any specifics on what the Boost really does?

I read somewhere that the boost switch gives a fixed amount of extra gain to the channel and boosts certain frequencies which overall fattens up the tone. Does anyone know exactly what is happening?

@bordonbert I'm sure you've had a deep dive into this circuitry?

Sorry to miss your post Proddy but, in truth, I'm not on here as much as I used to be nowadays.  The way things are going, factual technical help from an engineering perspective is not really what is wanted with too many members nowadays so I find the time I spend responding in a way which gives accurate info so that non-technical people could understand it is often pretty much wasted.  It's a tad disheartening to find that you actually piss people off when you sensitively point out that their ideas simply cannot be true based on actual real life evidence and quote that evidence to them!  "Never teach a pig to sing, it's a waste of your time and it annoys the pig" as Robert Heinlein once said.

Until you asked this question I had my idea of how the Boost works pretty much fixed in my mind.  Errm, it turned out it was time to learn a bit more or at least revise as it had slipped a little!  The Boost switch does not work in the circuitry direct, it only acts on the digital sections and gives a change in gain structure through them.  The actual switch simply sends a grounded control signal to the Master processor when it is engaged.  The Master CPU then sends out a 'Boost enabled' signal which goes to the digipot board and affects the values of a resistor and a couple of capacitors in the Gain control stage and changes their configuration.

The Gain control stage is an active solid state block with a synchronised twin type stereo control pot.  The pot in the input changes the signal level fed to the stage and a pot at the output of the feedback network changes the gain.  Both of these have shaping circuitry around them too and the stage is involved in the overdrive characteristics as well with a very soft diode action built in.  It is within the work I did on modelling the Gain section in Spice but I can't remember offhand what overall change it made to the response of the network.  One cap is across the top of the input pot like the treble bleed of a guitar.  The other components are part of a fairly complex feedback network with a number of changes made when they are switched.

I think you own description is pretty much on the mark though, "gives a fixed amount of extra gain to the channel and boosts certain frequencies".  The "fattening up of the sound" is down to your own ears.  I never really liked over use of that Boost mechanism it didn't seem to fatten up my system, just to make it more shrill.  That may be down to my own lack of empathy for high gain stuff though, I really don't have a feel for what "metal and above" players are looking for in their sound.  I do know when what they say contradicts the facts of what can be measured though! What I hear often doesn't seem to match up to their descriptions of what they do but that could just be different understanding of the terminology used to describe it.

Glad you're still here bordonbert. I hadn't realized you had been less active on this forum. I'm not really a regular either but for what it's worth I always find the feedback from yourself and others extremely helpful, especially when understanding the engineering theory behind the actual sound.

Thanks for digging into the workings on the Boost and the gain stages, much appreciated. Info like that is golden. I dabble in electronics myself (& built a few terrible foot pedals) so understand better how this circuit is shaping the sound now. Personally, I could never get to grips with the Boost on my GM40D and typically just stick to my tube screamer if I need slightly more umpf on my clean presets. Just for comparison, I had owned a Laney Ironheart once and really liked their variable pre-boost knob which simply drives the preamp tubes harder giving it a more natural boost sound.

"...and typically just stick to my tube screamer if I need slightly more umpf on my clean presets."  Now that is where the input buffer comes into its own.  Try this.

Turn off the TS.  Back off on your guitar volume to almost nothing.  Set up your amp on the Clean channel with a bit of gain to give it a little grit.  Slowly turn up the guitar in stages playing between and adjust your Master Volume down to match.  You should reach a point in level where the sound begins to come alive a little more even on the Clean channel.  When you have that then keep on going step by step until your guitar is on max.  You should now be moving into a much more lively and aggressive sound.

Now repeat the same test with specifically the Lead channel.  Set it up with the guitar very low and just enough gain on the channel to give you a little life.  adjust the guitar and Master Volume against each other in steps.  This time you should hear a real change at some point depending on your guitar output where the amp comes to life and starts to thicken up. If it gets too distorted then back off on the Gain control and keep going.

The input buffer is a really cleverly designed piece of electronics and it incorporates 3 levels.  The first is ultra clean and totally transparent (despite what some people will say), the second higher level is clipped on one side only to add even harmonic distortion adding more 'valvey' sound, and the third is a fully clipped TS type circuit acting as a full distortion pedal which gets more aggressive for the Lead and Ultra channels.  This is totally independent of any of the amp's controls and is managed only by the level of the input signal you push into it, your guitar or the last pedal output.  The amp has ample gain to cope with low level signals passing cleanly through the input buffer when you want it to stay clean or when you only want the valve stage distortion.  However, it allows you to, as you said, add 'umpf' to the sound at medium levels and real TS distortion at high levels once you know how to use it.  That still means you have the other internal distortion generating mechanisms of course but that first stage is a real boon and you may not need that TS in front at all if you can get to grips with it.

I do find that many people just hit the front end of the amp with all they have got in the bellief that that will generate 'valve sound' and try to tame it after that. This is a mistake with the H&K design as the front end can be forced to put in far too much clipped distortion on top of their chosen pedal distortions and the other mechanisms later in the chain. That's real tonal overkill even for the most aggressive of us.