input noise

[teejay246]

Hi

I have a setup in a hall where the input audio device is an archos in a separate room from the amplifier. The archos connects to two of the three phono inputs on a wallplate in the Hall ( The commonly available wallplate is suited also to AV ). The wallplate is connected via a digital screened cable over an eight metre length to a wallplate from which a phono cable connects with the CD input of a high end Panasonic integrated Stereo amplifier. The vid input on the archos is left unconnected and the wallplates are linked only to the two audio phonos.

When the system is switched on, provided that the Archos is "On" and connected to its wallplate, there is no audible noise through the speakers. However if the Archos is disconnected with the amp "on", there is a loud buzzing through the speakers.

When I disconnect the phono lead connecting the amp to the wallplate there, all is quiet.

It might be worth mentioning that one colleague who must have more sensitive hearing than I, can hear background "hum" even when music is played.

I am looking at replacing the cable between the two backplates with a Q.E.D type audio cable - about 8 metres in length. Do you think this will solve the problem?

Your input ( no pun intended ) would be appreciated. Thanks

[jamesperrett]

When the Archos is disconnected you have an open circuit input on the amplifier which will always be prone to picking up noise. If you want to avoid the noise then you could create a pair of shorting phono connectors by soldering a wire link between the tip and outer of the connector and insert these connectors whenever the Archos isn't connected.

 

Replacing the cable may help but on its own isn't really the best solution. You really need to find a way to add balanced inputs to the amplifier which should do more to improve things. A balanced input will help reject interference and also help with ground loops - either of which could be the cause of your hum.

 

James.

[Andrew Edwards]

Digital screened cable?

As in satellite/cable coax?

[ramdram]

Ref #2:

 

My very old Technics amp had phono/RCA shorting plugs supplied with it.

 

You might consider looking here:

 

http://www.hificollective.co.uk/components/rca_sockets.html

 

See this item (apologies for duff format)

 

 

RCA, phono sockets:

 

new product Cardas RCA C SP RCA Shorting Pin Caps

 

Signature Series RCA Shorting Pin Caps (RCA C SP SS) can be used to short out an input RCA. This is desirable on some equipment to lower the noise floor. With the RCA SP SS the user can decide to either short or not short inputs. Generally outputs prefer not being shorted. Billet brass, nickel plated caps fit over unused female RCA connectors to shield against RFI, EMI, dust and corrosion.

 

Ref the hum; this might be useful(?):

 

http://www.epanorama.net/documents/groundloop/

 

http://www.soundonsound.com/sos/1994_artic...roundloops.html

 

Suggest you read up on this topic...before you open your wallet...you might find a cheaper solution.

 

HTH

[teejay246]

Digital screened cable?

As in satellite/cable coax?

 

Actually I think it's called screened serial cable. It was used for a digital remote control unit. 9 leads inside plastic cable each individually screened and all encased together in foil winding. It's the reason I am considereing going to a more conventional audio interconnect over a shorter length.

Cheers

.

 

A concurrent post has been automatically merged from this point on.

 

When the Archos is disconnected you have an open circuit input on the amplifier which will always be prone to picking up noise. If you want to avoid the noise then you could create a pair of shorting phono connectors by soldering a wire link between the tip and outer of the connector and insert these connectors whenever the Archos isn't connected.

 

Replacing the cable may help but on its own isn't really the best solution. You really need to find a way to add balanced inputs to the amplifier which should do more to improve things. A balanced input will help reject interference and also help with ground loops - either of which could be the cause of your hum.

 

James.

 

Many thanks for that. Sorry I am a bit "tick" but how do I "balance" an input

[jamesperrett]

Many thanks for that. Sorry I am a bit "tick" but how do I "balance" an input

 

Use a balanced to unbalanced converter box on the input. They will usually either contain an audio transformer (if they are unpowered) or a small electronic circuit (if they need power). ART sell a range of suitable boxes. You can also buy cheaper ground loop eliminators which are actually small transformers but they often reduce the sound quality a little.

 

James.

[Cheeseweasel]

That's just what you'd expect to happen with a long, unbalanced cable only connected at one end.

 

Ideally you'd use a balanced connection, but there are plenty of AV setups in meeting rooms/classrooms etc that don't use them and don't suffer too badly from hum. A few things to try...

 

1. How is the gain structure set up? MP3 players usually output at less than domestic line level, which is bad news in your situation as it means you have to amplify the signal even more than, say, a CD player (and with it amplify any other hum and interference). Try get as much level out of the Archos as possible (turn off the hearing-protection setting if it has one and turn it up to max). The idea here is you won't have the amp turned up so much, so the hum will be quieter when the Archos is disconnected.

 

2. It may well be that your phono cable is picking up hum from a power cable or transformer. If the cable is running alongside a mains cable it won't be helping matters.

 

3. If you're using proper screened cable, one thing you could do is solder the 'cold' pin of each connector to the cable screen at the amp end of the cable, creating a 'pseudo-balanced' connection. This is partially successful at rejecting interference, but not as effective as using a fully balanced line. I've done this a few times when I've had to use long, unbalanced connections. Don't bother buying fancy-pants QED cables - they're just over-priced hifi-interconnects (just looked at the price - £30 odd for a 0.5m phono!). Buy some proper screened audio cable and make your own.

 

Hope that helps.

[teejay246]

Many thanks for that and to all contributors. It gives me loads to try out.

Best wishes

Teejay