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Virtual Ground Problem

post #1 of 7
Thread Starter 
I'm a newbie in Headphone Amp DIY. I just soldered two Amplifiers according to the projects in Headwize. One is A Pocket Headphone Amplifier (by Chu Moy) using OPA134 Op-Amp. The other one is "A class A MOSFET Headphone Driver" by Szekeres.

The Op-Amp based amplifier is grouded to Virtual Ground as in my case 6 V (I use 12 V power supply). The MOSFET driver ground is 0V, on the other hand. The Op-Amp amplifier and MOSFET driver use seperate power supplies.

The input/output of the Op-Amp amplifier are grounded to the virtual ground. While the input the MOSFET driver is grounded to 0 V. Since the MOSFET driver has a gain smaller than one, I use the Op-Amp amplifier to pre-amplify the signal.

I'm not quite sure if the grounding in this case is all right, although everything works when I connect them together.

The problem I have is that when there's nothing plugged into the input jacket of the Op-amp amplifier, there's a relatively high pitch noise. When I plug my laptop audio output into the Op-amp amplifier input, the noise disappears. The strange thing is that if I plug my Panasonic portable CD-player into the amplifier, the noise doesn't dissapear at all.

Could anyone explain me why this is happening and how to solve the problem.

Thanks!
post #2 of 7
You have a ground loop. The problem now is to figure out where it is and fix that, or to avoid the problem leading up to it.

First, I hope you're using an isolated power supply to run these amps? It probably wouldn't work if you aren't, but I have to ask...

Are you using the combined gain-stage and buffer from the Szekeres article appendix?



If not, why not? That circuit can be powered from a single supply and shouldn't have this ground loop. The most likely reason for your problem is running both circuits from separate supplies. The second most likely cause would be running them both from an AC supply you've built and you have a mismatch between earth ("safety") ground and power supply ground (the center tap of the transformer secondary, usually).
post #3 of 7
Thread Starter 
Thank you for your thread!

ground loop was really something I worried about. The power supply use the following circuit to create the virtual ground. I replaced the battery with a 12V DC power supply. It is just a normal cheap power supply you could buy from any store.



I didn't read the Addendum when I was buiding the amplifiers. Clearly the circuit with gain stage you suggested would be a good try. I will see if I have time these two days to hook it up. The disadvantage of this circuit is that the Op-amp is not driven by dual supply (with the virtual ground, the Op-amp sees the supply as a dual supply). So the reason I'm using two isolated power supplies is I want to take advantage of the virtual ground.

The connection of the two amplifiers do work fine when I plug them into my laptop audio output. I still don't quite understand the noise when there's nothing (or my CD player) plugged into amplifiers.
post #4 of 7
Quote:
It is just a normal cheap power supply you could buy from any store.
Then it's probably an unregulated supply, in which case it's isolated. The question remains what supply you're using with the Szekeres...

Quote:
The disadvantage of this circuit is that the Op-amp is not driven by dual supply (with the virtual ground, the Op-amp sees the supply as a dual supply)
You would need to use a dual supply or a virtual ground supply with the addendum circuit. You can't power it from an unsplit single supply.

I'm no Szekeres expert, but it seems to me that you could probably get away with using the CMoy virtual ground supply with this setup. You really should use a linear regulated supply to power the whole thing, though. Since you have to get a new supply, I'd recommend the Elpac WM080 which has a higher voltage than your current one, which will help you to avoid clipping due to the virtual ground shift you're likely to get with the CMoy power circuit.

Alternately, you could get the Elpac WM-071 or -072, which are dual-voltage supplies.

Quote:
I still don't quite understand the noise when there's nothing
Clearly the input ground to the CMoy is floating relative to some other ground point in the system circuit. When you plug the source in, you pin virtual ground to the system's ground, so the noise goes away. What the full path is is another question, but I advocate avoiding the problem by running the whole thing from a single supply instead of trying to find the loop and fix it.
post #5 of 7
Thread Starter 
I used a TLE2426 "rail splitter" to creat the virtual ground. The noise is gone. I think you were right, using resistors to cut the input voltage by half is not stable so the Virtual ground was floating around, which created the noise when there was no input source. The TLE2426 seems to perform really well. The only concern I have is its limited output current. I read some threads about putting multiple TLE2426's in parallel to increase the output current. I'm not quite sure if this is the best way to do it. I know by cascading a TLE2426 with a buffer would take advantage of the precision of TLE2426 and power of the buffer. So I guess that might be what I will try.

I checked the Elpac power supply. It seems to be the choice of many experienced DIYers. Before I get it, I want to try the buffer solutions with an ACOPIAN A24H1200 Linear Power Supply I just got. Since this power supply has an 24V output. I'm considering set the virtual ground at 12V and use the difference between 12V and 24V to drive the MOSFETs. The Op-amp can be driven with -12/+12V voltage. I wonder if there's any possibility to drive the MOSFETs with dual voltage supply too.

I put the Op-amp circuit in a BUD enclosure. I also soldered all the componets to a PC board. Before I was testing the setup on a test board. So I think part of the noise might be also picked up by the long leads of the components.

Thank you for your help!
post #6 of 7
Quote:
The only concern I have is its limited output current.
Do you know for a fact that this is a problem, or are you just hand-wringing?

Quote:
I read some threads about putting multiple TLE2426's in parallel to increase the output current. I'm not quite sure if this is the best way to do it.
I wouldn't.

Quote:
I know by cascading a TLE2426 with a buffer would take advantage of the precision of TLE2426 and power of the buffer.
Only if the buffer is a closed-loop type. With an open-loop buffer (HA-5002, BUF634, almost any discrete type...), you only get higher output current, not high accuracy. You need to wrap the buffer with an op-amp to keep the high accuracy of the TLE2426. See this article for details.
post #7 of 7
Thread Starter 
Quote:
Originally Posted by tangent
Do you know for a fact that this is a problem, or are you just hand-wringing?
I know this is a problem. That's what the MOSFETs are for. By having large input impedance for the MOSFET driver, the output current of the Op-amp can be limited to a small value, although I didn't calculate how small it would be.

I actually checked the link of the buffer circuits yesterday. It's very helpful. I didn't realize that it was your website untill just now. I really like the website.
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