Question about Zen Master

simeon

simeon

Axe-Master
just wondering why this drive has a variable clip type (and the other 808 derivatives don't) and whether there is a particular reason for it? does it relate to something on the original pedal?
 
hmmm...not sure...the manual says...

  1. Voice - “Tunes” the pedal to the amplifier and guitar. The Voice knob performs two functions:
    1. Limits or increases the total amount of gain in the pedal. To limit the gain, rotate counterclockwise. To increase the gain, rotate clockwise.

    2. Adjusts the bottom-end response of the pedal. To increase the bottom-end response, rotate counterclockwise. To reduce the bottom-end, rotate the clockwise. Remember to adjust the Gain knob to compensate for changes in the overall gain of the pedal which occur during the Voice knob adjustment process.
 
I've noticed both of those characteristics matching up to what the variable knob does ime, are you noticing differently?
 
No, to be honest i haven't played with that parameter much. I thought it was the bass parameter that equated to voice, but I'll definitely play around with it and see what it does. Thanks man.
 
No problem man, if I had to describe it it sounds almost like turning it cw kills the headroom on the distortion and consequently I like it in the 10-12 area
 
just had a play around with it. nice. 10-12 seems to be the sweet spot, as you say. thanks again!
 
I couldn't find any useful settings for Zen Master. It's so loud without any useful compression and very dull in terms of equalisation. Tried with different kind of amps and cabs and guitars. It's nothing like the original.

I still really believe that the weakest member of the AXE FX chain is still the drive block.
 
The Zendrive is 808-like in that it has clipping diodes in the feedback loop of the opamp. What does this do? Basically, the gain out of the opamp is controlled by a resistive divider that sends some of the opamp output off to the pedal output, and some back into the feedback circuit. The higher the resistance going into the feedback circuit, the more of the opamp output will go to the pedal output, and the more the opamp will increase it's gain to compensate. Once the voltage coming out of the opamp reaches a certain level the diodes will open up (as is their nature) and allow virtually 100% of the opamp output above that level to flow into the feedback input, which chokes off the gain from the opamp. This threshold voltage is achieved at the tops and bottoms of the waveforms of the input voltage, so this has the effect of leveling off the tops and bottoms of the waves -- clipping!

So when you turn up the "drive" on one of these pedals, what you are actually doing is increasing the resistance in the circuit that bypasses the diodes in the feedback loop, forcing the opamp to increase it's output, which will eventually cause the peaks (and valleys) of the waves to open the diodes and cut the gain dynamically.

EDIT: I had a closer look at the Zendrive circuit and I see that the biasing stuff (that I've deleted) is wrong. I forgot that the input signal is biased to +4.5V (which, of course it has to be since a 9V battery by itself can't supply a -ve voltage). I also see that the "biasing" on the negative feedback input goes through an RC filter, which means that it's going to tend to pull higher frequencies towards the 4.5V biasing point, which in turn is going to mean that the opamp is going to push more of them out. The "Voice" knob controls how much of this reduction of the high frequencies going into the negative feedback input is going to happen.

So the "Voice" knob is basically a tone control, but since it's in the feedback loop clipping circuit it has a slightly indirect effect on the output tone, and it also affects the amount of clipping on those frequencies that are accentuated. And all of the clipping and tone changes are softened out a bit because what you actually hear is the effect of them on the behaviour of the opamp - and there are limitations to how fast it can react to changes in the feedback input.

So, having got that figured out properly, I still don't know how the AxeFX drive parameters map against this.
 
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I had another look at the Zendrive schematic. I saw I had some stuff wrong, so I made some major changes to my earlier post.

I think it's right now.
 
not sure if "variable" is the correct clipping type. FuzzFace currently has "variable" as well which certainly isn't correct and the +Distortion has just been fixed in 2.03 also wrongly defaulting to "variable"...
 
The Voice control, on the real pedal, is a variable resistance between the inverting input of the opamp gain stage and it's ground reference. As there is a cap between it and ground this controls two things simultaneously. The overall gain amount of the stage and the high pass filter of the stage. At it's minimum setting you get the least gain and the most bass and conversely at it's max you get the most gain and least bass.

The clipping diodes in the real circuit are a mosfet in series with a Schottky diode parallel to a mosfet in series with 2 Schottky diodes. I would have to experiment a bit with the Clip Shape control to see if it does the same as the real pedal.
 
groovenut said:
The clipping diodes in the real circuit are a mosfet in series with a Schottky diode parallel to a mosfet in series with 2 Schottky diodes. I would have to experiment a bit with the Clip Shape control to see if it does the same as the real pedal.
Click to expand...

The asymmetrical clipping is supposed to be more "tube-like". It's one of the main differences between a Tube Screamer and a Boss SD-1. The SD-1 has asymmetrical clipping diodes.

I spent a lot of time some years back auditioning different types and combinations of types of diodes in an SD-1. I was looking to find a something to push my Dumble clone from crunch rhythm to lead tone. I can't remember exactly what configuration I ended up with, but I do remember that germanium diodes were involved. Using all silicone diodes tended to sound a little screechy.

Also interesting, that modified SD-1 into a clean amp doesn't really sound that good. It's too wooly, and doesn't sound very realistic. You get a much better drive tone with all silicone clipping diodes into a clean amp. But my Dumble clone has a really tight sound that I associate with a high level of negative feedback, and pushing it with the SD-1 with the germanium diodes with the drive set fairly low loosens it up a little bit, and allows it to really sing.

I haven't messed with the diode types that much in the drive block in the AxeFX, but if they react anywhere near the way that a real stompbox does with components switched out, I'd say that you really have to audition every option into an amp block set up the way you want it to find what works best.
 
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