Using Log Pot instead of RevLog (simple question)

http://www.audiomaintenance.com/acatalog/om-01-055_extended_info.html

http://www.audiomaintenance.com/acatalog/om-01-028_extended_info.html

http://www.audiomaintenance.com/acatalog/RU-01-022_extended_info.html

These will work.
Best,
Bruno2000

http://www.taydaelectronics.com/potentiometer-variable-resistors/rotary-potentiometer/anti-log-reverse/50k-ohm-c50k-50kc-anti-log-taper-potentiometer.html

joaquins said:

This does works, for rheostats, only for the antilog version. The log version only works as a pot.

The thing is, you are making the center point to where you want it, relative to the end. So, you want a 100k antilog rheostat. 500k lin pot, 120k resistor. 97k at max, close enough. at mid point you get 250k//120k it's about 80k. That's a 80% law,

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That's not what you want for a sweepable EQ, you want 10-20%, and this is not feasible, unless you find a stash of negative resistors.
Indeed, the rheostat law becomes somewhat revLog, but at the wrong orifice. In an EQ, we use the section between wiper and bottom (whatever side this bottom is) and this section it is impossible to make revLog, unfortunately.
The maths seem ok, the implementation doesn't agree.

ruffrecords said:

LOL. The story goes that people meeting on the road used to keep to the left so that they had their sword hand close to the  person passing them and as most people are right handed that means passing on the left.

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That makes sense actually, never  thought about it

ruffrecords said:

Either that, or we do it just to annoy the French.

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That even makes more sense, and everyone should do the same! LOLOLOL

Whoops said:

ruffrecords said:

Either that, or we do it just to annoy the French.

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That even makes more sense, and everyone should do the same! LOLOLOL

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1+
and
1+

abbey road d enfer said:

That's not what you want for a sweepable EQ, you want 10-20%, and this is not feasible, unless you find a stash of negative resistors.
Indeed, the rheostat law becomes somewhat revLog, but at the wrong orifice. In an EQ, we use the section between wiper and bottom (whatever side this bottom is) and this section it is impossible to make revLog, unfortunately.
The maths seem ok, the implementation doesn't agree.

Click to expand...

Ok, then it's log law but reversed, or the antilog wired the other way... And that's not possible to do.

Negative resistors, easy, Gyrator with resistors instead of caps... wouldn't it?

The other option is to label period instead of frequency and use a regular log pot.

There may be some ways around it but for new designs, not preserving copied schematics I guess. Probably easy to get the C pots anyway or swapping the wafers in the ones you can. I've done that for a guitar pedal which needed double 100kC pots for rate of a  univibe IIRC. It does work, you may need to give some to the bin till you get the way of doing it and end with a reliable pot but it's doable.

JS

ruffrecords said:

Yes, you can wire it reversed and it will work. However it will also work backwards. So, if full gain with a rev log was fully clockwise it will now be fully anti clockwise.

Cheers

ian

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Hi Ian,

I'm really confused now. Some people say it will work and some say you can't make a logaritmic Pot working as a rev log pot. ?

SV.L said:

Hi Ian,

I'm really confused now. Some people say it will work and some say you can't make a logaritmic Pot working as a rev log pot. ?

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Yes, there's a lot of confusion around.
First you have to understand that you're asking for a RevLog rheostat, i.e. a two-terminal device; traditional law-steering doesn't work here. Basically, for at least half the travel, the law will be unchanged. If you use a Lin pot, 50% of the rotation will be Lin (veeeery slow, with a variation of less than an octave), and the further you go, more cramped the settings.
It is indeed possible to "make a logaritmic Pot working as a rev log pot.", ...somewhat; that would be stupid. But working like a Rev log rheostat, never...

abbey road d enfer said:

It is indeed possible to "make a logaritmic Pot working as a rev log pot.", ...somewhat; that would be stupid. But working like a Rev log rheostat, never...

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Never say never. If you wire a log pot as a rheostat using the the'wrong' pair of contacts it works as a rev log BUT only if you turn it anti-clockwise,  This is a counter intuitive way to rotate a pot BUT if you place the scale underneath the knob the scale runs from left to right, the same as a true rev log pot would with the scale across the top.

Cheers

Ian

ruffrecords said:

Never say never. If you wire a log pot as a rheostat using the the'wrong' pair of contacts it works as a rev log BUT only if you turn it anti-clockwise,  This is a counter intuitive way to rotate a pot BUT if you place the scale underneath the knob the scale runs from left to right, the same as a true rev log pot would with the scale across the top.

Cheers

Ian

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Indeed. After all, it's a matter of POV. Who said the knob rotates under the action of the fingers, maybe it's the contrary, the whole world turns around the shaft... 
I wonder what would happen if we applied the same course of thinking to vehicle's driving wheels, though...

abbey road d enfer said:

Indeed. After all, it's a matter of POV. Who said the knob rotates under the action of the fingers, maybe it's the contrary, the whole world turns around the shaft... 
I wonder what would happen if we applied the same course of thinking to vehicle's driving wheels, though...

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I remember reading about a visual experiment many years ago. As we all know, the image on our retinas is inverted due to the action of the lens but as babies we soon learn to mentally invert the image so it is the 'right way up'. The experiment involved fitting an adult with a pair of spectacles that inverted the image (again) before it entered the eye to see if they could once again learn to mentally invert the image. Apparently they could.

Cheers

Ian

ruffrecords said:

I remember reading about a visual experiment many years ago. As we all know, the image on our retinas is inverted due to the action of the lens but as babies we soon learn to mentally invert the image so it is the 'right way up'. The experiment involved fitting an adult with a pair of spectacles that inverted the image (again) before it entered the eye to see if they could once again learn to mentally invert the image. Apparently they could.

Cheers

Ian

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That's right. There was also an experiment where the driving wheel of a car was reversed, and it didn't take very long to accustom. The problem is relearning each time you change car. In fact it's not much different than driving on the other side of the road; it just takes minutes to get used to it, except when you exit a roundabout, you often end up in the wrong lane.

ruffrecords said:

Never say never. If you wire a log pot as a rheostat using the the'wrong' pair of contacts it works as a rev log BUT only if you turn it anti-clockwise,  This is a counter intuitive way to rotate a pot BUT if you place the scale underneath the knob the scale runs from left to right, the same as a true rev log pot would with the scale across the top.

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Does this explain "backwards" control pots on Drawmer gear?

Andy Peters said:

Does this explain "backwards" control pots on Drawmer gear?

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Possibly. You would have to peek inside to find out for certain.

Cheers

Ian

Ive messed around with swapping wafers on dual pots before ,the back wafer is easy to swap,the front isnt, as usually ,on the mini alpha pots the shaft is sorta riveted to the wiper ,if your carefull though it can be done ,then you need to glue the shaft to the wiper after. There are some very interesting circuits using dissimilar dual pots . Theres a really old BBC designed preamp it uses one pot section to vary negative feedback to the grid ,and the other section rheostat connected as a padding resistor on the input. Incidentally I found a great page before with all the old BBC valve preamp circuits on it ,I cant seem to locate in anymore though ,I did archive all of it somewhere on a hard drive ,I might make a post once I locate the material again .

Just thaught id add this in ,
This guy does a good selection of alpha pots with reverse log taper ,
https://www.bitsbox.co.uk

> if we applied the same course of thinking to vehicle's driving wheels, though...

When I was young:

More radio volume was clockwise.

More fire volume on the kitchen stove was counter-clockwise.

Somehow we survived these arbitrary decisions.

That's not to mention how the BBC rigged mix-faders for decades.

I guess once conventions emerge they tend to be adhered to ,sometimes for very valid reasons ,sometimes just because.In the case of the upside down faders on British broadcast equipment, I believe the idea  was so that if the presenters sleeve happened to catch a fader on the way to reaching further up the board you didnt end up overdriving your transmitters. For me function trumps formality, it doesnt bother me if the pointer goes over or under ,or even if a pot  attenuates as you turn it clockwise ,sure its unconventional and quirky, but lifes like that sometimes.  I bought an amek/neve era broadcast console from a closing down broadcaster in the uk some years back  ,wasnt a big deal to flip the faders round and take out the overpress microswitches for the remotes,of course now my fader scale markings are meaningless ,but so what.

PRR said:

> if we applied the same course of thinking to vehicle's driving wheels, though...

When I was young:

More radio volume was clockwise.

More fire volume on the kitchen stove was counter-clockwise.

Somehow we survived these arbitrary decisions.

That's not to mention how the BBC rigged mix-faders for decades.

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True. Still, I hate the way light switches are wired, with the ON position being downwards; for me, the natural ON position for a switch is up. Of course it's irrelevant for multiway switches.

ruffrecords said:

Never say never. If you wire a log pot as a rheostat using the the'wrong' pair of contacts it works as a rev log BUT only if you turn it anti-clockwise,  This is a counter intuitive way to rotate a pot BUT if you place the scale underneath the knob the scale runs from left to right, the same as a true rev log pot would with the scale across the top.

Cheers

Ian

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That makes sence now!

And I guess when I wire a rev log pot as a rheostat using the the'wrong' pair of contacts it works as a CCW  log?