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Post by JohnH on Aug 31, 2011 6:53:09 GMT -5
A few comments I have:
C3, at 1.5nF, is a very small tone cap – is that what its for? I’m guessing r4 is for a 1M volume pot, and R5 is the amp input impedance? If so, to get a proper output from these types of tests, then the cable capacitance is also worth putting in, and 0.5nf in parallel with R5 would be a good value . Now to the series resistors, R6 and R7 – those are large values and likely to dull the sound a bit, but this analysis should demonstrate that visually, if you try without them for comparison. One thing about normal parallel wiring is that, if both pickups are getting the same amplitude of a given frequency, they don’t interact since they agree with each other, but when one pickup is getting different to the other, as in neck and bridge positions for example, they will fight each other and come to some in between value based on phase and impedance. It seems to me that these series resistors would change that, maybe in an interesting way, but the analysis probably won’t pick it up because in the calculation, V1 and V2 are both the same. It might be interesting to make one of them zero, then look at the contribution of just one pickup, in the presence of the other, with or without the resistance.
Just thinking... John
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Post by roadtonever on Aug 31, 2011 7:00:06 GMT -5
You want to add resistors in series with your pickups so that the inductance of each pickup has a harder time affecting the other? That's right. And then, you want some kind of equation to figure out exactly what's going on with each inductance and resistance? I believe most of the equation has been worked out, see reply #54. I just need to work out what sumgai said in reply #55.On a side note, I'm not sure what you mean in your schematic. R4 and R5 seem to be static resistors in parallel. Care to elaborate on what's going on in there? R4 is the volume pot, R5 is the amp input impedance. I'll be using a buffer so I didn't add capacitance past R4.
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Post by roadtonever on Aug 31, 2011 7:05:33 GMT -5
C3, at 1.5nF, is a very small tone cap – is that what its for? Yes V1 and V2 are both the same. It might be interesting to make one of them zero, then look at the contribution of just one pickup, in the presence of the other, with or without the resistance. I'd be glad to experiment with that!
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Post by asmith on Aug 31, 2011 10:31:10 GMT -5
I drew your pickups in parallel on a piece of paper like so: Effectively it looks like your two pickups are appearing as RL circuits to each other. Here's a good image from a Georgetown University electronics site I stumbled across. . I: Current V: Voltage Z: Impedence R: Resistance L: Inductance w: Angular frequency in radians per second (helpfully, w = 2*Pi*f where "f" is frequency) Greek Letter Phi: Phase angle of the AC signal 'Course, really these pickups have capacitance across them as well, so they're technically parallel RLC circuits. Have you taken capacitance readings across them? I'm not sure what you mean by wanting to acheive certain resonances, presumably that you want the minimum impedence at a particular frequency? Of course you don't want to use capacitors as in an RLC though, as they'll affect the tone of the other pickup. Capacitance aside, here's a very cool page on RL circuits and a calculator for different values. Remember R 1+R 2 in series = R total. EDIT: And here's a calculator for RLC circuits, if you want to use your values of 65pF and 54.5pF. Remember, (1 / R 1) + (1 / R 2) ... in parallel = (1 / R total). I'm only just beginning to get my head around your situation at the moment, and I couldn't begin to pretend I'm even remotely clued up on this subject. Either way, inshallah you'll find something useful in this post.
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Post by sumgai on Sept 1, 2011 12:01:01 GMT -5
Ace,
I'm not having one of my 'good' days... would you please take over for me?
Thanks.
sumgai
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Post by asmith on Sept 2, 2011 8:20:19 GMT -5
This is not Gospel.Alright Roady, as far as I can tell, the whole thread is based upon the idea that if you stick resistors in series with the pickups, they're going to act as some kind of 'gate' for frequency resonance. That's sort of true, and it isn't. You pickups are acting on each other - as well as the amp, the cable, all the other components - like parallel RLC circuits. When your two pickups are in parallel, just within those pickups you have two inductances in parallel, two capacitances in parallel, and a myriad of resistances in parallel. Let's look at your two parallel pickups as if they were one pickup, a " Thévenin equivalent." L total = (4*5.3)/(4+5.3) = 2.28H. R total = (8*10.3)/(8*10.3) = 4.5k. C total = 54.5+65 = 119.5pF. You're changing the fundamental values of what makes up a resonant circuit. That's what gives you your tone. A quote from the above link states it pretty clearly: Bingo. Essentially: you've quartered the inductance by sticking your pickups in parallel, and the resonant frequency of your pickups together is now twice as high. But you can knock that down a few notches by putting a capacitor in parallel with your pickups. After all, with all this talk of resistors, it puts me in mind of this helpful explanation of resonant circuits' selectivity and quality ("Q") of resonance. The important quote is: Emphasis in the quote was mine. I think the selectivity and quality concepts are the same for both series and parallel RLC circuits. Basically, with all this resistance being banded about, you're just going to dull the resonance, not use resistors as gates. "...the algrebra necessary to characterize the resonance is much more involved." Just before we go on, here's an explanation of some frankly batshit equations. Thankfully, there's a calculation page to play around with. You should be able to play around with the equations and find an appropriate capacitor value to rip your too-high resonant frequency down to where you want it to be. At least, that's how I see it. That was not Gospel. And if in somebody's qualified opinion it's garbage, quick, delete it before anybody notices.
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Post by roadtonever on Sept 2, 2011 22:42:55 GMT -5
This seems like the only way to achive what I wish to do. But you can knock that down a few notches by putting a capacitor in parallel with your pickups. I've considered that but wrote it off as it would make the tone control less versatile. That said the gate resistor might not be much better. As you said: Basically, with all this resistance being banded about, you're just going to dull the resonance, not use resistors as gates. I wanna see if that still holds true when external resistive load is reduced. Also I've gathered the resonance dulling could be avoided to some extent by eliminating cable capacitance, although I'm not sure how that works. I'm still unable to find an answer to my main query. I suppose R3 and R4 lower the effective resistance of R7. How can I calculate this? Thanks for the explainations and drawings btw they certainly helped me with stuff I've had trouble with in the past.
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Post by JohnH on Sept 3, 2011 7:02:07 GMT -5
I'm still unable to find an answer to my main query. I suppose R3 and R4 lower the effective resistance of R7. How can I calculate this? I think there is no answer to this, you cant simplify this circuit to make R7 alone match what the combination of R3, 4 and 7 does, withthe tone cap. Also, I had a further play around and I think the effect that asmith is refering to, ie the reduction of the peak resonance is the main thing that is happening here. I dont think there is any magical number to find, just different variations in tone which can be seen by testing your model but only really understod by physical testing. John
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Post by roadtonever on Sept 8, 2011 1:19:19 GMT -5
I guess I'm ready to write off the gate/decoupling resistor idea by now. By considering each pickup equivalent to a parallel RLC circuit it's easy to factor the controls. Using a target frequency for JohnH's equation, corresponding to the resonant frequency of each pickup the closest e12 value resistors to be added in series would be 220k for both pickups. This results in a considerable dampening of the resonant peak when the tone control is tuned down rendering it useless for me. Even with external resistance removed the resonant frequency is still below reference level.
I suppose the reason the resistors are used in the JD Tele wiring is for convenience and saving time otherwise required for adjusting the tone knob to match other settings on the pickup selector switch. Having selectable tone cap value looks like much more robust solution.
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