Charge pump w/ switch

[jeromeplank]

I've got a compressor that I wanted to build a charge pump into to run at 18v. Is there any negative to placing the charge pump on one side of a switch so I could run it at 9v or 18v? Something like this is what I was thinking with a DPDT toggle:


To charge pump (9v in)  ------[ ]       [ ]-------   From charge pump (18v out)

From DC adapter            ------[ ]       [ ]------   To effect 9v input

Jumper                                 [-]------[-]


I'm using a MAX1044 for the charge pump and I've read that these can be sensitive. Is there anything in this line of thinking that you guys can see which would create a problem?

[jkokura]

That looks like it work work fine to me. Would you connect the 9V and the 18V outs of the switch to the same pad/adjacent pads on the board?

What compressor are you thinking of doing this with? Also, it should go without saying, but make sure you use components that can handle the 18v...

Jacob

[stubish]

to add to this (and possible threadjack) what would it be like if you could switch between 9 - 12 - 18 volts? that would be a neat trick!

[newfish]

...or maybe run the whole thing at 18v, but with a 'starve' pot on the DC supply?

[Processaurus]

Sorry, but will the difference in sound be both noticeable and useful in both positions?  I'd guess that if one could have more headroom, they'd want it, the only thing that would happen with a typical compressor on a lower voltage is it would be more likely to clip (and 9v isn't likely to clip often from a guitar effect), and if that were the goal, there are more versatile ways to get a distorted compressor...

[jeromeplank]

Thanks for the input everyone. Jkokura, I was planning on only sending that middle lug out to the pcb power pad. Basically like a standard component swapping switch but this time with the DC power. I'm doing it with a Ross clone and all the parts can handle 18v just fine.

Processaurus: it's not so much about the difference in sound. I want the added headroom of the 18v and plan to run it there at all times. I've seen a number of threads that talk about MAX1044s dying for various reasons so the switch is just my emergency backup. If, for whatever reason, the chip in my charge pump crapped out I could just run the effect on 9v. I'm sure it's probably overkill but I had some spare DPDT toggles and plenty of space in the enclosure. I figured why not put it there just in case.

[Processaurus]

The LT1054 has been suggested as a more robust charge pump for pedal projects, I don't plan on using the max1044 ever again, as you said, because they are prone to fail (probably because they are only rated for 10v, 10mA max).

Kind of an interesting idea though, making backup systems for guitar pedals, like you would for serious electronics like life support and aviation.  Seems like you should be getting that for those ultra expensive but simplistic boutique pedals (even though you never would...), like two totally independent identical circuits where all the pots are dual pots, and you have a 3pdt toggle switch that switches the in, out, and power to the backup one!  Your backup pedal is in the other pedal.  That and something like RG's mosfet power polarity protection rectifier thing (or just a schottkey FWR) to accept any polarity power, or AC, and you'd have a road warrior.

[liquids]

The original idea will work fine.  I do think the LT1054 is simply superior to the MAX1044, FYI, unless you have a MAX on hand an don't have an LT in the stash nor an order on the horizon...and don't need but a few ma's of current.  =D 

No problems with the LT1054 dying, they are easier to work with in many ways, more tolerant of higher input voltages an can deliver more current without issue.   I've killed quite a few components in the last year or two, and don't think an LT1054 was ever one of them...but hey, Your Mileage May Vary.

Re: stubish / newfish comments...If you want more than 2 voltage options, your best bet is not a voltage starve.  You can get a charge pump voltage, and then feed that voltage to an op-amp based voltage divider. 

You can use a small value  (1k, 5k or 10k) pot and set resistors to function as a a variable voltage divider.  You'll get voltage drop from the charge pump and then again with the op amp, as most op amps can't handle or 'swing' or output to the full rail voltage they are being run off. But you could easily get within about -2v of the charge pump's available voltage if you use as large value electrolytic caps as you can fit, and shottky type diodes if/ where called for. 

Be sure to use a reasonably small pot / resistor combo.  If you use a large value pot and resistor combination, my experience is that it takes a fair amount of time for the voltage to 'settle in' and stabilize from where it was last, audibly.

Then again, real time variable voltage can a very cool thing to implement in certain kinds of pedals, but it can be overkill in others.  Higher voltage like 18v+ or +/-9v is nearly always my preference, but the one spot only comes in one 9.4v version, so charge pumps it is!