Importance of Guitar Cable Capacitance with AxeFX vs Tube Amp

THX

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In a traditional tube amp system, the guitar cable itself acts as a capacitor in the signal chain between the guitar's pickup and the amp's pre-amp stage (along with stomp boxes, etc ahead of the amp). This impacts the response characteristics of the pre-amp stage, etc.

IGNORING the resonance / eq / filter effects of the guitar cable's capacitance on the actual transmitted signal, does the AXE-FX model the interaction of being presented with different capacitances at the input jack and how the pre-amp stage responds to these variations?

It would seem the answer is "NO", since I'm assuming the input signal immediately undergoes the A/D conversion as soon as it enters the AXE-FX and that's why Cliff was gracious enough to provide us a plethora of pre-amp / amp adjustments. However, I was curious as to what the real answer is.

I'm a mechanical engineer by trade and my EE is a little rusty. This it the kind of crap I think about on my day off of work and I'm looking at buying a new guitar cable. Sorry, it's a curse; it drives my wife crazy :) !

Thanks in advance.
 
The incoming signal is already affected by the capacitance of the cable in conjunction wit the AFX's input resistance to form the passive filter created by the RC ckt created by the cable in conjunction with the device input. Modeling it twice (the passive RC filter is the effect of that capacitance) would not make any sense since it is physically there and IIRC, the AFX varies its input impedance in accordance with its signal chain. If you want to roll off highs further put a low pass filter block at the input. ;)
 
The input circuitry of the Axe FX does mimic the input impedance of different amps and pedals, but I'm not sure to what extent the reactive nature of that connection is recreated. I would think it would respond more like a tube amp with a buffer pedal in front.
 
There is no such thing as "interaction of being presented different capacitances". The pickups see a load. That load is the impedance of the cable and the impedance of the amp input. The typical input impedance of a tube amp is a resistance plus a small capacitance to ground (120 pF or so). The Axe-Fx input simulates this input impedance.

Some effect pedals present a different load. The Axe-Fx replicates this by switching in different load resistors and capacitors when a model of that effect is first in the effects chain.
 
Hi Cliff,

Thanks for the explanation. Please bear with me for a second, I'm just trying to make sure I understand:

Zc= 1 / jwC (I realize this is for steady state AC, just trying to keep it simple)

Where, Zc = Impedance of a capacitor, j = imaginary unit of the complex number, w = frequency, and C = Capacitance.

For the guitar pickup to the input jack, we have a circuit in series, and the total Impedance of the circuit, Ztotal = Z1 + Z2 + Z3 + Zn.

It is my understanding that guitar cables can vary in capacitance from 52 pF to 190 pF per meter **. So if I have a 10 meter guitar cable, the total capacitance of that cable can vary anywhere between 520 pF to 1,900 pF. For a given frequency, capacitance and impedance change inversely to each other. Therefore, if I increase the capacitance of the cable, the total impedance (Ztotal) "presented" to the input jack of the Axe-Fx or tube amp decreases (assuming everything else in the signal chain doesn't change).

For a tube amp, everything from the pickup all the way through to the speaker cone driver (and to a degree the speaker cabinet itself) can be thought of as one big "analog" electrical circuit. A change in one component can change the overall circuit.

For the Axe-Fx, the analog portion of the circuit stops at the A/D conversion. So are you saying that the Axe-Fx simulates how the rest of the analog chain in tube amp system would react to load / impedance / capacitance changes ahead of the input jack?

Sorry Cliff, I'm not trying to be difficult. I'm just trying to understand. There's a reason I went into mechanical engineering and not EE and you're seeing the results :D! It's been almost 30 years since I had my electronics and circuits classes and I killed off those brain cells long ago!

** = http://www.shootoutguitarcables.com/guitar-cables-explained/capacitance-chart.html
 
Cliff,

If I re-write my sentence to read:

"Therefore, if I increase the capacitance of the cable, the total impedance (Ztotal) at the input jack of the Axe-Fx or tube amp decreases (assuming everything else in the signal chain doesn't change)"

Is it correct now?

Thank you for the reference. I'll be sure to check it out.
 
The pickups see a load. The load is the cable capacitance (ignoring resistance and inductance) and the input impedance of the amp. The first tube is a buffer. Everything that happens after that is irrelevant. You are confusing passive circuits and active circuits. One of the whole points of an active circuit is impedance isolation. The Axe-Fx duplicates the input impedance of the buffer and everything after that is done in DSP-land.
 
Cliff,

YOU ARE THE MAN!!! Thank you!

Passive vs. active circuits is what was throwing me off! It's been a long time, but it's starting to come back to me now.

I did check out your reference book on Amazon. Looks like some light reading ;). I hope I'm up for it.

Got it! Thanks!
 
Slow down, Cliff.
If you type and explain things this fast, it makes it very hard for the competition to take notes.

Not really, yek. Cliff's explanation is somewhat basic electronic theory (just that I'm too dumb to remember). The REAL trick is this part:

The Axe-Fx duplicates the input impedance of the buffer and everything after that is done in DSP-land.

Much easier said than done! This is one of the areas where the Axe-Fx really shines and why it's so far ahead of everything else.
 
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Every time I read one of these technical responses you type Cliff, I can feel my IQ go up a little.

Me too!

Thanks for this Cliff:
The first tube is a buffer. One of the whole points of an active circuit is impedance isolation.

When you said that guitar pickups see the load of cable capacitance and amp's input impedance I instantly think in the other side of the circuit: the interaction between speaker and amp's output and I capture in a rudimentary manner the elegance of the whole picture:
Pickup coil (passive transducer)<>cable<>amp's input>amp (active)>amp's output<>speaker coil(passive transducer)

http://forum.fractalaudio.com/threa...etting-on-the-amp-blocks.109793/#post-1314131

Since the end of this chain is already modeled (speaker page, cab block), I think we axe users have to pay attention to its beginning because is the part that remains physical...since Cliff designed axe's input to behave like a tube amp's input the remain parts are the guitar electronics and the cable: changing a pot value or guitar cable affects the pickup response and the overall tone of the chain...consider this and compensate if necessary as zenaxe points.
 
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The Type of cable you use, being an artist, is totally subjective. I mostly use a van den hull, integration hybrid also mogami platium and van damme silver series are my two other choices. But, once again, personal pref.
 
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