speakers- and they didn't hum. All the circuits are well grounded, yes.
 
In addition to power ground problems I find that connecting an unbalanced output to a balanced input can cause pretty bad hum.

I'll look into that, thanks, - although I'm pretty sure that none of my gear is ' balanced '.
 
Monitors are susceptible to hum if they're connected with unbalanced leads. I had the same problem with my KRK Rokit 8 monitors until I switched them to balanced. Now they're dead quiet and sound great.
 
I will obtain a pair of balanced leads- but that does not explain to me why the 5" should hum with the Quad-Capture and not with the 1820m- using the self-same leads.
 
I will obtain a pair of balanced leads- but that does not explain to me why the 5" should hum with the Quad-Capture and not with the 1820m- using the self-same leads.

I just looked them up and both of your interfaces have balanced output. It may be that different ways of doing the balancing interact differently with your unbalanced connection.

Interestingly, the E-mu manual specifically says to use unbalanced cables to connect to unbalanced devices to avoid noise problems.

I looked up "Fostex 5"" and there were several models so I couldn't determine if the inputs are balanced. What is the exact model number?
 
I just looked them up and both of your interfaces have balanced output. It may be that different ways of doing the balancing interact differently with your unbalanced connection.

Interestingly, the E-mu manual specifically says to use unbalanced cables to connect to unbalanced devices to avoid noise problems.

I looked up "Fostex 5"" and there were several models so I couldn't determine if the inputs are balanced. What is the exact model number?

They are PMO.5n . I tried the Quad-Capture and the Fostex 5" with very good quality Lynx 2,5mm speaker leads- but they still hummed. They are silent when connected to the 1820m. It would be easy to write off the Quad-Capture as cheapo kit- but there's no hum when it's used with the Fostex 4" .

Thanks for your time, incidentally. Appreciated. I've reverted to using the 1820m but it's showing its age, I think.
 
I tried the Quad-Capture and the Fostex 5" with very good quality Lynx 2,5mm speaker leads- but they still hummed.

Speaker leads? Like this?That cable is for connecting an amplifier to a passive speaker. Your active speakers have the amps in them and so you need to use shielded line cables, preferably balanced. That could be the cause of the problem.
 
Speaker leads? Like this?That cable is for connecting an amplifier to a passive speaker. Your active speakers have the amps in them and so you need to use shielded line cables, preferably balanced. That could be the cause of the problem.

Could you post a link to these so that I can be perfectly sure of the kit you're describing ?
 
This is the cable I found using your description "Lynx 2,5mm speaker leads", available with 1/4" TS phone (6.3mm jack) connectors:

Lynx 2.5mm Speaker Cable, 72 x 0.2mm Strands | Andertons

This is the type of cable I think you need (select the appropriate length):

6.3mm TRS Jack - to Jack Balanced Patch Cable 2m | Andertons

Speaker cables have thick conductors side by side. Line cables have either a single conductor or a twisted pair of conductors inside a wire mesh or foil tube (the "screen"). They are fundamentally different in construction and application, even if they sometimes look superficially similar.
 
OK, I've ordered a pair. Wouldn't you think that cable type would be specified when buying active monitors ?

Incidentally, these are stereo. Should I have been using stereo jacks from the outset ?
 
I'm not sure what you mean by the monitors being stereo. The cables that jimmys69 confirmed are the correct ones are balanced lines, whereas the ones the you had were not. Though they use the same jacks as, for example, stereo headphones, they're not really stereo -- here in the States, we call them "TRS jacks," for "Tip, Ring, Shield." Here's a Wikipedia article that will tell you more than you want to know about balanced cables: Balanced audio - Wikipedia, the free encyclopedia

The short version is: balanced cables greatly reduce noise from external sources.
 
A stereo cable has 2 leads that carry the left and right signal and a common that both channels share. The common is usually a shield. A balanced line has two leads, one which carries the signal, the other which carries the signal inverted and a shield that is used as a ground or earth. As long as the stereo cable is shielded it will work as a balanced cable.

The vendor describes the cable as "stereo jack," not as as "stereo." In other words, the cable uses TRS jacks, instead of XLR jacks, which are the traditional connectors for balanced lines.
 
OK, I've got that now. Thanks very much for the explanations. I'll get back when I've fitted the new active monitor leads. ( There, that wasn't too difficult a description, was it :facepalm: )
 
"Stereo" is just one application of TRS. TRS can also carry a balanced mono signal, unbalanced send and return as in an insert or even two control signals as in a guitar amp switch pedal. And the cable is different. Stereo and insert cables should have two separately shielded coax cables inside while balanced should have a twisted pair inside a common shield.

Just to add to the confusion, the term "jack" means something different in the UK than it does in the US. In the UK a jack is any 1/4" connector. Here in the US a jack is a generic term for socket or female connector while a plug applies to almost any kind of male connector. "Jack socket" makes perfect sense in the UK but it's a contradiction in terms in the US.
 
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