Slowing down attack (taper) on Potentiometer

[peAk]

I have a A50K modulation pot that I need the initial attack to be a little slower. This is something that's used in real time and the modulation kicks in too quickly when I barely turn it. I would like the beginning of the ramp/taper to be a little slower. 

Is there a way to do this with adding a resistor (maybe a trim, so I can dial in better)

I can't switch out the pot, so it would have to be done to this pot.

Any help would be greatly appreciated!!!

[jimilee]

A resistor n wouldn't lower the value of the pot, but it would do what you want to do.if I'm hearing you right, you'd  want to raise the value of the pot instead in order to decrease the rotation of the pot.


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[Matt]

Google The Secret Life of Pots

[midwayfair]

Post a schematic. No one can tell anything without knowing what circuit you're working on.

[peAk]

Post a schematic. No one can tell anything without knowing what circuit you're working on.

I attached the schematic Jon but this isn't just a simple modulation depth knob on a pedal. This is the modulation wheel on a minimoog. From what I understand about this circuit is that the way modulation is handled is that modulation is always on and the modulation potentiometer actually shorts out the modulation. So it kind of works in reverse.

I just don't like how sensitive the wheel (potentiometer) is when applying modulation. I wish the first 5%-10% of the turn was a little more subtle.

The other thing that makes mod and pitch wheels different is that they only use 30-40% of the potentiometer rotation due to the limit of where the wheel sits under the panel and the mechanical stop on the assembly.

Also, I had to edit the original post, it's actually a A50K, not a B25K

[peAk]

Any ideas Jon?

I was thinking about putting a 5K or 10K trimmer in before the pot and see if I could kind of dial in the attack somewhat.

[Lubdar]

I'd say secret life of pots may give you some ideas of "modifying" taper, but what you've described in terms of 2 pots in series will presumably just allow for fine adjustment of the 2nd pot. I'd say Bust out your multimeter and check out how/if the resistance changes the way you'do expect.

[peAk]

I'd say secret life of pots may give you some ideas of "modifying" taper, but what you've described in terms of 2 pots in series will presumably just allow for fine adjustment of the 2nd pot. I'd say Bust out your multimeter and check out how/if the resistance changes the way you'do expect.

Yeah, didn't have much luck tonight. I am not giving up yet though. The original Minimoog modulation pot must have had a different taper than a standard log.

[midwayfair]

Hm.

First let's figure out what's happening with the pot. There may be a couple errors here, because this is off the cuff.

R376 (!!!!) is a current-limiting resistor for LED8, which blinks in time with the modulation signal buffered by Q5; the modulation signal is DC-coupled to T53 (!!!!!!), forming a current-controlled resistor. We see these a lot in some form or another. The FET in the EA tremolo or the Bearhug is the voltage-controlled version of this (because FETs), there's a transistor that's used this way in the Ross compressor, the Tap Tempo tremolo uses a PWM output to control a transistor buffer, etc.

The buffer Q5's base is pulled up well above ground (R160 is much much smaller than R164!), which should put the emitter voltage above ground as well (measure it ...), and the modulation, when it swings negative (BELOW ground) will reduce the resistance between the collector and emitter of T53, blinking LED8.

The modwheel forms a reference voltage via R163, creating a voltage divider between ground and -10V for the emitter of Q5, which is pulled up (because of the base's voltage) probably above ground (again, measure it). Turn the wheel up and the emitter isn't held near ground when the LFO output sing comes along and tries to wiggle it.

Okay, let's look at the modwheel itself.

As a 50KA pot, it will be at 5K when it's at halfway, and 500R in the first 10% of its rotation.

There's a blocking resistor, R163, that's 1K. This means that even at 0, it's like we're starting above the 10% portion of the pot.

So the first thing I'd do is drop R163's value, maybe even short it entirely. You'll get to about 9:00 on the modwheel before you exceed the original starting point.

If that doesn't work, then I'd start looking at the strength of the modulation signal, starting with increases to either R375, R162, R165, or R376.

... Good luck. :/

[peAk]

Hm.

First let's figure out what's happening with the pot. There may be a couple errors here, because this is off the cuff.

R376 (!!!!) is a current-limiting resistor for LED8, which blinks in time with the modulation signal buffered by Q5; the modulation signal is DC-coupled to T53 (!!!!!!), forming a current-controlled resistor. We see these a lot in some form or another. The FET in the EA tremolo or the Bearhug is the voltage-controlled version of this (because FETs), there's a transistor that's used this way in the Ross compressor, the Tap Tempo tremolo uses a PWM output to control a transistor buffer, etc.

The buffer Q5's base is pulled up well above ground (R160 is much much smaller than R164!), which should put the emitter voltage above ground as well (measure it ...), and the modulation, when it swings negative (BELOW ground) will reduce the resistance between the collector and emitter of T53, blinking LED8.

The modwheel forms a reference voltage via R163, creating a voltage divider between ground and -10V for the emitter of Q5, which is pulled up (because of the base's voltage) probably above ground (again, measure it). Turn the wheel up and the emitter isn't held near ground when the LFO output sing comes along and tries to wiggle it.

Okay, let's look at the modwheel itself.

As a 50KA pot, it will be at 5K when it's at halfway, and 500R in the first 10% of its rotation.

There's a blocking resistor, R163, that's 1K. This means that even at 0, it's like we're starting above the 10% portion of the pot.

So the first thing I'd do is drop R163's value, maybe even short it entirely. You'll get to about 9:00 on the modwheel before you exceed the original starting point.

If that doesn't work, then I'd start looking at the strength of the modulation signal, starting with increases to either R375, R162, R165, or R376.

... Good luck. :/

awesome Jon. This is a lot of info to sort through but I will start trying your suggestions ASAP and report back.

I just don't get it because I have watched a ton of videos of minimoog and the modulation even with the wheel half way turned is pretty subtle. I seem to get passed vibrato into deep modulation very early. I feel I am already at max modulation when my wheel is half way up. I would think it was a wrong value on my part but I know someone else who has built this with the same results. I am starting to believe that the original minimoog modulation potentiometer must have had a different taper. I tried two different A50K potentiometers (Bourns and Alpha) with the same results. 

Hopefully I can tame it a little with one of your suggestions.

Thanks for the help 

[peAk]

Hey Jon

After endlessly searching the web, I found this post on a forum from 2007

The mod-pot has only two wires. In full counter-Clockwise postion it should have 0 Ohms. Fully forward (full modulation) it should have 1,2 KOhm. This is a movement of 90 degrees - it´s a miracle, that this 50Kohm-Pot (audio/log) makes only 1,2 Kohms at the first 90 degrees...this is really special. So it will not work if you change it to a simple 50KOhm log Pot.
If the modulation begins too soon you can loosen the little screw of the modulation-wheel and turn the pot-axle some degrees- try out, in which direction (Sorry, i forgot it Wink). There is nothing that you can damage or make wrong - it is stone-age-elctric.
Just loose the two screws of the modulation-panel, pull out the small wood-block and remove the panel carefully. Before you kill the kabels, disconnect the 12-pole jack, so you can make your measurement very easy on your table. The jack only fits in one position - you dont have to mark it.

So I did seem to locate a place that has suitable replacement pots that they sell for $30.00 (+shipping) 

Do you think there is another way to achieve this with my current pot? I haven't tried any of the resistor swaps you mentioned on the board yet because that is going to have to disassemble everything to flip the board and desolder. Before I spent all the hours or work to do that, I wanted to get your take on this.

[peAk]

Okay, so I happened to have a Bourns A25K that got me to about 1.8K at 90 degrees.


This seems a lot better. I will do some more testing tomorrow.