Yambo Booster

Yambo Booster
 
 
Yambo Schematic
 
Yambo is a low power booster pedal with a high impedance input and a flat frequency response. The name is an acronym of Yet Another MOSFET Booster/Overdrive. If you want to boost certain frequencies, you can put your favorite tone control at the end of this circuit, or use an external eq pedal. The output voltage will approach 4.5 Vpp, so you may want to have the volume turned down all the way before adjusting it to the desired volume. Under some cases, this can act as an overdrive, especially if the output is connected to something that loads down the second stage.
 
This circuit is fairly simple. The 1st stage is a simple jfet source follower that acts as a high impedance buffer. I used large value capacitor to couple the first stage to the second to minimize reduction of low frequencies. R5 should be a trimmer pot and is used to bias Q3. R1 is used for gain control by adusting the amount of negative feedback to the gate of Q3. Q3 should be biased with gain set with R1 set to 100 kOhm. Turning the gain down will obviously alter the bias for Q3, but because the gain is reduced as the feedback is increased, this should be ok. Q1 acts as a simple current source. This allows the output voltage of this stage to more closely approach 4.5 Vpp. In simulation, it seems to also flatten the frequency response.  
 
This circuit seems to work wonderfully under Multisim. I do not yet have the tools to do a good real world test, but it sounds good to me. When I get a function generator, I'll run some real tests.
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