Due to a lot of work commitments, I don't know when I'll be able to apply the following mods to the basic Orange Squeezer. I'll cover the mods now in case any brave soul wants to give them a try.
There are five basic mods (actually six - but I'll leave the sixth for the moment) that can be done with pots and switches. You can do as many of the mods that you want or as many as room on your enclosure will allow.
The modded schematic:
The first two mods concerns setting the gain of the op amp. This basically equates to setting the threshold level on a compressor (the point at which the compression kicks in). In the basic version of the Orange Squeezer the gain of the op amp was set at 23 (if you used a 220 kΩ resistor) or 21 (if you used a 200 kΩ resistor), by adding a pot, R9a, in series with the R9 resistor, you can adjust the gain from 16 to 26; if you also add a switch across R9 you can reduce the gain from between 1 and 11. If you want to make the gain transistion smoother you could change R9 to 110 kΩ, this will give you a gain structure of:
Switch closed: 1 to 11
Switch open: 12 to 22
The second pair of mods concerns adjusting the values of the capacitor (C7) and resistor (R12) combination, the values of these two components set the release time of the compressor (the length of time after the signal drops below the threshold level when the compressor stops compressing). The basic circuit uses a 4.7µF capacitor for C7, Mark Hammer suggested changing the value of C7 to 10µF which increases the release time. By adding a second capacitor (C7a) and a switch you can now choose between two capacitance values, switch open 5µF, switch closed 10µF giving you a short and long release time. If you want to increase the release time even further, make C7a larger - an example C7a at 22µF and C7 at 6.8µF will give you a total capacitance value of:
Switch open: 5.2µF
Switch closed 22µF (I suspect this might be too long a release time)
The second mod suggested by R.G. Keen was to change C7 to 10µF and replace R12 with a 68 kΩ resistor and a 500 kΩ pot.
If you have room on your enclosure, it might be worth adding both of these mods for greater flexibility.
The last mod is basically changing the trim pot R7 to a pot which is mounted on the enclosure. R7 equates to the compression ratio of a compressor (it sets the amount of reduction of the signal going into the op amp). The problem with just replacing R7 with a 10 kΩ pot is that there is a dead area at one end of the pot's rotation - no signal at the input of the op amp. Ideally the pot should provide heavy to light compression throughout its full rotation. This is done by adding a resistor, R7a, in series with the pot. The bad news is that it is impossible to calculate the value of R7a - this is due to the very wide differences in the electrical characteristics of JFETs. The following method will ensure you get the correct value for R7a:
Take a 10 kΩ pot and connect lug 3 to b19 of the circuit board and lug 2 to ground.
Power up the circuit, plug you guitar in and adjust the pot until you get a low-level signal that you are happy with.
Without disturbing the pot setting, remove the pot from the circuit board (very important !!!)
Measure the resistance between lugs 2 and 3 to get the correct value for R7a.
OK, enough talk - onto modding the board:
To add the gain pot to the op amp:
Change the b7-g7 jumper to a c7-g7 jumper and wire the R9a pot to b1 and c1.
Dont forget to connect lugs 2 and 3 of the pot together.
To add a switch to the op amp feedback resistor:
Wire one side of the switch to lug 1 of the pot R9a and the the other side of the switch to k1.
To add a capacitor selection switch:
Instead of connecting the C7 capacitor between j16 and l16, connect it between j16 and k16.
Add the C7a capacitor between k13 and l13.
Connect one side of the switch to j19 and the other side of the switch to k19.
To add a pot to R12:
Remove the jumper from l17 - m17.
Run a wire from m19 to lug 3 of the R12a pot and connect lugs 1 and 2 to ground.
To change the trim pot to an external pot:
Remove the trim pot R7 from the board.
Calculate the value of R7a, as described above.
Connect R7a between a17 and b17 (damn - there goes my completely flat layout)
Run a wire from a19 to lug 3 of the pot R7 and connect lugs 1 and 2 of the pot to ground.
And there you have five mods for the Orange Squeezer.