9v Wall AC adapter? Not really 9v??

[gooseEL34]

Hey all.
I am a lawyer first, guitarist second, and tinkerer third. I can read a schematic, I can use a Fluke to test for resistance, current, and voltage (both AC and DC), I have built tube amps from schematics.   I am scared of the B+ supply in an old Ampeg SVT (for good reason). 
But that being said, an EE I am not.  I have been mostly self taught.  Recently I learned about P-Channel jfets and diagnosed a guitar pedal with a FET switching system and then built a test rig to watch a JFET "open" and "close" as a switch when voltages were applied (super neat!)


 Here is the issue....

Many of the guitar effects processors (called pedals) run on a 9V battery OR a 9v AC wall wart.  Normally, there is a 5mm barrel (female) on the unit, and it is sold with a 9v (normally it is center negative) AC plug unit that terminates in a 5mm barrel connector and 2.1mm center pin.  The female barrel is a switching connector and disengages the battery leads when a wall wart is connected. 

Here is my question:

There are MANY MANY MANY things that use a 9v center neg adapter supplying 200-500mA.  I grabbed three at the local goodwill.  They all had labels (or embossed lettering on the unit) which said they were 9v DC output (120vAC 50-60Hz input) and generally supply .3A to 1A (although 200mA and 300mA seem the most common).

Here is the rub.  Every one (albeit without load) showed 13v on my Fluke and the same on my scope (older tek).

On the scope (no load), the output of all three was giving me ripple free power, but I am unsure if these are built to supply 13vDC and will "drop" once connected to a load drawing 200-300ma current.  OR, do most non guitar things not care if there is an extra 3-4 vDC shown to them??

Thanks
Tim

[theatrus]

This is very common for the "transformer + cap + diode in a box" wall-warts. Once loaded, they will be much closer to 9V, but remain unregulated.

Modern switch-mode power adapters are regulated, lighter, and more efficient, but may have high frequency ripple.

[mariush]

As above, without any load those adapters usually output a higher voltage... 13v is a bit high, but not uncommon.
Keep in mind that 9v batteries are actually about 10v when fully charged, so any circuit using 9v batteries shouldn't be very sensitive  to the exact voltage.
I would guess the 9v is required because some use opamps that need at least +/- 3 v to work (6v) and because some opamps are not rail-to-rail, therefore they may want some room above that.

If you're still concerned about it, there are linear regulators with very low voltage drop that you could use to make sure the output won't ever be more than 9v.

For example:

LM2940-9 : http://uk.farnell.com/texas-instruments/lm2940t-9-0/ic-v-reg-ldo-9-0v-2940-to-220-3/dp/9493514  ( ~ 0.15v drop @ 200mA , 0.5v drop at 1A)
ba90bc0fp : http://uk.farnell.com/rohm/ba90bc0fp-e2/ldo-fixed-9v-1a-to-252-3/dp/2342873 ( ~ 0.3v drop at 1a, less at lower currents)

If your input voltage is higher than 9v + that voltage drop (which varies with the current used), the regulator outputs 9v. If your voltage is less, it just outputs the input voltage, minus the small voltage drop (so you can still put a 9v battery if you modify a pedal and put the regulator inside

[deth502]

seems your question was answered.

one thing not mentioned, is that once you do load them down and get your 9v output, its probably not going to be too clean.

i just made a linear regulated power supply capable of a few amps and ran several plugs off of it to power each pedal.

i came up with the idea myself, but i dont think it was anything groundbreaking, as i have seen several similar products on the market since.

[John Coloccia]

Most pedals aren't going to care about a little higher voltage, and those power supplies come right down to more reasonable voltages as soon as you load them a bit.  The noise is not much of an issue.  Any pedal will have relatively large decoupling caps in the power supply.  100uF is pretty common and wall wart noise usually not a problem.  If there's something important in a pedal that you really don't want to blow up, you would usually protect it with a zener.  For example, my next product uses a charge pump and they are VERY sensitive to overvoltage, so it will get protected.  Very few builders bother with regulators unless they're running some sort of processor that needs it.  Since these things are usually also designed to run on batteries, it had better work under a wide range of voltages.

[hamster_nz]

Many of the guitar effects processors (called pedals) run on a 9V battery OR a 9v AC wall wart. 

If it truly is an AC adapter (at it is, for at least a BOSS unit I looked at), then this is expected behavior.

Your meter is reading peak voltage, not RMS voltage. Because the AC is a sine wave (at first approximation) the RMS average value is 70% of the peak value. 13V * 0.7 = 9.1V.

It allows the pedal to convert it 9V DC with minimal parts - just a diode or two and a capacitor before the 9V regulator.

Mike

[John Coloccia]

The adapter should put out 9VDC.  The only unit I can think of off the top of my head that runs on low voltage AC are some of the Line 6 units, I believe.