The Humble Volume Control
Somewhere in the line that stretches from sound source to sound reproduction is a volume control. Very useful devices, but when dealing with hi-fi circuits, a little thought needs to go into the process.
Volume controls are usually implemented with a potentiometer configured as a voltage divider. The signal goes to lug 1, the output is connected to the wiper (lug 2) and the lug 3 is connected to ground.
As you turn the pot shaft, more or less of the signal voltage is sent to either to ground (quieter) or to the output through the wiper (louder).
This simple approach works well, but we are concerned with stereo, which has that pesky concept of two independent channels. But your standard pot only is a "single-gang" device: it only has one set of lugs.
Wait! What Value Pots Should I Use?
There are two basic types of potentiometers: linear taper and audio taper. Because our hearing is based on a logarithmic scale, the audio taper is the best choice for volume controls since it's rotation more closely approximates how our ears discern volume or loudness. So in general, you will choose and audio (also known as log) taper part. As for values, most circuits use 10k ohm values, although a 20k, 25k or even a 100k audio taper pot will work.
As far as power ratings go, almost all volume controls are at the preamp, or source side of the circuit. So anything in the 1/4 to 1/2 watt range will do.
Schematic Diagrams n' Stuff
If all you can procure are single-gang pots, then your volume control would look like this:
Of course, that would be silly because you would now have two volume knobs, one for each channel. First off it is inconvenient and second off, it would be difficult to get the levels of each channel correct.
Fortunately, electrical engineers are clever folks and they invented the dual-gang pot to solve this problem.
A dual-gang pot is actually two single gang pots lashed together sharing a common shaft. That gives you the convenience of one knob to turn, and enough lugs to implement two separate volume circuits. So here is the schematic for creating a stereo volume control with a dual gang pot.
Here are the pin numbers for a couple of different types of dual gang pots:
Let's Build One!
Here's a wiring diagram using RCA input and output plugs and a dual-gang pot.
So we are done right? That's all that we need to know for volume controls right? Well, not quite...
Issues with Dual-Gang Pots
Potentiometers are not exact devices. Their tolerance ratings aren't terribly high, usually in the range of 10%-20%. That means that you can grab two 10K ohm pots of the exact same part # and manufacturer and expect one pot to max out at 8k and the other at 12k. This really isn't a big deal for most applications--circuits are designed with part tolerances in mind.
But imagine that you are implementing a stereo input volume control with those variances. Each "gang" of the pot could be off by as much as 20%, and matching of both gangs with a given shaft position could be way off also. Many folks have commented on the abysmal inter-lug tolerance of dual-ganged audio/log taper parts as opposed to linear types and that does make sense in an empirical way. (For a great DIY way to convert the more accurate linear type of pot into a pseudo-log pot, check out this Decibel Dungeon article.)
The result is that by using a cheap dual-gang pot as a volume control you are actually building in an unintentional balance control also. You can never really be sure if the left and right levels being reproduced are what they were when recorded.
The problem is these variances can become quite pronounced when you are dealing a stereo volume control.
And it is an expensive process to create two potentiometer wafers (the disk parts that do the actual resisting) that have exactly the same resistance depending on where the single shaft is. Unfortunately, when equipment manufactures have to cut costs to lower prices, component types are usually compromised in terms of quality. In the world of stereo/hi-fi-/music reproduction equipment, part of this compromise is inevitably played out through the use of cheap crappy dual-gang pots for volume controls.
This imprecision is easy to test. Get out your multimeter and measure the the resistance across the outer lugs (1 and 3) of the first gang of the pot. Then do the same for the outer lugs of the second gang of the pot. Changes are about 99% that they won't match. So out of the box, the rated resistance doesn't match. This means that your stereo signal will not be balanced.
Then add in the fact that the shaft's position on the two wafers is pretty much guaranteed to never be exact. This is pretty easy to measure also (as long as you have two DMMs). Hook up lugs 1 and 2 of the first gang of the pot to the first DMM and set it to measure resistance. Then hook up lugs 1 and 2 of the second gang of the pot to the second DMM, again set to measure resistance. Turn that shaft. What happens? You won't get exact matches on both DMMs.
Add it all up and you'll see that by using a standard dual-gang pot for stereo volume control is hardly ideal. But there are solutions!
Solution 1: Commercial Grade and High End Dual-Gain Potentiometers
Dual-gang potentiometers seem almost to be an after thought for most top-tier manufacturers. If you dig enough you'll find fairly cheap parts that are carried by most of the big resellers. If cost and convenience are you watchwords, you can probably get by. What we are looking for is good overall tolerance (i.e. lug 1 to lug 2 uniformity across both gangs) and good Independent tolerance (i.e. lug 1 to 2 on gang 1 compared to lug 1 to 2 on gang 2). Unfortunately, most manufacturers do not include the "Gang Error" which is the tolerance mismatch between gangs.
Check out the Parts Page for some commercial and high end grade dual gang pots.
Solution 2: Stepped Attenuators
What if you entirely do away with the variances and vagaries of potentiometers altogether? Well you can, in the form of a stepped attenuator.
The stepped attenuator is a big rotary switch with a large number of discrete hi-precision resistors soldered to it. So instead of the continuous curve of a potentiometer, you have shaft that steps from one position to the next. Each position has a specific value set of matched resistors. So at any point in the rotation of the shaft, you have pretty damn near the exact same resistance on both sides of the stereo signal. The key attribute that makes a stepped attenuator work is the tolerance of the discrete resistors. Most top-end units use at least 1% MILSPEC parts.
So if you compare the 10% to 20% variance on potentiometers plus any inter-gang variance, and the .5% variance on a good stepped attenuator, the advantage is pretty clear.
Anyone trying to sell you a stepped attenuator with 5% or 10% tolerance values is really not offering you much value over a dual-gang pot. You should fart in their general direction.
Goldpoint makes some fantastic high quality stepped attenuators, available as kits or in completed form. Their line of passive preamplifiers is also top notch. A visit to their site is highly recommended.
Solution 3: An Optical Solution
A very clever fellow name George came up with an idea to use LDRs in a clever way to control volume. His resulting Lightspeeed Attenuator has gained quite a following. Check out his original thread on diyaudio.com for more info.
Solution 4: Digital Volume Controls
A number of chip manufacturers offer stereo volume controls in the form of digital or analog devices. These are typically the solution used in mass-produced and some high-end gear. Expect and article on building a Digital Volume control sometime in the future...
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