You are hereHomeSound Advice Q. How can I permanently stop mains noise in my studio?I’m currently working on a new project that contains sensitive analogue components and have rediscovered what many before me already know, that is that the VUSB 5V line that comes out from your computer’s USB hub is noisy. In fact I was surprised at just how noisy it is. There’s random noise at high and low frequencies. There’s persistent ripple and some mightily impressive spikes that sail through at seemingly random time intervals. If you’re running a sensitive component such as an ADC or DAC then you need a clean power supply and all this noise will really ruin your day. Anyway I thought I’d provide a practical example of how to clean up the power supply and eliminate all that noise. The result is a nice and stable VBUS line that you can use to feed your sensitive circuits. A hacked USB B connector I’l be using the hacked USB B connector pictured above and connected to a breadboard for testing.

I’ll be using my trusty old HP 500MHz oscilloscope and I’ll be probing directly at the component legs so as not to introduce noise from jumper wires. I should really be using a ground spring on the scope probe instead of a wire and clip but I haven’t got one those so I can’t. Firstly let’s establish the baseline by probing VUSB on a couple of USB ports. I have a number of root hubs exposed through the front panel of my case. The two blue ports on the left connect to a generic no-name PCIe USB 3 expansion card. The remaining four ports connect back into the motherboard USB 2 headers. I’ll be testing the circled ports For these tests I’ll be looking separately at VUSB on the USB 2 and 3 hubs on the left of the picture. The cables from the back of these ports has to travel quite a long way across the case to the motherboard header for the USB 2 ports and across the board to the slots for the USB 3 ports. Let’s get started by showing just how bad things are without any filtering.

Here’s a selection of screen grabs from the oscilloscope showing the noise on the supply from the two ports. The vertical resolution is 50mV/div in all the captures. I’m sure you’ll agree the noise levels are pretty awful with spikes of +/- 100mV. I don’t think your sensitive analogue components would thank you for connecting them to this supply. The noise on my USB 3 port is even worse than on the USB 2 port. I’ve got spikes, harmonics of spikes and even a pretty decent sine wave in there! Maybe it’s because the cable has to travel all the way across the motherboard or maybe it’s just because I bought a cheap USB 3 card. I’m going to filter out the noise using a selection of capacitors and a ferrite bead based on the values provided in this FTDI application note and if any significant noise is still present then I’ll tweak the values of the components until it’s gone. Here’s the schematic for the filter. It’s basically an LC filter with a selection of additional capacitors chosen to respond to different frequency ranges.

It’s a little known fact that the USB 2.0 standard mandates a maximum of 10µF in parallel with VUSB to limit the inrush current. I’ll admit to having broken this rule several times in the past and the world didn’t end but do bear this in mind if you’re designing a product for mass production. T Shirt Printing Murphy NcIf you do need significantly more capacitance than 10µF then the FTDI application note shows one way that you can design a slow start into your circuit to limit the inrush current.Pet Fox Puppies For Sale That’s the USB input on the left and the filtered output on the rightTop Down Bottom Up Cellular Shade Repair I have all of those components available in through-hole form. The long leads and the breadboard interconnects are not ideal from a noise point of view.

All those exposed wires will probably act as tiny little antenna picking up stray noise from the environment. Now let’s take a look at how the power supply line has cleaned up with the filtering components in place. Firstly here’s how the USB 2 supply looks with filtering: I’m sure you’ll agree that this is a huge improvement over the unfiltered supply. Noise is infrequent and limited to 50mV when it does occur. Again you can see that the filter has done its job very well with noise levels about the same as for my USB 2 port. The noise patterns are also very similar which makes me think that maybe this residual noise is being picked up by the leads in my circuit. The addition of a few small and cheap components can make a huge difference to the stability of the USB power supply. Noise is reduced to a low, tolerable level that should not present any issues for sensitive components. I hope you enjoyed this short practical demonstration. If I’ve saved at least one of you from having to bin a batch of PCBs and do a redesign to filter supply noise then I’ll be happy.

I recently purchased a Blue Yeti USB microphone for use in recording screencasts. I am using Screenflow to record with and while the sound is excellent in quality there is a nasty hum in the background that is not anything like ambient noise. I have tried different settings and randomly adjusted the gain, but I am a real noobie on this. Google searches yielded little or not help, so I am hoping to get some guidance here. Thanks in advance for the help. There are several possibilities: The noise is from the PC power supply. Try using a USB hub that has it's own power supply. If you're using a laptop, try running off of battery and the charger and see which one is better. As @Matt Jenkins said, maybe the mic is picking up the hum through the desk. I'd like to add here that Matt is talking about mechanical vibrations. The fans and hard drives in your PC/laptop could be vibrating the table which is coupling into the mic. Move the mic and see if the noise goes away.

The noise is caused by EMI/RFI. Try moving the mic around the room and see if it gets more or less noise when in different positions. Try a longer USB cable so you can move it around more. Once you've identified what is causing the EMI/RFI then you can do something about it. And there is always the possibility that the mic is bad. Try using it on a different computer, in a different building. If you still have problems then get a different mic. I am not a fan of removing the noise after the recording process. Or, rather, it is much better to remove the source of the noise than to try and filter it out later. Fixing it later is what we in the biz call, "Fixing it in the mix". Sometimes you have no other options, but when you do it is always better to do it right. Unless you think the microphone is faulty, In which case you should probably send it back, you should look at processing the signal in software. Audacity is a great cross platform Sound Editor that has a fantastic noise removal plugin, and it's totally free- Highlight a section of your track that just contains noise, then go to the "Effect" menu and click on "Noise removal", now click the "Get Noise Profile" button.

Now highlight the whole track and go back to the Noise Removal plugin and click "Ok", it's pretty darn effective :) Only pull it apart if you are sure that it's not covered under warranty and you've exhausted all other options. It might be that the noise level is normal in mic of that type, if you've not used a condenser mic before, the huge gain and noise can be above what you expect. It could be picking up the hum of your computer through the desk. Try holding it in your hand instead of standing it on the desk. I had the same problem and held the mic in the air while recording. The HUM went away! It was picking up vibrations from the table, How odd. I put it on a stack of rubber mouse pads and all is well now. Try grounding the computer case to a pipe or other large metal item close by like metal shelves or a metal desk. This will dissapate any ESD/RFI noise being picked up by the mic wire, you can also try plugging the mic into the front of the case away from the power supply, fan, etc...