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Old 15th June 2013, 04:06 PM   #1
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Default Virtual ground in Cmoy amp

Hello guys, i`m trying to put together a headphone amp by using Tangentsoft`s approach ( http://tangentsoft.net/audio/cmoy-tu...angent-sch.pdf ) but i fail to understand how to wire the virtual ground. I just can`t figure out where to connect the wires. I made a Fritzing design of where i am now. If anyone could tell me what to do with the wires hanging around in the picture( https://dl.dropboxusercontent.com/u/45794359/amp_bb.jpg ) i would really appreciate it. If someone finds any other mistakes please feel free to point them out.
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Old 15th June 2013, 04:56 PM   #2
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I didn't verified the whole scheme but "virtual ground" is the ground, the jack ground pin should be connected to it.
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Old 15th June 2013, 05:06 PM   #3
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The red arrow I added to your image is the virtual ground point. So that point needs to connect to all other points in the schematic with the ground symbol.

se
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Old 28th February 2015, 01:55 PM   #4
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I am also using a virtual ground, but on a much larger scale in an effort to 'rehab' a dynaco ST-120. (I also did one using 'updatemy dynaco' circuits, but for this one, I want to use chipamp.com's LM3886 boards, thus the need for bipolar ps.) So, I created a simple regulated PS using a virtual ground created off the diode bridge with two big resistors followed by two big capacitors followed by a simple regulator using IR240/9240. (I am traveling and do not have the ability to scan and post a schmatic, sorry.)
Anyway, my question is: how to manage the grounds? The ST-120 does not have a third wire power cord, but I'm not sure that makes a difference and I may add one. The chipamp board connects Input gnd, Output gnd, and Power gnd (+ and -) to the same fairly large ground plane. My plan would be to run twisted pairs for power +/gnd, powr -/gnd, input +/gnd, and output +/gnd, all isolated from chassis. The question then becomes WHAT/WHERE do I connect to chassis? My thought would be to use a heavy wire from the virtual ground on the PS board to chassis with no 'loop breaker'. Since signal and power gnds are connected on the chipamp board, I cannot 'lift' the signal gnd from the chassis unless I 'lift' the PS as well which strikes me as a bad idea. Of course in its present form, the chassis is not, itself "grounded" since it only has a two wire power cord.
Coments/advice welcomed!!
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Old 28th February 2015, 02:30 PM   #5
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What is wrong with using the centre point of the batteries as ground?
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Old 28th February 2015, 02:35 PM   #6
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Quote:
Originally Posted by JonSnell Electronic View Post
What is wrong with using the centre point of the batteries as ground?
I'm afraid you didn't read my post correctly. I am not using batteries. I am using the transformer from a dynaco ST-120 which does not have a CT, thus the need for a virtual ground.
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Old 28th February 2015, 04:06 PM   #7
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I'm not convinced that it's a good idea to run a chip amp with a "virtual" ground. I'd be willing to bet my lunch money that it would introduce stability and/or distortion problems when pressed into such an application.

If you must use the Dynaco transformer, the easier solution (which would probably give the best results) would be to modify the circuit for single sided supply. You still have the big output caps from the Dynaco, right?

Maybe a better solution would be to find a center tapped transformer from an old amplifier or receiver. I have one right now out of an old Pioneer receiver that I swear must have just been replaced before the unit was left at the electronics recycling dumpster. The outputs were brand new too, but fried. Anyway, some dumpster diving, garage sales, thrift stores, etc might turn up a better transformer.

There's people that collect those old Dynacos. I know a guy that would have a fit if he knew what you were doing.
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Old 28th February 2015, 04:18 PM   #8
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You can use the center point of the batteries as ground, Nothing wrong with that at all and should be how it is done.

The only exception is that if the batteries don't discharge equally than one may have a lower voltage then the other one and this can cause asymmetrical distortions and/or clipping of the waveform.

Using a the resistors assures that the ground would be exactly half of the supply voltage even as the voltage gets lowered when the batteries are draining.

This method is only good if you are only using a few stages of opamps, If you are using many more opamp's then it is advisable to use a Virtual Ground buffer (for a single ended supply) to assure a low impedance ground system for all of the common opamp stages.

This has been covered in great detail in many many and recent threads.

FWIW

jer

P.S. The above is for general use in opamp circuit's and not meant use for high current situations such as in a power amplifier.
In cases such as that a proper Bipolar Power Supply should be used.

Last edited by geraldfryjr; 28th February 2015 at 04:21 PM.
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Old 28th February 2015, 05:41 PM   #9
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I would like to see the schematic of this virtual ground circuit.

For this application, it would have to be large and fast. I picture a fast, precision voltage divider driving a high impedance input servo circuit. The buffer portion would have to be super fast and able to deliver large current, or else distortion would be introduced.

This seems impractical to me. I'd rather source another transformer, whether that means scrounging second hand stores and garage sales or dumpster diving, or else just ordering s new transformer.

I still think that a single ended circuit might be a cheap and practical solution. Do you still have the big chassis mounted caps? With them, the original transformer, and a handful of extra parts, you're good to go.

The datasheet should have an example of a single sided circuit configuration.
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Old 1st March 2015, 05:59 AM   #10
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Originally Posted by JonSnell Electronic View Post
What is wrong with using the centre point of the batteries as ground?
In case you were asking a real--not rhetorical--question, nothing's wrong with it, it is ground.

To the electricians who wired your house, "ground" is literally that, it's the planet earth. There's a ten-foot rod driven into the earth outside and that's ground.

But in electronics "ground" is arbitrary. It's a point you define as zero volts, and then all voltage measurements are relative to that point.
.

Last edited by bentsnake; 1st March 2015 at 06:07 AM.
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