The problem with the stock Orange Squeezer schematic is that it assumes you will use a battery, so it has no hum rejection built-in (or polarity protection). Adding a filter cap to V+ rail doesn't help that much in this circuit, in my experience. This is because the power ripple is being fed to the non-inverting input of the op-amp via the 390k resistor. That op-amp provides about 25x voltage gain, so any ripple is amplified by a factor of 25. Even a small amount of ripple will be noticeable. What it needs is either low-noise biasing or a regulated adapter. I use both, but either one on its own will probably fix you up.
Using low-noise biasing means you can use an unregulated supply. Unfortunately, it also means that your existing layout will have to be modified. The biasing resistors are the 390k and 470k that meet at the non-inverting input. What you need is a filter cap to ground from that node. (100 uF will give you a corner frequency of f=1/(2*pi*390k*100u) = 0.004 Hz, which will provide 66 dB attenuation at 60 Hz, and even more attenuation at the ripple harmonics.) But if you put a big cap there, you will bleed your input signal to ground, treble especially. So you need to put a resistor (220k) between the non-inverting input/signal (from the 47nF cap) and the bias resistor node. (220k is chosen because 390k in parallel with 470k is 213k, so 220k will provide roughly the same impedance to ground as the stock circuit.)
So the recipe is:
1. Lift the 470k and 390k from the non-inverting input and join them back together so that they are not touching the non-inverting input of the op-amp. Make sure the 47nF is still attached to the non-inverting input.
2. Put a 100u from the junction of the 390k and 470k resistors to ground. (Cathode on ground.)
3. Put a 220k from the same junction to the non-inverting input of the op-amp.
Low-noise biasing is a very useful trick and you can get a lot of mileage out of it. Every circuit I build gets modded for low noise biasing if it isn't already included on every gain stage. Buffers are not gain stages, but I do it there, too, though that may be overkill. If you want to try it and can't figure out how to adjust your layout, post the layout and we'll help.
Alternately you can just buy or build a regulated adapter. Fortunately you can turn a crappy 12V wall wart into a regulated 9V adapter with a regulator and two caps. Look
here for the datasheet schematic. Read further in that thread and also
here if you want to get more ambitious and build a regulated multi-supply. You can build the regulator circuit into the pedal, but then you'll have to remember to use a 12V adapter, as it won't work with 9V anymore because the regulator needs an input voltage at least 3V higher than the output voltage.
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