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Post by geo on Feb 26, 2014 15:58:19 GMT -5
Has anyone heard of uF-range variable capacitors? Something that can could vary by a few tens/hundreds of nF? I'd been rolling the idea over in my head of using variable capacitors for the tone circuit asmith posts here: guitarnuts2.proboards.com/thread/6056/splitting-tbx-tone-control?page=1&scrollTo=65033I can't find anything in the nF range. It seems like it'd be reasonable to vary the bass-cut cap, but the treble cap would be tricky. The question then also becomes, are variable capacitors up to snuff for tone circuit applications. |
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Post by ashcatlt on Feb 27, 2014 18:59:58 GMT -5
I haven't looked for specific values, but AFAIK, most of these things are physically too large to really be practical, and pretty darn expensive to boot. You could probably use a larger value variable C in series with a smaller static C to get some variability. Switching and/or capacitor/variable resistor combinations are usually much easier to implement, and usually work fine.
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Post by sumgai on Feb 28, 2014 17:00:46 GMT -5
geo, The problem is, capacitance is a function of the distance between two conductors, as separated by a "dialectric". Air is the most common dialectric material, but in order to achieve the desired numbers, any such capacitor in question will have to be pretty large. See this page for images and more info; en.wikipedia.org/wiki/Variable_capacitorMouser, Digi-Key, JDR and others all have tiny variable caps, but nothing ranges over a few picofarads (pF), perhaps upto a 100 or so. That's only a couple of percentage points of what we need in a guitar - most tone control caps run between 22 and 43 nF, though some folks like them somewhere on the order of 1 to 1.2 nF, for that "woman" tone. Also, this lower value is often preferred for treble bleed circuits. But it's still way above whatever is available at these "fine" electronics parts houses. There is another method of varying capacitance that isn't discussed in the above Wiki article, that of "switched capacitance". This isn't the digitally controlled stuff, it's a matter of taking a regular cap, and switching it into/out of a circuit with a timer chip. A better overview than I could provide can be found here: en.wikipedia.org/wiki/Switched_capacitorBut for you/us, the downside is the need for a battery in order to drive the timer chip. Still, it is an option.  HTH sumgai |
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Post by geo on Feb 28, 2014 18:09:25 GMT -5
I was hoping for something voltage-regulated, so that you'd have electric potential forcing the two plates apart powered by some battery. A varactor would do the trick, but I'd want separate leads to control the capacitance and for my tone signal to pass through. (i.e. two low-current voltage leads to define capacitance, two low-voltage current leads to pass your signal through)
Maybe I could plug a 9v into the varactor and use a pot to tune voltage, thereby changing capacitance? I get basic principles, but I know nothing about electrical engineering, so I'm worried that might overvolt something like the Clapton Midboost kit I've got installed, or that I'll sketch up a circuit that winds up using a 9v battery as a high-current voltage source.
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Post by sumgai on Feb 28, 2014 23:51:31 GMT -5
geo, A varactor is two-pole device, and is in fact a diode. Right there, you are clipping off half of the signal presented to it, and that's not gonna sound so good now, is it? Check with ash or John, they've experimented more deeply with various types of diodes for different distortion effects. Besides which, there are two voltages running through it - a micro-volt sized signal (usually radio frequency, or possibly microwave (radar)) and a bias voltage, which is DC. It's that DC component that ends up on the signal path that needs to be dealt with, and at that point, you're adding component complexity. Not to mention that final nail-in-the-coffin, these things are well below the nominal pico-farad range - not large enough for use in any audio circuits, let alone a standard guitar. Sorry about that. To the best of my knowledge, there are no 3-pole or 4-pole varactors out there.... but I could be wrong, who knows? HTH sumgai |
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Post by geo on Mar 1, 2014 2:48:20 GMT -5
OK, back to searching for exotic variable capacitors!
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Post by sumgai on Mar 1, 2014 15:55:26 GMT -5
OK, back to searching for exotic variable capacitors! Good luck! |
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Post by JohnH on Mar 2, 2014 0:57:55 GMT -5
OK, back to searching for exotic variable capacitors! Good luck!
I'm floating this out here, because I have not seen it anywhere before, nor tested it in real life:
I believe the following buffer circuit amounts to a variable capacitor within a tone control, controlled by a pot, with a continuously variable range of 0 up to to the value of C2:
Schematic and frequencies plot shown, which are familiar from looking at the result of changing a tone capacitor.
It works by having a high-impedance buffer sample the pickup output, at one end of the capacitor. the other end of the cap is not grounded like a normal tone cap, but is connected to the buffer output. If the buffer receives no input signal, due to R5 being at minimum, the cap works just as if it was grounded to a zero reference, at its full value.
But if the R5 is at maximum, the buffer is giving the same voltage to the lower end of the cap as received by the upper end. Nothing flows through the cap, it is like no cap at all. In between, the signal flowing through the cap changes, as voltage across it changes. We change the voltage across the cap, to control the current flow through the cap, as if it was the cap value was being changed. You can see on the traces how this works out, like a variable cap.
Note that this is not a buffered output, nor an active tone control stage. The signal path from pickup to output is all passive.
This version is taking a max cap value and reducing down to zero. If the buffer is inverting, we can also multiply a cap value. And if it has gain, we can actually create a negative capacitance. That last one can be used to do things like reduce unwanted capacitance in a long cable.
Chew on that! |
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Post by b4nj0 on Mar 2, 2014 17:48:46 GMT -5
Hmm, 3.8 Henry inductor. That's a big one!
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Post by JohnH on Mar 2, 2014 18:17:21 GMT -5
Hmm, 3.8 Henry inductor. That's a big one! The inductor and most of the parts there are for modelling the basic pickup, volume control and cable. The extra parts are just the opamp and the r5 pot, plus the tone cap |
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Post by b4nj0 on Mar 3, 2014 17:49:48 GMT -5
OK. Lumped constants. I get it now. I was just visualising Chesapeake and Potomac Co. 88mH coils and wandered off!
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Post by grapegranate on Oct 20, 2023 6:16:10 GMT -5
Has anyone heard of uF-range variable capacitors? Something that can could vary by a few tens/hundreds of nF? I'm sorry for reviving such an old thread but I think i invented one and wanted to check if anyone else had made it before me. I couldn't find anything, much like you. i was wondering if there was still interest. My design is just a proof of concept at this stage, so there's a LOT of optimization left to do on this scrap-components, 20cm long, 5cm diameter cylinder, but i've got it going from 1 to 10 nF already. I'm an electrochemical research technician but honestly i'm not well versed in analog electronics. I don't know what this capacitor could be good for outside of not needing such a large inductor to reach low frequencies. Is there any appetite in the analog world, maybe even outside of signal modulation? I'd love any thoughts you might have on the topic. Also this first one is pretty much whatever conductor and dielectric i had lying around. i don't want to share too much, but i don't see why this concept couldn't be extended to electrolytic capacitor levels, up to maybe tens of μF's. Really it's not a "smart" idea really it's ridiculously simple, it just looks like it's never been done before so i don't want it to get scooped. |
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Post by geo on Nov 15, 2023 11:17:56 GMT -5
If and when you're able to share more information, please do!
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Post by grapegranate on Nov 29, 2023 10:31:00 GMT -5
I was trying for weeks to figure out how best to get a patent, but today i figured out i can get most of what i wanna get done using a creative commons license. Working on that now. I'll make sure to ping you again when i finish it. Shouldnt take too long.
Actually, since i dont have a lot of activity here yet i cant DM yet. could you send me a dm, i can share some early prototypes to see if theres any glaring issues like ESR or not quite sinusoidal enough responses.
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