# HELP ! how do I get 16V a.c. out of 12V D.C.?

Discussion in 'The Projects Forum' started by tom foxe, Apr 8, 2008.

1. ### tom foxe Thread Starter New Member

Apr 4, 2008
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I run a 12V sound system, from a car battery, but my mixer uses 2 x pp3 9v batteries or a mains adapter supplying 16V a.c/1100mA. I'd like to use the car battery as the only power source, and I don't want to use a 12V/mains invertor, the whole point of my sound system is that it's low voltage & safe. I'm an ex-tv repaiman so I can build a circuit. I'd prefer to buy something readymade, but all the inverter stuff seems to have 110 or 230v output. I'm guessing a chip which produces sinewave could drive a transformer & give me 16VAC at 50/60hz. Tx 4 any help, Tom

2. ### S_lannan Active Member

Jun 20, 2007
247
2
use that dc input for your 2x9v batteries

nominal car voltage while on = 13.8v
nominal voltage for 2x9v batteries = 8.4x2 = 16.8v

not too much difference there... give it a go... it should work.

3. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
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I wouldn't under-voltage your device, especially since battery voltages fluctuate a lot in a car!

Perhaps you can do a 'semi-custom' DC-DC converter from National Semiconductor. They have this power bench tool where you enter in your parameters, then in many cases, they can send you a kit with a generic PCB and parts/partslist so that you can solder it.

http://www.national.com/appinfo/power/webench.html

You would need an input voltage range of about 8V to 16V to be conservative, then a 16.8V-17V output. Judging by your AC requirements,you'll need roughly 1-1.2A of current. You can enter these parameters and go with their recommended parts. I don't think the kit is too costly.

Steve

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
Wait a sec....
Do you have a schematic for your mixer? Or a part# and manufacturer?

I suspect the mixer contains opamps which require a bipolar supply. The two 9V batteries would then be used as +9 and -9 rails, with ground reference coming from the audio input shields. If that is the case, attempting to run the mixer from either straight 13.8v or a single 18v dc-dc converter wouldn't work, as both are referenced to chassis ground.

OTOH, if the DC-DC converter had dual outputs (-9 and +9) it could be made to work. This option would increase complexity and costs, naturally - but it may be required.

I suggest that we don't have enough information yet.

Tom, check your battery connectors, and see if one connector's + and the other connector's - have continuity (<1 Ohm) to GND (either the case or the audio input shield). If so, it requires a bipolar supply (+9 & -9). If only one - terminal has continuity to GND, then it needs a single-ended positive supply (+18). If only one + terminal has continuity to GND, then it needs a single-ended negative supply (-18).

If you don't get continuity to GND from any battery connector, we'll either need a schematic, or have to open up the case to do some circuit tracing. Just the part numbers from some of the ICs may be enough, particularly around where the 16VAC is - looking for voltage regulators, which may be TO-92 cased critters or perhaps SMT/SMD's.

5. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
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Yes, Wookie is 100% correct, that is a very likely assumption!

Steve

May 16, 2005
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Dec 27, 2007
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8. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
Actually, I think thingmaker3 came up with a really cool plug-in solution! It might even be possible to add a 12VDC connecting jack (say, a 2.5mm jack) to the outside, and put the DC-DC converter in the battery compartment. Slick!
If the mixer uses standard 9v PP3 "transistor" batteries, I'd be really surprised if the mixer required more than 30mA per rail. At that rate, your typical 565mAH alkaline 9v batteries would be dead in under 19 hours.

9. ### tom foxe Thread Starter New Member

Apr 4, 2008
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Of the 2 x 9V terminals, one +ve and 1 x -ve are shorted to the chassis, so I do need a +9v/0/9v- supply. It's a Peavey RQ200, have mailed Peavey for a circuit but no joy yet. I'd prefer an external solution because I have 2 mixers, and would rather not alter the case or circuitry. Really appreciate the help I've been getting, thanks.

10. ### tom foxe Thread Starter New Member

Apr 4, 2008
4
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Should have read all posts b4 my last reply. I agree with Sgt W re pp3 drain, the mixer has run for 8 hrs without two new alkalines going flat, so I think I'll go for the Thingmaker 3 fix. The mixer has 22V phantom supply for condener mics. also a headphone output, neither of which will get much use, so 1.1A is way superfluous to reqs. will let you know how I get on. Thanks again, Tom

11. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
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That's really strange for them to spec such a high power source relative to your actual needs. Nonetheless ,the battery results speak for themselves. You can measure the typical current draw if you were so inclined, thus confirming the solution as aadequate.

Steve

12. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
Have you looked at the various power requirements in the manual? Alkaline battteries only!

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13. ### SgtWookie Expert

Jul 17, 2007
22,183
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http://www.peavey.com/search.cfm?c=0&term=RQ200

With a +9/-9 supply, you will only have 8v phantom power for mics. If you are not using mics that require phantom power, that is of no consequence.

If the mixer will perform all of the functions you require when powered with only two 9v alkaline batteries, then Thingmaker3's suggested dual-output dc-dc converter will be more than adequate.

Radio Shack stocks 9v battery connector "pigtails"; they're normally used to connect a 9v cell to a circuit board. There's no reason you can't use them in reverse; connect the pigtail to the outputs of the DC-DC converter. Caveat here: the colors of the pigtail will be reversed if used in this manner. Also, you will want to label the pigtail from the +9v output of the dc-dc converter differently from the -9v output so that you don't connect them incorrectly. You will need to connect the +9v output to the Peavey battery connector that has the negative battery clip grounded. I suggest a dab of red paint or nail polish will work just fine. Connecting the supply up backwards would very likely cause immediate damage to the mixer's amplifiers.

The RQ200 manual pdf file has a flowchart on page 11; while not precisely a schematic, it would be quite helpful if you ever had to troubleshoot internal problems.

Let us know how it works out.

14. ### pedalpa.org.uk New Member

May 23, 2008
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Tom - I run a very similar system - username=web address... ;-)

The way I got around this is to go inside the audio amp which has an internal inverter (to get +/- 40v, say, for the audio amp) and tap that off at the smoothing capactor - that can go straight into voltage regulator ICs to give you your +/-9v (my mixers use +/- 20v), but I used dropper resistors to do some of the word for them.

this way 0v stays 0v and also audio ground, and you get as much current as you need...

good luck!