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    Looking for the signal chain in the GM 36/40

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    epivox

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    Looking for the signal chain in the GM 36/40

    Post by epivox on Fri Jul 07, 2017 5:08 am

    Hi All,

    I'm new in the forum and wand to ask a question about the GM Signal Chain (not a english native so I hope it's the right term).

    First of all I want to give you some Information. Actually I am the owner of a GM 36 and a GMD 40. I want to drive the GM with a T.C G-System where I use the 4 cable method. Now I want to adjust the levels in the G-System in an optimal way. for this there are some amps and attenunators inside. If I want to use the FX-loop I need to know exactly what is going on in the GM. So what will help me is an overview of the signal chain. This means e.g.

    Input - Gain - EQ - Boost - Volume - FXsent - FXreturn - FXsection - poweramp - output.

    Do anyone know exatly the signal "way"? is the channel Volume before or behind the FX-Loop. Where is the booster? and so on.

    It will help a lot If you can give me an answer.

    I searched for the Information but I didn't find it. Embarassed

    thanks in advance
    Dieter
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    bordonbert

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    Re: Looking for the signal chain in the GM 36/40

    Post by bordonbert on Fri Jul 07, 2017 11:05 am

    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! Wink )

    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.
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    epivox

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    Re: Looking for the signal chain in the GM 36/40

    Post by epivox on Mon Jul 10, 2017 6:48 am

    Thank you very much bordonbert,

    Wow - this was much more I expected. I didn't asked for such deep dive, but it was a very interesting explanation.

    The background of my question was more about the FX loop and where the Channel Volume level is. But as you wrote - it is before the FX sent and so it influences the G-Systems Insert Return.

    Hope I'm right.

    cheers
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    bordonbert

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    Re: Looking for the signal chain in the GM 36/40

    Post by bordonbert on Mon Jul 10, 2017 8:18 am

    You are right but I wonder if you are viewing the Volume plus Master Volume issue clearly. It is a slightly unusual setup to have in an amp.

    The Master Volume after the Fx Loop is obviously there to set the overall level you are playing at just as normal. It is a plain old twisty pot independent of MIDI controlled selection. It stays exactly the same for any patch you apply.

    The Volume control before the Fx Loop is a very different beast. There is so much control over settings in the amp and so much variation of gain available that it is difficult to keep different patches balanced in level against each other. A Clean patch can be effectively drowned out by an Ultra patch needing the Master Volume to be altered to balance the two, obviously impossible live. The Volume control is a MIDI controlled pot and its selection is therefore stored with the patch. With a high gain Ultra patch you can drop the level with the Volume control and that is remembered, while a Clean patch can have its Volume set higher up to bring the two into balance against each other. When you select the Clean patch the Volume setting increases and with the Ultra patch selected it decreases keeping them in step with each other.

    That way you can balance patches and still use the Master Volume to affect all patches equally to set your overall level. Now I'm sure you are aware of at least some of this. Considering that the Volume level is stored with the patch I'm not sure where your problem is in having the Fx Loop signal altered by it before feeding it into the G-System.
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    epivox

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    Re: Looking for the signal chain in the GM 36/40

    Post by epivox on Mon Jul 10, 2017 9:41 am

    Maybe I set a wrong impression. but I'm very familiar with using and setting up a GM. I Have a GM36 since around 2,5 years and and using it with a FSB 432 III and a Grossmann ISO Cab. I love this amp and I have bought a GMD40 to compare and decide which will stay. The only thing I don't like in the in the overall setup is the inflexibility of the FSB 432. From my perspective it is very poor, that you only can do Midi PC or Midi CC and this is a huge challenge to do everything with patches (yes, I know you can use extended switches, but it is not a smooth solution).

    So I decided to buy a T.C G-System and this combination is KILLER.

    But to get the best sound out of it it is a little tricky to setup the G-System. because you can adjust Input Signal, Loop in and out, output Signal and so on and so far. Many variations. And to do the right setup I needed to know if the Volume Level influences the FX loop or not - and it does. That Information you gave to me. Laughing

    Now I can optimize my setup. My plan is to get a very good sound with the RedBox so that I can leave the speakers at home when we have a gig. Just two Bags and the guitars. But maybe it doesn't work (Sound) , not sure actually but it's more or less an experiment.

    From programming and switching perspective the G-System is like heaven. Switch programs and switch everything you want (also all GM functions).

    Thanks for your help. You are very responsive and an expert as I think. Maybe I have further question.
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    bordonbert

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    Re: Looking for the signal chain in the GM 36/40

    Post by bordonbert on Mon Jul 10, 2017 10:38 am

    Not an expert at all in a lot of what we talk about here EV.  We have a phrase in England: "teaching your granny to suck eggs".  I fear I have just tried to do that.  My apologies, I missed the fact that you knew the GM36 before the GM40D even came along.  I can offer sound technical advice on electronics design and implementation in the engineering sense but the use of some of the gear people here know about goes over my head.  My own rig is very very simple, it doesn't even have a pedalboard nowadays.  ("If you can actually play a guitar all you need is a decent axe, a decent amp and 3m of decent cable! - the one and only Clive Trimby).  I have to admit that it is just that complicated set of permutations in most modern gear which I don't want.  KISS rules in my world!

    Can anyone else wade in with some experience and ideas here, the rest of you guys kjnow a lot more than I do about what is possible in this area?

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