Hum

Posted by: docmark on 28 March 2018

Hello - I have an Astell & Kern SP-1000 player.  Currently I use a stereo mini out of it, to a cable terminated with RCA, into Analogue 2 of my 272.  Sounds great.  However, I want to try optical out from the DAP to a DAC.  When I unplug the cable from the headphone/optical out of the DAP, I hear an immediate hum from my system.  This is even with the DAP not playing, completely turned off.  When I plug the RCA to mini cable back in, the humming stops.  it's not very loud but it is annoying.  Any idea what might be going on?

Posted on: 28 March 2018 by docmark

Anybody?

Posted on: 29 March 2018 by james n

It's normal - the now un-terminated cable will cause a small amount of hum if that input is selected when you disconnect it from the DAP (high impedance input on 272, cable picks up stray fields when open). If you touch the tip of the 3.5mm plug it will get louder and as you found, this will go away once plugged in to your DAP.

Posted on: 29 March 2018 by docmark

Hi James - thanks for your input.  Would deselecting that particular analogue input on my 272 be enough to get rid of the hum?  Or is it more involved than that?  I suppose, too, that once there is music playing, I really won't hear the hum.

Posted on: 29 March 2018 by james n

Hi Mark - Yes just deselect it. If you are going to use the optical input instead then it's best to disconnect the RCA>3.5mm cable from the 272. It won't cause any harm if you don't, but it's good practice to have cables you're not using  unplugged from the pre-amp.

James

Posted on: 29 March 2018 by Huge

James is absolutely right - unplug any unused cables from the 272.

The optical fibre won't cause hum if you unplug it from the AK, but still leave it connected to the 272 (it's a fibre, not a cable - so no electrical contact to transmit stray fields).

Incidentally if in the end you decide to stick with an analogue connection, using a 3.5mm jack to DIN5 would be significantly better than 3.5mm jack to RCA phono and is likely to have less of a problem with hum.