Darkglass front end filtering question

[slowferrari]

Can anyone help me wrap my head around this filter that directly follows the fet booster stage of the microtubes pedals? Taking a WAG, it looks like it's shelving high and low frequencies. I'm basing that on the fact that it looks a bit like the contour control on the marshall stomp boxes or the filter circuit on a shin ei FY-2, both of which scoop mids, but reversed, and with two shunts to ground, instead of one.

So like, where are the corner frequencies though?

I'm trying to figure out how the tone is being shaped before it hits the cmos inverter because I've been messing with a lot of cmos circuits and I'm trying to figure out why the darkglass pedals sound so much better with bass than my experiments have been.

No hard feelings if I'm the only one who finds this an interesting topic.

[antonis]

Hi & Welcome..

Don't get me wrong but it should be more convenient to post a more widen part of scematic..
(e.g. there should be a series resistance before C5, even if it's a couple of ohms, or else that part could be disregarded..)

[slowferrari]

Sure sure. I cropped it because I figured it would make it more clear which part I was asking about, but this is the full schematic.

That said, now that you mention it, there *isn't* a series resistance there, and there isn't a coupling cap from the previous gain stage either...

I never noticed that, and it seems weird.

[antonis]

I shouldn't involve into pencil calculations due to many interactive HPFs/LFPs, not only in the area originally posted but also in IC_2B respective filters..

P.S.
IMHO, too much trouble  for a minor result..
(but, it's just me..)

[PRR]

Welcome.

....there *isn't* a series resistance there, and there isn't a coupling cap from the previous gain stage either..

There is a LOT of series impedance. That stacked JFET with the C from G to S approximates to "infinite output impedance".

You could simulate that thing but waste of time. If you listen and think you want lower pitch, increase all capacitors the same proportion, or vice versa.

The lack of blocking cap will give it a pop when switched?  ---- oh, only 220pFd, only a small tic.

[FSFX]

That said, now that you mention it, there *isn't* a series resistance there -

As mentioned above, the mu-amp configuration of the two JFETs has a very high output impedance. In this case, with J201s, it is around 300k ohms at 1khz.
Due to the capacitor from its source to gate, which is a bit like a 'bootstrap', the load presented by the upper JFET is approaching infinity and so the effective output impedance is just down to the output impedance of the lower JFET and that in turn is also affected by the operating current and the value of the source bypass capacitor and the signal frequency.
Normally a mu-amp configuration like that would be followed directly by a buffer to overcome the limitations created by the extremely high output impedance.

R G Keen explains the mu amp in one of his articles:
http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

[Rob Strand]

So like, where are the corner frequencies though?

I'm trying to figure out how the tone is being shaped before it hits the cmos inverter because I've been messing with a lot of cmos circuits and I'm trying to figure out why the darkglass pedals sound so much better with bass than my experiments have been.

No hard feelings if I'm the only one who finds this an interesting topic.

The part of the circuit you extracted is a notch filter.  The three cap passive notch is called a Hall Notch (1955). The notch frequency is around 300Hz.   It's just the common notch you see in the 200Hz to 600Hz region on bass effects and amplifiers.   You can implement a notch in a number of ways.  The Hall notch is most prolific in Ampeg bass amplifiers.   There are other notch circuits with less parts which sound slightly different.   The Fender/Marshall tone stack also has an intrinsic notch characteristic in the same frequency region.

While the output impedance of the mu-amplifier *and* the parts following the notch filter do affect the response, the response is what it is.  The effect of the surrounding impedances would only be a "issue" if you try to analyse notch circuit alone (by hand with maths without those part as you expected the parts to have a small effect), or, they were excessive enough for the circuit to not work normally.   For a 300Hz the output impedance of the mu-amplifier is bordering on low enough but still has an effect

The circuit around the 2xJFET mu amplifier has quite a bit of frequency shaping - from the input cap and source caps.  There's also frequency shaping after the notch and in fact throughout the whole circuit.

Circuits which sound good are usually the result of someone spending a lot of hours hand tweaking all the frequency points and parts.


Here's a simplified simulation of the parts around the mu-amp and notch.  Not the whole ckt.



FYI,
Hall Notch
https://www.kennethkuhn.com/electronics/design_and_applications_of_the_hall_network.pdf