Simple law-faking advice...

[darkmoebius]

I've read all the posts and Nuuk's site for info on law-faking linear pots, but there is one question I haven't been able to answer:

Is the final resistence of the pot equal to it's initial value divided by the law-faking resistor value?

e.g . a 100k pot with a 5k law-faking resistor produce a 20k result?

Also, most posts around here(and Elliot Sounds article) seem to indicate that that the faking rsistor should be between 1/10th and 1/20th the value of pot. Is this true?

 

[glennb]

>Is the final resistence of the pot equal to it's initial value divided by the law-faking resistor value?

No, the input resistance is the pot resistance at min volume setting. At max volume, it is the faking resistor, pot resistance and the output load impedance in parallel.

10% is a good compromise for Rfake / Rpot. I've even used 15 - 20% and its seems OK.

The output resistance (ie. as seen looking back by the load) is zero at min volume setting. At max volume it is the faking resistor in parallel with the pot resistance (at max volume) and the source impedance.

HTH,
Glenn.

 

[rabbitz]

Rod's info shows that a ratio of 6.67:1 should be maintained. For 100K pot the resistor should be 15K (100/15=6.67).

As far as I know, the pot remains the same, 100K pot remains a 100K pot when adding the resistor from wiper to ground. From info on the Leach Amp site, to alter the resistance of the pot you add the resistor from the input to the wiper.

If I'm right, I'm right, if I'm wrong I'm sure someone will tell me.

 

[darkmoebius]

Thanks for the info guys...

Great explanation, Glennb. I think I've got the hang of it now.

Rod's info shows that a ratio of 6.67:1 should be maintained. For 100K pot the resistor should be 15K (100/15=6.67).

It seems Rod changed his opinion on that ratio slightly in a later article(under "Changing the Law of a Pot") , where he stated :

Using pots can be done in the conventional way, or you can get adventurous and achieve a lot more. A good example is the "Better Volume Control" shown in Project 01. The other ideas presented also show how you can make modifications to the way a pot behaves, just by adding a resistor (R). The "ideal" value by calculation is 22k for a 100k pot, and this gives a maximum deviation of +1.58 and -1.7dB from a real log curve. This is contrast to the original article, where 15k was suggested, and although the error is greater (+2.89dB and -1.12dB), the overall behaviour is almost ideal in listening tests.

But, it seems that I could easily test different ratios between 5-10% by ear and go with the one that sounds best in my system. Afterall, a few more resistors won't make that bg of a difference in cost after all the upgrades I'm doing.