the fabled FET guitar preamp, something I bought for £10 in a bargain bin before it got all famous. I mean it is a good device but some of the crazy prices on eBay that it goes for are just, well - crazy
the reason of course is because 1. famous people use them and 2. they can't be made anymore because the monolithic preamp ICs inside it were discontinued however I have cloned my FA-1 replacing the preamp ICs with 741 ICs and honestly, it sounds exactly the same and you get the same voltages at important test points within the 5% tolerance range set by Boss on their original service manual schematic. and this is because the sound isn't in the IC it's in the circuit! the filter stages and so on. obviously different ICs have different gains and bandwidths but in the case of a guitar effect unless the IC has a significant gain difference you won't hear a difference. which is why I get so annoyed with people who say they have "modified" a guitar effect by changing an op-amp or when they describe a certain op-amp as being "warmer" anyway I won't go into all that crap.
here is the layout and my schematic for the Boss FA-1 - I included my schematic because even though you can readily find the original, it leaves out important information like op-amp inputs, I had to figure out whether connections went to inverting or non-inverting inputs for example.
someone else worked the circuit out using a dual op-amp however he was reluctant to share the schematic for people (there were comments asking but he never posted it and you just have to think - what a dick)
The FET - in both my schematic and layout I used the MPF102 however using this particular FET will add a lot more clipping and if you wish to have a cleaner sound go for the original 2SK246 - however I warn you it has a different PINOUT so you may have to do some leg bending.
VOLUME POT - you need to use a LOG pot not a Linear one
|spot the difference|
|someone on twitter asked me how to add a switching jack so I drew this - I thought I may aswell add it here|