USB Isolation without lots of money? - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: January 14, 2018, 07:46:08 pm
content: I finished a mini-project to sort out my desktop audio./ 47 DIY Amplifer input -> headphonesUSB -> PCM2707 DAC 3.5mm Y Splitter \ Numark powered speakersIt works a treat and has got rid of MOST of my horrid display/cpu/pc noise as none of the audio circuitry is inside the PC case.However there is a tiny bit left which can be heard if you ramp the volume up on the headphones. It has to be coming in on the USB, probably the ground. Either that or it's 'airborne' RF noise leaking from the PC.Other than expensive USB isolators is there any easy way to remove it? It's not ground hum it's most likely the display/video card noise. It's not even one frequency, it's all over the place depending on what I am doing with the mouse at the time.I seen people selling filters, but as this is in the audible range and multi-frequency I don't want to filter as it will also filter the audio.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 14, 2018, 08:17:08 pm
content: What do you call expensive usb isolators? They're €30 on eBay (from the UK).If you really care you should get an S/PDIF headphone DAC anyway.

author: Signal32
author_rank: Frequent Contributor
timestamp: January 14, 2018, 08:23:33 pm
content: Quote from: Jeroen3 on January 14, 2018, 08:17:08 pmThey are as cheap as $10 from China (https://www.ebay.com/itm/NEW-USB-Isolator-1500v-isolator-ADUM4160-USB-to-USB/401143526490).I think all of them in this range are based on the ADUM4160 chip. Not sure about compatibility with OP's scenario, the chip definitely has some limitations.

author: paulca
author_rank: Super Contributor
timestamp: January 14, 2018, 08:31:31 pm
content: I looked into it the other night and seen a few videos suggesting the cheap isolator IC are too slow and cause issues. However if it's been put into a USB device maybe it will work.When I look for USB Isolators a lot of the branded product ones are £100+I'll maybe give the chinese one a try, just to see if it works.SPDif. My dac has an SPDIF out, not in. Besides is SPDiff not RCA, how would that solve the problem of RF leaking over the wire? I "could" use an optical link, my mother board may actually have an optical audio output, but my DAC nor amp has an input for it.This may turnout to be a fools errand as I could use a fully isolated DAC/Amp and still have noise.

author: Electro Detective
author_rank: Super Contributor
timestamp: January 15, 2018, 12:14:34 am
content: I'm amazed that the TooHungLo marketing team haven't whizzed out a -WORKING- 'Ten Dollar, Free Ship' in-line USB isolator cable by nowPerhaps if they slowed down on the de-velopment of disposable cheapass multimeters and assorted bin worthy devices and accessories for a minute..and bang out a useful product a lot of consumers actually need and will BUY...Guaranteed order from me for 5 or 6 of them if they can handle USB 2.0 without drama

author: viperidae
author_rank: Frequent Contributor
timestamp: January 15, 2018, 12:52:04 am
content: Quote from: paulca on January 14, 2018, 08:31:31 pmSo get an sp/dif adapter. No noise issues with optical cables and coax can be isolated with a transformer. Some sp/dif inputs/outputs are already isolated.

author: cdev
author_rank: Super Contributor
timestamp: January 15, 2018, 12:58:39 am
content: The cheap USB isolators on ebay are all USB 1 -

author: David Hess
author_rank: Super Contributor
timestamp: January 15, 2018, 02:57:20 am
content: I do not see where USB noise should create a problem unless there is a design mistake with your PCM2707 DAC. Is the noise present if the powered speakers are not connected?

author: cdev
author_rank: Super Contributor
timestamp: January 15, 2018, 04:04:33 am
content: Paul, what have you done other than this USB device to eliminate the hash?Have you used clip on common mode chokes? (NOT on your ground if things are grounded)It often works well. If your USB cable is small enough and your ferrite core is big enough you can thread it through multiple times and often get substantial reductions.Also, make sure the cases of your unbalanced audio devices are grounded to one another using as short and direct a path as possible. Copper raid and crimp connectors works well. test its resistance, it should be the resistance of the wire, and no more, ideally a tiny fraction of an ohm.make a probe of some kind, a detection device to detect changing magnetic or electric fields..Why don't you take a picture of your setup and post it?

author: David Hess
author_rank: Super Contributor
timestamp: January 15, 2018, 04:23:39 am
content: Quote from: blueskull on January 15, 2018, 04:13:34 amIncreased jitter or latency may screw up the PCM2707's buffering and sample rate following which requires a PLL. The datasheet does not specify enough to know and this is a common weak point in USB DACs. If it does affect it, then I would consider the PCM2707 design defective which is a possibility.Given the isolated nature of the headphones however, I suspect the problem is with the application circuit. Noise of any kind through the USB connection should not have any effect.

author: massivephoton
author_rank: Contributor
timestamp: January 15, 2018, 04:28:43 am
content: Quote from: free_electron on January 25, 2015, 09:03:11 pmDoes anyone know if the cited usb isolator is/was available?It's based also on the ADuM4160.

author: paulca
author_rank: Super Contributor
timestamp: January 15, 2018, 08:01:55 am
content: Quote from: cdev on January 15, 2018, 04:04:33 amThe setup at the moment is still a bit "how you doing". The PCM2707 is on a bare board (£5 Chinese module) lying on the desk.I had this issue before with the off-the-shelf Project HeadBox USB headphone amp.If I use a normal analogue amp the PC noise is unbearable at high volumes. With the USB DACs is a minor annoyance that is only audible if your music stops while the amp is near MAX. This suggests I have got further away from the noise source, but not far enough. It's almost certainly the brute of a video card in the PC. Maybe I needed heavier, shielded HDMI cables. But it's there and that urks me.I will try and isolate things by double checking I don't have any unnecessary connections, disconnecting the powered speakers, etc.I have a few ferrite rings I found the other night, so I can try and put the USB cable through a few of them.As I say, it's a minor annoyance, but if there was an easy option to kill it, I would.

author: cdev
author_rank: Super Contributor
timestamp: January 17, 2018, 08:37:02 pm
content: This is likely the problem right there. Its possible you have portions of the electronics acting as antennas - Within your own situation - if you want the best results, you should try to follow the same rules that any designer would follow about grounding and circuit routing, your wires should be as short as possible, and above a common ground plane. Use a piece of PCB to hold the module and provide a flat GP underneath it. You can also use the flat clip on kinds of ferrites on any ribbon cables, etc. If that doesn't do enough, buy some copper flashing and make some small fences between the different parts of your setup and route non-signals like DC power (and even the audio output between the two stages) via feed through capacitors through them, soldered in. That may help stop RF radiated from one part from getting into other parts.Do any of the noise levels change if say, you grab the HDMI cable? If so, there are probably some changes you need to make in how the shield is handled - I dont know much about HDMI but I bet its easy to find out more about. Search on USB and HDMI shield grounding conventions and try to follow them.Also, try using an RTLSDR to see if you see this noise.. If you can see it that will help you in your attempts to reduce it.They are cheap and helpful in this kind of situation. Tune around until you see its interference. You'll likely be seeing a harmonic but it doesn't matter. Think about getting a bunch of ferrites in different form factors. They are super useful to have around. You almost can't have too many of them, they always will find a use.Also what blueskull is saying about power supplies and audio is really important. Using USB power limits you to using fairly modest levels of filter capacitance..USB will crap out if the inrush current is too high. It will not supply power as it then thinks the device is shorted. If you must use the USB power, you will need to use a resistor to limit the inrush current on any large caps, or smaller polarized caps for that reason.It might make more sense to not use the USB power, use a better regulated external power source - or even a wall wart and then additionally regulate and filter it. There you almost can't overdo it, the more filtering the quieter it may well become. Or more likely, then you will get to see what other parts of the setup need attention...Quote from: paulca on January 15, 2018, 08:01:55 am

author: paulca
author_rank: Super Contributor
timestamp: January 17, 2018, 09:07:04 pm
content: I found some of it. The "some" is indicative though. I believe it's just pick up on the audio signal cables from the PC. I shortened my routing. As an idiot I still had my feed to the powered speakers passing through the Headbox amp pass through. Sending it through two sets of cables instead of one. I removed that necessary link and the noise level dropped significantly. It's just perceptible still.It drops even more if I remove the powered speakers completely. So I'm guess it's the signal cable passing around the back of the PC and monitor to the powered speakers that is the issue.If I reverse the balance L for R on them and move the powered one that takes the RCA in to "this" side of the desk the signal cables can be kept away from the back of the monitors and PC completely.Now I have another dumb ebay purchase to sort out. I bought a 3.5mm jack Y splitter cheap. It's already fissling and humming when moved and takes a while to get it to settle quiet with both channels working. I ordered a Belkin one for £5.99, without spending £££ it should, I hope, have better contacts than this £1.99 jobby.

author: cdev
author_rank: Super Contributor
timestamp: January 17, 2018, 09:20:17 pm
content: You should make sure its clean, maybe clean it with some IPA on a paper towel and if you see any tarnish, try rubbing it a bit with a pencil eraser.Unless the y-connector has some kind of visible problem ..(do you see tarnish, or rust or anything like that?) Gold plated connectors are less prone to tarnishing, but many of them are fake.. I think there is a form of titanium or similar that looks golden in color.Quote from: paulca on January 17, 2018, 09:07:04 pm

author: paulca
author_rank: Super Contributor
timestamp: January 17, 2018, 09:22:51 pm
content: Quote from: cdev on January 17, 2018, 09:20:17 pmIt's problem is it's cheap Chinese tat. One of the ports was incredibly stiff at first and is now incredibly loose, so probably the contacts bent. One of my jacks is slightly larger so if I keep it in the loose one it works after a fashion.Another device on the "things to see what they do in the microwave" list.

author: bson
author_rank: Unknown
timestamp: January 18, 2018, 04:06:40 am
content: Just create a small board that replaces the computer generated Vbus +5V with your own linearly regulated one, and perhaps put a ferrite bead on Vbus on the DAC end (if possible).

author: cowasaki
author_rank: Frequent Contributor
timestamp: January 19, 2018, 08:31:57 pm
content: I've just built a USB isolator as part of a circuit board to give my new arbitrary wave generator an isolated power supply and USB. I had the boards made in China literally just to test out them for quality before I buy them. If you want one you can have one if you send me a stamped addressed envelope or cover the postage :-)If you choose to use the power side of it then it does work but do so at your own risk plus..... The silk screen is wrong! The caps labelled 470uF should be 100nF and the positive rail 2200uF cap's silk screen is backwards BUT it does do the job {I will draw on the board the correct polarity for the 2200uF that needs rotating just ignore the polarity of the other 3!)