Resistors and Potentiometers

7/27/2019 Resistors and Potentiometers

1/30

Resistors and Potentiometers:A Practical Guide

Second only to capacitors, resistors (and their variable cousins, thepotentiometer) are the most common passive components found instompbox designs. So here's an overview on types, what they do, and howto use them.

Note: This content is about the practicalapplication of resistors and potentiometers, asused in guitar and audio circuits. So I will avoidre-hashing all the basics of Ohms law, readingcolor codes, or other topics that are readilyavailable on the web. See the resources section

at the end for links.

A resistor is a pretty simple beast: it is made up of a compound that resiststhe flow of current. It has two leads and is not polarity-sensitive; in otherwords you don't have to worry about the order of the leads.

First up, a brief list of things to know about resistors:

Resistor values are quoted in Ohms ().

Resistors have various power ratings, signifying the power they canaccept. For stompbox use (typically 9-18 volt DC supplies) 1/4 wattpower rating is fine.

If you like puzzles and memorization, you can learn and memorizethe color-coding of resistor values. Or, if you are like me, youcancheat.

The most commonly used resistor type in stompboxery is carbonfilm. You can use other types, but carbon film is cheap, reliable andeasily obtained.

If you lack a specific resistor value, you can use the formulas forResistors in Parallel/Series to cobble together a specific value.

Most resistors have a specified tolerance (for example 1% or 5%).So it is rare that a standard resistor will measure out to exactly itsclaimed value.

Resistor Types and Compounds

Resistors come in a wide variety of types and ratings. The most common guitar effect resistortype is carbon film, axial leads, 1/4 watt, 5-10% tolerance. You can use this type across prettymuch every type of guitar effect/stompbox design you come across.

Type Typical

Tolerance

Comments

Carbon Film 5% This is the most common type of resistor
http://www.beavisaudio.com/techpages/Pots/#Cheating_on_the_Codeshttp://www.beavisaudio.com/techpages/Pots/#Cheating_on_the_Codes


2/30

used in guitar and effect circuitry. Common,reliable and inexpensive.

Metal Film 1% A bit more expensive, but you get a closer tolerance. Additionally, metal film as acompound generally has lower noise and

better temperature stability.CarbonComposition

5% An older compound, found in vintageelectronics. Sometimes employed byboutique builders for mojo or tone.

Which Type of Resistor Should I Use?

In general, carbon film and metal film are absolutely fine for stompbox work. And resistors arethe most inexpensive passive components you'll buy. So you can stock up on metal film partsfor very little money.

There are some that say the older carbon composition resistors add mojo to a circuit. In my

experience, this isn't really noticeable, unless you are building high-voltage tube circuits andwant to remain faithful to original designs.

It's also worth thinking about specifications for tolerance and noise:

Guitar pedals are not high-fidelity devices. Indeed, the most common circuits are foroverdrive, distortion, and fuzz which are all about distorting the input signal. And then thinkabout your guitar pickups and their inherent noise, finally terminating in a noisy tube ampcranked up way to loud. With all these factors, is the noise rating of a resistor really going tochange things? Probably not. Regardless, if you use metal film resistors, that resistor, in andof itself, is not going to add an appreciable amount of noise to your circuit.

So what about tolerance? Don't I need to use 1% parts to get as close as possible to anoriginal design? Well, if you look at the inside of commercially built pedals, you'll find aplethora of 5% parts. The circuit was designed with these variances in mind. So your attemptto use 1% tolerance parts to match a design that was perfectly happy with 5% parts may notyield the results you are looking for. But as with noise, use metal film resistors if you want.

Cheating on the Codes

I'm useless at codes and puzzles and memorization. So I gave up long ago on trying toremember the resistor color code guide. What I found to be very easy is to buy a big batch of

resistors, and organize them into envelopes with the value written on them. Makes part-picking and PCB populating much quicker.

And when in doubt, you can always pull out your multimeter and test the values. Rememberthat the tolerance of the part means you won't get an exact value, but it will get you closeenough to know whether you are looking at a 10K part or a 12K part. Finally, a little tip: whenmeasuring resistors, don't hold the leads with your fingers--that will add resistance that maycause an inaccurate reading. Instead, use test-clip leads, or lay the resistor on a non-conducing surface and measure it there.

All About Potentiometers

Potentiometers (or pots, as well call them) are incredibly versatile devices. They can act asvoltage dividers, or as variable resistors. There are tons of resources on the web about pots,


3/30

so what Im going to cover here are the basic types, operation, and uses specifically for guitaraudio.

Types of Pots

Pots come in all sorts of shapes and sizes. The most common type we use in pedals andamps are usually of the 24mm or 16mm round metal can type. There are also multi-gang pots(which stack multiple independent pots on one shaft), slider pots, trimmer pots, etc.

In the case of a standard pot, as shown above, we have a round case with three connectorsand a shaft that turns. Heres what it looks like in a schematic:

Lugs and Numbers

One of the first (and most common) mistakes in using potentiometers is misreading the frontversus the back and the lug numbers.

The pot has three lugs and by convention they are numbered 1,2, and 3. Pin 2 is called thewiper.


4/30

These numbers map to the schematic symbol like this:

Tapers

Pots come in different tapers. The taper defines how the resistance of the pot changes inrelationship to turning the shaft.

Linear Taper: The simplest form. The rotation of the knob directly corresponds to theresistance change in linear fashion.

Audio/Logarithmic Taper: This taper compensates for how the human ear perceiveschanges in volume. It has a different curvethe resistance change as you turn theknob is not linear. Note that audio taper is the same thing as logarithmic (or log asyou will sometimes see it). Just different names.

Reverse Audio/Log Taper :This has the same curve as the audio taper, but in

reverse.The following diagram shows the relation of resistance change as you turn the pot knob


5/30

Which leads us to the most common first question about pots: when do I use linear and whendo I use audio? The answer is, do whatever the schematic or layout you are working fromspecifies.

Volume controls typically use an audio taper because it is designed to change the resistanceon a curve that is smoothed out given how our brains interpret changes in volume. Lineartapers are usually used for other parts of the circuit such as tone controls, distortion levels,etc. But not always!

The good news is that you can interchange linear and audio tapers in a pinch. If a projectcalls for a 100k audio taper and you only have a 100k linear taper, the linear one will work. Inother words, the exact same changes in resistance will be available on audio/log and lineartapers, it just depends on where you have the knob turned to. In general, stick to the specifiedtaper for best results.

Codes and Numbering

Of course it would be too easy if manufacturers would simply print the value and type of taperon a pot in plain English. But they dont they use codes. Very simple to figure out though.

The code is:

A single letter, A for audio/log, B for linear

and

A Numeric value, i.e. 10K

So a 100k linear taper would be B100K. A 1k audio taper would be A1K. There, now youknow the code. It should be noted that back in the old days the A and B were reversedif

you are working some old mystery pots, the above coding scheme may need to be reversed.


6/30

Ok, Why Three Lugs?

The most confusing thing to me about pots was that it had three lugs. Why? I mean a resistoronly has two connections, shouldnt a variable resistor also have just too connections? Wellsure, and there are variable resistors (called rheostats) that only have two connections. But if

you add a third lug, the humble rheostat becomes the mighty potentiometer.

As you turn the shaft, the resistance between the wiper and lugs 1 and 3 changes inversely.For example, if you turn the shaft clockwise, the resistance between 1 & 2 increases while theresistance between 2 and 3 decreases.

So if you think about it, the resistance between lugs 1 and 3 never changes. If you have a100k pot, the resistance between 1 and 3 will always be 100k no matter how you turn theknob. It is the wiper (lug 2) that changes.

The Trimmer Resistor

Lets look at one of the most common uses of a potentiometer: as a trimmer resistor. Hereswhat the schematic looks like:

Here we have a potentiometer where lug three is the input, and lugs 1 and 2 are connectedtogether to form the output. As you turn the shaft, the resistance decreases. You are forming

a simple resistor whose value is variable.

Wait a minute! you might say. Why do I need to connect lug 1 to lug 2 if all I need is avariable resistor, like this:

Well that certainly works the same way. But what happens if the potentiometer fails for somereason (age, poor quality, dirty, etc.)? If the wiper (which is the rotating part of the componentand probably most prone to failure) shorts out, it will let the full amount of signal through. Byattaching lug 1 to lug 2, we are building in a fail-safe. This ensures that the circuit is nevercompletely openthere will always be some resistive path in case the wiper goes south.

A Volume Control

Now lets look at a more interesting example: a volume control. Assume we have a simplestompbox that does distortion or overdrive or something else interesting. At the end of thecircuit we have an output. Wouldnt it be nice to control the output level (or volume) of thepedal?


7/30

Here we have the signal going to lug 3, the output coming out of lug 2, and lug 1 connected toground. To see how this works, assume you have the shaft turned all the way clockwise, i.e.turned all the way up. In this configuration, there is little if any resistance across lugs 2 and 3so the maximum output signal goes to the output.

As you turn the shaft counter-clockwise, the resistance across 2/3 increases and theresistance across 1/2 decreases. This causes more of the signal to be dumped to ground.This dumping essentially sends the signal into oblivion, thereby lowering the overall output

level.

So if you think about it, you are never really turning the volume up! The volume or level in thecircuit is always running at full tilt. What you are doing the above volume control is actuallyattenuating (making smaller) the full volume that was there to begin with.

Ok, so what are trimmers?

The pots weve looked at so far are the typical panel-mount variety. They are mounted onthe equipments enclosure and usually attached to a knob so you can control the circuit.Trimmers are potentiometers that are typically directly mounted to the circuit board. They are

a set and forget type of device. When a circuit is built, there is sometimes the need to tune itcorrectly. For example, if you have a stompbox that uses certain types of transistors, youllwant the transistors to be biased properly. This relies on resistance. So a trim pot would allowyou to dial in the correct voltage, close up your stompbox and be on your merry way.

On a philosophical note, Im generally opposed to using trimmers where a panel-mount potwould offer some control over the tone. For example, some modulation pedals have trimmersto control the range of an affect. These are often fine-tuned by the manufacturer with trimpots. However, by burying that control inside the box, you may be missing out on someinteresting tones. Of course, there are places where trim pots make sensei.e. when theyare used to tune a circuit and any variance beyond the factory tuning and anything else is notmusically useful.

What About those Pesky Tabs?

Most pots come with a small metal tab that sticks up in the same direction as the pot. Theseare used for anchoring the pot firmly in the enclosure. If you are so inclined, you can drill anadditional small hole in your enclosure to use this tab. If you are like most of us DIY folks,you'll just snap the tab off before mounting the pot.


8/30

Fun with Pots

Ok, lets have some fun with pots. First off, well build a voltage sag circuit. This allows you tosimulate a dying battery which can yield some interesting tones in certain types of circuits,especially fuzz effects. Heres the general way a battery is connected to a circuit board:

To sag the voltage, well introduce a pot between the battery positive connection and thecircuit board, like this:

Now, as we turn the potentiometer, we increase the resistance between the battery and thecircuit, thereby lowering both the current and voltage.

A Passive Mixer


9/30

Generally you want any mixing circuits to be active (i.e. to include opamp or transistorcomponents to match and balance the inputs). However, you can squeak by in some caseswith a passive mixer. Although the passive design can load down the input devices, it willwork fine in many cases.

Here we are going to use an audio taper pot for each input channel:

A Fine Pot

You may have seen pedals with a speed or time knob. Typically speed is for LFO (lowfrequency oscillator) such as in a phaser or chorus. And time knobs are in time-based effectssuch as delays. And sometimes, you may have seen an arrangement of two knobs for speedor time; one to control the coarse value and one for fine value.

In other words, the coarse pot makes big changes in values, the fine pot makes smallchanges. The fun part here is that it is super easy to implement such a scheme: just use twopots instead of one. In the following example, we augment an existing 100K pot with a new 1kpot:


10/30

Make a Tone ControlYou can also use a pot and a handful of components to build a simple tone control that youcan retrofit into any pedal that lacks one. Look at the following drawing:

The input accepts the signal. It passes through a R1 and C1 which form a filter network. R2and C2 form the other side of the network. (Note the different component values). As youmove the pot shaft, more signal is sent to one side or the other of the tone network. The wiperof the pot (lug 2) is the output. This is a simplified explanation of the tone control, but it showshow the wiper allows you shift the signal from one side of a circuit to another.

A Note about Math, Science, and Technical Stuff: Itseems the norm

Wrapping Up

Pots are everywhere. Because they are so useful I guess. Here are some additional


11/30

resources on the web for further research:

The Secret Life of Pots: http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

Beginners Guide to Pots: http://sound.westhost.com/pots.htm

The Secret Life of Pots

Copyright 1999 R.G. Keen All rights reserved.

As electronics tinkerers, we all use potentiometers, or "pots" for short. We count on them to control all our

musical gear, and quite often get frustrated by their limitations. As in all relationships, a little understanding goesa long way. Let's take a look at how pots work so we can use them better.

Back in the dim reaches of electronic prehistory when electricity, let alone electronics, was poorly understood , anumber of researchers were trying to figure out how this mysterious force worked. They had no meters, nooscilloscopes, not even very good batteries, and had to literally make their own parts to get anything electrical to

work. In this era, a fellow named Ohm settled a controversy. There was general agreement in the electrical

research realm that the ratio of a voltage to the resulting current in any chunk of material was dependent on thematerial itself and the value of the current raised to some power, or

V=k * I^x

The raging discussion was over the value of x. There was general agreement that the value of x was something

odd, 3/2 or 4/3 or some such value, and the electrical intelligentsia spend a great deal of work trying to measurethis value ever more precisely. Ohm astounded his fellows by proposing that the value of x was simply one, not

some odd value. For this work we remember him every time we use Ohm's law, that the voltage across anyresistor is equal to the resistance times the current through it.

The reason I brought all that up is that the resistors and controls that were common in Ohm's day were verycrude, simply bars or strips of some resistive material, often carbon or charcoal. To get a variable voltage, Ohm

and his colleagues would impress a voltage across a length of resisting wire or carbon, and touch the conductor

part way along it to obtain a voltage intermediate between the voltages at the ends of the resistor. As you mightexpect, this sometimes led to sparks and burned fingers.

Despite all our sophistication, we still do that with our pots. They all work the same way, a contact sliding along

a strip or coil of resistive material. Let's have a bit of boy-genius fun - let's tear one open to see what makes ittick - then we can compare to Ohm's variable resistances.

The following picture is representative of how modern pots are built. I used a carbon composition pot made byAlpha, a linear 10K, 25mm diameter control.
http://www.geofex.com/Article_Folders/potsecrets/potscret.htmhttp://sound.westhost.com/pots.htmhttp://www.geofex.com/Article_Folders/potsecrets/potscret.htmhttp://sound.westhost.com/pots.htm


12/30

To get inside it, you notice those little metal tabs bent over the housing on the top? I took a fine bladed

screwdriver and bent them up....


13/30

When they're all bent up, you can lift the shaft and phenolic wafer assembly out of the back housing like this.

All the fun stuff appears to be inaccessible under that black plastic cover. Actually, that cover performs a couple

of very useful functions; it serves to hold the wiper in place, and also interacts with the stop on the case to set themechanical rotation limits. If you remove the tab on the plastic disk and reassemble the pot, it will now rotatefreely 360 degrees. Here's a little better view of the shaft/wafer/wiper assembly.


14/30

Looking closely, you can see that the shaft is held onto the plastic disk by being mechanically squashed into

place. That is quick, and makes for cheap pots, but it can't be removed and successfully reassembled. That is a

peculiarity of this brand, as some pots can be disassembled and reassembled successfully. In the next picture,

I've used a big pair of diagonal cutters to remove the plastic disk and the metal wipers. This exposes the bareconductor assembly.


15/30

The whole works of the pot, excepting the rotating wiper are printed on the phenolic wafer. The connections to

the ends of the resistive material are a conductive metal material, as is the circle of conductor that connects to the

center contact. We can now begin to see our control's kinship to Ohm's variable resistors. The resistive material

is a circular band of carbon-containing gunk that is printed onto the phenolic wafer, overlapping the metalizationto the two outside lugs at either end. The wiper is just a sliding bridge from the center metalization ring to the

outer resistive ring. The contact to the metalization is fairly reliable, but the contact to the resistive ring may beintermittent, so the connection to the outer ring is made by three contacts in parallel, as shown by the three tracks

the contacts make in the top of the resistive material. Those gouges and scratches are a product of the delicatemanner in which I removed the plastic wafer, so ignore those.

With this view, a couple of things fairly leap out at us. This is a linear pot, which is exactly what we should havefrom a continuous, homogeneous strip of resistive material. That is to say, the wiper traverses an equal amount

of resistance in ohms per degree turned. If we wanted to make a non-linear pot, where the amount of resistanceper degree turned were not linear, we could mess with the resistive strip to make it nonlinear. This is in fact how

it's done. The strip can be made to get skinnier toward on end or the other; a narrower strip has more resistanceper unit length and so the resistance per unit degree turned will change. This is not desirable from reliability

reasons, as we'd still like to have three contacts to the resistive strip, and that isn't possible if the strip getsskinnier. We could also change the thickness or composition of the material from one end of the resistive strip to

the other.

It turns out that printing a varying thickness is very difficult to do cheaply. Also, varying the composition in anykind of smooth fashion is very hard, not a repeatable manufacturing step. What makers of pots actually do is to

use straight-line approximations to a nonlinear resistive curve, and print in sections. For instance, the first 1/3 of

the circumference of the pot's resistive band may be printed with one resistive material, the second another, andthe third yet another. This allows the maker to come reasonably close to some taper curve.

Other things are that we can see the way pots wear out. The wiper literally wears a path through the conductivestrip. When all three contacts get all the way through, they don't make contact any more, and the pot quits

functioning. During the normal life of the pot (that is, the wear-out process) the bits of resistive material gougedand work from the resistive strip stay around and can actually lift the wipers from the resistive strip. If there is a


16/30

DC voltage across the pot when this happens, the wiper loses and then regains contact at a different DC level

than it left, so it makes a scratch or click.

Pots used to be made with taps part way around. This was just a fourth terminal connected to a strip of

metalization running under the middle of the resistive material part way round. The metalization on the ends thatthe wipers touch when the pot is turned all the way to one end or the other can be extended further around the

path of the resistive band so that the resistive band can start later in the pot's mechanical rotation and/or endsooner. The maker can print metalization "dead zones" in the middle of the rotation where nothing much

happens resistively. It's very flexible.

Tricks you can play

This simple structure means that there are lots of non-standard things we can do with pots.

if you have a brand of pot that can be reassembled, you can swap wafers between pots to repair a

broken or worn out one

if you have a dual pot body, you can assemble custom dual pots with wafers from other single pots of

that same brand

if you have to repair a pot and don't have another wafer, any connection you can make between the two

disconnected sections of pot will work

you can bend the wiper contacts sideways to let them contact new, unworn areas of resistive material

you can bridge over breaks in the resistive material with copper or silver conductive paint, or make a

custom tap point

I have heard that you can use copier toner dust to melt over a section of worn resistor material and

repair it.

You could hypothetically scrape material off the resistor band and make a higher resistance, odd taper pot, but

there are better ways to do that.

MBA's, Stock Pots, and Resistors.

There is an affliction loose in the business world that I call the MBA Disease. Old timers will recall a time when

Radio Shack was really a place to get electronics parts. Radio and TV repair places had electronics parts, anddistributors had a large variety of odd miscellany. You could usually find parts, even if they were not hot sellers,

because the parts suppliers ordered a range of parts that they thought would appeal to their customers, and keptstock items because they were useful, even if they didn't sell for a long time.

Enter the MBA. The creed of the MBA is that everything I have in my business should be making the best return

on my money *right now* and *continuously*. If some item I'm selling is not returning as good a rate of profit

as some other things, I can make more money be not selling the less profitable items and concentrating on onlythe things that are hot sellers with high rates of return. This does work. Businesses that adopted it were more

profitable than their slower peers were, and in many cases the slower peers went out of business.

These are the seeds of destruction for the electronics tinkerer, though. A lot of times tinkerers want devices that

are not used in millions. This means that increasingly, it's almost impossible to find odd values, odd tapers, orunusual terminal arrangements in pots. Reverse log pots have always been a specialty item, but it's getting hard

to find ordinary audio/log taper volume pots, and values that are not decade multiples of 1 or 5K are getting rare.Distributors will not stock pots that aren't high sellers; this leads to some self-fulfilling prophecy. Eventually, the

manufacturers note that odd value or taper pots are not selling - pesky distributors don't order them anymore -and they quit making the odd ones. This leaves us where we are today. The manufacturers have gotten good at

making a run of 10K of any pot value, any lead/leg setup, any taper, and any case. They'll happily makewhatever you want, if you want enough of them. The few of us that hand solder stuff together have to make do

with whatever the distributors do order. And today, that is increasing only linear pot, maybe a few audio tapers.


17/30

Which leads us to tapering.

What is taper? It's just the ratio of the resistance already passed as the pot turns to the total resistance of the pot,described as a curve. For instance: we want to make a variable power supply with an adjustment pot that

smoothly varies the voltage from one to ten volts, so we want a control that lets us do that. We have no idea

whether we'll want mostly low voltages or high voltages, so we want to adjust it equally well anywhere in the

range. In this case, it's most natural for the control pot to have an equal change in resistance or voltage dividedper unit of rotation - we want the control to feel linear. This much of a turn is one volt, no matter whether it'snear 0V or near 10V.

Volume controls are different. The human ear does not respond linearly to loudness. It responds to the logarithmof loudness. That means that for a sound to seem twice as loud, it has to be almost ten times the actual change in

air pressure. For us to have a control pot that seems to make a linear change in loudness per unit of rotation, thecontrol must compensate for the human ear's oddity and supply ever-increasing amounts of signal per unit

rotation. This compensating resistance taper is accurately called a "left hand logarithmic taper" but for historicalreasons has been called an audio or log pot. In these pots, the wiper traverses resistance very slowly at first, then

faster as the rotation increases. The actual curve looks exponential if you plot resistance or voltage divisionratios per unit of rotation.

If you used an audio/log taper pot for the control of the power supply we mentioned, the output voltage would

increase very slowly at first, creeping up to maybe 10% of the final output at 50% of the pot rotation. It wouldthen blast the other 90% in the last half of the rotation - very hard to control. Likewise, if we used a linear pot for

volume control, the volume would come up dramatically in the first half of pot rotation, and then do very little

change in the last half.

The dark horse taper is reverse audio, or more strictly "right hand logarithmic" taper. This taper traverses

resistance very quickly at first, then more slowly as it is turned further. It's the inverse of the audio taper. This isused in some bias circuits and in controlling the speed of certain RC oscillators, which is where the audio

tinkerer runs into it most.

The following diagram shows the three main kinds of pot tapers, along with one common approximation to an

audio taper. Curve 1 is linear taper. If we clip one lead of our Ohmmeter (Hey! There he is again!) onto theleftmost lug, and the other lead on the center lug, then the resistance we read as we rotate the pot clockwise will

fall on the curve that goes diagonally upwards. The proportion of the total pot resistance we traverse as we turnthe pot is linearly proportional to the amount of rotational travel we turn.

Curve 2 shows what happens with an audio or logarithmic taper. As we turn the shaft, the proportion of

resistance we traverse increases slowly at first, more slowly than the percentage of rotation. As we get past halfthe available rotation, the rate of resistance traversed speeds up as we get closer to the furthest rotation. This

compensates for the human ear by increasing sound levels very slowly at first, then faster as the ear's sensitivity

falls off at higher sound levels.

When we buy "audio taper" pots, we usually get something like Curve 3. For less expensive pots, manufacturers

use a two or three-segment approximation to Curve 2. It's not perfect, but it usually works OK. Curve 4 is thetypical resistance versus rotation curve for reverse log pots. In real life - that is, if you ever found one of these in

real life - it is usually a two or three segment approximation, too.

If you have an unknown pot, you can figure out what taper it is. You measure the resistance from end to end,

then turn the pot exactly to half its rotation and measure the resistance from the counterclockwise lug. Thecrosses on curves 1, 2 , and 4 show the most probable values. If the resistance is 50% of the total resistance, then

the pot is linear. If you measure only 10% to 20% of the total resistance, the pot is an audio taper. If you measure80%-90% of the total resistance, the pot is a reverse log taper.

At this point I should probably explain what a counterclockwise lug is. Of the three contacts on the pot, the

wiper is easiest to pick out. If you turn the shaft fully counterclockwise, the wiper lug will show very small

resistance to one of the other contact lugs. This is the counterclockwise or "cold" terminal. Turning the shaftfully clockwise, the wiper will show very small resistance to the most clockwise lug, also called the "hot" lug.


18/30

There are other tapers, but they have very specialized uses.

Our problem is this: If we need a specialized taper to recreate some effect, how do we get it if we can't go buy

one? That leads us to tapering resistors.

If we set up the easiest, simplest pot to get, a linear taper pot of resistance R, and then connect a "tapering

resistor" across the wiper and CCW lug, we get the situation shown in the following diagram. For clarity, I'veseparated the resistance above the wiper and the resistor below the wiper into two separate resistors. It makes the

calculations much easier.


19/30

We're assuming the total pot resistance R is split into an R1 at the CW side and R2 on the CCW side, with R3paralleled with R2. We'll let "a" represent the fraction of the total resistance R that the wiper has turned, and "b"be the fraction of R that R3 is. When we get out the algebra books and do the math, we find out that we can show

that the ratio of output voltage to input voltage is that odd looking fraction in the picture. When we calculate outthe results, we find that the divider ratio of Vout to Vin is shaped something like a true logarithmic tapered pot if

we pick the right value for b. If b happens to be 1/4 to 1/5, the resulting voltage division is remarkably close to atrue logarithmic pot, probably closer than a two segment approximation that we could buy! Wow! No more

waiting for volume control pots!

Here's what we see when we do the math:


20/30

Unfortunately there's a gotcha in there. It's true that the voltage division ratio of this rig is arbitrarily close to that

of a log taper pot. However, neither the load seen by whatever drives Vin or the source resistance as seen by theinput of whatever is connected to Vout is close to what would exist for a real log pot of value R. In fact, the load

on Vin varies from 1/(1+1/b)*R up to R. That means that if we're trying to do a log taper with b = 1/4, the loadon Vin will be as much as 0.2* R. This may be OK, but you have to keep it in mind.

In general, if you have a voltage source that can drive a load of 1/4 to 1/5 of R and a load on Vout that has aninput impedance much higher than that same 1/4 to 1/5 of R, this is a good replacement for an audio or log taper

pot.

In a bit of good fortune, if you hook up the tapering resistor from the CW or hot side of the pot to the wiper, the

pot emulates a reverse log pot, just as well as it did a log pot when hooked to the CCW side.


21/30

There are two ways to hook up a pot. You can hook it up as a three terminal voltage divider as we've seen above,

or as a variable resistance, the two-terminal connection (sometimes called a rheostat connection for historicalreasons).


22/30

In the two terminal connection, the resistance through the pot is what we're interested in, not the voltage divider

ratio.

It turns out that the tapering trick works here, but only partially. If we want to make a reverse log tapering pot,

we're in! However, there is no way to get a simulation of a log taper pot in the two terminal connection. For that

we have to buy real audio taper pots.

Here's what you get if do the series resistor connection:


23/30

Note that the CW terminal is unused. This is usually tied to the wiper, although that is not seen here. You can't

simply put the tapering resistor from the CW terminal to the wiper and get a log taper pot emulation, like youcould with the voltage divider connection. That just gives you the reverse of this graph, with the resistance

starting to decrease slowly and then faster. The two terminal connection is non-polar; it looks the same however

you hook it up. The only thing that changes is which end of the graph you start from.

In the math examples I left b, which is the fraction of the pot resistance that the tapering resistor is, as a

parameter rather than making it a fixed ratio. Usually, people pick a value of b of about 4 or 5. Those curves are

close to the classical mathematical description of a log or reverse log pot. I left b a parameter to show you thatyou can make your own taper by selecting a different value of b. For a semi-log taper, use a b of about 2.

Szia


24/30

sajnos ez nem ilyen egyszer, ha az volna nem gyrtank a logaritmikus potmtereket. Ilyen mdszerrel nem lehet azegyik tpust a msikba vinni. Viszont az ltalad lert mdszerrel igen j pontossggal el lehet rni egy vals logaritmikusfggvnyt (pl. 100k-s potmter s 22k-s ellenlls), amirl sokak azt lltjk sokkal jobb a gyri karakterisztiknl.

dv,Oszi

Csatolmny Mret

pots-f8.gif 4.69 KB

pots-f4.gif 7.62 KB

Szia Oszi,

Akkor ezek szerint a 22%-os szably az rvnyes?Nekem 12% derengett rgi emlkeimbl.

dv:Krisztin

Hali, sok erstt csinltam mr. A 12% szerintem jobb. Itt van egy cikk is, ahol meg a 15%-ot tartjk megfelelnek.Vglis csodk nincsenek, ahhoz, hogy normlis legyen a HANGERSZABLYZS, valami ilyesmi mindenkppen kell:http://sound.westhost.com/project01.htm

gbenyov

Better Volume (and Balance) ControlsRod Elliott - ESP

Additional Material Provided by Bernd Ludwig

More Sharing ServicesShare|Share on facebookShare on myspaceShare on googleShare on twitterBetter Volume Control (Pt 1)

The volume control in a hi-fi amp or preamp (or any other audio device, for that matter), is a truly simpleconcept, right? Wrong. In order to get a smooth increase in level, the potentiometer (pot) must belogarithmic to match the non-linear characteristics of our hearing. A linear pot used for volume is quiteunsatisfactory.

Unless you pay serious money, the standard "log" pot you buy from electronics shops is not log at all, but iscomprised of two linear sections, each with a different resistance gradient. The theory is that between the
http://elektrotanya.com/files/forum/2010/11/pots-f8.gifhttp://elektrotanya.com/files/forum/2010/11/pots-f4.gifhttp://sound.westhost.com/project01.htmhttp://addthis.com/bookmark.php?v=250&username=rodehttp://addthis.com/bookmark.php?v=250&username=rodehttp://addthis.com/bookmark.php?v=250&username=rodehttp://addthis.com/bookmark.php?v=250&username=rodehttp://sound.westhost.com/project01.htmhttp://www.addthis.com/bookmark.php?v=250&winname=addthis&pub=rode&source=tbx-250&lng=hu&s=myspace&url=http%3A%2F%2Fsound.westhost.com%2Fproject01.htm&title=ESP%20-%20A%20Better%20Volume%20Control&ate=AT-rode/-/-/513456fd636853e3/2&frommenu=1&uid=513456fdac7712ea&ct=1&pre=http%3A%2F%2Felektrotanya.com%2F%3Fq%3Dhu%2Fcontent%2Fhangero-szabalyzas&tt=0&captcha_provider=nucaptchahttp://www.addthis.com/bookmark.php?v=250&winname=addthis&pub=rode&source=tbx-250&lng=hu&s=google&url=http%3A%2F%2Fsound.westhost.com%2Fproject01.htm&title=ESP%20-%20A%20Better%20Volume%20Control&ate=AT-rode/-/-/513456fd636853e3/3&frommenu=1&uid=513456fd7755596f&ct=1&pre=http%3A%2F%2Felektrotanya.com%2F%3Fq%3Dhu%2Fcontent%2Fhangero-szabalyzas&tt=0&captcha_provider=nucaptchahttp://sound.westhost.com/project01.htmhttp://elektrotanya.com/files/forum/2010/11/pots-f8.gifhttp://elektrotanya.com/files/forum/2010/11/pots-f4.gifhttp://sound.westhost.com/project01.htmhttp://addthis.com/bookmark.php?v=250&username=rodehttp://sound.westhost.com/project01.htmhttp://www.addthis.com/bookmark.php?v=250&winname=addthis&pub=rode&source=tbx-250&lng=hu&s=myspace&url=http%3A%2F%2Fsound.westhost.com%2Fproject01.htm&title=ESP%20-%20A%20Better%20Volume%20Control&ate=AT-rode/-/-/513456fd636853e3/2&frommenu=1&uid=513456fdac7712ea&ct=1&pre=http%3A%2F%2Felektrotanya.com%2F%3Fq%3Dhu%2Fcontent%2Fhangero-szabalyzas&tt=0&captcha_provider=nucaptchahttp://www.addthis.com/bookmark.php?v=250&winname=addthis&pub=rode&source=tbx-250&lng=hu&s=google&url=http%3A%2F%2Fsound.westhost.com%2Fproject01.htm&title=ESP%20-%20A%20Better%20Volume%20Control&ate=AT-rode/-/-/513456fd636853e3/3&frommenu=1&uid=513456fd7755596f&ct=1&pre=http%3A%2F%2Felektrotanya.com%2F%3Fq%3Dhu%2Fcontent%2Fhangero-szabalyzas&tt=0&captcha_provider=nucaptchahttp://sound.westhost.com/project01.htm


25/30

two they will make a curve which is "close enough" to log (or audio) taper. As many will have found out, thisis rarely the case, and a pronounced 'discontinuity' is often apparent as the control is rotated.

As with all pots used as volume controls, the first 10% of rotation causes a very large variation in level(essentially from 'off' to quietly audible). A 'true' log response over the full range of perhaps 100dB is notreally useful, because most of the time the gain is varied over a relatively small range. 25dB of variation is a

power ratio of 316:1 - this will normally be the range over which any volume control is used.

Figure 1 - Circuit of the Log Pot Approximation

Take a 100k linear pot (VOL), and connect a resistor (R = 10k - 15k, 12k used to produce Figure 2) asshown above to achieve the curve shown. It should be a straight line, but is actually still far more logarithmicthan a standard log pot. For stereo, use a dual-gang pot and treat both sections the same way. Use of a 1%resistor for R is recommended. Different values can be used for the pot, but keep the ratio between 6:1 to10:1 between the value of VOL and R respectively. While 8.33:1 (as shown) is close to a real log curve, itmay still allow excessive sensitivity at low levels. Higher ratios than 10:1 can be used, but will causeexcessive loading of the driving stage, or necessitate the use of a pot whose resistance is too high.

Figure 2 - The Transfer Curve in dB

Provided the gain structure of the preamp is set up properly, a good approximation to true log pot operationis obtained over at least a 25dB range, which is sufficient for the normal variations one requires.

The gain structure of the preamp is correct when the pot spends the vast majority of its time between the 10

and 2 o'clock positions. If the volume is often below or above this range, consider changing the preampgain. The gain can be switched to give a 'two-stage' volume control, so that the optimum setting is alwaysavailable.


26/30

The other advantage of the 'fake' log pot is that linear pots usually have better tracking (and powerhandling) than commercially available 'log' pots, so there will be less variation in the signal between left andright channels. This is improved even further by the added resistor, which will allow a cheap carbon pot toequal a good quality conductive plastic component (at least for accuracy - I shall not enter the sound qualitydebate here).

Make sure that the source impedance is low (from a buffer stage) and that it can drive the final impedancewhen the control is fully advanced (it may be as low as 9k Ohms with a 100k pot). Use of a high impedancedrive will ruin the law of the pot, which will no longer resemble anything useful.

Better Volume Control (Pt. 2 - Further Ideas)

Originally designed by Peter Baxandall (of feedback tone control fame, amongst many other designs), thereis also an active version of the 'Better Volume Control', which uses an opamp and a pot in the feedbackloop. The log law is almost identical to that for the passive design above, but it can provide gain as well asattenuation. An example of this design may be found in Project 24, and the circuit for the basic idea isshown in Figure 3.

Figure 3 - Active Logarithmic Volume Control

The buffer (U1A) enables the inverting stage (needed so the circuit can work) to have a very high inputimpedance. This would otherwise not be possible without the use of extremely high value resistors, whichmay increase the noise to an unacceptable level. The maximum gain as shown is 10 (20dB) and minimumgain is 0 (maximum attenuation). The input impedance is variable, and is dependent on the pot setting. Atminimum gain, input impedance is the full 50k of the pot, falling to about 27k at 50% travel, and around 4.3kat maximum gain. The impedance is much less than that of the pot itself because of the feedback from thefinal opamp.

These impedance figures are similar to (but a little lower than) the simple passive version (if a 100k pot is

used), and again, a low impedance drive is required or the logarithmic law will not apply properly. Theactual value for VR1 does not matter, and anything from 10k to 100k will work just as well, although it willinfluence the input impedance. The error at 50% of pot travel is less than 5% with values from 10k to 100k.


27/30

Figure 4 - Response Vs. Rotation Of Figure 3

Note that the additional benefit of improved tracking does not apply to the active version (at least not to thesame extent), so use the best pot you can afford to ensure accurate channel balance. With 20dB of gain atmaximum, this will be far too much for many preamps. 10dB of gain is normally sufficient. Increase R2 toget less gain (3.3k will reduce the gain to 10dB, close enough). Doing so will also increase the worst-caseinput impedance.

Better Volume Control (Pt. 3 - Mono Version)

The following trick has been used in a few guitar amps, but because it uses a dual-gang pot isn't suitable forstereo because 4-gang linear pots (well, any4-gang pots) are next to impossible to obtain. Theapproximation to log is very good over at least a 30dB range, but it's only marginally better than the version

shown in Figure 1, but requires a dual-gang pot to get there.

Figure 5 - Log Approximation Using Dual-Gang Pot

The response vs. rotation is shown below. Across the final 25dB range it is almost a straight line (i.e. trulylogarithmic). This is a good way to get a smooth response from the pot, but as already noted it's only reallyusable for a mono system. This rather limits its usefulness.


28/30

Figure 6 - Response Vs. Rotation Of Figure 5

Better Volume Control (Pt. 4 - Multi-Channel Version)

For anyone needing a multi-channel true logarithmic volume control, see Project 141. The project usesTHAT2180 VCAs, and can be set up as anything from 1 to 8 channels (or more if you have a use for morethan 8 channels). It's ideal for home theatre systems, and you only need to include channel switching for acomplete preamp. The VCA also provides gain, so is essentially a complete preamp as described.

Better Balance Control (Contributed by Bernd Ludwig)

Bernd, a reader of The Audio Pages, has contributed a useful variation - in this case, a "better balance"control. Note that the configuration described requires a high impedance load, and the passive "BetterVolume Control" cannot be used in this circuit. Used in the manner shown, it is a very similar concept to thebetter volume control of Figure 1, except it is (in a sense) the same idea in reverse.

Bear in mind that many (especially early Japanese) designs use a specially designed pot for balance, andthese are not suitable for the circuits shown below. These pots commonly have a centre detent, and theresistance of each track remains very low from the centre position to one end (or the other) of travel. These'special' pots are characterised by the level remaining constant in one channel or the other as the balancepot is moved. The overall law of these controls is (IMO) unsatisfactory for hi-fi.

A standard configuration of Balance/Volume control using conventional pots (1 channel) is shown below:

Figure 7 - Conventional Balance / Volume Control
http://sound.westhost.com/project141.htmhttp://sound.westhost.com/project141.htm


29/30

BAL = 2.5 * VOLFor example: VOL = 10k log, BAL = 25k linear

Adding a resistor 'R' gives opportunity for two interesting improvements of the standard balance-volume-control networks. Note that the switch is optional, and may safely be left out (i.e. shorted).

Figure 8 - Improvement With Added Resistor

A) R = VOL (for example, 10k)

The BAL-pot is 'virtually absent' when in the centre position:

In the centre position the resistive track of BAL only affects the load for the previous stage, since there is nocurrent through the sliding contact (so you could open switch 'Sw1' without changing anything at all - if youplease). This seems to be reasonable: As long as you don't manipulate the balance control it is virtuallyabsent from the circuit (no signal passes through its sliding contact). Hence quality (or age) of the BAL-potdoesn't matter at all then.

Sonic detriments can only come into play for two reasons:

If the resistive tracks of BAL are not absolutely symmetrical current through atleast one of the sliding contacts will not be exactly zero at centre position(adding the switch 'S' would cure this entirely - but I doubt that there is anyneed for it).

If track resistance of a carbon pot (worst case scenario!) changes due tovarying pressure of the sliding contact (induced by acoustical resonance, justlike in the carbon microphones of veteran telephones), the load on theprevious stage will change (but I suspect it might be really difficult to find a

stage that will 'feel' it).

Thanks to 'R', the balance control operates conveniently slowly near the centre position and overall volumeis affected significantly less than without it. This leads to another option:

B) R = 4k7 (R = ~0.47*VOL)

The balance knob works without affecting the overall volume

This will give best operating convenience since the sound stage then moves from the left to the right withoutsignificant overall volume change. Input voltage on both channels constant and equal, the sum of Left andRight channel power remains approximately (0.2dB) constant across about 80% of the dial (which still

works conveniently slowly about the centre position). I decided on the .47-factor after some PC-simulationand checked it by implementation in my preamp afterwards:


30/30

It works as expected indeed (there is just a slight increase of overall volume at the extreme right and leftpositions). I don't want to miss out on having a balance control any more, since there are in fact recordswhich suffer from severe channel imbalance. Moving the armchair or the speakers is not a convenient curefor that. Moving the soloist two feet to the left or right without changing the overall volume, just by activatingthe balance knob, is the way to go.

Any compromise between 'golden-ear -' and 'maximum-convenience -' versions is possible by selecting asuitable 'R/Vol factor' between 1.0 and 0.47 .

The impedance of these "enhanced" networks is approximately that of 'VOL' alone (if R = Vol and BAL ~ 2* VOL), so you can add BAL and R to any "purist's" design without changing critical parameters of the circuit(4-6dB attenuation by R will occur, of course, so you will have to add about 5 or 10 degrees of arc on thevolume dial in future). Even when BAL is set to the extremes there is only a moderate change of load(max.: -30%) which will not upset any reasonable preamp.

If there already is a standard network in your amp, it is easy to add the additional resistors ... Just solderthem across the corresponding pins of the balance pot (on one channel from centre to the left and on theother from centre to the right!) The volume pot is not involved.

Bernd Ludwig

Resistors and Potentiometers

Documents