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Old 28th June 2008, 03:00 PM   #1
Archwn is offline Archwn  United Kingdom
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Default Pros/Cons on Split supply vs. Virtual Ground

Hi everyone,

I have seen so many design on headphone amplifiers using virtual ground or ground channel driver.

OTOH I see none of them so far in normal speaker amplifiers. The laters seem to use split supply instead of virtual ground.

If the first one is good, why we don't see it in our speaker amplifiers? And if the later is good, why we still see people using virtual ground/ driver in their headphone amps?

Or if they are both have pros and cons?

I'm seeking for the explanation of what would be the difference between these implimentations and what will be pros/cons comparing these together.

Thanks in advance for your answer.

AK
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Old 28th June 2008, 06:57 PM   #2
KSTR is offline KSTR  Germany
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Split supply is a passive bridge circuit (easily seen when you draw it in its completeness, without using GND or supply symbol shortcuts). "Virtual ground" is an active bridge circuit which is not driven by the signal. This is not very economical for high power designs and also has some drawbacks (you only add the distortion of the bridge if you don't drive it). There are some clever design which drive the "virtual ground" with the error signal from the main amplifier to lower the effective distortion, though. But then the "virtual grounds" of different channels cannot be connected.

- Klaus
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Old 28th June 2008, 08:48 PM   #3
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Quote:
Originally posted by KSTR
Split supply is a passive bridge circuit (easily seen when you draw it in its completeness, without using GND or supply symbol shortcuts). "Virtual ground" is an active bridge circuit which is not driven by the signal. This is not very economical for high power designs and also has some drawbacks (you only add the distortion of the bridge if you don't drive it). There are some clever design which drive the "virtual ground" with the error signal from the main amplifier to lower the effective distortion, though. But then the "virtual grounds" of different channels cannot be connected.

- Klaus
The first power amp I have seen using it was QUAD 306. I have used it in many of my designs.
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Old 28th June 2008, 09:36 PM   #4
Nordic is offline Nordic  South Africa
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Relating to your question... simply those units are mostly used with portable sources with floating grounds too...

Single sided is realy usefull from a simplicity point of view though, but I agree maybe not for a big poweramp... I am busy makeing an fx pedal tonigt, the same DPDT switch switches on the unit, and status light as well as swithing the signal from bypass to FX...
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Old 28th June 2008, 10:07 PM   #5
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Default Re: Pros/Cons on Split supply vs. Virtual Ground

Quote:
Originally posted by Archwn
I have seen so many design on headphone amplifiers using virtual ground or ground channel driver.

OTOH I see none of them so far in normal speaker amplifiers. The laters seem to use split supply instead of virtual ground.

If the first one is good, why we don't see it in our speaker amplifiers? And if the later is good, why we still see people using virtual ground/ driver in their headphone amps?
For a headphone amp powered by a single battery, the virtual ground circuit is a simple way to derive split rails and thereby avoid a big coupling cap on the output. In short, it is the easy/elegant solution.

For a poweramp, the virtual ground circuit gets more difficult to implement since the currents involved are so much larger. The split supply is the easy/elegant solution in this case.
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Old 29th June 2008, 01:17 PM   #6
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Default Re: Re: Pros/Cons on Split supply vs. Virtual Ground

Quote:
Originally posted by leadbelly


For a headphone amp powered by a single battery, the virtual ground circuit is a simple way to derive split rails and thereby avoid a big coupling cap on the output. In short, it is the easy/elegant solution.

For a poweramp, the virtual ground circuit gets more difficult to implement since the currents involved are so much larger. The split supply is the easy/elegant solution in this case.
The current through the loudspeaker returns through the supply capacitors not the center tap of the transformer.

Not difficult to implement a virtual earth at all, DC current is a few milli amps or about as high as the NFB current in the amp.

And no, a split supply is no more difficult than a single supply, one uses two transformer secondaries while the other two capacitors.
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Old 29th June 2008, 03:18 PM   #7
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Default Re: Re: Re: Pros/Cons on Split supply vs. Virtual Ground

Quote:
Originally posted by Nico Ras
The current through the loudspeaker returns through the supply capacitors not the center tap of the transformer.

Not difficult to implement a virtual earth at all, DC current is a few milli amps or about as high as the NFB current in the amp.

And no, a split supply is no more difficult than a single supply, one uses two transformer secondaries while the other two capacitors.
You seem to be suggesting that a simple passive virtual ground is adequate for a power amp, which goes against EVERYTHING I have read on the subject.

You might want to look at http://en.wikipedia.org/wiki/Virtual_ground
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Old 29th June 2008, 04:26 PM   #8
KSTR is offline KSTR  Germany
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"Ground" is a virtual concept anyway (electrons don't know about it), so much discussion about it is more or less semantic.

In that sense, between split supply vs. single supply there is no difference in principle, both are passive bridges, for the single supply the bridge function degenerates close to and at DC, owing to the capacitors.

I repeat my recommendation of explicitely drawing the circuit topologies and then have a look where exactly the currents flow in circles, additionaly discriminating between three time domains: DC, AF and HF. For audio, AF (Audio Frequency) is the main concern, obvoiusly.

- Klaus
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Old 29th June 2008, 08:13 PM   #9
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Default Re: Re: Re: Re: Pros/Cons on Split supply vs. Virtual Ground

Quote:
Originally posted by leadbelly


You seem to be suggesting that a simple passive virtual ground is adequate for a power amp, which goes against EVERYTHING I have read on the subject.

You might want to look at http://en.wikipedia.org/wiki/Virtual_ground
Been around before wikipedia and op-amps.

As Klaus points out draw the current flow then it will become clear.

If you analyze the circuit you will see that your ordinary amplifier has a virtual ground most of the time since the rectifier diodes only conduct for a small percentage of the time.

While the diodes are switched off your whole amplifier relies on the potentials across the power supply caps being the signal ground reference.
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Old 29th June 2008, 11:39 PM   #10
KSTR is offline KSTR  Germany
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Now I found that drawing again I made a while ago to show the mentioned current flow. As Nico points out, when the diodes are off, it's only the caps that are at work, closing the loop. And when the diodes conduct, the currents are rather uncorrelated to the signal.

With a little imagination and using some black box circuit equivalents for cap arrangements etc, one can also redraw a cap output amp and will end up with exactly the same schematic, with the only difference -- wrt to the output stage -- that the center tap is always floating (while with a split supply it's only floating most of the time).

When we speak of virtual ground in the context of power amp (speakers or headphones) then we have an active return point, some sort of output stage. This changes the situation a bit in the details, but not in the basic idea that the load currents always go through the supply caps in some way or another (exept for constant current class-A bridge circuits).

- Klaus
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