Decent volume control

[Zebidee]

Decent Volume Control

I just finished installing a volume control in my latest arcade cab project.

This volume control project has zero need for programming or configuring, and is:
- analogue
- modular, meaning easy to change/replace with equivalent devices (within reason)
- simple, easy to install
- cheap, effective

All that is needed is a simple linear potentiometer and some audio cables. The exact pot value doesn't matter too much, it could be anything from 10K to 100K, but I find that the B50K pots, middle-of-the-range, work well with PC audio. So I've used a B50K pot in this example. They must be dual-gang pots, 2x3=6 pins, for stereo (two channels). Pots should only cost a few dollars.

In terms of wiring a simple linear pot for volume control, this isn't hard at all. There are 3 pins for each channel (x2 for stereo). The ground goes to one outside pin, and the input to the other outside pin. The middle pin is called the "wiper", and that will be the output.



Typical pot wiring for volume control


Do this x 2, total six pins, for dual-gang pots. With dual-gang pots, make sure to connect the two ground pins with a little extra wire. Done.

In some cases there is an EASY WAY, with no very little soldering.

For the easy way, buy a kit of plastic headers. I got this one some time ago, seemed like a good idea at the time, cost maybe $10 or less. It has a bunch of standard sizes and pins.


   
The pins are a perfect fit for standard 5-pin header



As luck (and standarisation) would have it, and because the pot pins are spaced at 5.08mm (2/10 inch) and the header pins are spaced at 2.54mm (1/10 inch), the 3 pot pins fit into the 5-pin header SNUGLY and PERFECTLY.

If you don't already have a decent crimping tool, spend another few dollars (probably a little more than $10) on one. Then you can just make the female pins, insert into the headers, and away you go. Plug n play.

So, no soldering required EXCEPT you must connect the two ground pins. Easier to solder them at the pot rather than at the cables. There are convenient little holes at the base of the pot pins, you can just run an old resistor leg between them.

I used some blue and green heatshrink to remind myself which cables are input (green, from PC) and output (blue, to amp).

In testing, this gave me full range of volume, from really loud to super quiet. It is pretty good (though not perfect), would satisfy most people's needs. However, I note that most of the volume change seems to occur in the top half or third of the pot's range. More on that soon.



 
Complete, except: ground pins (blue) not yet connected on left; ground pins are connected on right


 
Installed, front and centre. Can't see it? Good, it is meant to be subtle, though not hidden.



Still there? Well strap in, it is about to get exciting.

This pot will give your reasonably good outcomes, but. I know what you're thinking. "This is a linear pot. All fine and good, but you need a log pot to properly reflect the range of human hearing in a smooth way across the range of the potentiometer" 

Well, the theory says you are absolutely right. So let's dive into that with the next installment, which will be on turning this into a log (or anti-log) potentiometer.

Want to see more on this project? Check out my Aussie lowboy build thread.

[Zebidee]

DECENT VOLUME CONTROL - PART II

POTENTIOMETER MODDING

Logarithmic pot

Linear pots are called "linear" because, if you chart out their responses over range, you will get a straight line (see chart below). However, this is not how we hear. Human ears are more sensitive at the middle-low range of hearing. With a linear pot, you would mostly just use the top 60-90% of the range.

A logarithmic pot is theoretically better for a volume control - it dampens the pot's responsiveness in that middle-low range, giving you more precise control and opening up the full range of the pot.

Fortunately we can approximate a log pot using a linear pot. Yes, I could go and buy a log pot or volume control pot, but they are relatively expensive and this modification may actually be better than typical commercially available log pots.

Here is a simple schematic of how to make a log pot:



Make linear pot into log



All that is needed are a couple of resistors. The exact value is up to you, but works better with values around 10-25% of the pot. For this example, I used a B50K pot (measured maximum resistance = 45.5K) and 6.8K resistors.

The resistors go between the GROUND and WIPER (output).

If we plot this out on a chart, we get a concave response curve (refer chart below) that approximates a log pot.

This log modification worked exactly as advertised - however, in my case, I found that this did not improve the volume response. For whatever reasons, I found that it made things worse, making the middle-low volume even harder to hear and the pot was less responsive.

I discovered that I needed the OPPOSITE of a log pot, an anti-log pot instead.


Antilog pot


Making an antilog pot is very similar to making a log pot, the main difference is that the resistors go between the INPUT and WIPER (output).



Make linear pot into antilog


Decided to experiment with some different values. It is very easy to experiment with different resistor values, just bend the legs and shove them into the back of the headers. Plotting out the antilog response, over the pot's range, shows a convex curve. Lower value resistors make the curve more pronounced (refer chart below).



This is just temporary and for testing!


As it turns out, antilog seems to work best when the resistor value is about the same as the pot's maximum range. In my case, as the 50K pot (when I tested/measured it) maxed out at 45K, so I chose 47K resistors. Then I installed the 47K resistors directly onto the pot itself. I used small holes, at the base of the pins, to secure/solder the resistors. If look carefully below/left, you can also see how I joined the two ground pins with an offcut resistor leg. The formula is the antilog math.


 
Resistors attached/soldered to tiny holes at base of pins


This made the antilog response quite gentle and focused on the middle-low part of the pot's range, which is perfect for my needs. On testing, it sounds perfect!

One small drawback of this mod is that it has reduced the pot's maximum/range (the B50K linear pot is now a 25K antilog pot), such that when volume is adjusted to minimum I can still hear the audio faintly. It is not a big issue, I am still happy with it as I can control volume as needed, and if I need volume to go lower I can still adjust it on the amp directly.

If I feel motivated later, I can replace the B50K pot with a B100K pot with same form-factor (I have some ordered) - all I would need to do is put the 100K resistors on and plug it in.



Voltage change over potentiometer range: linear, log and antilog


A spreadsheet (Excel format) is also attached as a zip file. You can use this for your own projects/analysis. Just plug in your own pot and resistor values.