Promethium Filter Question

fractal33

Active member
Hello, I'm new here and to pedal building in general and I just recently put together a Promethium distortion successfully. It sounds great, but one thing I noticed was a little reduction in the low end with the distortion knob turned all the way down compared to my original HM-2. I'm aware that there are always going to be differences when dealing with clones and tolerances of components compared to the original pedals, but I did notice one small deviation in the Promethium schematic vs the original HM-2 and some other clone schematics.

It appears on the promethium that C5 and R10 are in reverse order compared to the original HM-2 (and every other clone schematic I've seen) and I'm wondering if this is creating a high pass filter when the distortion is at zero that could potentially account for this slightly reduced low end compared to the original? Let me emphasize that I am new to this and I'm sorry if I'm completely off track here. Thanks!
 

Attachments

  • Screen Shot 2021-09-14 at 10.51.01 AM.png
    Screen Shot 2021-09-14 at 10.51.01 AM.png
    161.9 KB · Views: 12
  • Screen Shot 2021-09-14 at 10.53.33 AM.png
    Screen Shot 2021-09-14 at 10.53.33 AM.png
    196.8 KB · Views: 12
They are equivalent circuits. Since the components are arranged in series, the order does not matter. The difference you're hearing is most likely due to component tolerances and actual values. Note that the taper of the Boss unit is slightly different than the A taper used in the promethium. That wouldn't factor in with the control at the extremes, but it would affect the sweep. (D-taper is a more extreme version of an A-taper.)
 
If the C1R1 and R2C2 combos were not in series, but like this:

mnm_Passive_Second-Order_RC_Band-Pass_Filter.jpg



... then it would make a difference.
 
Thanks everyone that is what I was thinking too but I got confused because when I was reading a breakdown of this circuit it was noted that there was a high pass filter being formed on the other side of the pot with C8 and R16 when distortion is at max, is that in parallel on the other side? I understand the difference between series and parallel but I’m still kind of figuring all of this stuff out and it isn’t clear to me here.
 
In your first diagram, my understanding is that the "Distortion" pot at full clockwise (pinout & rotation-direction noted on schematic) would null & void R16 & C8, and R10 is being added to the pot's resistance for a total 250150 Ohms — all going to ground. With C5 before it, it forms a HPF.

Turn the Distortion all the way off (CCW), and it prevents C5 & R10 from performing any filtering, basically takes them out of the circuit — but then you've got the Dist pot's 250k added to R16 going to ground with C8 forming another HPF.

Add 'm up & crunch 'm down...
Fully CW: 250150 Ω + 10µ = 0.1Hz
Fully CCW: 297k Ω + 0.047µ = 11.4Hz


A 0.1 Hz and 11.4 Hz corner frequencies... Dist at min or max, you'll hear no tonal difference, just the variation in distortion itself. How much it distorts will introduce different partials/overtones, so I would guess this is the source of most tonal differences involved with this part of the circuit, not the HPFs. More distortion, more overtones.

If I'm off base on any of this, I welcome any correction.



How I learned to stop worrying and love the bomb...
 
Thanks everyone that is what I was thinking too but I got confused because when I was reading a breakdown of this circuit it was noted that there was a high pass filter being formed on the other side of the pot with C8 and R16 when distortion is at max, is that in parallel on the other side? I understand the difference between series and parallel but I’m still kind of figuring all of this stuff out and it isn’t clear to me here.
Yeah, I could see that R16, C8, and R18 are all going to the inverting side of the TL072 op amp. This makes that stage an active inverting high pass filter.

Although, I think it's actually technically a bandpass filter because of C9 being there too. This is more advanced than the rudimentary circuits they taught us dumb mechanical engineers.
 
Back
Top