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Old 20th May 2013, 09:01 PM   #1
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Default OP AMP SINGLE VOLTAGE SUPPLY

I'm trying to figure out how to power an OP amp with one 9-volt battery. Looked at a Texas Inst application note, talked about "virtual ground", sill confused. Looked at a guitar effect pedal, has a stereo input jack see pic). Can someone explain how this is done? Thanks Jeff
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Old 20th May 2013, 09:13 PM   #2
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Op Amp Power Supply Considerations: split, single, virtual ground, etc. - a tutorial - YouTube

Here is a quick video that describes the split supply, which is probably what you are looking for. Have fun!!
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Old 21st May 2013, 01:46 AM   #3
Minion is offline Minion  Canada
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On the MXR schematic you will notice that there are 2 1m resistors in series between +v and Ground , the point between these two resistors is your virtual ground , this point is at half supply voltage (4.5vdc) and is used to bias the opamps input to half supply so that the output can swing positive and negative in relation to the bias point .....
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Old 21st May 2013, 06:15 PM   #4
Mooly is online now Mooly  United Kingdom
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Both those circuits are conventional AC coupled designs with biasing to suit. I wouldn't refer to the opamp circuit as a virtual ground system. Ground for both is the negative of the battery.

A virtual ground system would refer both the input and the output ground to the arbitrary "mid voltage" reference which is the junction of the series 1meg resistors that Minion illustrates. If that were done it would allow for a true DC coupled design (if the designer wished) and such an implementation could then be called a "virtual ground" with the supplies then appearing as plus and minus 4.5 volt rather than a single plus 9 volt.
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Old 22nd May 2013, 04:12 AM   #5
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Thanks all for the info. I'm still can't figure out the input jack....Looks like a stereo jack, but the guitar is mono. Or is it just a power-switching jack? And is being "AC coupled" (with the .01 on the input and the 1uf on the output?) to keep dc bias off the signals in and out? Thanks
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Old 22nd May 2013, 06:52 AM   #6
Mooly is online now Mooly  United Kingdom
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The input jack is used to switch the power whenever a plug is inserted. You could use a separate switch if needed from the battery negative to ground or, just the same, wire the battery negative permanently to ground and put a switch in the battery positive lead.

Yes, those caps are needed because the opamp input and output pins are at around 4.5 volts DC (half supply voltage). That voltage has to be blocked on the output side so that it is not passed to the next audio component in the chain such as a pre or power amp. The cap on the input side is needed to prevent anything from altering the steady state DC bias of 4.5 volts provided by the 1Meg resistors.
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