Had I been publishing articles in prior years, I undoubtedly would have mentioned my ongoing quest for a better way of enjoying my rockin' tunes. A couple of years ago, I finally replaced my over-a-decade-old Cambridge Soundworks FPS2000 Digital Speakers with a pair of Yamaha HS50M studio monitors fed from the computer via a FiiO E7 USB DAC. The upgrade in sound quality and clarity was astonishing!
A little while later, I added a Yamaha HS10W powered subwoofer to fill in the gap in the audio spectrum. And god damn, does that thing pack a punch. However, throughout all of these changes, there was always a noticeable hum from the monitors (and even the sub) when the music was stopped or at low volume. Eventually, I added a Technics SL-1200MK5 turntable with a BBE RIAA phono amp and a Berenger XENYX 802 mixer into the...mix. Lo and behold, if I unplugged my computer from the mixer and just ran the turntable, there was no hum from the speakers!
For a while, I just put up with the noise. Between being busy working (often away from home) and just being lazy, I let it continue for quite a while. If I turned the music up loud enough, I couldn't really hear the hum anyway. But after my most recent move, the hum became unbearable. I put my basic electronics knowledge to work and started hunting for the cause. My first step was to isolate the possibility of a ground loop. I had been keeping my computer UPS on a separate outlet from my audio equipment, but I rearranged things and plugged both sets of electronics into the same outlet. And what do you know, a lot of the hum disappeared.
Okay, so ground loop eliminated. Right? Well, there was still a very noticeable hum when I wasn't playing anything through the speakers. The hum went away if I disconnected the audio cable between the USB DAC and the mixer. Similarly, if I unplugged the USB DAC from my desktop computer and connected my phone audio out to the DAC line in, I had no hum. So I figured that the USB DAC probably wasn't the source and the mixer definitely wasn't the source. But what about the USB cable from the computer to the DAC?! I noticed it didn't have any chokes installed, so I went to Radio Shack and got a couple of chokes. I applied one to either end of the cable and...nothing. Damn.
Well, there's more than one way to shave a pussy, amiright? Undaunted by the inefficacy of the chokes, I decided to go a different direction. Since adding a low-pass filter didn't work, I started thinking that perhaps the noise was of a lower frequency than I thought. Residual ground loop perhaps? One lesson I've learned well from working with machines that incorporate high-voltage variable-frequency drives and delicate analog sensor signals is that having everything ground together can result in tons of motor noise in the analog signals. And even if we're dealing with items that have separate grounds, sometimes those grounds get inadvertently connected via shield wires across various components. One of my more useful coworkers advises to wire a system without shields and then connect as needed, rather than connecting all the shields and then trying to track down all the ground loops. With that in mind, I decided to perform surgery on my USB cable to sever the shield connection.
Snip snip, clip clip, taking care not to pierce the insulation of the inner wires, I severed the braided shield and clipped it back on either side of the incision. And you know what? It didn't make a damn bit of difference. Well, let's just get crazy and try putting the chokes on the newly unshielded cable! Yeah, nothing doing there either. The hum persisted.
I went back to the googles and saw that some laptop DJs were complaining that their laptops and USB DACs produced noise in their speakers as well -- but only when the laptop was plugged in to mains! Ooh, now we're getting somewhere. When running from smooth DC voltage (ie, a battery pack), these people don't experience hum in their systems. But when plugged in to the wall via that brilliant switched-mode power supply, the hum appears.
I (very) briefly entertained the idea of acquiring a linear power supply for my computer. That was a dumb idea. As I continued to read about people having laptop hum issues, I found that some had been successful in using a USB isolation device between the computer and the peripheral. As USB devices share a ground connection with the computer, any noise being piped into that ground connection (from, for example, a switched-mode power supply) gets passed on the peripheral.
I found a few options for USB isolators online, at various price points and with mixed reviews. And then I came across the Circuits@Home USB Isolator. This device isn't the cheapest of them all, but it's definitely on the lower end of the price spectrum. And even without a case, it looks a lot better than the shrink-taped Olimex offering. But what I liked most about the Circuits @ Home device was the nature of the site. If you read the about page, you'll see that the site is pretty much one dude sharing his electronics knowledge and experience. He blogs about software and hardware, he shares designs for his creations, and he offers a modest amount of products from an online store, sometimes as build-it-yourself kits, and sometimes as built, tested devices. I'm all for supporting a guy like this who shares so much with the maker/hacker community. So I placed an order and waited nervously, hoping this would solve my problems.
Just a short couple days later, the device was in my hands. I didn't want to get too excited, in case I plugged it in and nothing changed. I dug around in a bin for a proper power supply (the device doesn't come with one, but it's got a standard center-positive DC jack with a forgiving 6-25V input range). Then I tracked down a USB A-to-B cable (like printers use) so I could connect the device to my computer. I then plugged my USB DAC into the USB isolator and verified that the computer recognized the device. No problem at all.
Alright, moment of truth. I turned on the mixer and speakers and ... nothing. I guess I forgot to plug something in. NO WAIT, IT'S JUST THAT FUCKING QUIET NOW. I couldn't believe it. This little USB isolator effectively eliminated all noise between my computer and my speakers. I have to nearly max out the level on the mixer before I hear any static, and I wouldn't even consider playing audio at that level (I'd blow something, be it a speaker or an ear drum). This thing was the silver bullet. A+++ would buy again. And finally, I am content with my computer audio. Well, at least for now.
Interested in checking out the results yourself? Here are recordings of my computer speakers without the USB Isolator in use and with the USB Isolator in use. Both clips were recorded at same input and output levels on my DAC, mixer, speakers, and Zoom H1, which remained in the same position in both recordings.