Non-linear Pots

[Happy Jack]

A long-standing irritation (for me) resurfaced yesterday while chatting to another Basschatter ... why are pots (especially volume) so non-linear?

With most of the volume controls I've ever encountered on passive basses, virtually the entire change from 100% to Zero is encompassed in the first quarter-twist of the knob, and sometimes it's an eighth of a twist. That's what we professionals call an On/Off Switch.

That usually leaves another at least half twist (i.e. twice as much travel) to get to the end stop, none of which has any appreciable effect. That's what we call a Waste Of Time.

I've encountered this on new and old, expensive and cheap, so clearly it's supposed to be like this.

But why? Why not make them linear?

IME they're routinely pretty linear on active basses, and presumably that's some sort of clue, but I have absolutely no understanding of electrickery so it's not a clue that I can use.

[KiOgon]

Here's a short read for you

CTS pots have been found on most high end guitars over the years, and for the most part, are still seen to this day.


The main aspect of all CTS pots is that they’re made in the exact same way as they always have been, so if you’re looking for that vintage tone, these should do the trick. This tried and tested design, with very few mechanical components has created an absolute classic guitar part – the simple, rugged design has means that CTS pots offer unrivalled reliability, with many of the original examples still going strong after nearly 60 years.
From a player’s perspective, your average CTS pots will have a much firmer action thanks to the old fashioned design which utilises non-degrading grease under the wiper blade – this ultimately means that a CTS pot will take the occasional accident knock and not spin off under its own weight.
[b][u]Shaft style[/u][/b]
The shaft is the protruding, rotating part of the pot which will eventually be covered by a knob.
In a move away from the modular designs you normally see with electronic components, CTS developed 2 styles of “shaft” on their pots. This has a direct influence on which knobs can accommodate which pots.
The first model CTS employed used a solid shaft – fairly easy to spot, it is literally a solid lump of brass of polished zinc, with a smooth outer edge and a diameter of about 6.3mm or ¼”. These most commonly turn up on guitars with set screw mountings on the knobs. Normally, you’ll find the suitable knob will have a grub screw to hold it in place against the solid shaft.
A later model has come to be known as a “split shaft” – again, it’s all in the name – it will either be a knurled (toothed) edged piece of brass or zinc, with a large split down the middle and a diameter of 5.9mm. These turn up almost exclusively on guitars with push on knobs – it’s very common to see the suitable knobs have no visible means of fixing, but an internal knurling that will mesh with the toothed edge of the shaft.
[b][u]Taper - Logarithmic or Linear?[/u][/b]
Taper is simply a technical term for how a pot moves from 1-10, or from 0-100%, depending on how you think of it. Different tapers have different roles within the guitar, and in most cases, they’re interchangeable to give varying results.
Linear pots give a true representation of the output, (so 1 on the dial is 10% of the output, 5 is 50% and 10 is 100%). For this reason you’ll find them used all over the place, (rightly or wrongly) doing a variety of jobs,
Logarithmic pots are a little bit more specialised. They offer very narrow control range. Electrically speaking, by running though 1 – 6 on the dial, you cover 0% - 15% on the output, then tracking from 6 – 10 covers a much larger range from 15% - 100%.
Probably the easiest way to explain the whys and what for’s, is to break down the pots into real life situations you’ll find in your guitar.
[indent=1][b][u]Volume control[/u][/b][/indent]
[indent=1][b][i]Linear[/i][/b][/indent]
[indent=1]As a volume control, you’ll mostly find linear pots on Import guitars. In this case, a true representation of output lends itself really well to bedroom playing. It’s purely aimed at a target market looking to keep the guitar quiet and controllable, hitting low volumes during practise. They also turn up fairly often on bass guitars with 2 pickups, but no switching – giving the player a fairly easy to follow blending system.[/indent]
[indent=1][b][i]Logarithmic[/i][/b][/indent]
[indent=1]You’ll mostly find logarithmic pots on higher end instruments acting as the volume – in these cases the guitar is obviously targeted at gigging musicians. The guitars never going to be played quiet for extended periods so a log pot actually gives a nice “sweep” at the top end which lends itself to moving through different volumes whilst playing (moving from 10 to 8 on the dial will pretty much half the volume). This gives a great effect when used in combination with true amp overdrive.[/indent]
[indent=1][b][i]Relation to our hearing[/i][/b][/indent]
[indent=1]Obviously if you play on 10 constantly, it’s not going to make much difference; the arguments only really start when you’re changing volume.[/indent]
[indent=1]The main point to take into consideration is that the human ear works on a logarithmic scale. So whilst moving through volumes during playing, it pays to try to match the way the ear works. The problems start when you consider that our hearing is far superior to anything electrical, so the “curve” of a log pots output never really matches up perfectly, so although the sweep feels more natural, it’s still not perfect. Speaking personally, I try not to think of the curve as matching our hearing, instead I think of it as a “sensitive” control that allows better control at high volumes through less movement.[/indent]
[indent=1][b][i]The pitfalls[/i][/b][/indent]
[indent=1]The downfall of this “theory” is that it flags up the log pots main cause of concern. Most of the numbers on the dial (1-6) only give access to a very low output. This leads us back to the way the ear works – you can hear a pin drop just as well as you can hear a jet engine, the ear should be able to pick out the lower volume sound the same way it picks the higher volume, and your brain should tell you it’s quieter.[/indent]
[indent=1]Unfortunately, it’s not that clear cut, and this isn’t a perfect world, and although some people can either live with this feature (even use it to their advantage), others find the lower volume settings next to useless and the sudden drop off (usually between 6 and 7) quite annoying (some describe it as acting as a kill switch – basically muting the guitar)[/indent]
[indent=1][b][i]So linear volume?[/i][/b][/indent]
[indent=1]On the back of this – the linear pot seems the natural solution, but as with most things, it just isn’t that simple. Although the linear pot is great for low volume playing, and it’s easy to see where you are etc – they sweep in a way that isn’t conducive to human hearing. It sounds like its jumps from one setting to the next, and the ear finds it rather unnatural.[/indent]
[indent=1][b][i]But not all is lost[/i][/b][/indent]
[indent=1]Fortunately, “sweeping” from low volume to high volume still sounds perfectly fine on a log pot, but moving around at low volumes is still something that’s never been quite perfected as yet. Luckily, it’s not something most of us do.[/indent]
[indent=1][b][i]In Conclusion[/i][/b][/indent]
[indent=1]In closing on the volume topic – if you like making use of volume control whilst playing, then logarithmic control is probably your best bet thanks to the more natural curve of the output, but if you’re looking for a straight forward control solution at set volumes, where dial position represents the output, then linear pots are a great solution.[/indent]
[b][u][color=black]Tone[/color][/u][/b]
[color=black]The tone pot on most guitars is a fairly neglected control nowadays sadly. The tone of any given pickup at anything less than 9 seems somewhat muddy and woolly, so more often than not the tone pot is stuck on 10 and left there.[/color]
[color=black]In our experience, most complaints with pots come purely from the control that the stock tone pot is offering to the player.[/color]
[color=black]The reason seems to be that most tone pots (contrary to popular belief oddly) in any production guitar will in fact be a Logarithmic taper. This will make it behave exactly the same as the volume control (at least to our ears).[/color]
[color=black]As we discovered earlier, a Log pot will have most of its control at the higher end of its sweep – which works absolutely fine for volume control – however, with a tone control, when you’re making a “selection” – it leaves a little to be desired, purely because each setting is not equal to the ear. [/color]
[color=black]Some people are absolutely fine with this setup, and will use the tone pot to make minute tweaks in the higher numbers.[/color]
[color=black]An alternative would be to use a linear taper pot rather than the standard Log version – this will give you much more accurate control over the tone of your pickups.[/color]
[b][i][u]Resistance [/u][/i][/b]
Potentiometers, in the most basic of terms, are variable resistors – and whilst not strictly true – if you think that when they are resisting at 100% they are letting nothing through, and when they are not resisting anything, they are letting everything through – you know about as much as most people will ever need to.
At Axesrus, we try to offer a decent range of the most common resistances for guitar applications, and we’ve worked closely with CTS over the last decade to get the very best from the pots that we carry.
[indent=1][b]250K or 500K?[/b][/indent]
[indent=1]As a very quick rule of thumb, its 250ks for single coils and 500ks for anything that’s either noise cancelling or humbucking.[/indent]
[indent=1]The logic behind it is no great secret – as any pickup gets hotter (which will be caused by a rise in impedance and inductance) it also becomes darker (warmth and darkness are two sides of the same coin in reality, but how we perceive it is a matter of personal taste – one man’s mud is another man's hot!)[/indent]
[indent=1]Because humbuckers are, in essence, 2 single coils, wired in series, they are normally hotter, so to keep them balanced, need brightening up just a little bit to make them appealing to the ear.[/indent]
[indent=1][b][i]What happens when you mix?[/i][/b][/indent]
[indent=1]When you’re running a Humbucker in the same guitar as a Single Coil, you normally will only have access to one master volume unfortunately. This is probably the most important pot in the circuit when it comes to the resistance, so you’ll normally find that it’s a compromise when it comes to which value to pick.[/indent]
[indent=1]Within the industry, you’ll find preference is always given to the Humbucker and a 500k pot will be used, but in home builds and modifications, its completely down to you player.[/indent]
[indent=1]When testing, we’ve found that 250k will suit a single coil perfectly, so it’s ideal for single coils and lipsticks giving that beautiful, chiming, chirpy, soulful twang that you typically associate with single coil guitars. Humbuckers became very “warm” with a lot of the definition lacking in the top end.[/indent]
[indent=1]With a 500k pot, the humbuckers absolutely shined, it was bright, spanky, clear, with a nice definition in the bottom end as well as the top, which gave the impression of a depth in the tone.. Single coils by comparison sounded very shrill, with a very weak bottom end. Not without its charm, but certainly not that USA 50s and 60s rock tone, much closer to the 70s and 80s metal and rock tones.[/indent]

[iiipopes]

My personal preference is to bridge the wiper and output of a 500 kohm audio taper (aka logarithmic or log taper) pot with ether: a 1 megohm resistor to get effectively a 333 kohm hybrid taper sweep; or a 500 kohm resistor to get effectively a 250 kohm hybrid taper sweep; all to take into account what is set forth above, and to fine-tune the loading for the desired resonant peak of the particular pickup and the desired tone.

[Happy Jack]

KiOgon, that's just what I needed to read. Cheers.

[BigRedX]

IMO the CTS is quote is only half the story.

Volume pots go between the pickup and amp. While the input impedance of the amplifier is normally so high is to be irrelevant, the impedance of different pickups varies massively and in relative terms much closer to the resistance of the volume control, yet two values of volume pots (250k and 500k) are supposed to cover all situations. It is hardly surprising that most of the time the change in volume is not smooth across the whole travel of the potentiometer. In an ideal world the volume pot (and the tone pot) would be matched to the impedance of the pick ups(s) they are controlling. In practice manufacturers just stick whichever of the two common values is the least worst and leave it at that.

Sometime just by chance the controls happen to be matched to the pickups and the volume change is smooth and consistent across the whole range. On my black Gus bass this happens - but only when both pickups are selected and in humbucking mode. All the other combinations are less good as regard volume smoothness.

Active basses don't suffer as badly because everything is buffered and volume control should be affecting the gain of the circuit rather than having the signal passed directly though it.

Edited July 28, 2015 by BigRedX

[Woodinblack]

This is the reason I don't like passive instruments.
The volume isn't non linear, it depends on how you load it. If you plugged an active instrument in you would find that the same volume pot was linear.
All of my passive basses volumes work fine, but that is just down to the amp I use.

[chris_b]

Does anyone use the volume controls when gigging?

And if so, why?

When we're playing my volume controls are on the end of my hands.

[ikay]

I generally play with the volume on the bass maxed so, while I can see the irritation, does it really matter? Unlike guitarists who use the vol pot for fine adjustments of tone and character etc. I wonder how many bass players use a volume in this way? I expect I'll now find out the error of my ways!

[Woodinblack]

I used to start about 3/4 of the way up to give me some more headroom to turn up, but now I use filters, that isn't so useful.
I tend to use a volume pedal for any swell effects, its easier to get to than the volumes. But then, I tend to gig active basses so it isn't really an issue. Just need to put a preamp in my squier P.

[Happy Jack]

On a straight Precision it doesn't really matter. On any two-pickup bass the situation is quite different.

Unless there is one tone, and one tone only, that you actually like, you will want to use the controls provided.

[grandad]

Thank you KiOgon.

[ikay]

Ah OK, with a VVT setup of course that makes sense I've never had a problem backing off one of the pups a touch with either of my passive Js though. Maybe they both have log pots.

[planer]

[quote name='grandad' timestamp='1438117188' post='2831869']
Thank you KiOgon.
[/quote]
Seconded. Top post!

[brensabre79]

This is why I can't get on with VVT, I have a rotary 5 position switch instead with pre-set resistors at each position to give the 5 most useful combinations of the two pickups, I can instantly dial in a pre-set combination of the two pickups and I know exactly what I'm getting! Takes a little fiddling to set it up but once it's done it's done forever.

[bartelby]

Cheers KiOgon!

When I replaced a jazz pickup with a musician one in my bitsa bass I was a little disappointed in the muddiness.
After reading your post I realised it still had the 250K pot from the Jazz. Just fitted a 500K one and the difference is remarkable!

I'm going to keep the 250K pot in for the neck MM pickup as it would be a bit too harsh if it brightens up as much as the bridge MM did.

[BigRedX]

[quote name='chris_b' timestamp='1438103963' post='2831693']
Does anyone use the volume controls when gigging?

And if so, why?

When we're playing my volume controls are on the end of my hands.
[/quote]

When I am using a bass with a smoothly acting volume control I use it to fade out over-long sustained notes at the end of the song. Something you can't do with just your fingers.

[Ghost_Bass]

[quote name='BigRedX' timestamp='1438182215' post='2832354']
When I am using a bass with a smoothly acting volume control I use it to fade out over-long sustained notes at the end of the song. Something you can't do with just your fingers.
[/quote]

Palm mute just over the bridge saddles

[Grangur]

Thank you KiOgon.

Great thread.

[tauzero]

[quote name='chris_b' timestamp='1438103963' post='2831693']
Does anyone use the volume controls when gigging?

And if so, why?
[/quote]

Yes. So I can turn it up or down a bit if I want to without going to the amp. I normally set the amp to somewhere a little louder than I'm likely to need, then back off the volume on the bass to the level I want. I'm happy to do that these days, with low noise bass electronics and amps - in the badder old days, I'd want the entire signal chain down to the master volume to be at full whack to keep the signal to noise ratio as good as possible.