USB isolator with Magus?

#1

There is an audible 60 cycle buzz when I plug in my Magus via USB. It’s pretty inconvenient when testing to have to keep switching back and forth between USB and wall power, so I decided to get a USB power isolator. I got this thing:

DSC_0389.JPG492×744 108 KB

However, it does not work. When I plug the Magus into the isolator and the isolator into my Macbook, the Magus does not fully turn on. Instead the screen is blank and all of the lights in the bottom two rows turn on at a uniform half-strength green.

I see in another thread that people have successfully used USB isolators with OWLs. Did I get the wrong isolator? Am I doing something wrong?

#2

Have you tried it with switch on full-speed?

Other than that, it consumes 200mA from USB port. Green LEDs is what I get when insufficient power is provided, too.

#3

Btw the Olimex isolator mentioned in another thread can be powered by external (unisolated) PSU. So it should be possible to use like that, even though this is probably the same as just powering Magus with USB and its own PSU.

#4

though this is probably the same as just powering Magus with USB and its own PSU

You mean plugging in the USB and the DC power at the same time? Does that work? For some reason I thought that was unsafe.

#5

Just consider how many users would have ended by up with fried devices by now if something like that would be unsafe :wink: But yes, that works.

#6

I just tried with an Olimex adaptor and it’s not providing enough juice either without an external power source. Actually the patch runs, but the OLED screen doesn’t…

The Magus draws quite a bit of USB power, up to 500mA to power all those Eurorack level CV points. But yes, running it with external power and USB both connected is absolutely fine.

#7

Right, I’ve measured USB power consumption if it’s powered by USB PSU providing up to 1 A on its ports. Device consumes 450mA according to DMM.

Also, it looks like most affordable USB isolators are built around the same ADUM 3160/4160 chips, meaning that it’s unlikely to find anything providing more power. So we just need a confirmation that isolator + PSU works fine and kills that USB noise.

#8

Did some testing with headphones plugged into the out jack and using a “Silence” patch:

  • Magus powered by USB alone: Very pronounced 60 cycle hum, Macintosh can talk to it
  • Magus powered by DC alone: Basically silent but see below
  • Magus powered by DC, USB plugged in: Sounds same as DC alone, Macintosh can talk to it
  • Magus powered by DC, USB plugged in to Adafruit isolator: Sounds same as DC alone, Macintosh cannot talk to it

So, neat! I can power from wall and plug into USB with no audible noise. I can’t use the Adafruit isolator, but it doesn’t help, so who cares.

Then I discovered something weird…

I did some testing with this patch. This patch takes its input and plays it to the output after a delay; the idea is you can create a delay-echo or looper by connecting the right output to the right input. If you turn down parameter 16 “Monitor output mix” to 0 and listen only to the left output, what you are hearing is the left input fed directly into the left output.

If I run on DC power only, run this patch and plug in nothing except headphones into the “OUT” output, I hear a very loud 60 cycle hum, almost as loud as the USB hum. In other words, when powered from DC power there’s no hum in the headphone OUT, or if there is it’s so quiet I can barely hear it, but if I play the inputs to the outputs with headphones plugged in it appears there is quite audible 60 cycle hum on the inputs. (This is ruinous for the delay looper because when you feed back it layers on itself louder and louder). The hum gets noticeably louder whenever one of the buttons (that is the press-down knobs) is held down.

If this pronounced hum is not seen with other units, one possibility is that the power in my building is just very dirty. The building is very old and there’s only one power plug in this room into which are plugged many electronics. I’m also hearing a pronounced 60 cycle hum on my electric bass and I was assuming there was something wrong with the bass, but maybe the problem is the ground in this building is just noisy. It is also maybe possible(?) that I messed something up when I disassembled and reassembled the unit to upgrade the firmware. I’m using the “mass power” AC adapter that came with the Magus.

Do you have any advice on this? Does the loud input hum with the “pure delay” plugin reproduce on your own units? If the problem is the Magus is failing to clean dirty building power, is there anything I can do about that?

#9

Mine has noisy inputs even when powered from PSU. I don’t think we’re in the same building.

Slightly reducing volumes makes it unnoticable, but that won’t help with feedback problem. Unrelated, but you should make sure that your input from audio channels is processed by DC blocker, you may be getting a DC offset as Magus doesn’t use AC coupled inputs.

#10

Input is powered from Eurorack, but I get buzz even when input is direct out from Magus or there is no input.