7055-C or 7055-B?

[simonom1]

I am getting an intermittent, low-level “chuffing" sound, a sort of fast and steady "shh-shh-shh" noise, from my computer through my speakers (iMac>Denafrips Iris DDC>Denafrips Pontus DAC>Eastern Electric preamp>Meridian power amp>Harbeth speakers). Sometimes it happens when my audio system is first switched on, and then it dies away after 5 mins., and on other more bothersome occasions, it occurs only when music is playing and can be terminated by switching the DAC off and on or, more quickly, changing the DAC’s phase setting.

I am currently using a Hifime Hi-speed isolator between the iMac and DDC, and that has definitely improved the situation, because without it there were times when the noise was both invasive and unstoppable without rebooting the computer (other than pulling out the USB cable, of course, from the computer).

Before the Hifime isolator, I tried the Topping HS01 isolator, which had zero affect on the problem.

The Hifime isolator provides 3kV RMS isolation; the Topping one 1kV RMS. The 7055-B provides 5kV RMS, which makes me think it might cure my problem. Do you agree or do you think I would be better buying the lower-noise 7055-C, even though it provides only 1kV RMS isolation between it and my iMac's USB 3.0 socket?

Thank you in advance,
Simon O'M

[Daniel (Intona)]

It looks like isolation won't help and an intona isolator won't help either. The error description sounds like a synchronization problem. The iMac outputs a packet of audio data every 125 us. The amount of data varies and adapts to the clock of the device (here your Iris DDC). To do this, the device must send information to the Mac. More precisely, how many samples have passed during two USB transactions. And this doesn't seem to be working properly for you. Sometimes the cable is bad. Audio-grade USB cables are notorious for this. Try a printer-grade USB cable from the supermarket. In most cases, however, this is either a configuration problem (audio-midi control app on the Mac, settings in the device) or a bug in the firmware in the device. Looks like the device uses a stock XMOS chip/firmare. Old versions have (to me) known bugs. Apple is less likely to be at fault, as we have developed OEM devices that can transfer 256 channels at 96 kHz bidirectionally on USB and work flawlessly on any Mac.

If you still want to try it, take a 7055-C.

[simonom1]

Thank you, Daniel, for your detailed and very helpful reply. It's much appreciated.
I'll try changing the USB cable, although what I currently have can hardly be described as fancy; it's the entry level Audioquest one.
I've read some reports on the web about synchronisation issues with the Pontus. That would be more problematic, if that is what I'm dealing with, because I like the sound the DAC.
I might yet try the 7055-C, if only to see if I can get the results some reviewers have reported. That would be nice! For now, I'll continue to monitor the situation.
Kind regards,
Simon