Using a pot to switch/blend between two caps...can it be done?

[skiraly017]

Communicating this idea is easier said than done, so I'll apologize up front.

I have a PT2399 based delay circuit. There's a cap that to my ears affects the high end roll off and I've found two caps that affect the high end roll off differently. One offers a little roll off and a nice warm sound, the other offers more roll off and a gritty sound, like dirty tape heads. I know I can install a switch to toggle between the two caps, but is there a way to do it with a pot allowing me to go from one to the other and all points in between. I'm not trying to create a tone control, but a cap select/mix control for lack of a better term. The only reference I can make is like the balance knob on a stereo. Thanks for any ideas.

[John Lyons]

Easy as pie.
Input  goes to the pot wiper/pin 2. One cap on the outer leg, the other cap on the other outer leg. Connect the caps unconnected legs together and this becomes the output. Try a 250k pot fifrst and see what you get.

            /-----I I-------I
            /                  I
In -------                  I-------OUT
            /                  I
            /------I I------I

John

[rockgardenlove]

^Yep! 

[caress]

this is great!  i wish i knew about this earlier...  will it work with electro caps as well?

[skiraly017]

A problem that has been stumping me for awhile seems so simple when someone else explains it. Thanks for the help.

[Auke Haarsma]

this is a simple great idea, which I'll try to use on my pt2399 delay!

thanks for asking and thanks for answering

[Michael]

If i did the same in a fuzzface (with the input and output caps), would i need a 250 K pot or a different value?

It would be a cool mod but im unsure if i need a 250 K in that circuit.

Michael

[Mark Hammer]

I don't know that he "invented" it, but certainly a big nod of appreciation has to go in Joe Gagan's direction for making the use of pots and parallel caps popular in the context of his various digressions on the Fuzz Face theme, such as the Bronto Boost and other NVN designs/products.

[John Lyons]

Try a few different pot, I'm not really sure what will be best. 100K, 250, 500K? The bigger the value the slower the blend speed while turning the pot.

See if you can follow this for a fuzz face blend cap, taken from the Skyripper. Thanks to Joe Gagan for this!!!!!! Check out his stuff.
I have some sound samples of a lot of it here www.mrdwab.com/john/soundclipspage.html
     
                    10uf
            I------I I-------I
            I                    I
            /----I              I
            /----I              I-------OUT   
   IN-----/    100K         I
            I                    I
            I------I I-------I
                    .005uf

Hmmm hard to dray here but the pot is a 100k linear with the top and middle legs connected as a variable resistor.The input connects to the lower leg. This works well to give your fuzz face etc a fat and squishy input with the 10uf cap or an articulate and pick responsive input with the .005.

[Michael]

Thanks a lot

Im a bit unsure of the difference between the two drawings you made... not so much the reading of them, but the function...

Is it that in the first you blend between the two values and in the second drawing you add?

Whats the difference in how the blend works?

This can be done with both electrolytic or normal caps?

[John Lyons]

Michael

The difference in the drawing is that the wiper/lug two is the input of the first diagram and the second one has the input at the bottom lug and the two top lugs are connected together. So yeah, you are adding the capacitance (caps in parallel add, and in series subtract)
I didn't really think about it at the time but yeah...they add in the one position...

With the first diagram the caps are sort of switched by the pot from one to the other. I guess the do add though since in the middle of the pots rotation they are parallel with some reisistance between them. SO maybe more reisistance is better...?

You can use any kind of cap. For electrolytics the + side is closest to the top of the pot. (+ to in, - to out)

A guy asked me offline about switching caps in a tubescreamer. I fthought I'd post it here as well.

If you switch between two capacitors and like it then the blend pot will work similarly. the blend pot is basically the next step from using a switch between two caps.  What I would do is try out a few different caps, see what each one does to the sound. Since you can blend between two caps you may as well take it to extremes with the cap vlues and somewhere near the middle of the pot travel will be a "normal"  range of tones. Then at the extreme settings there are the more wild tones...

It helps to have a few test clips (alligator clips with a several inches of wire in between). This way you can clip in different caps and try them out having without soldering them in. Once you settle on what sounds good you can use those values to build you blend pot.

Hope this helps..

John

[Michael]

It does indeed!

Does the electrolytic caps need to be reversed (from what you explained) if im using it in a PNP circuit?

Thanks for your help, it is much appreciated.

Michael

[John Lyons]

No, the cap can stay the same either NPN or PNP. The reason some caps need to be changed of PNP.NPN is because the power is reversed from the battery or supply. SO any caps that are in the power or to ground positions need to be oriented acording to whether the "ground" is positive of negative. The caps in the input blend as just sort or floating and do not reference to ground or the supply voltage. Is that clear?

John

[Michael]

yes thank you.

I will try this out.

Michael

[Skreddy]

The input/output is the two pads where the red cap is connected to the board.

Note: this doesn't address the original question; it only adds bass response by bringing in a bigger cap in parallel.  If you have two caps that differ in their fundamental tone and high-end characteristics, I think really the best way is just to use a switch instead of a pot.