+9V/0V/-9V DC power supply

[greiswig]

Hi, all,

This is likely beneath most of you, but I can't think of a group I'd rather send this question out to. I have an old Raven Labs preamp, and the power supply finally died. I can use batteries in it, but I'd rather use a power supply if possible. The supply has a 300ma rating, but it doesn't say whether that's on both the + and - sides. It has three leads out: -9VDC, ground, and +9VDC.

Any suggestions where I might buy such an animal? Or a circuit that would help me make it with decent regulation?

Thanks!

[ToneMerc]

Hi, all,

This is likely beneath most of you, but I can't think of a group I'd rather send this question out to. I have an old Raven Labs preamp, and the power supply finally died. I can use batteries in it, but I'd rather use a power supply if possible. The supply has a 300ma rating, but it doesn't say whether that's on both the + and - sides. It has three leads out: -9VDC, ground, and +9VDC.

Any suggestions where I might buy such an animal? Or a circuit that would help me make it with decent regulation?

Thanks!

Should be able to find a dual polarity ps kit on eBay. If not, there's lots of dual PS schems floating around the web if you want to roll your own.

http://www.ebay.com/itm/DUAL-ADJUSTABLE ... 0695205897



TM

[greekie]

If it can run on batteries, it can probably run of a MAX1044 (or LT1054)

Check it out here - then you'll just use your usual 9V wall wart with it.

http://www.geofex.com/circuits/+9_to_-9.htm

PS: Thanks goes to R.G. Keen!

[Cliff Schecht]

IMHO the MAX1044 is not a good part for audio applications. Even the ones that switch at 30-60 kHz are asking for trouble! Although people typically say Nyquist for humans is around 20-22 kHz, there are higher order harmonics that get mixed down to lower frequencies and sound like "digital hash" on top of your signal. Maybe not everyone can hear it but I sure can.

There are much better power solutions out there that are smaller, switch at a much higher frequency and are true switching power supplies instead of charge pumps. Not that charge pumps aren't useful but they are bad supplies and can't hold regulation worth a darn (meaning output voltage varies proportionally to output current)! Also they are low efficiency and tend to be noisier than SMPS's but in the end all of that might not matter in your application.

[Cliff Schecht]

Also FWIW the way ToneMerc posted to get dual polarity is the classic circuit for doing so. Simple and easy to use (i.e. not a lot of tricks to get this to work). Sometimes you'll see the linear regulators replaced with Zeners but IMHO the linear regulators are much better because they are both short circuit and thermally protected meaning they are hard to blow up. Also the output voltage doesn't vary with input voltage or current draw and the linear regulators will have lower noise.

[chief mushroom cloud]

There are much better power solutions out there that are smaller, switch at a much higher frequency and are true switching power supplies instead of charge pumps.

Can you recommend any?
Thanks

[Cliff Schecht]

For such low current draw the old MC33063 can be used. This is a super cheap boost/buck/inverting controller that doesn't require a lot of external parts to work well. It's got a built in switch and requires maybe 6 or 7 external components to work.

Also the TL497A (an ANCIENT part) could be used but it's not the smallest or simplest solution. It'll handle about 500mA continuous current and only switches at 100 kHz (compared to 500 kHz and a 1.5A switch for the 33063).

I'm only mentioning solutions that would be easy to implement with stuff from around the house (maybe just my house.. ). While you might not have a TL497 laying in anything anymore, the MC33063 can be found in just about every cell phone car adapter in the USA. The diode and inductor can be salvaged as well meaning you need a few resistors and caps to get the job done.

[LeftyStrat]

Sorry to highjack this thread, but I've been struggling with a dilemma.

I want to run +9v over the ring of a stereo cable to supply power to a guitar with active pickups. I can easily build a breakout box, but I started thinking how nice if amps had this built-in.

The problem of course is when someone sticks in a mono 1/4 plug, the +9 supply is now connected to ground.

There are now a range of pedals that support this:

http://dcvoltage.net/p3

Just wondering how someone would go about switching the 9v supply off when a mon plug is used.

[Cliff Schecht]

The easiest solution is to have a switch over the jack that turns the 9V on and off. Maybe not the most full-proof method but it would work.. Some sort of LED on a DPDT would make it very obvious when the 9V is on.


The more elegant solution would be to find a SMPS controller with pulse-by-pulse current limiting and enough self-protection to not harm itself when short circuited (a very easy task given you are asking maybe a few mA from this supply). This way when you short 9V to ground the supply is smart enough to stop trying to supply current and when unshorted, knows to come back up. There are many modern controller ICs that could implement this sort of functionality safely.

[LeftyStrat]

Thanks Cliff. I'll investigate that further.

[Cliff Schecht]

Check out the bottom of page 21. Something like this is all you need to protect the circuit in a short circuit situation, especially at such low current levels. A part like this isn't even getting out of bed (let alone breaking a sweat) powering a guitar preamp.

http://www.ti.com/lit/ds/symlink/tps54040.pdf

[Jana]

Lefty,

I suspect that the active pickups are pretty low with current draw. If so, then if you had a resistor in series with the voltage supply in the amp--a value that would not drop the voltage any appreciable amount when running the active pickups but would serve as a safety measure if the power supply was shorted out with a mono plug. That way, instead of a dead short, it would be dumping its voltage across the resistor.

If I was going to do this I would first measure the actual current draw of the active pickups. Then decide what a reasonable value for that resistor would be--let's say 220 ohms. That will be about 40 to 50 ma. that the power supply will be dumping across that resistor with a mono plug. Use a 2 watt resistor to be extra safe. So, now all that is left is to calculate how much the voltage is going to drop across that resistor with the active pickups and raise the voltage of the power supply by that amount to get a full 9 volts to the pickups.

Having said all that, I am wondering if potential damage could be done to the other parts of the active pickup circuitry or to other non-active pickups. When a mono cord is plugged in, there is going to be a brief spike of voltage as the tip of the cord goes past the ring connector. If the cord is already plugged into a guitar, then that spike of voltage is going into the pickups if the volume of the guitar is not at 0.

[Jana]

Cliff, Well that's all fine and dandy--but what about the brute force, using a hammer solution I suggested? (sarc)

[LeftyStrat]

Guys, I'm starting a new thread as to not hijack this one:

http://ampgarage.com/forum/viewtopic.ph ... 980#203980

[greiswig]

I knew you guys would come up with the goods! Thanks for the tips. I'll look around a bit at the solutions you are suggesting.