using a pot to pan between caps?..

[deadastronaut]

hi guys..

how can i use a pot to pan between two caps..so we have an adjustable cap value.

1nf-22nf...so i can also get the ranges in between too..?.

ive tried my signal going to lug 2 then a cap on 1/3 then joined and out...but it only really works at each end of the pot..hmmmm..

this is coming off a 'T' filter btw...

[DDD]

This idea works decently in some circuits. For example, it controls low frequencies in the input circuitry of stompbox if the caps, etc.
But it's impossible to get "pure variable capacitor" with two caps and one pot.

[deadastronaut]

hmmm thought so...

cheers..

[mth5044]

I've seen circuits having the input and one side of C1 connect to lugs 3/2 of a 100k pot, then C2 connects to lug 1. C1 and C2 connect together and that's the out. I believe it's used for variable input cap and I also saw it in Madbean's wah pedal for the sweep cap instead of a rotary.

EDIT: I guess a drawing could help. R1 would vary between 0 and 100k or something.


In ------- C1-----------Out
      |             |
      R1          |
      |             |
      -----C2----

[samhay]

sorry Rob - has been a busy week.

No idea if this will work, but you could try putting the two caps in series and then running a pot (R) as a variable resistor parallel to the smaller cap (C2). The size of the pot and the taper will matter.

---C1----|-----C2--------|-------
            |                     |
            |-----R----------|

Edit  - I don't think this will work in your filter.
I think you may have to make your peace with a switch - it could be rotary if you prefer a knob over a toggle (wow, there is some potential for innuendo there).

[midwayfair]

Using a pot as a variable resistor is, essentially, "panning" between two cap values. As the pot value gets lower, you get more of a larger capacitor, increasing the capacitance. More capacitance at lower settings, less at higher settings. It works best with a log pot.

You can take a look at the treble control on Fender amps to see an example of using the pot as a voltage divider instead of a variable resistor. In that, a series resistor is used with the larger capacitor before it gets to the blending pot, to avoid the control sounding much louder at the higher capacitance. I don't really see much added value in that arrangement, since it uses a much larger pot (250K) and a fairly large fixed resistance (100K), which is pretty loss-y. Doing the variable resistor is better in that regard.

Samhay's version is sometimes done as a treble control in compressors. The cap bypasses the resistor, but I don't think that you change the cutoff frequency as the resistor changes. The cutoff of the low-pass should remain constant based on the resistance to ground in whatever follows, while the resistor controls how much of the lower signal gets through.

[deafbutpicky]

The easiest way would be the input bass control of the easyface. I used it at fuzzes with good results but you might
have to increase/decrease the caps values to make it more effective.
The down side is:
it will mess with the gain and frequency response of the following input stage depending on it's impedance which should be
on the higher side as this tone controle will give you a slightly sweeping peak added to the bass roll off.

[deadastronaut]

cheers guys, i'll give them a go and see how i get on.

if all else fails then i'll probably have to resort to a switch arrangement...which i was trying to avoid...but hey.

nice one..

[deadastronaut]

yep, think i'm gonna have to go the switched route...

cheers though guys..

[R.G.]

Good place to talk about the differences between resistors and caps.

Both resistors and caps will impede (impede... get it?    ) the flow of electricity. However they do it different ways.

Imagine driving your car down a road. The road surface changes from hard asphalt to gravel, to dirt, to mud. The change in the road is a changing -resistance- to your car's rolling forward.

Now imagine driving across a smooth grassy field. The resistance to your rolling is determined by the water content of the dirt, right? That's resistance. A strong wind comes up and pushes you directly sideways so you have to fight to keep headed across the field. That's capacitance. Capacitance acts at right angles to resistance.

That's why the combination of resistance and capacitance can cause different amounts of impede-ance to electricity, and why varying the resistance can change the apparent overall impedance. But if what you want is the pure change of capacitance, no amount of messing with the resistance will affect it.

[deadastronaut]

well explained...cheers R.G.

now how do i attach an on/on/on switch to a car driving on chicken soup at 190mph going passed me in torrential rain Whilst blasting out ''walk dont run' by the pink fairies creating a doppler effect?...  ...

[R.G.]

well explained...cheers R.G.
now how do i attach an on/on/on switch to a car driving on chicken soup at 190mph going passed me in torrential rain Whilst blasting out ''walk dont run' by the pink fairies creating a doppler effect?...  ...

Well, OK.

For that, we have to start doing analogies using quantum physics as examples.   

So, if you consider that you have an 11-dimensional Riemann space with...   

[duck_arse]

if you were playing "roadrunner" instead, rob, you could try C1 in series with C2, pot cw to left-hand end of C1, ccw to right-hand end of C2, and wiper to junction C1//C2. might work if you can dc-de-couple it.

beep-beep

[GibsonGM]

Good place to talk about the differences between resistors and caps.

Both resistors and caps will impede (impede... get it?    ) the flow of electricity. However they do it different ways.

Imagine driving your car down a road. The road surface changes from hard asphalt to gravel, to dirt, to mud. The change in the road is a changing -resistance- to your car's rolling forward.

Now imagine driving across a smooth grassy field. The resistance to your rolling is determined by the water content of the dirt, right? That's resistance. A strong wind comes up and pushes you directly sideways so you have to fight to keep headed across the field. That's capacitance. Capacitance acts at right angles to resistance.

That's why the combination of resistance and capacitance can cause different amounts of impede-ance to electricity, and why varying the resistance can change the apparent overall impedance. But if what you want is the pure change of capacitance, no amount of messing with the resistance will affect it.

Nice!   Great analogy, R.G.

Set up your tensor matrix, time for some n dimensional dialog

[pappasmurfsharem]

Check the bass control of a Timmy, works well enough.

[R.G.]

Check the bass control of a Timmy, works well enough.

Sure. The right-angle-to-a-resistor stuff doesn't keep you from doing variable controls where the necessary result is a variation in AC current flowing or a variable AC voltage divider.

But changing the value of a pure capacitor is not easily amenable to a variable resistor and two caps. This puts a premium on knowing whether your circuit can work OK with a mixed resistor-and-cap impedance or whether only a capacitance will do. There's been almost a century of electronic thought expended on ways to get around the limitations of inductors and caps. In particular, there has been a lot of effort on the issue of making tone controls with only variable resistors, not variable caps and/or inductors.

Example:  the emitter capacitance of the second transistor in a Vox-style wah is electronically varied by varying the wah pot. The wah pot varies the effective gain, which varies the apparent capacitance of that cap by the Miller effect. The cap doesn't change, but it looks like it does to the inductor and rest of the circuit.

[deadastronaut]

just to clarify, this is the bit i'm working on..

i was trying to get C1 to go from say , 2.2nf to 22nf...thin to fat.  (with ranges in between)