Chuck D. Bones
Circuit Wizard
I'm been following some of the Grover / 5Lb Bag build reports and thought it was time to breadboard something. As others have stated, it's basically a Si PNP FF driving a Zen Drive. There seemed to be a few extra parts, which often happens when one sticks two circuits together. After a couple weeks of fiddling, this is where I landed.
Knobs (L-R): LEVEL - BASS - TREBLE - VOICE sw - GAIN
I kept the basic topology, but removed a few redundant parts, tweaked the voicing and replaced the treble-cut control with a Baxandall tone stack. Taking a cue from the Diamond Peak, I used low-HFE transistors (Q1 & Q2) in the FF section and added a couple of caps (C5 & C6) to smooth out the very top end. The main difference between the Grover & the 5Lb Bag is the resistor and cap(s) between the input & Q1-B. Folks have complained about squealing with the Grover and that was mostly down to the 100nF cap at the input. The VOICE switch allows the user to select either the 5Lb Bag's fat voicing or a tamed version of the Grover's bright voicing. As is the case in many Lovepedal designs, the GAIN control also varies the bias. This causes the symmetry to change as the GAIN knob is rotated. When setting the Bias Trim, check the tone across the full range of the GAIN knob.
I figured out that the main characteristic of MOSFET body diodes when used in soft clippers is the relatively large capacitance. The two BS170s I used measured around 50pF each on my CCTT. 2N7000s are a little higher. The capacitance varies with reverse bias, which can also affect the voicing. I tried a few different clipping arrangements (notice the forest of spare diodes hanging off of the lower left side of the breadboard). Replacing the MOSFETs with 1N4148 or BA482 caused the tone to be slightly brighter. I ended up keeping the MOSFETs. I found that a BAT46 and a 1N34A in series, or two BAT46's or a BAT46 + 68Ω all sounded the same. Replacing the BAT46 + 1N34A with a 1N4148 (D2) also sounded about the same, so I went with that.
I thought the tone was a bit muffled, so I added R12 & C9 to create a little "presence" above 1KHz.
The VOICE switch up front and the Baxandall tone stack provide a broad tonal palette. Cork-sniffers may notice that the circuit inverts the signal. BFD.
One interesting facet of this design is that the gain from Q2-C to U1-1 is approx unity. The designer (Jeorge Tripps?) deliberately drives the soft clipping stage very lightly. The soft clipping stage is about voicing & dynamics, not gain. The majority of the sustain comes from the FF stage.
As always, all component values are negotiable.
Knobs (L-R): LEVEL - BASS - TREBLE - VOICE sw - GAIN
I kept the basic topology, but removed a few redundant parts, tweaked the voicing and replaced the treble-cut control with a Baxandall tone stack. Taking a cue from the Diamond Peak, I used low-HFE transistors (Q1 & Q2) in the FF section and added a couple of caps (C5 & C6) to smooth out the very top end. The main difference between the Grover & the 5Lb Bag is the resistor and cap(s) between the input & Q1-B. Folks have complained about squealing with the Grover and that was mostly down to the 100nF cap at the input. The VOICE switch allows the user to select either the 5Lb Bag's fat voicing or a tamed version of the Grover's bright voicing. As is the case in many Lovepedal designs, the GAIN control also varies the bias. This causes the symmetry to change as the GAIN knob is rotated. When setting the Bias Trim, check the tone across the full range of the GAIN knob.
I figured out that the main characteristic of MOSFET body diodes when used in soft clippers is the relatively large capacitance. The two BS170s I used measured around 50pF each on my CCTT. 2N7000s are a little higher. The capacitance varies with reverse bias, which can also affect the voicing. I tried a few different clipping arrangements (notice the forest of spare diodes hanging off of the lower left side of the breadboard). Replacing the MOSFETs with 1N4148 or BA482 caused the tone to be slightly brighter. I ended up keeping the MOSFETs. I found that a BAT46 and a 1N34A in series, or two BAT46's or a BAT46 + 68Ω all sounded the same. Replacing the BAT46 + 1N34A with a 1N4148 (D2) also sounded about the same, so I went with that.
I thought the tone was a bit muffled, so I added R12 & C9 to create a little "presence" above 1KHz.
The VOICE switch up front and the Baxandall tone stack provide a broad tonal palette. Cork-sniffers may notice that the circuit inverts the signal. BFD.
One interesting facet of this design is that the gain from Q2-C to U1-1 is approx unity. The designer (Jeorge Tripps?) deliberately drives the soft clipping stage very lightly. The soft clipping stage is about voicing & dynamics, not gain. The majority of the sustain comes from the FF stage.
As always, all component values are negotiable.
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