help - potentiometers..

[j.y]

hi guys..

i have read the secret life of pots article written by R.G. and i understand that.. when i use a taperin resistor on a linear pot.. i can get a audio taper pot.. but the resistance of the pot will be much lesser..

i used a 1M linear pot and placed a 120k resistor in between lug 1 and 2.. if im doin it correctly.. i shld be gettin a around 100k audio pot.. is that right?? i tried it on my breadboarded red fuzz pedal.. and changed between a 100k log pot and the 1m pot..  im gettin max volume when i turn to 90-100% of the 1m pot..below 90%.. i get very soft volume.. im sure theres something wrong...

can any one guide and help me?? 

thank you very much.

[R.G.]

when i use a taperin resistor on a linear pot.. i can get a audio taper pot.. but the resistance of the pot will be much lesser..

It will not only be less than the pot, it will vary with rotation. For this reason, tapered volume controls should only be used with circuits where the minimum pot resistance (that being the tapering resistor) is more than 10x the output impedance of the circuit, and where the input impedance it feeds is more than 4x the un-tapered pot value.

i used a 1M linear pot and placed a 120k resistor in between lug 1 and 2.. if im doin it correctly.. i shld be gettin a around 100k audio pot.. is that right??

Not quite. The ratio of the basic pot to the tapering resistor controls the "suddenness" of the pot sound. The best approximation to equal volume per degree of rotation is where the tapering resistor is between 1/4 and 1/5 of the whole pot. So for a 1M linear pot tapered to a standard volume control, you'd want 200K to 240K as a tapering resistor.

Let's talk about the suddenness of volume change.

The human ear wants to hear an exponential increase in signal level as "linear" increase in volume. In electrical terms, the signal increases only slowly for the first 1/2 to 2/3 of the pot travel, then rapidly increases for the rest of the travel. If you use a linear taper pot, the ear interprets the loudness as happening too soon in the pot travel. All of the perceived loudness increase is bunched up in the first half of the pot rotation.

A log-taper pot or a pot plus tapering resistor changes this so that the increase in electrical signal is delayed into later portions of the pot rotation. The electrical signal increases slowly at first, then more suddenly. The smaller the tapering resistor compared to the whole pot resistance the later and more sudden the increase in volume level is in the pot rotation.

If the tapering resistor is more than 10X the basic pot resistance (and all the way up to infinity!) it's like there is no tapering, so the apparent loudness increase seems all bunched up in the first half of the pot.

As you decrease the tapering resistor value down to maybe 1x the basic pot resistance, the apparent loudness per degree of pot rotation unbunches and spreads out to later portions of the pot rotation. When the tapering resistor gets down to 1/4 to 1/5 of the basic pot resistance, the loudness per degree of pot rotation seems to our ears to be equal per degree of pot rotation.

As you make the tapering resistor even smaller compared to the basic pot resistance, the apparent loudness per degree seems to get bunched up towards the top end of the pot travel. This is what you are doing. A 1M pot with a 120K tapering resistor is a ratio of 1/8. So the apparent loudness will be very delayed. It will hardly come up at all for most of the pot travel, then increase very rapidly at the top end of the pot rotation.

Do this: change to a 220K tapering resistor. That should help the volume change per degree of rotation a lot.

[j.y]

ok..

thanks.. i read and understood ur article..

but i was readin this article mins ago at http://sound.westhost.com/project01.htm and they said usin 6.7:1 like usin a 15k resistor with 100k linear pot is close to a read log curve.. that got me confused..  i downloaded a pot taper excel file from this forum and i use it to guide me.. it too showed me nice log curve.. is the article at the above website correct?? or did i interpret it wrongly..

thanks for helpin me..