Input Impedance of Tonebender MKII

[fuzzy645]

Using the schematic below from fuzzcentral as a reference, I'm wondering if the original circuit (as great as it sounds), might suffer from a low input impedance.  This begs several questions:

1. Is my hunch accurate, and would anyone care to calculate approx input impedance (or better yet, to teach how to calculate it)?  

2.  Has anyone ever tried a buffer in front of this circuit?  I have heard circuits like the fuzz face do not play nicely with buffers, and I'm wondering if the same is true for this circuit?

3.  Assuming a buffer will mess negatively with the sound of a MKII, I would assume the order this sits in a pedal board is critical with no other buffered pedals in front of it.

Thoughts?

[ashcat_lt]

Well it cant possibly be any bigger than the 100K to ground before the transistor.  That's pretty darn low for connection to a passive pickup.  You'll have serious dampening of the resonant peak at the top of the pickup's response at the very least.  I don't think, though, that its as low as a fuzzface.

But then "suffers" in this context is a subjective thing.  As best I can tell this type of effect really kind of depends on that loading of the pickup to sound the way it should.  With a higher in-Z you'd have more treble into the fuzz, and the output might come out harsher or "fizzier" than you might like.

[PRR]

> suffer from a low input impedance

"Suffer"? How does it sound? Is it suffering?

But FWIW: right-away the 0.01u (10,000pFd) cap is like a 300 foot guitar cord. Yeah, highs are clobbered-off before mangling starts. That's fairly common and musically useful.

Calculate the input? Do you know transistor theory? If so, you'll note that Q1 is apparently _OFF_. No source loading, no signal passes. It's not that simple. A OC81 will not go dead-off. It leaks, more or less depending on temp and vintage and moon-phase. Large signals will force it more-on. Input impedance varies from high to low over the cycle. Hey, that's distortion!

[fuzzy645]

> suffer from a low input impedance

"Suffer"? How does it sound? Is it suffering?

Thanks for the clarification guys.  I do regret the choice of the words "suffer" but I'm not much of a wordsmith. I think you get the essence of my question in that low input impedance is generally perceived to not be a desirable attribute.

[ashcat_lt]

Low-Z is not preferred when you want to get the full bandwidth and all the peaky, ear bleeding "treble" that you can out of a passive guitar pickup.  In many cases you don't.  The highest frequencies that a typical pickup can pass (which I consider to be more "high mid" than "treble") really are just non-harmonic noise.  It's harsh and nasty sounding on its own, and we're really most used to hearing it after it's been mellowed some by the limited bandwidth of a typical guitar speaker.  If you go and add harmonic multiplies of those frequencies on top...well most people don't like the way that sounds.  Trent Reznor, maybe, but it ain't gonna get you to Jimi-land!

Pretty much all of the popular ways to distort a guitar address this somehow.  A Rat or Tubescreamer, for example, will present a fairly high in-Z, apply a bit of an LPF, and then apply less gain to the treble than to the midrange signal, then distort the thing, and then often roll off some of the newly produced high harmonics.  And that's all before the limited frequency response of the speaker.

But, hell, the guitar and cable constitute an RLC based LPF for free!  This class of fuzz box takes advantage of that.

[electrosonic]

3.  Assuming a buffer will mess negatively with the sound of a MKII, I would assume the order this sits in a pedal board is critical with no other buffered pedals in front of it.

I wondered as well how people incorporate these into a pedal board. Do people feel it needs to be the first pedal on the board to sound right?  How about putting the AMZ pickup simulator in the same box as the tone bender so the pedal order is not an issue?

Andrew. 

[PRR]

> add harmonic multiplies of those frequencies on top...

Not just harmonic OVER-tones. Since there are multiple input frequencies, there will be Intermodulation, resulting in new tones both higher and _lower_, with no simple relation to the input tones.

IM is inevitable and can be useful, but as you say, we usually want to trim-off most of the string harmonics before adding new over- (and under-)tones.

[fuzzy645]

Speaking of buffers, I have noticed many of the buffers posted on the Orman site use a 10uf cap on the output (as in the schematic below).  I'm wondering if there is a "best practice" regarding choice of input and output caps on buffers since (I would assume) the goal of a buffer is to preserve the highs and not color the sound.  Would the particular choice of cap here (.1 uf input an and 10 uf output) be chosen for that reason, or is there some other reason?

[ashcat_lt]

Yeah, those are just "plenty big enough for any application" values.  In fact, the 10uF on the output is huge.  In most cases you can get away with quite a bit smaller, though the rolloff is dependent on what is loading it.

[mac]

A rough calculation for Q1 @ DC.
Let's say you have a NPN Germ leaking 100ua=0.1ma and gain of 60, fairly common in this pedal.
And let's say there is a 0.1v drop from base to gnd.
The 100k to gnd draws a little current from the base, ib=0.1v/100k=0.001ma.
Since the base is not connected to vcc this current has to come from the collector-base leakage. The amount of leakage that is stolen by the base is ib*hfe=0.001ma*60=0.06ma. (*)
Thus the "available" leakage across the 10k is 0.1ma-0.06ma=0.04ma. The voltage drop is 10k*0.04ma=0.4v and the collector is at 8.6v.
Believe me or not, I put some germs with this specs in the breadboard and I got this values.

Now, the base-emiter resistance is given by Rbe=0.026v/ib. The term 0.026v is given by K*T/e at 300K, where K is the Boltzmann constant, T the temp in Kelvin degrees, and e the electron charge. This is called the thermal voltage.

For this germ Rbe=0.026v/0.04ma*60=39k.
There is a 100k at the base so 100k//39k is 28k.
The input caps impedance are freq dependent, one series, the other in parallel with the 28k.

(*) Adding a 100k or so from base to gnd is a nice way to steal leakage on a FF Q1 without changing the low Z-in much.

I do regret the choice of the words "suffer" but I'm not much of a wordsmith.

I speak one language, spanish... and very bad!

mac