Overclockers Australia Forums
OCAU News - Wiki - QuickLinks - Pix - Sponsors  

Go Back   Overclockers Australia Forums > Specific Hardware Topics > Electronics & Electrics

Notices


Sign up for a free OCAU account and this ad will go away!
Search our forums with Google:
Reply
 
Thread Tools
Old 1st May 2012, 9:34 PM   #1
Thalyn Thread Starter
Member
 
Join Date: Jun 2005
Posts: 355
Default USB headphone amplifier: static?

From the input I received last time, I set about designing and building a 2-stage, single-supply amplifier based off two BB OPA2134A op amps, being powered off a USB port and fed audio from a PC soundcard. All in all, the project has been a resounding success - much improved from the basic CMoy design of its predecessor. However, I've encountered a bit of a problem.

When I power the device off my PC and provide it audio from another source (such as my phone), it's crystal clear. If I power it from an outside source (my phone's wall charger) and feed it audio from my PC, it's again perfectly clear. If I power and provide the signal from my PC, though, I encounter a noticeable amount of static in the background, whether there's sound playing or not. My immediate thought is: ground loop.

Problem is, I'm not entirely sure what I can do about it. As I mentioned in my previous thread, I'm piecing this together mostly from educated guesses and I've learned almost as much from this second attempt as I did from the first. So while I'm of the impression that isolating the earths would work I don't know how I would go about that.

The following are the basic schematics for what I've built:

Power supply
Fairly standard (from what I can tell) active voltage divider. Most of the values chosen are, if I'm perfectly honest, fairly arbitrary - I realise they're important but I don't know enough about them to choose correctly. L1, for example, just happened to be the biggest Inductor I could get in PCB-mount form, which I chose mostly because it was the same price as the smallest (and I figured the bigger one would offer a lower LPF frequency).


Amplifier (mono)
If I'm right, this is a fully AC-coupled cascading amplifier, using the second op amp as a buffer/voltage runner. As they're both OPA2134s I'm not entirely sure whether it's actually achieving anything, but at least I know the design works if I should get different amps and try again.

With that, what have I missed? Should I be connecting the input and output earths directly to each other, separate from the supply earth? Should I AC couple the in/out earths to the supply earth? Am I going to somehow have to fully isolate both in and out signals using transformers? Or is there another solution out there that I simply don't know about?

Ideally I'm hoping the solution is something I can easily splice into the existing circuit. If not, I guess I can always take another try - quite possibly one without three HPFs (do away with C3 and R4 entirely) if I can help it, and potentially using the second op amp in parallel with the first, rather than in series, to double the output amperage capacity.
Thalyn is offline   Reply With Quote

Join OCAU to remove this ad!
Old 1st May 2012, 11:18 PM   #2
2xCPU
Member
 
Join Date: Jan 2005
Location: melb
Posts: 2,794
Default

Sounds like an ground-loop to me.
What you want is less grounds, not more. Try unsoldering the grounds on the audio inputs and let them float. You will probably get nothing out when powering with a wall-wort, but should work fine powered and fed from the PC.

2.
__________________
If "building a computer" doesn't involve powertools, you're just assembling spare parts.
----
The universe is made up of atoms and empty space, the rest is mere opinion. Democritus, 400 BC.
2xCPU is offline   Reply With Quote
Old 2nd May 2012, 12:39 AM   #3
Thalyn Thread Starter
Member
 
Join Date: Jun 2005
Posts: 355
Default

Gave it a quick try with a couple of patch leads. Unfortunately, no joy. The output remains unaffected either way, including both sound I want and sound I don't.

Though one interesting thing I discovered along the way (at least, I found it interesting): If I just connect the earth from the sound jack, without the left or right signal lines, than there's no static. This is with all of the outputs wired up as normal. I suspect that's somewhat less interesting to people who understand it better than I do, since it seems like the kind of thing there's a good explaination for.

*edit*
What about adding another Inductor on the supply earth? Again, I'll admit that's just a wild stab in the dark - but it's the only thing I can think of.

If it helps, the noise I hear changes based on what my system is doing. Of particular interest is that moving the mouse results in an audible change, but harddrive access and even graphics load make changes; though still only when source and power come from my PC.

Last edited by Thalyn; 2nd May 2012 at 6:18 PM. Reason: Question added
Thalyn is offline   Reply With Quote
Old 3rd May 2012, 1:54 AM   #4
Piroteknik
Member
 
Piroteknik's Avatar
 
Join Date: Jun 2002
Location: Bat Country
Posts: 380
Default

have you tried an audio decoupler/isolation transformer? this will isolate ground.

we use these when installing loyalty systems in pokie rooms, as alot of them have poor mixers/amps and so hooking our pc to the pa system creates all sorts of horrible sounds/hums e.t.c.

these usually fix it:

http://www.carbonblack.com.au/buy-ca...mer-12368.aspx

i think we get them from electus, so jaycar should have them.
__________________
DJ SpeedRaver
Its the one, the only, THE off tap!
You want some more? This - is - my - boomstick!

Last edited by Piroteknik; 3rd May 2012 at 1:58 AM.
Piroteknik is offline   Reply With Quote
Old 3rd May 2012, 2:41 AM   #5
Thalyn Thread Starter
Member
 
Join Date: Jun 2005
Posts: 355
Default

I actually have one of the Jaycar versions on-hand, believe it or not. Well, not quite on-hand - it's inside this current amp's predecessor. I'd used it to isolate the source ground from the amp's ground because I hadn't AC-coupled the amplifier stage. It didn't even occur to me to try it, mostly because I'd deliberately taped up the old one so I wouldn't be tempted to play with it more (my intention was to start from scratch, rather than trying to "shore up" a flawed design).

Unfortunately, it works. I say "unfortunately" because this means I've wasted a lot of time trying to re-design the amp so that I no longer need it (the isolator). It also means I can possibly fix the previous one by adding a few low-capacitant ceramics to the power supply and an inductor on the positive voltage rail. Kind-of makes this new amp redundant, though I will admit I have learned more from it.

Still, at least if I go for a third iteration I know I can combine the two designs into something which is hopefully exactly what I wanted in the first place.


BTW, if you were curious, inside those isolators you'll find two transformers and two capacitors. The transformers are 170ohm input and 100ohm output, while the caps are 47uF polarized electrolytics. It doesn't seem to need the caps to work - I pulled them from the first one to remove them from the audio pathway.
Thalyn is offline   Reply With Quote
Old 3rd May 2012, 3:20 AM   #6
Piroteknik
Member
 
Piroteknik's Avatar
 
Join Date: Jun 2002
Location: Bat Country
Posts: 380
Default

you know you can also get decouplers as a pcd mount design as well. could help with a future revision.
__________________
DJ SpeedRaver
Its the one, the only, THE off tap!
You want some more? This - is - my - boomstick!
Piroteknik is offline   Reply With Quote
Old 3rd May 2012, 3:31 PM   #7
Thalyn Thread Starter
Member
 
Join Date: Jun 2005
Posts: 355
Default

While I wasn't able to find PCB mount versions of the decouplers (I'll just harvest another one of the Jaycar jobs and build it onto the board), I have come up with a new plan of attack - an evil hybrid of the two amps I've already built. Before I go harvesting anything (including amp #2), though, I'd just like to get a second opinion and some basic value checks:


Power stage
Slightly modified from before. Mainly moved C3 to after the op amp, since I doubt the op amp itself will ever have need for it beforehand. It may after, though, since I'm no longer fully AC-coupling the amps. Also dropped C6 entirely, though I may wind up using one of the soon-to-be-spare 1mF caps in place of C1 to compensate.

Note that "ground" is now equivalent to half the supply voltage, since I'm no longer AC-coupling the amps and am now decoupling the source signal.


Amp stage (mono)
Here's where it gets fun (for me, anyway). This is completely different to what I was going for earlier. T1 will be harvested from the Jaycar isolator. Now, if I'm right than the following should be true:

- ~2Hz HPF (C1, R1)
- ~160KHz LPF (C2, R3)
- ~4 gain (R2, R3)
- 2x output amperage potential

That much I'm fairly sure about. What troubles me, though, is that I'm taken to understand that R1 should be roughly the same as R2||R3, which they clearly aren't in this case. Is that likely to have negative repercussions? I've also read having such low values on the feedback loop is bad for current usage, but I cannot source a small enough, non-ceramic capacitor for the LPF to use larger resistors. Naturally I believe this to be true based on the good ol' V=IR, but is this going to be something of consequence?

I'm also unsure of whether R4 and R5 have the correct values (I'm taken to believe they should be the same, but the only advice I've found on them is "about 50 ohms"). Also, should I be running U2's feedback from after R5 instead of before?

Further to this, I'm unable to figure out what the output impedance is going to be (I'd like it to be 2ohms or less). I know that the output resistance is R4||R5||(R3+R2), but I'm lead to believe that having the feedback after R4 and R5 will lower the effective impedance by some amount determined by said feedback. In fact, the "formula" I read on it was Zout = Rout / Amount of Feedback. Lacking any quantative measure for "Amount of Feedback" makes that rather difficult for me, however.


So, what do you think? Am I barking mad for even trying something like this with my lack of background knowledge, am I over-complicating things, or have I pretty much hit the nail on the head - even if bending it a little in the process?


*edit*
Actually, the more I think about it the more I think I'm barking mad.

One of the more electronically-inclined might know for certain whether this will be the case, but I'm wondering if the effort I've gone to in order to double the output current of the amp stage will be wasted on account of the current capacity of the ground line. After all, an LM741 is only capable of 25mA, as compared to the 70mA of the parallelled OPA2134s. Wouldn't this mean that one side of the signal is getting all that extra grunt, while the other side is not only limited to a smaller amount but also has to share that with the other channel?

Last edited by Thalyn; 3rd May 2012 at 8:56 PM. Reason: Afterthoughts
Thalyn is offline   Reply With Quote
Reply

Bookmarks

Sign up for a free OCAU account and this ad will go away!

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time now is 6:35 AM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd. -
OCAU is not responsible for the content of individual messages posted by others.
Other content copyright Overclockers Australia.
OCAU is hosted by Internode!