Voltage Question: +15/-15V Supply

[poojalooba_cow]

So I've got a mixer with four mic preamp inputs, but it's got 12 total inputs - 8 additional line-level inputs. So I'm looking into building some simple preamps in order to fit more mics into the mixer so we can record the whole band live. The basic schematic is below. I won't be using phantom power in order to avoid complications - we'll run our dynamic mics through my DIY preamps and our condensers through my mixer that already has phantom power equipped. There will also be passive tone controls after the output on this schematic for some simple tone shaping.

So here's the problem: the INA217 op-amp calls for "+15V" and "-15V" supply voltage here. I'm not a total noob to DIY audio and electronics - I've built dozens of stompboxes, including RG Keen's Neutron Envelope filter that uses a +9V and -9V and 0V scheme. However, when I think of things with voltages labeled like this, I get a little confused. The way I think of the Neutron, for example, is "+18V" and "0V" and then there's a reference voltage of 9V via voltage dividing in the middle where everything is "grounded". I've provided the appropriate part of RG's Neutron schematic for reference to what I mean.

So when the schem calls for +15V and -15V, do I really actually need a 30V supply and then I'll just make a voltage divider to get the ground reference? Also on a related note, where the freak do I find a 30V DC wall wart? Mouser has a 28V one that'll be fine, I guess, but still...

Preamp schem:

(This is the "Famous $5 Preamp" Schematic)

RG's Neutron power supply section, with ground in the middle.

[njkmonty]

what about this?.....

[hymenoptera]

The usual way is with a stepdown transformer, bridge rectifier, LM7815 & LM7915, and some filter caps.

I put "15v bipolar supply" into google images and came up with a bunch of schematics.

If you're not used to messing with mains voltage though, don't try this.

[greaser_au]

To add to my buzzy colleague's post above... 

No mains experience?   16VAC 1A wallwart. Readily available cheaply because of the household security industry... UL/CE approved and well isolated (just be sure to use a unique connector that will not fit in a 2.1/2.5mm pedal plug so your best mate helping you with load-in doesn't fry your vintage <whatever>!!).   Inside your gear, use 2 diodes, with a couple of 220/470/1000uF25V electros and you have a +/- 22V supply good for a couple of hundred mA ready for regulation.

david

[duck_arse]

what they ^ said, but also: that diagram from RG: the +9/-9 on the left also need 0/gnd for them to make sense. unless it's an 18V battery.

[poojalooba_cow]

^ duck_arse, Ground is in the middle between the two voltages. It's the downward-facing triangle. I hooked it up between two 9V batteries in series.

Hey, thanks for the replies so far, it's starting to make a lot more sense. I tried googling "+/- 15V power supply" forever and never found any info on anything (something to do with the syntax of the +/- part), but "bipolar power supply" works a lot better. I guess I'll be buying parts for rectifiers and regulators and adding a bipolar power supply to my project. Now the hard part is finding a 14-16VAC wall wart. Looks like Mouser has exactly one available, and Digi-Key has a couple of expensive ones. And Amazon is useless because they don't differentiate so they all end up being DC output.

When I made the Neutron filter, I used two 9V batteries in series, and the ground was the junction between the two. That's what originally made me think of using a voltage divider, because isn't the ground reference "technically" getting 9V between the two batteries, and the whole circuit runs on 18V?

[hymenoptera]

I've been saving wall warts for > 20 years from various consumer junk, etc.

When I need one though, and I don't have it, I usually buy used from ebay or something. I've found them at thrift stores and yard sales. And 16VAC isn't too uncommon.

Smallbear even stocks a few 18VAC ones that might work
http://smallbear-electronics.mybigcommerce.com/wall-warts-1/

[anotherjim]

You can use a voltage divider - but only if ground current is low - a few mA. Where the current drawn from + and - supplies is reasonably equal, ground current can be quite small. If they were always exactly equal, ground current would be zero  - that's unlikely in practice. You can improve a voltage divider ground with an op-amp.
This article...
http://tangentsoft.net/elec/vgrounds.html
... is talking about the subject with in respect of portable headphone amps, but it covers the right ground (no pun intended). Here's the op-amp scheme from it...

Op-amp can be a 741 - should be good for up to 20mA ground current.

[davent]

Jameco have a few 14-24vac none of which are very expensive.
http://www.jameco.com/1/3/black-ac-to-ac-adapters

[anotherjim]

BTW, the R3 in the op-amp output above should probably be much less than 1k for better control of higher ground current - can even be zero ohm.