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How To Have A Better PC Volume Control
I don’t know if it’s only me but I have a beef with the Windows volume control. You know this sliding cursor that you actuate with the mouse.
I don’t say that it is not working – of course it is working – but it is not convenient at all. Imagine a typical situation: You are watching a video (a technical video of course!) and you had to crank up the volume because the presenter’s voice has very little volume. When the video is over you click to another video and here it comes, a blasting sponsor’s advertising, full audio power. By the time you grab the mouse, click onto the little speaker icon, wait for the volume control cursor to appear – it takes its time, usually a few seconds – by then you have woken up the wife, the kids, the cats and the dog… I know there are shortcuts but they are usually slow or not convenient. I wanted a control where I am the one controlling the volume instead of the computer doing it.
If you have an external speaker/amplifier as I do, it might have a manual volume control, so the problem is solved. You can skip this article, unless you want to read more about ancient but elegant ways of controlling the volume…
The external speaker I use has a volume control but it works only if you connect it through Bluetooth. It is a pair of digital buttons + and – , very nice but, again, not very convenient. Because of this and for other reasons I use the cable connection between the laptop and the speaker, and decided to make a manual, ergonomic and efficient volume control. All what was needed was to insert a potentiometer between the laptop and the speaker. As simple as that! But let’s do it on a practical and professional way:
All you need is a small project box, a couple of 3.5mm audio jack sockets, a potentiometer, a nice old style knob and pronto! You can also prepare a nice little overlay so it will look good:
It is easy, done in no time and it works perfectly well. Controlling the volume by turning a very accessible knob is much more natural; it has a great feeling and reacts instantaneously.
And it’s the opportunity to have a closer look at this almost forgotten component, the potentiometer. We don’t see them very often these days, except maybe as adjustable potentiometers (trim pots) soldered on PCBs.
The old “pot”, as we called it, was the way to control the volume of your granddad’s radio. But there is a twist, there are different types of potentiometers: linear and logarithmic! The one used in this mini-project is labelled “B50K”, meaning 50KOhm linear. The granddad’s radio volume control pots were all logarithmic. Why this difference and how did I get away using the wrong type of potentiometer for this function? Let’s first remember the basics of this humble component:
The figure above shows the characteristics of both, linear and logarithmic potentiometer. In the case of a linear potentiometer the output voltage Vout is proportional to the potentiometer rotation. This means that half way, at 50 % of the rotation the output voltage in this example will be 0.5V (50%). In the case of a logarithmic pot, the curve is no longer linear. For example at 50% of the rotation the output voltage is only around 10% (0.1V). Why do we need this? The reason is physiological, it’s because the sensitivity of the human hear is not linear, but logarithmic. The ear is more sensitive to lower sound levels than to higher ones. This is why we can hear such a wide range of sound levels, from murmurs to airplanes reactors… Controlling the audio volume with a logarithmic pot feels much more natural, this is why those log pots were used in grandpa’s radio.
To make those potentiometers they needed to manipulate the density of the resistive track in order to obtain the desired characteristic. This was quite an expensive process but it didn’t matter much as the demand for logarithmic pots was big. Today, there is not such demand anymore and the log pots have practically disappeared from the market. Audio volume is controlled with digital encoders or control remotes. Hence most of the potentiometers you would purchase are linear, the salesman will not even ask which type you want because he/she probably wouldn’t know the difference. But what if you want to repair grandpa’s radio and need a log pot? You might find it online but if not, there is another way around: the loaded potentiometer…
In our previous example we assumed that there was no load at the output (Nothing connected to Vout terminals). This was almost the case in grandpa’s radio because the input of the vacuum tube audio amplifier following the volume control had very high impedance. But see what is happening when we load the potentiometer with a lower value resistor:
The calculation of V2 is no longer depending only on the ratio between Ra and Rb but must consider that RL is now in parallel with Rb. For each step of the potentiometer we must calculate the value of RL parallel with Rb, which we called Rp in this example. Then we calculate V2 using Rp instead of Rb… This is hard work if you want to plot V2 for each position of the potentiometer. Thus we will use Excel so we can enter the formulas once only in the first row, then drag it down. And see what we find:
By connecting a 10K resistor at the output of our 50K potentiometer we transformed the linear pot into a near log pot! I selected this value of 10K because it is the input impedance of my speaker system. I didn’t have to change anything to get a kind of logarithmic volume control that feels very nice and natural.
So here is your solution, if you have to repair granddad’s radio and cannot find a logarithmic pot. Just use a linear pot and connect a resistor between the cursor and the ground. The value of the resistor will depend on the value of the potentiometer but you can use the example above, i.e. about 20% of the value of the pot might be fine. The lower the value, the curvier the characteristic will be; it is worthwhile to test a few values and select the one that produce the most natural volume control.
There are indeed more than only two kinds of potentiometers, some with specific characteristics to suit various analogue functions in scientific instruments. Some companies are still making “tailor made” potentiometers to suit any application. For a price obviously! For us, the standard linear is doing the job perfectly well.
I hope that you enjoyed this trip into the past…
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