|
|
Post by Yogi B on Jul 23, 2020 9:53:05 GMT -5
Often cable capacitance can be seen as an elusive ne'er-do-well, stealing our precious treble on its journey from guitar to amp, especially if we have the audacity to decide to roll down our volume controls. However, what if our goal is to roll off treble, as with a regular tone control? For about the first half of the rotation (10 down to 5) a standard tone control is not really adding a parallel branch (the tone capacitor) of a new capacitive LPF, but rather reducing the parallel branch of the already existing inductive LPF formed by the pickup(s)'s inductance and the potentiometers resistance. The trouble is that the cut-off frequency of this (1 st order) LPF is for the most part above that of the (2 nd order) LPF caused the pickup(s)'s inductance and our old foe cable capacitance, so there is little evident treble roll off over most of this range. My idea is a simple one, what if instead of trying to reduce treble by only introducing a new LPF, we also modify that pre-existing LPF that is (in part) due to cable capacitance? The simplest way we can achieve this is by adding series resistance between the volume control and the output jack, something like this: I chose 100k as the value for the second potentiometer gang as it should be large enough to make a noticeable difference in cut-off frequency at around ten to twenty times the resistance of a typical pickup (and roughly equal the maximum output impedance of a 500k volume control, at 125k), but also it should be small enough that it doesn't cause too large a reduction in volume considering the typical 1Meg input impedance of the following amp/stompbox/interface. A Log taper pot is specified such that the majority of the additional resistance is introduced within the first half of rotation (10 down to 5), i.e. where the regular tone control needs the most help in order to actually cut treble. As an alternative to a 100k logarithmic element, a 500k linear element could be used in parallel with a 120k resistor to give similar results. Or if you wish to experiment further, that parallel resistor could be replaced with a trimmer. This would allow changing the taper/maximum resistance, in addition to providing a way to return to the standard tone control behaviour by setting the trimmer to zero ohms, thereby shorting the second gang. There are two issues I can anticipate: - firstly this may well upset certain fuzz pedals with low input impedance;
- and secondly because of the location of the additional resistance when both the volume and tone controls are at zero, the tip of the output jack will no longer be directly grounded -- instead it is left floating 100k above ground, meaning it will likely be susceptible to quite a bit of stray noise, shielding notwithstanding.
|
|
|
|
Post by sumgai on Jul 23, 2020 16:22:31 GMT -5
There are two issues I can anticipate: And number three would be, RF cables are gonna have a tough time contributing much of an effect to this circuit. .... make a noticeable difference in cut-off frequency.... Oh goody, I always did want to see how the cutoff frequency for a xPF (anything pass filter) could be changed by jiggering the resistance. See reTrEaD 's response below, in Reply #4. sumgai |
|
|
|
Post by JohnH on Jul 23, 2020 17:08:08 GMT -5
This idea needs to be investigated in SPICE.
But what id expect is that Tone 1 acts like a series treble bleed (Kinman style)
Tone 2, with the volume pot, forms a three-legged 'star' . At a given setting we can imagine that alternatively in an equivalent 'delta' arrangement of three resistances. Its a standard transformation. Visualised that way, it equates to a load resistor across the pickup and then a normal pot of a new higher value
I think what it will mainly do is to flatten down the usual treble peak.
But a plot would reveal the truth....
|
|
|
|
Post by reTrEaD on Jul 23, 2020 21:04:23 GMT -5
Oh goody, I always did want to see how the cutoff frequency for a xPF (anything pass filter) could be changed by jiggering the resistance. In the case of a first order low-pass RC filter where the resistance is in series with the output and the capacitance is in parallel with the output, the corner frequency has an inverse proportional relationship with the resistance. fc= 1/(2πRC) |
|
|
|
Post by sumgai on Jul 23, 2020 22:06:48 GMT -5
Oh goody, I always did want to see how the cutoff frequency for a xPF (anything pass filter) could be changed by jiggering the resistance. In the case of a first order low-pass RC filter where the resistance is in series with the output and the capacitance is in parallel with the output, the corner frequency has an inverse proportional relationship with the resistance. fc= 1/(2πRC) I spoke a bit too soon, or perhaps, a bit too broadly. In the purely ideal world, a resistor is totally frequency independent. But here in the real, physical world, at some point a resistor will exhibit both inductance (in series with the resistance) and capacitance (in parallel with the resistance). Where this can get interesting enough to need attention is in the radio frequency spectrum. Since we're dealing with audio frequencies, the real world resistor can be safely said to have no discernible affect on frequency. Need I post beaucoup links to various sources aboot the web?  And one does have to wonder, if it were this easy to alter the corner frequency, and thus alter the tone one hears out of a guitar, then why don't manufacturers hook things up to get this effect? Why do we only get a reduction in the level of those frequencies above the inherent corner that is dictated by the value of the chosen capacitor? In fact, I daresay that many, many modders have attempted to do exactly this, to no avail. I know I was on of them, way back when. sumgai |
|
|
|
Post by Yogi B on Jul 24, 2020 0:27:25 GMT -5
RF cables are gonna have a tough time contributing much of an effect to this circuit. This is why I think all wireless devices specifically designed for guitars should include a couple of switches to add a choice of caps to their input. Lets say 270pF, 470pf, both parallel, or neither, in order that they might simulate cables of different lengths and to better accommodate the possibility that you might wish to use a wireless unit with a guitar that has a treble bleed installed. But what id expect is that Tone 1 acts like a series treble bleed (Kinman style) Tone #1 is wired just like a regular tone control, with the cap to ground, it's at an odd angle (compared to the way I'd normally draw it) so that it is orientated the same way as the other gang. I've updated the image with bolder lines and an increased the gap between the tone cap's ground and the output wire to hopefully make that clearer. Plots! First a regular 500k tone control attached to a 500k volume control at full volume, the 6-part SD-59 model as used in your GuitarFreak, 500pF of cable capacitance, and 1Meg of input impedance of the following stage. (Each line represents a 10% increment in the position of an idealised Log taper.) Next we have a plot of the dual-gang tone control presented in this thread. You can see that compared to the standard tone control the volume is reduced slightly as the control is lowered -- for the given parameters at zero this reduction is around 0.7dB. Finally a plot overlaying the two, standard in red and the dual-gang in green, additionally the plot for the tone at 10 (which is the same for both circuits) is represented in yellow. The plots of the dual-gang circuit have been offset to eliminate the volume drop noted above, in order to give a better comparison of frequency response. Additionally, to avoid crowding, the tone controls have only been plotted at 25% increments. God I hate the way some fonts choose to represent pi. (This is much nicer π) |
|
|
|
Post by JohnH on Jul 24, 2020 3:32:43 GMT -5
Nice plots! Thanks for remembering my model.
It looks like the new design covers similar territory in terms of overall tonal range but gets more of the available tone change quicker as you dial down from 10?
|
|
|
|
Post by Yogi B on Jul 27, 2020 11:03:27 GMT -5
Thanks for remembering my model. When I wan't something a little more accurate, the SD59 is my go-to 6-part model as it's a nice middle ground between high-Q single coils and low-Q hot humbuckers, plus it's one of the pickups for which GF has specific split & parallel versions. Arguably a model based on specifically matching the impedance of the coil rather than the voltage output (e.g. what went on in this thread) would be better. Models with accurate voltage output are good for highlighting differences between pickups, but for highlighting differences between wiring arguably impedance is more critical, though ideally we'd want both to be accurate to get the full picture. But all of that is really only relevant if we were trying to model a specific pickup, not just a general case, and I'm not trying to derail my own thread, honest! Mainly, that itself not being a bad thing (in my opinion). The other difference is that it works a little bit more like a cutoff frequency control than a gain control of a shelving filter. I wanted something that would complement the behaviour of the 'tweed bright cap' style of treble bleed -- in which a parallel treble bleed's capacitor is routed through the normally unused upper half of the (regular, single-gang) tone potentiometer -- but would give that behaviour even when when the volume was at 10, thus the treble bleed shorted. |
|
I chose 100k as the value for the second potentiometer gang as it should be large enough to make a noticeable difference in cut-off frequency at around ten to twenty times the resistance of a typical pickup (and roughly equal the maximum output impedance of a 500k volume control, at 125k), but also it should be small enough that it doesn't cause too large a reduction in volume considering the typical 1Meg input impedance of the following amp/stompbox/interface.
