Having run into what Koz loves to call “frying mosquitoes” with my Behringer UCA-202 integrated into my Home Theater PC & Stereo system, I did a bit of searching to see if there were any products that might help to isolate USB audio interfaces from electrical computer noise. Since there are numerous posters on this forum having this noise show up in what are otherwise well regarded USB interfaces, I figure my findings might be of interest here.
If you do a Google search for “USB isolator” you’ll get a ton of hits. The vast majority of these are quite expensive and intended for industrial applications were you need to put a USB device in an electrically hazardous location (like on 480 Volt building mains). However I found Amazon selling this device:
The price was reasonable so I purchased one. (Though I’ll admit it’s more expensive than the UCA-202 it’s helping).
Being an Engineer the first thing I did after getting it was to open it up (which is not hard, it has a very simple snap-together plastic case):
It appears to have been reasonably well designed. My unit appears to have been hand-soldered, so they are not mass-producing these.
As you can see there really isn’t too much to the unit. At it’s heart is an Analog Devices ADUM4160 ( data sheet for the engineers out there: http://www.analog.com/media/en/technical-documentation/data-sheets/ADuM4160.pdf) This unit is an integrated USB isolator that uses transformers to couple the USB data from the input side to the output side. The device is rated of many hundreds of volts between the output and input sides.
To provide power to the down-stream side a transformer coupled 5V to 5V DC-DC switching converter is used (the larger rectangular block at the top of the board). The converter is rated at 200 mA which is probably enough for most smaller audio interfaces. (For example the Blue Yeti is rated at 150 mA by its spec sheet) But this is a fair bit short of the 500 mA that a USB 2.0 port is expected to be able to supply, and so might not be enough for some devices.
On the downstream side of the DC-DC converter there is a simple LC filter – 4.7 uH and 22 uF. This does little to stop audio frequencies (more on that in a bit) but does provide a fair amount of additional isolation from the 100 KHz switching frequency of the converter. The 5Vs is regulated down to 3.3V for the output side of the Analog Devices part.
Having a look at the bottom side of the PC board:
You will see there is a neatly milled slot in the PCB under the center of the Analog devices unit and the DC-DC converter. Also the bottom layer of the PCB is only used to provide ground planes on input and output sides (and the two are well isolated).
So my initial test was to put the unit into my HTPC setup to see if it would cure the issue at hand and the answer is “Yes! it does”. Unfortunately I’m coming up against the 4 attachment limits, so I’ll report on my testing in a followup post.
My first test with the USB isolator was to install it in series with the UCA-202 in my Home Theater PC and stereo system. Here are two recordings from the UCA-202 (with the stereo receiver’s phono input selected and muted by the turntable).
Both recordings have been amplified 50 dB from the original raw recording to make the noise level evident. Audibly you’ll see that the whine is quite apparent in the test without the USB isolator and not audible in the test with the isolator. Checking with the spectral analysis tool shows that the 1 kHz component is still there in the second recording but it has been lowered about 12 dB.
Now I should stress that the reason the unit helps in this situation is that without it there are multiple ground paths between the UCA-202 and the stereo amplifier. There is the direct path along phone cables between the UCA-202’s inputs and outputs and the amplifier, but then the amplifier is also connected to a TV, and a DVD player. Both of which are in turn connected to the computer both by video cables and by antenna cables. It is the multiple ground path situation that causes the noise level in the UCA-202 to rise to this level and by breaking that connection on the USB port with the isolator it gets better.
I did some more tests with the USB isolator on my workbench. I injected a 1-10 kHz signals directly into the USB power on the input side of the isolator unit, and then looked for those signals in the USB power on the output side of the unit. Lo and behold signals at those frequencies passed through pretty much unscathed. Which is unfortunate, I had hopes that the regulator in the DC-DC converter might have enough bandwidth to provide some suppression. The RF frequency “hash” on the USB power supply is very well suppressed by the combination of the converter and the LC filter.
I rigged up a simple test to determine the capacitance between the input ground and the output ground of the unit, and came up with a value of about 45 pF. The Analog Devices unit is spec’d at 2.2 pF so the bulk of that must be the DC-DC converter.
So the this device probably isn’t a “cure all”. It will certainly help in any situation where multiple grounding paths are the problem. It might help in situations where the USB device is suffering from high levels of RF noise on the USB power supply. I think it could probably be significantly improved by upping the DC-DC converter so that it can supply something much closer to the 500 mA that USB 2 allows and at the same time add wider-bandwidth active regulation or filtering to improve the blockage of audio frequency noise riding on the USB power.
I don’t have enough information to say, and would actively solicit anyone with a mosquito infested Yeti to give this device a try and tell us. The listing does include the statement “Full refund if it turns out not to work with your equipment. 1 year warranty”.
It seems unlikely that problems with a Yeti would be due to multiple ground paths (unless the yeti has metal body and you put it on a metal desk).
If the problems are being coupled by RF noise then it should help considerably. But if the problem is audio-frequency noise in the USB power then I would not expect it to help much at all.
One other note. This device is 12 Mb “Full Speed” USB1.1 ONLY. It will not work with low data rate devices like keyboards or mice. It will also not work with higher speed USB 2.0+ devices. This is a limitation of the AD part – it does have a hardware-selection for 1.5 Mb/sec vs 12 Mb/sec, but that selection has been hard-wired to 12 Mb/sec in this unit.
Actually it’s the exact same chip. Only difference between the ADUM3160 and the ADUM4160 is the voltage the isolation is rated for.
They have used a somewhat beefier DC-DC converter (400 mA vs 200 mA) so that might be a good thing, hard to tell if it offers better isolation from AF noise on the USB power or not without buying one for test. Wonder if they have a US distributor.
They claim they do, but when I tried to use it, they shunted me off to an Asian location, whose shipping is, oddly enough, cheaper.
And yes they did say in the dialog that the restricted list of equipment this will help with is an ongoing problem. No, you probably can’t run your USB3 popcorn popper.
Hi Greg,yes the USB isolator can remove those
kind of noises. The drawback is that it doesn’t work with
all devices, as it doesn’t support USB High Speed 480mbps
which is required by DACs that accept sample rates above
96/24 or multi channel dacs with preamps. There are no other
isolators that we are aware of that supports USB High speed
480mbps. Note USB Full Speed 12mbps is often confused with
High speed 480mpbs. Both are (or can be) USB 2.0.
some reviews and comments on amazon Amazon.com
but as you can see, it has mixed reviews, as it doesn’t work with all equipment. We are
trying to communicate better what equipment it is not
expected to work with.
My product has shipped. Hongkong Post Tracking Number:
