'Virtual Ground' question
post-1390551
Thread Starter
Post #1 of 10

Migroo

Head-Fier
Joined
Apr 20, 2005
Messages
63
Reaction score
0
Joined
Apr 20, 2005
Posts
63
Likes
0
Hi everyone.

Having almost completed (everything apart from enclosure) my first venture into DIY audio I've got a question.

Whats the difference between a virtual ground and a .. 'real' ground??

I've seen that the PPA has a 'ground' channel, if you can call it that.

Can someone explain it to me in laymans terms?
 
     Share This Post       
post-1390569
Post #2 of 10

Migroo

Head-Fier
Joined
Apr 20, 2005
Messages
63
Reaction score
0
Joined
Apr 20, 2005
Posts
63
Likes
0
I noticed that Tangent had a page on it here (link) but it doesn't really answer my question. Interesting reading though.
 
     Share This Post       
post-1390632
Post #3 of 10

morsel

1000+ Head-Fier
Joined
Mar 22, 2002
Messages
1,372
Reaction score
10
Joined
Mar 22, 2002
Posts
1,372
Likes
10
Classic amplifier design uses dual supplies with a common ground. Power supply ground must sink and source the returns from the left and right channels.

Virtual ground design uses a single supply and synthesizes a ground reference 1/2 way between the rails. The ground channel buffers this reference. It is the ground channel, rather than the power supply that must sink and source the returns from the left and right channels.
 
     Share This Post       
post-1390656
Post #4 of 10

Migroo

Head-Fier
Joined
Apr 20, 2005
Messages
63
Reaction score
0
Joined
Apr 20, 2005
Posts
63
Likes
0
Thanks Morsel!
 
     Share This Post       
post-1390670
Post #5 of 10

Megaptera

500+ Head-Fier
Joined
Mar 7, 2004
Messages
891
Reaction score
12
Joined
Mar 7, 2004
Posts
891
Likes
12
"Real" ground can mean a couple things:

There's "earth ground" which is a a connection that basically sends return currents to, literally, the ground, via a big metal rod, or metal plumbing, buried in the earth.

Then there's the "power ground", which is a safety mechanism that indirectly connects metal chassis to the earth ground, in case of an equipment failure that would energize it. That's where the third prong on your AC power cords goes. The neutral wire in your AC stuff is usually connected to this somewhere, and it's supposed to be pretty close (from a voltage perspective) to earth ground.

"Signal ground" is whatever your circuit is using as a reference for voltage. Since saying something has a voltage implies "between points A and B", the signal ground acts as a common point B for all the different point As you might have in your circuit. This often gets connected to power ground or earth ground somewhere along the line, and is usually created by the transformer. In a lot of cases it's the same thing as the negative power rail.

Audio electronics often need a dual power supply. Since single power supplies are often easier and cheaper, "virtual grounds" are basically a way of saying some point between the positive and negative power rail is going to be the new "0 volts" as far as the signal is concerned. This makes it look like you've got two pairs of power rails: the positive rail on the negative side is the negative rail on the positive side. That's basically what you're looking at with a dual power supply too, but here the difference is that the virtual ground is maintained by circuitry, instead of a physical ground connection. It's usually connected to signal ground.

And finally, some amps like the PPA use a ground channel with an active output; it keeps return currents from your headphones from getting back into the signal ground, and thus into the signal itself. Basically, the "output ground" is not really a ground at all, but your headphones can use it as one. Since it's the "output" of the ground channel circuitry, it should never be connected to anything other than the headphones or else you wind up with big problems.

Does all that seem logical? I hope it's not too oversimplified; it's one of those topics that you can usually get away without knowing a whole lot about but it's a really good idea to understand what's going on with it.
 
     Share This Post       
post-1390741
Post #6 of 10

tangent

Top Mall-Fi poster. The T in META42.
Formerly with Tangentsoft Parts Store
Joined
Sep 27, 2001
Messages
5,969
Reaction score
56
Joined
Sep 27, 2001
Posts
5,969
Likes
56
...and to summarize Megaptera, "ground" is whatever the circuit designer says it is. (Or whatever the circuit designer says they are in the case of amps like the PPA, where there are multiple grounds.) As long as it makes sense, people will go along with it.
 
     Share This Post       
post-1391058
Post #7 of 10

Migroo

Head-Fier
Joined
Apr 20, 2005
Messages
63
Reaction score
0
Joined
Apr 20, 2005
Posts
63
Likes
0
Wow, thanks guys.

I suppose I started off on the wrong foot. Coming from car audio, all the wiring I do in the car called 'ground' is actually the negative power rail, or "return path"..

Looking at the cmoy I guess its really simple. Without the ground rail nothing would work. Its the connection between all the circuitry and without it we'd have an open circuit, right?

Metaptera thanks for your post. Not oversimplified at all. I think I got most of what you're saying. I don't understand the PPA bit (ground channel with an active output) but I'll take your word for it and try not to get confused!!

I'll take a look at the PPA schematics and see how I go!


Tangent - cheers.
 
     Share This Post       
post-1391307
Post #8 of 10

__redruM

100+ Head-Fier
Joined
Dec 8, 2004
Messages
396
Reaction score
0
Joined
Dec 8, 2004
Posts
396
Likes
0
Quote:

Originally Posted by Migroo
Looking at the cmoy I guess its really simple. Without the ground rail nothing would work. Its the connection between all the circuitry and without it we'd have an open circuit, right?


Its more complicated than that. The opamp would power up fine without the virtual ground, because it runs off of v+ and v- only, it's signal needs a ground to return too, but signal is A/C, not D/C, and I'm already over my head here.

the Cmoy is the first amp people usually make, so it the amp the most mistakes are made on. One common mistake is not hooking the ground channel up correctly. This produces an amp with a lot of static and noise, but music can be heard.
 
     Share This Post       
post-1391608
Post #9 of 10

Megaptera

500+ Head-Fier
Joined
Mar 7, 2004
Messages
891
Reaction score
12
Joined
Mar 7, 2004
Posts
891
Likes
12
Quote:

Originally Posted by Migroo
Metaptera thanks for your post. Not oversimplified at all. I think I got most of what you're saying. I don't understand the PPA bit (ground channel with an active output) but I'll take your word for it and try not to get confused!!

I'll take a look at the PPA schematics and see how I go!



Glad you found it helpful -- for my own part, I find nothing helps me understand something like explaining it to somebody else


Don't worry too much about the PPA stuff right yet. It's not really a "first step" kind of concept, and eventually you'll have enough of the more basic stuff down that it'll click.
 
     Share This Post       
post-1391818
Post #10 of 10

Migroo

Head-Fier
Joined
Apr 20, 2005
Messages
63
Reaction score
0
Joined
Apr 20, 2005
Posts
63
Likes
0
I know what you mean!! I'm a bit like that with car audio.

I think I'm getting there though. Need to do a few things on my cmoy - reduce the gain is definately one of them!!
 
     Share This Post       

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top