I breadboarded a simple and basic booster which began as a MXR Microamp clone. I was looking for something to push the amp into a certain sweet spot and the Microamp wasn’t quite right. I thought it was too farty, frizzy, and prone to clipping for what I wanted. I changed many of the part values, moved the pot to the feedback loop, and adjusted frequencies a couple of times to have a wide-band-pass filter which increasingly loses treble as the gain is increased. The gain of the booster is also decreased to 8 (18 dB), versus a gain of 20 (26 dB). This is to prevent clipping the op amp to an extreme with modern humbuckers. If a person is using vintage pickups, the values can be changed to get greater gain, but it isn’t really necessary. I used it with my humbuckers in pseudo-single coil, parallel wiring and it made them pop right out, adding clarity and girth.
Whereas the Microamp is meant to boost literally everything being produced by the guitar, the McBooster has less bottom to prevent blocking distortion, and less treble to save my ears from ringing. As the gain increases, it goes from an open 45 kHz (gain of 1) to a more distortion-friendly 2.2 kHz (gain of 8) cutoff, by way of the low pass filter created in the feedback loop.
The cutoff frequency of the bass fluctuates. The input is very high impedance, so that filter is basically letting all of the bottom pass (roughly .2 Hz). The high pass filter created by the RC to ground from the feedback loop is 88 Hz, which makes 176 Hz the point at which the bottom begins to fade from the signal being boosted. This tightens up the sound, but leaves the 200 Hz to 300 Hz area unaffected for palm muted chugging and keeping the meat of the sound intact. The “out” is cutting off below 72 Hz, so 144 Hz is the point at which it begins the fade.
All of the frequency cuts put the guitar’s central voice into the amp and removes the excess which could make the sound muddy or sharp as it gains volume. At low-to-mid gain settings, the sound is sharper. During the last 60 degrees on the gain control, it begins to really get loud and needs the edges taken off.
If a person decided to run it from an 18V supply, the gain could be increased back to 20. With 9V, the op amp hits the rails at about a gain of 10 or so and quickly begins to increasingly clip the signal into squared waves. It’s cool for making an amp sound like a fuzz, but I didn’t want that. I needed something more surgical.
I used the Dual Rectifier as the main point of reference for shaping the tone of the pedal. I also briefly ran it into an old Harmony amp. It caused the little 8 “speaker some pain, but sounded pretty good for a cheap, old, PoS amp.