How do i know if i can run a pedal on 18v?

[simpleobserver]

I don't totally understand how to tell if i can run a pedal 18v or not.  I just built the byoc reverb that, according to them, can run on 18v, but i also have a belton reverb pedal i built from the sample schem on their site.  The belton schem says 9v but can i run it 18?  What exactly is the difference and why?  What would i have to change in the belton schem to make it run 18v?  What's the general rule to apply that in other pedals?  I figure there's a thread somewhere on here that explains all of this, but i couldn't find it.

[JRB]

I don't know the circuits myself but in general one thing you need to be sure of is that the components used can handle the higher voltage.

[slacker]

What JRB said is pretty much all there is to it.
Once you've checked that your components are good for 18 volts, generally for opamp based circuits no changes are needed, for transistor circuits you may need to tweak components to bias them correctly for the higher voltage.

I would say for something like this there is no point running it at 18 volts though, the Belton chip runs off 5 volts so all you are doing is increasing the headroom of the dry signal. If your signal is hot enough to distort the Belton chip at 9 volts it will still distort it at 18 volts.

[Mark Hammer]

Some things that SAY they run at 18v actually don't.  Rather, they may run at 12 or 15v but 18 is needed for the onboard regulator/s to work.  In some instances omething more than 9 is needed, but having batteries that deliver a full 18v is merely helpful, not critical.

[simpleobserver]

Thanks guys.  Basically (i know it's not a good reason to run anything 18v but...) i've run out of room on my dunlop brick for 9v pedals so i'm trying to figure out what i can run 18v so i can use all the outputs available. 

I guess what i'm asking is it's just the transisters and the ics that i have to worry about, right?  I just need to make sure that the ones that i'm using in whatever pedal i have can handle 18v, that's the rule of thumb?

[amptramp]

You have to worry about any electrolytic filter capacitors as well.  Some may be rated for 15 volts and be across power to ground.  Most non-electrolytic capacitors are rated for at least 25 volts, even the leadless surface mount ones.  You may occasionally find resistors that are overstressed at 18 volts.

[JRB]

With pedals we use so little power you don't really have to keep a eye out for the resistors but some of the caps can have a low voltage requirements.

So what you want to keep a eye on is the caps, IC's transistors and of course the diodes.

And running 18V isn't any worse reason than running 9V its probably even better since you got more head room.

[R.G.]

A couple of observations:

- Some commercial pedals use caps rated for 10V. Most are 15/16V. Some are 25V.
- If you're out of spaces on your power brick, you *might* consider using a daisy chain for more than one pedal per output right out of your existing brick.
- I know of at least one 9V adapter that has many thousands of users, many of whom make their living playing guitar, running all their pedals on one daisy chain, and haven't died (or had hum, gritz, gratz, or other ugly, nasty sound come out) from it. Just a thought. The 'one output per pedal' thing is the solution to two specific problems I can think of, but it's by no means crucial and the problems are much less common than they used to be.
- 'head room' is one of those things we inherited from the audio fringe that arguably was not a good thing. It has become IMHO almost meaningless without a long string of qualifiers. Headroom for voltage regulators? Caps? Before distortion sets in (on a distortion pedal, no less)? Output volume/level/size?

And unwinding back to the original question, how can you tell if a pedal can run on 18V? Two ways: either (1) know the parts, schematic, and look up the datasheets for all of them, then determine that the biasing points are OK, nothing gets over powered, and so on, or (2) try it and hope nothing blows.

[Mark Hammer]

Once upon a time, voltage-rating differences  were predictive of both size and price, with higher ratings being noticeably bigger and pricier.  So, if you were making pedals in the 1960's you often wouldn't use anything rated for more than 10 or 16 volts.  These days, the average 100v electro is often smaller than the average 25v electro from the golden era, and often available quite cheaply.  There is precious little reason for anyone using through-hole components to want to empoy anything rated at less than 25v.  I'm not saying they never do, but it may be more a function of what the inventory is than of what will keep production costs sane in the long run.

With that little preamble, often the biggest concern about use of other higher supply voltages is, as others have said, the semiconductors involved.  CMOS chips tend not to endure anything above 15v.  TTL chips want 5v, but then you wouldn't find them unless the 9v were regulated down to 5v.  Most op-amps are quite comfortable with 18v and even 36v (+/-18v).  I'm sure there are likely some transistors that are not comfortable in that range, but the various new stock silicon JFETs and bipolars we tend to use will handle 18v safely.  The circuit itself may be designed in anticipation of 9v or less, but won't emit magic blue smoke if you choose to go above that.

As for headroom, please b aware that sometimes douling or even tripling the supply voltage will yield no difference in headroom.  Let'ssay we took a Tube Screamer and fed it 18v.  In the total absence of the clipping diodes, with gain set to minimum (around 12x), higher supply voltage might yield more headroom.  But consider that the diodes in the feedback path are conducting around 500mv come hell or high water.  So, whatever headroom you get with 18v in their absence vanishes in their presence.  Will the added headroom of the second op-amp stage and the input/output buffers matter?  Not very likely.