What Is The Philosophy For Reverse LOG Pots?

[Paul Marossy]

So what is the philosophy for using reverse LOG pots on stuff like speed controls on a phaser? Why wouldn't you use a linear pot in that sort of application?

[midwayfair]

Speed controls tend to bunch up at the CW end, especially in transistor-based phase oscillators in my experience. Also, a lot of people tend to want to dial in the fast settings more finely than the slow settings, and a reverse log helps with that regardless of how the oscillation is achieved.

You can use a linear pot that's too big and shrink it with a parallel resistor, which gives something close to a reverse log. This also lets you dial in the slowest possible setting without locking up the oscillator. I did this on RG's Vibratone (brownface) trem recently and it worked great, especially because there's no such thing as a dual gang 65K reverse taper pot .

[Paul Marossy]

Speed controls tend to bunch up at the CW end, especially in transistor-based phase oscillators in my experience. Also, a lot of people tend to want to dial in the fast settings more finely than the slow settings, and a reverse log helps with that regardless of how the oscillation is achieved.

I see. Makes sense now. Yeah, I noticed on my Phase 45 clone that there is scarcely much happening with the speed control until you are about 3 o'clock.

You can use a linear pot that's too big and shrink it with a parallel resistor, which gives something close to a reverse log. This also lets you dial in the slowest possible setting without locking up the oscillator. I did this on RG's Vibratone (brownface) trem recently and it worked great, especially because there's no such thing as a dual gang 65K reverse taper pot .

Yes, that is precisely how I was going to create a reverse LOG pot for msyself. I was just wondering if it was really necessary. I think that I will try it.

[zombiwoof]

You can get the same effect by using a regular Log pot, and wiring it in reverse.  It will of course work backwards (you turn it counter-clockwise instead of clockwise to increase volume/whatever.), but you will get a reverse log taper.  Similar to what Whirlwind does on the Rat pedals for the tone control.

Al

[Paul Marossy]

You can get the same effect by using a regular Log pot, and wiring it in reverse.  It will of course work backwards (you turn it counter-clockwise instead of clockwise to increase volume/whatever.), but you will get a reverse log taper.  Similar to what Whirlwind does on the Rat pedals for the tone control.

Yes, I know about that also. I guess it would work just fine for a variable resistor type of application. My problem will be trying to find 500KC PCB mounted Alpha pots however. I'll probably have to do some sort of workaround for that....

[Mark Hammer]

Reverse logs are also frequently appropriate for gain controls, when gain is being set by a varied resistance on the ground leg of a non-inverting op-amp (see Distortion+/DOD250), or when the gain is set by a parallel path on the ground leg of a single transistor stage (see AMZ MosFET booster or Alembic Stratoblaster).

In those instances, it is often the case that not very much is happening for a while at the largest resistance values, but you want greater dialability for the small values.  Reverse log will quickly skip you through the large resistance values so the smaller values can be distributed across more pot rotation: the same principle that makes them handy for LFOs.

[Paul Marossy]

it is often the case that not very much is happening for a while at the largest resistance values, but you want greater dialability for the small values.  Reverse log will quickly skip you through the large resistance values so the smaller values can be distributed across more pot rotation: the same principle that makes them handy for LFOs.

That's a very good way of putting it. Mainly it comes down to convenience for the end user. 

[R.G.]

That's a very good way of putting it. Mainly it comes down to convenience for the end user. 

I'd say it more positively than that. Humans tend to work well with linear-responding controls, where the output you get corresponds linearly with what you do on the controls, at least over some usable range. The point of pot tapers in toto, I think, is to help linearize the result as seen by the human twisting the pot so that you get about equal change per unit of pot rotation.

We all can learn to use nonlinear controls where little happens until some threshold is hit and then a bunch of change happens for a small change in controls, but it's hard, needs skill and attention, and is prone to accidental errors. "Touchy" controls can be learned, but it's a pain.

Imagine the "how much power to make" control in a nuclear reactor being quirky and touchy with sudden jumps in power output for just a ...little... more rotation. It pays to use some fancily "tapered" controls to make that variation smoother to the operator. 

[Mark Hammer]

Or even more succinctly, sometimes you have to use a non-linear electronic taper to achieve perceptual linearity.

[R.G.]

Or even more succinctly, sometimes you have to use a non-linear electronic taper to achieve perceptual linearity.

Oh, sure, say it more simply and more easily comprehended... 

That's what I was trying to thrash my way around to.

[Balthazar]

You can use a linear pot that's too big and shrink it with a parallel resistor, which gives something close to a reverse log.

Unfortunately that doesn't work if you want it to have full resistance at the CCW position, and then abruptly fall as you turn it CW, and have it ease out towards zero resistance near the CW end.

Perhaps you can get one of those that have a slider instead of a turn knob, regular logarithmic, and physically mount it the other way. Or, you can get a regular log, and mount it so that the shaft comes out on the LEFT side of the stomp box, and connect it to terminals 1 and 2. Then, as you turn it FORWARD the resistance sinks, as you turn it backwards the resistance gets higher....

[Mark Hammer]

Or even more succinctly, sometimes you have to use a non-linear electronic taper to achieve perceptual linearity.

Oh, sure, say it more simply and more easily comprehended... 

That's what I was trying to thrash my way around to.

Just a pinboy*** in the bowling lane of life, eh?  You set everything up just so, so that somebody else can fling a ball, make a big noise and knock 'em all down.   




***For the young....  "Pinboys" were regular features at bowling lanes.  Prior to the development of automatic devices that would sweep away the knocked-over pins and mechanically set up a new frame, the work was done manually - usually by teenage boys, who waited in the sunken area at the end of the lane, behind the pins.  Often you would only see their feet, calves and forearms at work  It was considered suitable work for a boy that age.  Maybe not as glamorous as pumping gas, checking fluid levels and wiping windshields, but a step up from a paper route. A little more dangerous than either of those, since you couldn't always rely on bowlers to wait long enough for you to get out of the way before flinging their ball.

[digi2t]

***For the young....  "Pinboys" were regular features at bowling lanes.  Prior to the development of automatic devices that would sweep away the knocked-over pins and mechanically set up a new frame, the work was done manually - usually by teenage boys, who waited in the sunken area at the end of the lane, behind the pins.  Often you would only see their feet, calves and forearms at work  It was considered suitable work for a boy that age.  Maybe not as glamorous as pumping gas, checking fluid levels and wiping windshields, but a step up from a paper route. A little more dangerous than either of those, since you couldn't always rely on bowlers to wait long enough for you to get out of the way before flinging their ball.

Although not forum related, interesting nonetheless. As always, this 49 year old is going to bed less stoopid tonight.... again.

Thank you, once again.

[Paul Marossy]

I'd say it more positively than that. Humans tend to work well with linear-responding controls, where the output you get corresponds linearly with what you do on the controls, at least over some usable range. The point of pot tapers in toto, I think, is to help linearize the result as seen by the human twisting the pot so that you get about equal change per unit of pot rotation.

Yeah, I suppose that is a good reason to use a reverse LOG control. 

Imagine the "how much power to make" control in a nuclear reactor being quirky and touchy with sudden jumps in power output for just a ...little... more rotation. It pays to use some fancily "tapered" controls to make that variation smoother to the operator. 

Well, in THAT case, I guess it might be a little more important. 

[Mark Hammer]

***For the young....  "Pinboys" were regular features at bowling lanes.  Prior to the development of automatic devices that would sweep away the knocked-over pins and mechanically set up a new frame, the work was done manually - usually by teenage boys, who waited in the sunken area at the end of the lane, behind the pins.  Often you would only see their feet, calves and forearms at work  It was considered suitable work for a boy that age.  Maybe not as glamorous as pumping gas, checking fluid levels and wiping windshields, but a step up from a paper route. A little more dangerous than either of those, since you couldn't always rely on bowlers to wait long enough for you to get out of the way before flinging their ball.

Although not forum related, interesting nonetheless. As always, this 49 year old is going to bed less stoopid tonight.... again.

Thank you, once again.

My pleasure.  You may find this book of interest; by the former Parliamentary Poet Laureate no less:  http://www.cormorantbooks.com/titles/pinboy.shtml