Bright Knob vs 3-Way Bright Switch

[rhys]

I'm trying to understand the different functions of a bright cap on the front end of a pedal (or amp). 

Let's say option one is a 500pf cap wired to a pot on the front end so that you could sweep between no capacitance and full 500pf. 

Option two is a three way switch with caps at 100 250 and 500pf. 

Aside from the obvious differences in having a sweepable pot versus 3 fixed settings, are these two options roughly the same or not?  For example, would the bright pot at 1/2 sweep sound like the 250 cap or would the characteristics be much different since the cap values are different? 

[Mark Hammer]

I think there are a few qualifiers that need to go into your query.

Do you mean a pot in series with the cap?
Do you mean a pot that provides a variable end-run around the cap?  And would that end run go through a cap of a different value?
Do you mean a cap that sets the input low-end rolloff as close to the input jack as possible?

The clearest, and most instructive, answer for you will need a somewhat clearer question to get things started.  So "breadboard" the question a little bit more, then pose it again.

[aziltz]

a bright knob would be like this:

smallish cap in stock position with a larger cap blended in with a pot. ( a pot in series with a big cap ) both are in parallel with a small cap.

or, a toggle for caps.



the switchable settings lends itself to pops.  there's really know way around it. we hacked it to death a few weeks back.

i prefer "bright knobs".  what it really does is cut bass though.  sometimes its called "Range" if its on a treble booster.

[Mark Hammer]

Hmmm, were there pictures in the OP that I could not see?

[chi_boy]

I've wondered about this myself. I've built a few FF's from GGG and thought about the input caps on the Easy Face versus switching. Sounds very similar to your query in regards to switching input caps or rolling between them with a pot. One of these days I'll have to experiment too, but it would be nice to know what to watch for.

[rhys]

There were no pics in the OP.  Let me try from this angle. 

If I stick a 500pf cap between the input and the circuit I get less bass, so I'm calling that a bright cap b/c it makes it sound brighter.  You could affect the amount of bass attenuation by changing the values of the bright cap--e.g. 100, 250, 500pf. 

You could also blend a cap with another using a pot like aziltz said.  (Could you also just bleed one cap to ground so that pot varies how much capacitance affects the signal?  Not sure about that.) 

So my question: Is the pot implementation just a more incremental version of the 3-way switch, or would there be more fundamental characteristics between having a 250pf through the switch versus having a 500pf rolled half way off.  Put another way, could you get a bright knob to sound like it's rolling off a lot of bass (as if there were a 500pf) and a lesser amount (like there were 100 or 250pf) by sweeping it.  Or would the sweeping the knob affect things some other way? 

As to Mark's questions, I'm not really sure what I envisioned except yes, setting the rolloff close to the input jack. 

[Mark Hammer]

At this point, I refer you to the various Fuzz Face derivatives that the clever Joe Gagan came up with.  These use a small-value input cap, and a much larger-value cap with a series pot in parallel with the small-value cap.  The user can adjust how much full range signal bypasses the smaller cap.  It is essentially a "bass bypass" pot.

The advantage of this is a) a wider array of tones attainable, and b) NO popping when cap values are switched (because there is no switching).

A reasonable question to ask is whether the filtering function provided by a continuous control like this can provide the exact same tone-shaping as what a switched-cap arrangment can do.  The answer is "Not exactly".

Note that most of the guitar signal "lives" in the bass.  What that means is that when you trim the bass with a smaller-value cap, you also present the input with a lower overall amplitude signal.  So, fading in the bass with a continuous control not only alters the tone, but alters the level hitting the first stage.  That may be what you want, but maybe not.

With a switched arrangement, the user/builder could use additional passive components to hold overall amplitude constant when switching between input tone settings.  I.E., less bass but same overall loudness.  That may, or may not, be important to you.

[Vitrolin]

if mark or someone could be as nice as to post an image that visualize this i would be most grateful

[frank_p]

if mark or someone could be as nice as to post an image that visualize this i would be most grateful

One of the options (that was used by Joe G.and that Mark refers to initially in this topic -I think but I am not sure-) could be seen in a topic posted not long ago:
http://www.diystompboxes.com/smfforum/index.php?topic=77474.0

WoW Subjects goes down so fast.

[Vitrolin]

Thank you very much it made it all clearer