yaux0005
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Post by yaux0005 on Jun 25, 2009 16:07:47 GMT -5
According to the Geofex guy, R. G. Keen, series resistance at the input of a Fuzz Face allows a certain kind of "clean-up" on the fuzz face that is unlike the clean up that is achieved by the guitar's voltage divider-type volume control. Series resistance between the guitar and the effect supposedly makes the guitar look more like a current source than a voltage source, and reduces saturation of the input stage of the effect. Volume controls can be altered to function as rheostats providing such a series resistance. My question is this: does it matter whether the resistance is between the hot side of the pickup and the output versus the ground side of the pickup and the signal ground? If so, why?
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Post by sumgai on Jun 25, 2009 16:46:48 GMT -5
Ben,
Yes, it matters where you insert the input current limiting resistor. Long story short, you want to limit the current going into a device's first active component, which is obviously not tied to ground (or properly speaking, the signal return line).
As much to the point, why would you consider controlling the input circuit of a stompbox (or an amp, or....) from the pickup of your guitar? Besides getting bogged down in complex arithmetic when calculating the proper value (a resistor here would affect many other factors, not just the input current), it makes more sense to control the current at the input of the device itself, so that any instrument could be plugged in, no?
HTH
sumgai
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yaux0005
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Post by yaux0005 on Jun 25, 2009 17:08:51 GMT -5
Well shucks, you lost me there in the first paragraph. As I understand it, the first active component of most if not all modern effects devices is tied to ground by a large (1 to 10M) resistor in order to avoid popping sounds when the effect is bypassed. I understand that a feature of fuzz face and similar circuits is that the amount of distortion can be effectively controlled by "rolling back" the volume at the guitar. I would take this to involve reducing the voltage presented to the input stage of the device. I have been under the impression that series resistance functions in an alternate fashion to make the guitar appear as a current source rather than a voltage source. But now I'm getting beyond my level of comprehension and can only refer you to this article: www.geofex.com/Article_Folders/fuzzface/fffram.htmHere is the relevant quote: "If that last one seems a little odd, think back to what I said about the low input impedance. With a low input impedance, the input loads a guitar significantly; the base can only move a few tens of millivolts before cutting off or saturating the first transistor. If you put a resistor in series with the guitar pickup, it raises the apparent source impedance of the pickup, making it look more like a current source (albeit a tiny one) and less like a voltage source. A signal from a current source lets the input of the effect seek it's own voltage level, and merely supplies a varying current. This can be much more linear than a voltage source drive. As a result, the variable resistor allow you to radically clean up the distortion that the FF produces, producing subtle shades of softer distortion. " Just wondering if series resistance at the guitar depends on whether the resistance is on top or underneath the pickup, so to speak. Ben
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Post by sumgai on Jun 25, 2009 17:46:24 GMT -5
Well shucks, you lost me there in the first paragraph. As I understand it, the first active component of most if not all modern effects devices is tied to ground by a large (1 to 10M) resistor in order to avoid popping sounds when the effect is bypassed. Uh oh..... First off, you mean "the input of the first active component...." That's often true, but it's not to clean up anything in the incoming signal, it's to set both the stage's input impedance and the bias point. I think we're in trouble at this point. Let's see if we can get you back on the right foot. Strictly speaking, not true. Due to impedance considerations, a guitar is a voltage source to whatever external device is connected to it. You can't convert it to a current source merely by adding one resistor in series. If that were the case, then every crappy cable ever made and sold to an unsuspecting guitarist would have been instantly transformed into a "Gawd of all Tone" device, and equally instantly priced out of this world. (Oh wait, that last part's already been done.....) No, sorry to say, we'd need a current mirror for that to happen, in the proper sense of what you're attempting to accomplish. This isn't the first time I've run afoul of what Mr. Keen has to say. I happen to think in a much more classical vein, and I don't agree with his interpretation of how a resistor can "raise the current" such that it can vary more significantly than otherwise. He's correct, a transistor is (normally) a current controlled device, but IMHO, he's attempting in mix the input current, the input voltage, and the bias current all at the same time. That's not easy to do in 25 words or less - I certainly wouldn't try it! I ask again, why would you not put it at the input of the device, in this case the Fuzz-Face, instead? Why limit the Fuzz-Face's "clean-up-ability" to just your one guitar, instead of letting it be useful for every guitar you might plug into it? Oh, were you thinking of making this universal, so your guitar could clean up any device you might plug it into? Well, I'm not sure, but let me ask you this: Don't you think that most guitar makers haven't given this a thought or two by now? Why aren't they putting in a half-cent resistor if it would give players an additional feature? Or does it have to be a pot, which will run them about 15 to 20¢ and maybe double that for the installation labor? I posit that they have given it some serious consideration, and rejected it for lack of usefulness, not cost. Remember, guitarists will pay the damnedest prices, if they perceive a benefit, that's a given. HGH sumgai
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yaux0005
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Post by yaux0005 on Jun 26, 2009 2:08:41 GMT -5
Ri right, the input--just what I meant. Better yet the base of the first transistor. Even better, the guitar side of the DC blocking cap between the guitar and the base of the first transistor. Yes yes I know I know. Nothing to do with bias but rather to prevent the free side of the cap from drifting to a high static charge when the effect is "true bypassed." Maybe you are thinking of a bias resistor and I am talking about a so-called "tie-down" resistor.
In any event, I'm not that interested in cleaning up brand X effect pedal. just wondering if it makes any difference which side of a pickup a series resistor is on. I think not. I forget why I asked in the first place now.
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Post by ChrisK on Jun 28, 2009 17:20:38 GMT -5
While his website explains things in a semi-fantasy manner, adding a series resistance to a voltage source will effect a varying current source of sorts in that the rheostat varies the current into the load.
This can be effective with effects that have under-engineered input structures, typical of the design practice of the past. While this resulted in some truely wonderful examples of effects, it also resulted in many crappy ones as well. Each unit would have to be calibrated to the exact transistors instances used, and is not to be desired in a mass produced product.
Note that while this series resistance will reduce the current contribution into the effect input stage, if it is located within the guitar, the ability to drive the cable crapacitance will be seriously compromised and tone will suffer.
Adding the resistance in the signal ground side of the pickup structure will effect hanging coils, which are less than fondly considered by many.
Adding the series resistance in the ground lead itself will effectively eliminate the shielding of the guitar and is not to be considered.
This varying series resistance properly belongs in the effect input, after the cable crapacitance, in the signal (not ground) path.
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