Knobs

Do size matter?

Yes! — That's what she said

Preface

A trend over some years in Eurorack have been to make modules smaller, more narrow not to waste HP (horizontal pitch). "The hunt for more modules and functions in smaller cabinets."

Small knobs

This comes with the obvious cost of making modules less comfortable to navigate and control. But there is another aspect of this not too often being discussed — The loss of control that comes with it — often rendering modules less useful.


How is it we lose control?

As this discussion talks about analog potentiometers maybe a better description could be loss of mechanical control. Or a even better term for this could be loss of resolution. How is this? The potentiometer after all is a rotary device with a fixed size independent of size of the knob?

The best way to describe this is to compare size of a knob to travel length of a slide potentiometer, or fader as its often called. With some exceptions of course depending on quality of resistor path and wiper size inside the potentiometer.

Resolution

As an exercise let us divide up the travel length for for an given size knob into 1024 steps. This equals 10-bit resolution typical used in ADC control scanners for many synthesizers.

A large 38mm knob will for each of the 1024 steps have a short travel length of approx 0.1mm. Now with a 6mm knob these step are narrowed down to 0.015mm.

If compared to digital resolution this is not far from going down from 10-bit to steped 7-bit which for some controls can work fine but for other not so much. Please see table for more examples.

Pot travel length

Hunting for the Sweet Spot

Many have experienced how challeting a short 25mm fader can be when trying to set accurate LFO speed or sliding through filter resonance. Often missing the sweet spot wanted and near impossible to position the fader accurate.

A rotary pot with 6mm knurled shaft are even harder to set accurate with a pratical travel path of only 15.7mm.

While 100mm long faders on a mixing desk just feels smooth and accurate and joy to slide into position.

Similar analogy can be transferred to knob size for rotary potentiometers.

Idea for a BIG knob in a narrow panel

One could mount potentiometer similar to how an expression wheel in a synthesizer is mounted. But using a full 360° circle disc not limited to the usual half circle 2/5 turn.

Knob size and travel length

Chart show potentiometer with 300° rotational angle.
All measurement in millimeter.

Knob
Diameter
Travel
Length
1024 Step
Length
128 Step
Length
6 15.7 0.015 0.122
9.6 25.1 0.023 0.196
12 31.4 0.031 0.245
16 41.9 0.041 0.327
18 47.1 0.046 0.368
20 52.4 0.051 0.409
32 83.8 0.082 0.655
38.2 100 0.097 0.298
50 130.9 0.127 1.022
100 261.8 0.255 2.045

Math used here

Circumference of a circle are found by calculate pi * diamter (πd).

Typically a rotary potentiometer have a rotational angle of 300°. This can be represented as /1.2

Travel Length = πd/1.2

3,1415926535897932384626433832795 * 10mm / 1.2 = 26.18mm

Dopher A111-5

While knobs with longer circumference gives better control they also can be problematic for other reasons.

A-111-5 Mini Synthesizer's shown in photo knobs where replaced with some vintage style knobs (not Doepher original). To compare size look carefully at the silkscreen around the scale surrounding the knob. As one can see there are very little difference compared to the standard knob size used on the right side.

Sadly those vintage style knobs made this module more difficult to control. Reason being their size are uniform from top down to the bottom. Original knob size are cone shaped. Course they are much smaller at the top where finger grab one can twist the knob full turn with one fingergrip. With the vintage style knob one have to grip the knob from each side since row knob spacing are more narrow than the column spacing. Then take a new grip every 20-25° turn or so.


Comments

Your comment are welcome

Name:


Email: optional, will be hidden


Comment: