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Old 30th December 2012, 07:34 AM   #11
Mooly is offline Mooly  United Kingdom
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This is the simple way to get you started. The two resistors must be equal in value and around 10K. They connect across the supply and junction becomes half supply. The cap is any small electrolytic of around 47 to 100uf. That will bias the opamps to around 6 volts.

Tho output MUST have a cap to AC couple it and for headphone testing add a resistor as mentioned.
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Old 31st December 2012, 04:25 AM   #12
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Thanks so much for the help! I made a quick spit supply and threw a 220k resistor on the output of a unity gain circuit and it works!

I'm a bit busy for the next few days but hopefully I can update my full circuit to run on a split suppll and get that up and running soon. Assuming I wire the split supply correctly, does the existing circuit look good?
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Old 31st December 2012, 08:07 AM   #13
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Your circuit looks OK electrically.

When you say you "made a split supply", was that a real dual split supply or a virtual split supply (as in my drawing) ? A real split supply could be made from two 9 volt batteries.

Not sure what where the 220K resistor you mentioned fits in (thats really high for a series resistor and it wouldn't drive headphones).

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Old 31st December 2012, 01:06 PM   #14
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Yeah.... I'm not sure why I put a K there... it's only 220 ohms... =)

The split supply I made was just a voltage divider. My final circuit will have a cap like your drawing. I've seen a few schematics where there's a cap for the 12V and the 6V, but it seems to me that the 12V side wouldn't need a cap at all. I guess it's just to make it look symmetrical?


Question on your drawing, though. It looks like the first half of the circuit is grounded to the virtual ground (6V), and the second half is grounded to the real ground (0V). Am I reading that right?
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Old 31st December 2012, 02:25 PM   #15
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Quote:
Originally Posted by SmackaMuta View Post
Question on your drawing, though. It looks like the first half of the circuit is grounded to the virtual ground (6V), and the second half is grounded to the real ground (0V). Am I reading that right?
I drew it that because of you using headphones as a load thinking you would take the headphone output ground to the power supply zero. Both the power supply zero and the virtual ground are the same point electrically at AC but the "high" load of the headphones could modulate the virtual ground. That was the thinking

To be absolutely correct all the signal grounds should go to the virtual ground and that becomes the main ground in the system. The power supply zero volts line would go to the opamps and nothing else other than the virtual ground components.
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Old 31st December 2012, 03:25 PM   #16
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That makes sense. For my clarification, all signal grounds to the Virtual ground, including the output to the amp?

Also, for my purposes, do I need to worry about the virtual ground shifting voltage or is that really more of a problem for systems that use more power?
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Old 31st December 2012, 05:34 PM   #17
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Yes, the virtual ground becomes "proper" ground for the circuit. Its very hard to explain all this in a few words.

The virtual ground... if we take that as our ground and reference point then by definition it is fixed. For it to vary we have to say it varies with respect to something else. That something else are the power rails, the power supply zero point and the 12 volt point. That is the "problem".

As a line stage you have no worries with the simply virtual ground. Just make sure that the caps (we should really use two equal value caps, one across each resistor rather than the economy of a single cap) are large enough like the 47 to 100uf I mentioned.
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Old 31st December 2012, 06:04 PM   #18
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makes sense, and sounds easy enough! I really appreciate the help. I'm learning quite a bit on this build... =)
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Old 6th February 2013, 05:56 AM   #19
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Well, all was great when hooking it up to headphones. I'm now trying to hook it up to a small class D amplifier, but when I do the circuit stops working. All I get out the speakers is a quiet hum. When I disconnect the amplifier and hook the headphones back up, everything works perfectly (only bass coming through). I know the amp works fine because I can hook normal sources and everything plays well.

The amp and preamp run on the same 12VDC power supply. Could the amplifier be grounding the virtual ground to the power supply ground and therefor pulling the virtual ground to 0? Do I need to ground the amplifier's circuit board to the virtual ground?
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Old 6th February 2013, 07:42 AM   #20
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Using the circuit in post #11, if your using the same supply for both amps then you need to use the black zero volt line as ground and this connects to the input ground of the Class D amp. The audio input and output grounds now connect to this black line instead of the blue virtual ground.
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