1) that is a STRONG low pass filter and is NOT common or needed, at all, in normal SS guitar amps.

2) you do not show where they lead to, but a few amps or preamps or pedalboards, digital or analog, include similar 24dB/octave or stronger low pass filters as "cabinet simulators".
Normal load for a guitar amp is a guitar speaker which has a built-in "mechanical" low pass, usually above 4kHz or so.
If signal is fed into a recording or PA mixer or into a powered cabinet which have way wider response, tweeters or drivers, etc. , sound is *unbearably* buzzy, so such a filter is added to mimic guitar speaker rolloff.
Your circuit is incomplete but I guess it either feeds headphones/line out or somebody complained about buzzy sound and designer included it for good measure.
Again, not "common" by any means, you do not *need* it.

As a real world example, Hughes & Kettner Red Box: a  signal attenuator -direct box - cabinet simulator, including such a filter:
https://elektrotanya.com/hughes-kettner_red_box_classic_sch.pdf/download.html

The filter section is indeed low-pass filtering. It's using the Sallen-Key topology which creates a two pole filter (-12 dB per octave roll-off).

The good old Okawa Denshi site has a calculator for this.

To use a calculator, we need to interpret the capacitor values. The schematic gives some as two digits followed by a p, so 47p is clearly 47 pF and 470p is 470 pF.  I believe that the unsuffixed codes have to be interpreted the same as capacitor markings. For instance 473 means 47 with three zeros or 47000 pF (= 47 nF, = 0.047 uF).

The prefixes C and M give tolerances, I think: C means +/- 0.25 pF, and M means +/- 20%.

For the first stage, we use the first calculator presented on the above page.

The first stage has slight peak (boost) around 10 kHz, after which it rolls off at the -12 dB per octave rate. The C prefixes on the capacitor values, I think, are giving the required tolerance: +/- 0.25 pF, which is rather stringent.  So this filter not only rolls off after 10 kHz, but also provides a little presence boost there.

The second stage has gain in addition to the filter, and so we use the third calculator which is for a circuit that incorporates the same feature.

That filter has a cut off frequency of around 8.6 kHz. No bump a the knee.

There is also an RC filter between the two stages consisting of R15 and C16. This has a cut-off frequency of also around 8.6 kHz, rolling off at -6 dB per octave (one pole).

Thus the RC filter and second stage are intended to create a three-pole low-pass filter, rolling off -18 dB per octave after around 8.6 kHz.

This makes sense if you're coming from a high gain circuit that generates a lot of fizz, and then going into a clean power amp. Why four poles though? The answer is almost certainly that this is for an amp that has a tiny 8" speaker. I don't think you'd want this much roll off going into a cabinet with 12" speakers, especially if their resonant frequency is below 100 Hz.

In my rack, I have an ADA MP-1 pre-amp, and clean power amp going into fairly bright 12" speakers. In between there is a 31 band eq and a splitter/mixer/filter device of my own design which has a Sallen-Key low pass filter with a variable frequency control knob.

I need to use the high frequency bands {6, 8, 10, 12, 16, 20} kHz on the equalizer and my low-pass filter to have good control over the high end.

This summer Laney released the Lionheart Foundry series.  A trio of 60w amps.  The LF60-112, LFSUPER60-112, and LFSUPER60-212.  All three sound great in the video clips I have listened to, and while I tend to shy away from the term "tube sound", I do agree that these sound pretty fantastic.

From Guitar World: "The LF60-112 ($429/£279) is the most affordable of the trio, offering the core spec for minimum outlay. Next up is the LFSUPER60-112 ($539/£349), which adds tremolo and chorus effects, with controls for Rate and Depth. Most intriguing, however, is the LFSUPER60-212 ($699/£429) – a vertical 2x12 combo."

https://www.guitarworld.com/news/laney-lionheart-foundry

Quote from: phatt on September 11, 2024, 11:08:52 PMThanks Joe,
Yeah, If I didn't already have a sound/tone pedal board that I'm quite happy with then I would certainly check this out.

Using Analog circuitry for cab sims and amp parts is likely part of the reason it sounds good. Full bore dirt is a bit weak in the low freq but hard to judge when it's recorded on a phone.
Reverb sounds ok.

Certainly far more usable than a lot of other pedals. (what happened to the thumbs up emoji??)  

The one sound a lot of these fancy pedals can't seem to crack is that magic sweet Santana lead tone, which delivers a huge cut through without the brittle irritating top end fizz. I'm yet to hear that done.
Phil.

I totally agree.  Also I fixed the emojis   

The circuits around IC5A & B look like gyrators (rather than low pass filters per se) to me.
Resonant filters.
There's probably online calculators to derive their response, and teemuk's book will have the circuit explanation / analysis / equations.
https://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf

Thanks Juan,
            At least I have several recording of my wife singing that we recorded some years back. I'm so glad that I persuaded her as she was a trained singer with a unique tone and range. I was able to play one of her recording at her Funeral which got a huge response.
Now I just sing at open mic jams, the music keeps me sane.
Phil.

Thanks Joe,
Yeah, If I didn't already have a sound/tone pedal board that I'm quite happy with then I would certainly check this out.

Using Analog circuitry for cab sims and amp parts is likely part of the reason it sounds good. Full bore dirt is a bit weak in the low freq but hard to judge when it's recorded on a phone.
Reverb sounds ok.

Certainly far more usable than a lot of other pedals. (what happened to the thumbs up emoji??)  

The one sound a lot of these fancy pedals can't seem to crack is that magic sweet Santana lead tone, which delivers a huge cut through without the brittle irritating top end fizz. I'm yet to hear that done.
Phil.

Blackstar's Debut lineup has been quite impressive, and their new little amplifier is no exception.  The Debut 30E sports a 10 inch speaker and 30 watts of power.  Small enough to practice at home, large enough to practice with the band.  The official demo shows plenty of beautiful cleans, old school overdrive, punk rock distortion, and even some fairly thrashy tones.


MSRP is $209 USD

https://www.musicradar.com/news/blackstar-launches-the-tape-echo-equipped-debut-30e-practice-amp

Last month Quilter released the DirectAmp.  This is a new take on their existing SuperBlock amps, this time incorporating Fender, Vox, and Marshall sounds into one box - but not including a power amp, this is a preamp only.  The video linked in this review does show some decent tones, but I'm not sold yet and really want to get my hands on it myself.  Take a look yourself, what do you think?

MSRP is $399 USD and has stereo XLR output, headphone output, effects channel, and bluetooth input.

https://www.guitarworld.com/news/quilter-direct-amp

I´m impressed by the designs and the looks, congratulations.

This is a DREAM opportunity for somebody wanting to build up a small Factory, either a Tech who wants to take the next step or an investor, who wants to multiply his money and hires a Tech to do the soldering and assembly, he takes care of sales (and collecting money ).

The amps can be assembled and sold as-is in those futuristic head-shells or rehoused in more conventional ones, his choice.

The see-through illuminated front panels look breathtaking on any stage (or Album cover ), of course conventional punched and painted (or polished) Aluminum can also be used.