Arcam DAC - Mains Hum When Connected to iMac

[Kellytm272]

Hi everyone, hoping for some advice.

I recently purchased the Arcam FMJ D33 Super dac at a bargain price (new).

It is connected to my imac via usb, and is plugged into the same 6 way Tacima mains power block as the iMac.

It is connected to my Tisbury Audio Passive Pre amp via 5m long rca cables, which in turn are connected to Cyrus Mono X pair and Dynaudio DM 2/6 speakers.

The problem I'm having is that I get a mains hum through my speakers, the second that I connect the usb cable to the usb 2.0 socket on the dac. Through the usb 1.0 socket on the dac, I don't have the problem.

Reading the Arcam manual, it states that the usb 1.0 socket is galvanically isolated, whereas the usb 2.0 socket is not.

I have so far tried 3 different approaches to fixing the problem:

1. Cutting the power cable in the usb cable. Made little difference.

2. Also cut the ground cable in the usb cable, so that only the data wires are working. Made no difference, in fact the connection became unreliable.

3. Purchased an Aqvox USB Isolator - which supposedly cuts the usb power and replaces it with direct mains power, as well as supposedly helping with ground loops. This made no difference at all.

Does anyone have any experience of this type of problem, any proposed solutions?

For the record, I previously owned an Audiolab Q Dac, which did not exhibit this problem at all - setup was the same otherwise.

Thanks

Tim

 

[Chris Firth]

Computers are electrically noisy things.

Does your iMac have a S/PDIF output?

If it does try it.

USB connections can be plagued with issues. In truth they're not the best way to shovel music data from your computer into a DAC.

The S/PDIF from most Macs tends to be via the headphone output - it's a dual use socket that has an optical transmitter inside it.