Power Supply Question

[DocAmplify]

I have a box of old power AC to DC power supplies (I think I see them referred to as "Wall Warts" on the forum).  

The supplies have a wide range of specs; most are +ve center and -ve sleeve, so I plan on reversing that, but I'm curious what is the ideal rating for a stomp box?  

For example, I have one here that's 7.5v 1000mA.  Is that too low to power a stompbox?  

Let me revise this.  I got my calculator out.

I'm using 9v and my circuit has 100K of resistance.  That means my circuit has .00009 amps of current (.09mA) - right?

If I use a 7v power supply, I could reduce my resistor to 77K and keep the same current.  

I guess my question is, do I need to keep a certain voltage, or is this all about the current?  I'm deciding that I don't want to do this because if I ever choose to use a power board with a 9v source, I'm not going to pull it apart and change the resistor before I do it, but I still want to make sure my math is correct.  

Edit #2 - I'm on a roll, so I'll keep asking.  The amps listed on the power supply; is that the maximum it can produce, and the stompbox just takes what it needs up to that point? I think I read that in another post somewhere.  I thinking that the voltage (the push) is constant, and then the resistance of the circuit regulates the current.  Am I right on this?

[Seljer]

The amps rating is a maximum, its guaranteed to give out at least the specified voltage up to that current (and if you go to high you risk the possibility of burning something...). So yes, you're right about the second thing.

1000mA is plenty of current capacity, enough for an entire pedalboard.

The 7.5 volt rating is slighty different. No its not too low to power a stompbox. 7.5 volts is basically what you'd get from an almost dead 9volt battery. Some pedals play nicely with lowered voltages (mostly pedals based around opamps) and the only thing you lose from 9 volt power is clean headroom. Transistor based pedals usually aren't as cooperative with varying voltage. The bias points have to be precisely set for optimum amplification and whatnot and lowering the voltage with adjusting things often leads to gated fizzy sounds (which people sometimes happily exploit in various fuzz pedals for more sonic options)

Another thing worth mentioning is that power supplies have an unavoidable internal series resistance. You can think of a supply as an ideal voltage source (it always puts out the specified number of volts) with a resistor wired in series. If you measure the voltage on the output of the powersupply without anything connected you get the voltage of the source. However, if you hook up a load, the load draws a current, the current creates a voltage drop over that internal resistance and thus, the voltage if will be lower on the output of the power supply if there is a load connected.
If your wallwart is unregulated it would suffer from this phenomenon. The rated voltage is usually specified at maximum current, with no or little load the voltage is most likely higher, maybe even the range of a couple of volts above the rated voltage. Get a multimeter and measure it. Regulated power supplies have electronic components that guarantee even as the current rises the output voltage stays the same.

[DocAmplify]

Excellent reply; that answers my questions.  Thanks.