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DIY Stompboxes => Building your own stompbox => Topic started by: nirvanas silence on March 31, 2004, 02:57:46 PM

Title: Negative voltage from AC adapter?
Post by: nirvanas silence on March 31, 2004, 02:57:46 PM
I am modding an Xotic Robotalk for an AC adapter and it has 2 batteries.  The positive of one battery and the negative of the other both go to ground.  So how could I get the negative voltage?  18V adapter, 9V and a small circuit, two 9V adapters?  I'm curious what you guys have to say.  Thanks.
Title: Negative voltage from AC adapter?
Post by: Samuel on March 31, 2004, 03:28:37 PM
Check out the MAX1044 charge pump. The part is available from smallbear, and schematics for different configurations are available over at General Guitar Gadgets.
Title: Negative voltage from AC adapter?
Post by: Mark Hammer on March 31, 2004, 04:04:03 PM
So the Robotalk has a bipolar supply.  These are tricky, though not impossible to have adapters for.

Samuel's suggestion could work in principle (and the MAX1044 is used by many to provide the +/-9v needed for a Mutron from a single battery - see RG Keen's www.geofex.com site for info on how to do this).

Another alternative is to build a small AC/DC adaptor to provide +/-9vdc from an AC input.  This would use a pair of diodes, a pair of big caps, and a pair of 3-pin 9v regulators to derive +/-9v from, say, a 12VAC wallwart.  So, the wallwart itself provides AC and the regulation is provided onboard the effect or on a little daughter board.  Of course I say this not knowing a) how much room you have in there for an adaptor socket or daughter board, or b) whether you feel like sinking a hole into it.

While yo have the back popped off, can you tell us what chips are in there?
Title: Negative voltage from AC adapter?
Post by: nirvanas silence on March 31, 2004, 04:17:17 PM
I'll take a detailed pic for you guys since you are so helpful, but all the chips and transistors have scratched off labels.  I will find out the pinouts to narrow down the possibilities though.

The problem is I am doing this for someone else so I don't want to sink alot of time and money into it.  The AC idea could work though.  Thanks for the ideas.
Title: Negative voltage from AC adapter?
Post by: nirvanas silence on March 31, 2004, 05:03:29 PM
Ok, Im going with an ICL7660.  What value cap should I use in the feedback section of it?

Also, the robotalk has five 8pin ICs, and not one of them use pins 1 or 8.
Title: Negative voltage from AC adapter?
Post by: Mark Hammer on March 31, 2004, 05:53:17 PM
Quote from: nirvanas silenceAlso, the robotalk has five 8pin ICs, and not one of them use pins 1 or 8.

So, definitely no dual op-amps in there.  Both internally compensated single op-amps AND CA3080's do not use pins 1 and 8.  The CA3094 *does*.  So, my guess is its using an OTA-based filter like the Maestro S&H.
Title: Negative voltage from AC adapter?
Post by: Sic on March 31, 2004, 10:40:50 PM
yeah, the robotalk is based off of the maestro S & H from what i have read.

i can't wait till i have the skill to build one
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