The info is there Dieter but probably not in one place. And the question you are asking is NOT a simple one. The GM series is a complicated beast and people won't like some of what they are about to read but it works, and works incredibly well so please, no moaning about "the purity of all valve circuits" and how "we've been duped into thinking the GM was a valve amp" from anyone. You listens, you likes, you buys, you no likes, you leaves. It's as simple as that.
Firstly I need to say that I can only speak on the GM36, I have never seen a GM40 in the flesh let alone played one. They really don't interest me beyond my GM36 based on what I have heard so far online.
Let me say up front, there are opamps in the signal path at a number of places. GOD NO!!!! People on the outskirts of the music business generally do not know what they are talking about when they whitter on about the audibility of opamp circuits. If designed properly they introduce less than 0.01% distortion under any conditions which is absolutely inaudible in any way in a guitar amp. Tone suck? It doesn't exist nowadays for the competent designer, and H&K are very competent designers! In this level of audio work there is absolutely no "magic opamp" either. I have a document produced by a very good audio electronic engineer named Samuel Groner where he tests and compares 59 opamp types from the lowly TL071 to expensive Analog Devices and Linear Technology OP and OPA types. With top flight measuring equipment they clearly show differences in the raw opamp types outside of their use in feedback loops with the more expensive being better. However, within the guitar audio spectrum, with feedback correctly applied, with internal clipping prevented under any circumstances, with frequency and slew rate limits adhered to, with loading restrictions adhered to, they plain cannot be identified. It is only under error conditions that the better ones outperform the cheaper and if you prevent any error conditions from applying..... Save your money and use a modern solid general purpose device over some expensive exotic or magic component, you won't hear any difference under proper test conditions or during live playing. I can supply Groner's report as a PDF to anyone who is interested but it is dry reading if you are not in the field. (But no, to too many people Groner could not know anything about the real world side of this as he is only an engineer not a "music techy" who can clearly hear the differences when he knows which is which!
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Ok, on to the circuitry.
The input is fed to an opamp input buffer/overdrive circuit. This is designed so it can produce 3 phases. First, with low input it acts as a perfectly clean buffer. Second, with medium input one side begins to clip introducing assymetric even order harmonics and distortion. Third, with high input it clips on both sides producing a harder distortion. It's the equivalent of having a better Tubescreamer type pedal inside your amp which you control with your guitar volume knob.
The signal is then fed to another opamp stage acting as an active Gain control. It has a twin pot with one part feeding the opamp and the other after it. This stage also introduces a degree of tone shaping and overdriving.
The Gain stage feeds the valve stages. They have a front end triode used by all of the channels, a second stage used only when in high gain channel mode, a third stage common to all feeding a DC coupled Cathode Follower which introduces a lot of the characteristic valve overdrive quality. All of these valve stages are set to offer different gains and frequency responses according to the channel chosen. This signal is then returned to the microcontroller board.
It then feeds the tonestack which is pretty much standard and is buffered by another opamp stage. This opamp stage has the Volume control in its feedback loop altering the stage gain. Again values in the stack and buffer shaping are switched according to channel selection.
Next we have the Fx Loop with a Send opamp buffer. The Mute control and Noise Gate both act on the signal innediately before the Send buffer.
Another opamp buffers the Return signal before it is shared off to the Modulation effects and blended back in when it returns at the input of the Resonance and Presence control block, (two more opamps).
The Reverb is added next around another opamp acting as the preamp output buffer then it is off to the Preamp Out.
The Master Volume fits in here. It is an active design around an opamp stage with the usual full overdrive protection to prevent it from clipping internally.
Finally, an output buffer with a little channel selective tone shaping and off to the power amp.
So that's the basic signal path. You get a lot for your money with a GM. You could do worse than have a look at the mammoth modelling job I did on most of the circuitry in the amp reported in another thread here. The opamp circuitry is magnificently designed. At no time is any opamp allowed to clip internally as any overdrive or signal level protection is performed by external components keeping levels inside the opamps below the rails at all times. Source and load impedances are very well chosen and matched to again keep the opamps in their comfort zone and producing no discernible unwanted signal modification, unlike many of the ultra expensive boutique pedals I look at which use opamp stages! These and a couple of other constraints prevents the characteristics of the opamp from affecting the signal quality in any way.
The character of the opamp stages is always neutral, all of the basic tone and overdrive quality is provided by the valve stages. The reason for using so many opamp stages is to maintain that ability for each stage to do a single job without stress and not imprint itself on the sound in any way. Those people who believe that just having an opamp sitting on the table near a valve amp somehow alters the sound in "tone sucking" ways do not know what they are talking about. You either like or dislike the sound quality of the amp, it is very specifically chosen and produced in a very complex way, but there is nothing wrong with the circuitry in any way which I have seen.