Mains hum!

[wilro15]

After a recent spate of box swapping and general mucking about I have developed mains hum in my system - argh! I'll try to explain what I have done, its a little complicated.

I bought a Speakercraft LL-One automatic line level switch. This takes 2 line level inputs, switches between them automatically and has 1 line level output. The idea is that I can use 1 power amp for both my music and AV setup, thereby only requiring one set of front speakers.

The first input into the LL-One is my Marantz NR1602 using its Pre Out. The second input comes from a Musical Fidelity M1 SDAC using its Pre Out. These both connect to the Speakercraft LL-One.

The LL-One then outputs to a power amp, a Musical Fidelity M1 Pwr. The M1 Pwr is then connected to my front speakers. The LL-One uses the Marantz by default but will switch automatically to the M1 SDAC if it detects a signal.

This setup introduces mains hum from the front speakers. The hum sounds like a low frequency constant note.

 

[wilro15]

I forgot to add that the M1 SDAC is connected to a Mac Mini via USB.

 

[MakkaPakka]

I've had long interconnects pick up hum before - it got worse when my laptop power supply was near them. Not a permanent setup so I didn't really investigate solutions but ferrite clamps maybe or switching to shielded.

 

[drummerman]

I've had long interconnects pick up hum before - it got worse when my laptop power supply was near them. Not a permanent setup so I didn't really investigate solutions but ferrite clamps maybe or switching to shielded.

Ferrite clamps are a good way of limiting RFI/EMI interference. I use them exctensively with all cabling and component internal. The only one I haven't fitted any to it is my external phono stage, must do that.

Interesting to see that Arcam and Cyrus, to name only two (on their 30th anniversary £17'000 system) use ferrite cores. A cheap and effective treatment as is copper/metal shielding of streaming components and digital boards in amplifiers and DAC's as well as iron/steel shields around transformers.

Most effective on my AV receiver (used in 2ch mode exclusively) and my Turntables, especially the SL-7 which had all small signal cabling post tonearm exit copper shielded but even on the cyrus system which has two regulated external power supplies the difference is audible.

Benefits far outweigh the small cost/work involved.

regards

 

[andyjm]

I've had long interconnects pick up hum before - it got worse when my laptop power supply was near them. Not a permanent setup so I didn't really investigate solutions but ferrite clamps maybe or switching to shielded.

Ferrite clamps are a good way of limiting RFI/EMI interference. I use them exctensively with all cabling and component internal. The only one I haven't fitted any to it is my external phono stage, must do that.

Interesting to see that Arcam and Cyrus, to name only two (on their 30th anniversary £17'000 system) use ferrite cores. A cheap and effective treatment as is copper/metal shielding of streaming components and digital boards in amplifiers and DAC's as well as iron/steel shields around transformers.

Most effective on my AV receiver (used in 2ch mode exclusively) and my Turntables, especially the SL-7 which had all small signal cabling post tonearm exit copper shielded but even on the cyrus system which has two regulated external power supplies the difference is audible.

Benefits far outweigh the small cost/work involved.

regards

Ferrites may work for RFI (radio frequency interference), but mains hum is 50Hz and by no means RF - ferrites only work at much higher frequencies.

The problem with mains hum is it is in the frequency range of the desired signal - you can't filter it out without removing parts of the signal you want to keep. You need to avoid it in the first place.

In the case of the O/P, try disconnecting one device at a time until the hum stops. You have then found the culprit. It may be a dodgy cable, or a device with inadequate shielding.

 

[drummerman]

Yes, I should have added that ferrites will not address the issue of mains hum.

regards

 

[Covenanter]

In any interconnected system you can get mains hum if any of the elements has a slightly different earth than the other elements. It only needs to be a few ohms of resistance and that can arise from something as simple as a poor connection. So the advice given above is good. Try to identify by elimination which element is causing the hum. Then try to improve the earth connections of that device by cleaning and tightening any earth connections you can see. Sadly that might not solve the problem because it can be inside a soldered connection (what is known as dry joint). If it persists that you will need technical assistance.

They can be b****ers to sort out!

Chris

 

[wilro15]

It seems to be the switch itself causing the problem. I don't think I can do what I am trying to do anyway. Back to the shop it goes.

 

[GCE]

Try to plug all your component in a socket without earth

 

[davedotco]

Try to plug all your component in a socket without earth

Not quite.

First simply go back to a single amp system, the A/V amp say. Play as normal and check that there is no hum.

Then add the switching box but not, as yet, the second amplifiers. Does it hum?

If it does then disconect the earth on this amplifier at the mains plug.

If, as I suspect, it does not hum until you add the second amplifier then drop the earth on that.

Ie only one of your amplifiers connected to earth at the mains plug.

Try that and report back.

 

[GCE]

........ First simply go back to a single amp ......

If, as I suspect, it does not hum until you add the second amplifier then drop the earth on that.

Ie only one of your amplifiers connected to earth at the mains plug..........

...........Try that and report back.....

I agree..