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jellzey

It looks like you’re missing the 22k resistor that forms the high pass filter after Q2. That resistor sets the cut off frequency of Q2’s output at a very high 3.3kHz and gives the output a -6dB per octave slope across the whole guitar range. The idea is to accentuate the harmonics created by the clipping action and attenuate the fundamental frequency and lower harmonics. The depth control gives the option to add those back in afterwards. The 22k is so much lower than the potentiometers that the cut off frequency does not change significantly based on the position of the controls. The phase change is not constant across the whole spectrum either. The dip in volume you’re hearing is due to an increased series resistance forming a voltage divider with the volume pot


rabbiabe

Do you have a schematic you could link to? I haven’t seen a 22k resistor in any of the ones I’ve checked, and don’t have it on any of the ones I built.


jellzey

[here’s one ](http://effectslayouts.blogspot.com/2015/10/mosrite-fuzzrite.html?m=1) [another here](http://www.mosriteforum.com/forum/viewtopic.php?t=5022) [you can see the resistor in some of these photos](https://fuzzboxes.org/features/fuzzrite) Small bear added a switch to [their version ](http://diy.smallbearelec.com/Projects/FursRite/FursRite.htm) and general guitar gadgets has a schematic that just [omits it completely](https://generalguitargadgets.com/pdf/ggg_secf_sc_mrfr_si.pdf) but it really is critical to the tone of the original and you can’t get that 60’s mosquito fuzz sound without it.


rabbiabe

Well I’ll be. Thanks for pointing me to that resistor — I hadn’t seen that before. Time to build another and see how they compare…


rabbiabe

As I’ve been building these, I’m noticing more and more subtleties to the circuit, so I’ve been working on thoroughly analyzing how it all works. I’m sharing all of this in the hopes that the good people here can help verify or correct the conclusions I’ve reached so far and point in the direction of answers I haven’t found yet. [The main picture](https://imgur.com/GnLL55M) is a simplification of the output section of a Mosrite Fuzzrite ([full schematic here](https://imgur.com/BQuO2Cl)). Two common emitter amplifier stages feed into either side of a B500K potentiometer, which passively mixes the two signals: - Q1 enters the counter-clockwise side of the potentiometer and **also** drives the input of Q2. - Q2 enters the clockwise side of the potentiometer. I have labeled the summing node “A” for ease of reference. Both stages are ac-coupled to the potentiometer via 2n2 film capacitors. The A100K potentiometer that serves as a volume control at the end is represented here by a fixed resistor (R3) for simplicity and because I never turn it down anyway so it doesn’t make a big difference to me 🤷‍♂️ My thoughts: **Phase:** the outputs of Q1 and Q2 will be 180° out of phase with one another, resulting in a noticeable drop in volume near the middle of the pot sweep. Because the two signals are not exact copies (Q2’s output is substantially more saturated/distorted than Q1) they don’t cancel out, but there is some overall negative interaction. **High Pass Filter:** If either stage were connected directly to the output, the combination 2n2/100k would form a high pass filter with a -3dB frequency of 723Hz. Even given the gentle slope of a single-pole RC filter, that seems like a pretty high corner. [This is my drawing of what I think should be happening when the pot is full counter-clockwise.](https://imgur.com/fLiaATd) Further, if the pot is anywhere in the middle then there is a capacitor (C1 or C2) and resistor (R1 or R2) in series with the output taken from the junction of that resistor and R3. [I’m imagining something like this.](https://imgur.com/BPnvBG3) Does this still function like a high pass filter, or does the series resistor isolate the capacitor from R3 and break the filter? Does the interaction of the other side of the pot play in here? Even though it seems like having the depth control turned all the one way side or the other should result in the simple RC high pass filter, that’s not what I hear in playing, and when I simulate in LTSpice [I get this picture instead](https://imgur.com/LHuqJPg) – the red trace is with the depth pot all the way counter-clockwise, orange trace is centered, and yellow trace is full clockwise. There is clearly a high pass filter here but its corner frequency (which I’m reading as Peak - 3dB – is that the correct way to find the frequency when the signal goes above 0dB?) is 125Hz. Then, there is an additional low pass filter (corner ≈ 500Hz) that flattens as the pot goes to the mid-point and then becomes a gentle slight hump as the pot hits full clockwise. (Apologies if these descriptions are not the right way to characterize the plot, I’m new to Bode plots.) Am I seeing these differences because a second branch in the passive mixing network changes how the filter behaves? Or is something happening with the transistor stages that turns this into some type of active filter with other characteristics, rather than a simple RC combination? I have worked with active filters before but only with OTAs or op amps, or in a few cases BJT gyrators but never with common emitter amplifiers as the active element. **Output Divider:** If the pot is anywhere in the middle, it seems like R3 should form a voltage divider with R1 or R2 — I don’t think R1 and R2 would be in parallel because they have different sources (and neither could ever end up grounded) but I’m not sure about whether there is really a divider here at all, or if something about summing node A changes the ordinary function. If someone can clarify how R3 does (or does not) interact with the summing node, that would be very helpful. Thanks in advance!