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Old 16th May 2008, 10:17 PM   #1
oraz is offline oraz  United States
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Default dual 9v supply confusion

I was trawling through some op-amp pdfs and saw a diagram for a dual 9v battery supply where the ground came from a resistor divider, as is in "split" supplies for single battery setups.

wouldn't it be better, though, just to ditch the divider, and use the center tap between the batteries as ground?

I thought this would allow more current to be drawn and increase headroom, but maybe there is some drawback that I'm not aware of?

any pros/cons to either config?
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Old 17th May 2008, 12:35 AM   #2
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The solution you suggest is certainly easier to build. The current to be drawn remains the same, so don't expect more headroom. If you want more current, you need to connect batteries in parallel. But don't, because they are never charged at the same level and discharge each other trying to find an equilibrium.

The drawback of your idea might be that you don't have a symmetric power supply, when the batteries are not equally charged, e. g. if one of the batteries is empty and the other is full. The divider with two batteries in series would maintain the symmetry even in that situation.
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Old 17th May 2008, 12:45 AM   #3
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Yes I use the second method for my BB op amp head amp, just using 2 9v batteries with centre tap. when the sound starts degrading I change both batteries, but I've had the thing for 2 years now and only have changed the batteries once. (use an 18v wall wart mostly).
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Old 17th May 2008, 04:28 AM   #4
oraz is offline oraz  United States
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I thought I noticed an improvement in sound quality, though, without the divider. I guess it isn't current draw, but could anything else about this setup be benefiting the sound?
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Old 17th May 2008, 11:07 AM   #5
AndrewT is offline AndrewT  Scotland
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The divider is drawing current. That loads up the PSU (batteries).
Reducing the current draw gives the PSU an easier job and rewards you with improved sound.
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Old 18th May 2008, 04:35 PM   #6
oraz is offline oraz  United States
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so the opamp actually, gets less current with the divider?
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Old 18th May 2008, 05:46 PM   #7
AndrewT is offline AndrewT  Scotland
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Old 18th May 2008, 06:57 PM   #8
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The 9V cells in series works great, use the CT (ground connection) of the batteries as circuit ground..

If this was a speaker amp, you get better bass by using the real ground instead of a resistor divider.

However, OpAmps don't make much difference with bass because of the lower currents, but using split supplies IMO gives cleaner sound, because the input of the amp goes directly to 0V point, instead of resistors which could pick up noise easier, which is why they usually filter a resistor divider with a capacitor. Also, it uses less parts and IMO is easier to build as well to just use the batteries instead of the divider.

IMO the biggest reason to avoid a divider is that it adds extra drain to the batteries to shorten their usage.
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Old 19th May 2008, 01:58 PM   #9
oraz is offline oraz  United States
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yup, it wasn't the divider that was affecting the sound, after all
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