Voltage divider+series resistance

[Taylor]

This is one of those questions that is really basic, but actually much harder for me to figure out than more "complex" stuff.

I want to have an expression pedal controlling an MS20-style filter's cutoff frequency, but I also want a panel-mounted pot for filter cutoff. I want them to work together, so the panel-mount sets the range, and the exp. pedal sweeps the filter.

The filter frequency is controlled by a voltage divider. You feed it 9v. I tried to wire them both as voltage dividers and my exp. pedal pot burned up.

Should I wire one of the pots as a variable resistor, and one as the divider?

Here's the schem:

http://www.geocities.com/tpe123/folkurban/fuzz/sk.gif

[Cliff Schecht]

Did you get the ratios right on your voltage dividers? If you're simulating a pot, you need to guess the pots resistance between the middle lug and each outside lug and choose your resistor values accordingly. With a 10k potentiometer in the middle position, it's two 5k resistors.

In the case of this circuit however, you need to adjust both filters at the same time. Your best bet is to limit the range on the footpedal mounted pot and make an external "coarse" control that sets the filters initial frequency. A simple resistive mixer will get you exactly what you need. Here's the schematic with what I'm talking about:


P1 is the potentiometer inside of your wah casing - the resistors on either side of the outside lugs act as voltage dividers with the pot to limit the output voltage range. I can't easily choose these values for you so you'll have to rig up something that allows you to play around with these range setting resistors (or maybe even place pots and tweak them until the pedal rotation controls the frequency range you want). P2 is the panel mount pot that controls the initial frequency. I would play around with pot tapers to find the best feel, the external range pot (P2) will probably be best as a log pot and I have no clue for the wah casing one (reverse-log?).



Sorry to anyone if I wasn't allowed to modify and repost this circuit :-x

[Processaurus]

This does what might be more along the lines of the original poster's goal, to have an expression pedal control the filter and the panel pot control the max it can sweep, but with nothing plugged in it sweeps the filter like normal, two voltage dividers in series.  V+ is divided by the panel pot, then that voltage is divided again by the pedal pot.  It's handy for the pedal to have a minimum adjustment pot, so with heel back you don't have to go all the way to zero.  Cliff's schematic mixes the pedal and panel pots equally, two voltage dividers in parallel.  That would be useful for something where the 2 pots need to be democratic.  Though the 1k resistors to mix them would load each pot with the other pot, what about sending each pot seperately through it's own 100K resistor to the current control input on the ota's, and the CV's are mixed there?

http://www.diystompboxes.com/smfforum/index.php?topic=45777.0

[Cliff Schecht]

If he wants a "normalized" connection scheme (which he didn't mention but is a great idea) then he just needs to used a normally-closed switching jack. I think I spotted a few in the BUMS thread you posted.

[Taylor]

Well, the whole circuit is going to be built in a wah enclosure, so I don't need normalization, Thanks for this info, guys.