The objective here is to build a portable guitar preamp that I can carry around and gig with different people. It can also replace the preamp section of my guitar amp, which is a Peavey, and borrow the sounds of Fender/ Marshall/ Vox from its tonestack.
Step 1: Schematics
I did not design the schematics myself. Since my objective is to make this preamp portable, I searched for the simplest preamp design and found this from www.redcircuits.com . This is called a "Solid-state Fender Blackface Preamp", which is a transistor version of the original valve circuit from the "Fender Blackface".
I then borrow the tonestack from the Tonemender that is found at www.runoffgroove.com . The Tonemender is a booster with a very flexible tonestack, which can re-create the Fender, Marshall and Vox responses.
Details of these schematics can be found at the links above, which explain clearly what these circuits do and how they work.
Step 2: Layout
Merging the 2 circuits found, I laid out the board of this little preamp. One thing to note is that pinout of the FET can be different, so it is wise to check the spec sheet from the manufacturer. If the pinout is the same as the ones I use, you should be able to build this project by using the same layout.
It took me some time to draw the layout (which was my first time), but it actually saved a lot of time for me to build the board.
I also matched the FETs by following the instructions found here:
Step 3: Building the Preamp
Not much to explain here. Just solder the parts according to the layout. It is always a good idea to test the board before putting the off board parts all together.
I used shielded wires for the signal path. More work and time is needed in soldering and grounding, but I think it surely worth it.
The preamp can be powered by 18 volt. The 18 volt battery clip is made by using some old 9V batteries.
Step 4: Finishing
Just put all the wires nicely and carefully into the box. Use some cable ties whenever suitable. Label the switches and knobs. Yes. I am a Dymo lover!
Step 5: Testing
Finally, the preamp was connected to my Peavey that has the preamp in/ out at the back. Remember to turn the volume all the way down before switching on anything, just in case.
I also tested the preamp using a pair of headphones and it worked fine for me, so it can also be doubled as a little practice headphone amp. A bonus.
The only problem I found with this preamp is that it has too much gain, so only about 1/5 of the volume knob is usable in my settings and adjusting the volume to level with others' instrument could be a problem. But I also doubted that it depends on the power amp, so I have not made any changes to the part values yet. Any suggestions on this are welcomed.
I hope that you would find this instructable useful!