Replacing Voltage Divider with Expression input problem

[lars-musik]

Hi there,
I hope you are all OK in these pandemic times. Hearing about the bigger problems on the radio, maybe you could help me with a smaller one.
I am trying to add an expression pedal input to a pedal. But however I wire it (in my last iteration I even switched all three poles by a 3PDT and soldered a pot to a TRS-Jack to exclude a faulty expression pedal), the sound is not nearly identical to the one I get with the internal pot (humm, buzz, distortion, all kind of dirt AND nearly no change).
I feel downright studip that not even a complete decoupling with a 3PDT works. I mean, really, isn't a voltage divider a voltage divider?

Here's what I come from




Here's what is not working

[idy]

It might interest some to know what kind of circuit this is.

Some expression pedals might not be wired with TRS the way you want. Are you sure yours is wired like that? Tip to wiper, sleeve to terminal 1, ring to 3, or something else? (oops)

I would not bother with 3pdt. Why not send out 9v and ground on two wires, attached to the 1 and 3 of the pot, and then just switch between the wiper of the existing pot and the wiper of the exp. pedal? Maybe you waste some current that way but not too much... a 100k pot with 292k(?!) to 9v and 150k to gnd, almost 450k. So don't forget to add those two resistors to your new CV/exp pedal jack, they are selecting a range for the CV.

And if you do this, with one conductor not ground but 150k above ground, you will need to use a jack that is not grounding to the enclosure, one of those plastic insulated jobs.

[knutolai]

In some cases I've experienced picking up interference with expression pedals. Maybe it could be worthwhile to try a simple low pass filter on the Tip/wiper when the expression pedal is connected (cap to ground)?

[lars-musik]

Thanks for your ideas.

Obviously I perceived the voltage-divider thingy wrong. The value of my expression pedal's pot is 10k and that changes the voltages arriving at u6 significantly. Now I'd like to be able calculate the  values for R13 and R31 for a 10K pot as VR1 to generate a voltage range from 2.4V to 3.1V (from +9V to GRD). My brain just doesn't do the maths.

Could somebody help out or point me in the right direction?

Thanks!

[antonis]

Lars, don't get me wrong but you're tragic..

There is no need for advanced maths (not even for elementary ones..)

Just scale down R13 & R31 ten times..!!

[lars-musik]

Lars, don't get me wrong but you're tragic..

Haha. No, actually it makes me smile.

Thanks for that.

Now, sorry to be persistant, but the actual voltage range of the 100K that is originally in this cicuit  is 2,5-4,1V. It is a log pot and the usuable range is 2,5-approx 3V (measured with my DMM, approx 3/4 of the pot's travel). Because regular expression pedals seem to utilise linear (often 10K) pots, I'd like to adapt the resistor values, so that I land within these 2,4-3,1V ballpark and hope that the sweep is still usable. At the moment I am in trial-and-error mode, which is OK - but maybe I'd even learn something from calculating the necessary values.

[antonis]

First of all, we must shake hands on what counts on a voltage divider is resistors values ratio and not absolute values..

We also must shake hands on sweep has nothing to do with Full-CW & Full-CCW voltage..
i.e. you'll have the same voltage range for 29k2+10kpot+15k with that of 292k+100kpot+150k..

What you might haven't is indermediate pot setting voltage as previously..

So, for using linear pot instead of log one proceed with sweep altering as R.G. makes clear enough:

http://www.geofex.com/article_folders/potsecrets/potscret.htm


Some calculators might make your math life easier: 

https://www.diystompboxes.com/analogalchemy/emh/emh.html

https://sites.google.com/site/garydavenportelectronics/java

[CodeMonk]

For some of the noise issues, you might want to try an electrolytic cap to ground on your divided voltage output.
Anything from 22uF - 100uF should do.

[anotherjim]

I go with Antonis. Make the onboard pot 10k. Make the resistors in the CV path a tenth of those values. Use a switched TRS jack to bypass the onboard pot when the expression is plugged in. The input to the opamp +input is a high impedance, so from the wiper to that input you can add an RC filter to kill any noise the expression pedal adds without altering the control voltage (and it'll catch the momentary rise in voltage when that range switch is flipped).

The #1 pot lug can have a cap to ground so that the screen of the expression pedal can at least have AC ground on it which will further reduce noise pickup.

[lars-musik]

Antonis and Anotherjim: Thanks, that I will do. I overlooked the idea of also chaning the onboard pot to 10K in antonis' post. That is very smart indeed.