Logarithmic pot - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: January 05, 2020, 07:39:28 pm
content: Is there a way to use a simple, passive circuit, with very few parts, to turn a linear, or a badly made "log" pot, into an audio pot? From some charts I have seen it seems we are looking for something like the below:Code:potentiometer positionexperimental log scale0010.220.6324551062073385498010100

author: Prehistoricman
author_rank: Regular Contributor
timestamp: January 05, 2020, 08:25:49 pm
content: You can add a resistor from the tap to either end of the pot to affect the curve. See pics.The graph shows the behaviour of a pot with a resistor 10% of its value to ground. X is the pot setting, Y is the tap output.

author: Whales
author_rank: Super Contributor
timestamp: January 05, 2020, 09:15:53 pm
content: +1 for "10%" recomendation. I have read suggestions of 50% elsewhere, but some analysis I did a while back showed 10% was a better fit.IIRC: this gives you about 20dB of db-linear audio range (used as an attenuator, although I think I was also putting a resistor above?). The last few degrees at the far edges of the pot will be distorted and give you a lot more than this.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: January 05, 2020, 10:09:04 pm
content: Quote from: Whales on January 05, 2020, 09:15:53 pmResistor above would make an anti-log taper.And yes, you're right about the edge distortion. Here's a quick excel plot of a real log taper against this hack.But a real log pot will probably contain two linear sections of different resistivity to roughly approximate the ideal log taper (source: internet, no validation has been done).

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 05, 2020, 10:22:02 pm
content: Yes I've done a least-squares fit between the ideal exponential curve and what you get from a fixed resistor in parallel with a linear pot as Prehistoricman says. The optimum value (least error) is at resistor = 11.6% of pot. This assumes that there is no other load and the source impedance is zero. It looks like this:

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2020, 11:18:46 pm
content: Quote from: Prehistoricman on January 05, 2020, 10:09:04 pmA real log potentiometer has the resistive material deposited in such a way as to make it a true log. The more common way of using two sections is a "fake log".https://sound-au.com/pots.htm#taperIt's also possible to cascade two potentiometer sections to get a log behavior:https://sound-au.com/project01.htm#s3

author: akis
author_rank: Frequent Contributor
timestamp: January 06, 2020, 10:17:35 am
content: I have found the power function the one most closely tracking the experimental audio scale as in the chart below. Both the "pot hack" and the exponential curves are too far away, and audibly, "it has all happened" before we even reach 5, with very little change afterwards.The range 0-4 is critical and it has to be very, very slow, as the chart would indicate.

author: ormaaj
author_rank: Contributor
timestamp: January 06, 2020, 02:00:28 pm
content: I guess everyone probabally knows about using a BJT for this... but for completeness...http://falstad.com/circuit/e-logconvert.html. I used a similar circuit that worked well enough for linearizing the input to a '4046 VCO with a linear pot.If precision is needed then I dunno. An OTA-based solution might perform ok. Maybe I'll try to get that to work.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 06, 2020, 02:42:52 pm
content: Quote from: Prehistoricman on January 05, 2020, 08:25:49 pmI've used that almost exclusively to implement analog volume pots for audio circuits for ages. It can be made close enough to a log behavior for this purpose, and has an additional benefit IMO when using mechanical pots: it adds a current path from the wiper to ground, which limits a lot electrostatic buildup that can lead to scratching noise when the pot is rotated. Very effective in practice IME.

author: TimFox
author_rank: Super Contributor
timestamp: January 06, 2020, 04:09:56 pm
content: Another advantage of using a resistor-loaded linear-taper pot is when you have multiple controls and want them to track each other closely, trading off absolute log accuracy. A good linear pot will be more reproducible from unit to unit than most log pots.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 06, 2020, 04:20:27 pm
content: Quote from: TimFox on January 06, 2020, 04:09:56 pmYup. In a lot of applications (especially audio), exact log is not critical anyway. And if you think a typical "log" pot will match a log function better than the above, you could be in for some surprise as well... unless you buy very expensive shit.Audio applications in which it could be a bit more problematic is the typical large linear pots (not in the sense of linear vs. log, but in the sense of linear vs. rotative) used on studio mixing consoles, as accuracy for a given position could be a bit more important there. But that said, even on analog consoles, it's not uncommon to see linear pots controlling VCAs with a log transfer function anyway... so...In any case, it answers the OP's question fairly well, and is actually used in real products and is not just a thing for hackers.

author: akis
author_rank: Frequent Contributor
timestamp: January 10, 2020, 10:32:51 am
content: Can't shunt to ground with passive guitar pickups, the pots need to have many hundreds of KOhms. Typical passive pickups pots are 500K. If you shunt this with say a 100K resistor, you'd be dropping the value seen by the pickup to <100K, and then you'll lose the high end.While the standard audio guitar pots are "OK" for the volume control, they are wholly inadequate for the tone control circuitry, as I recently found out. On the tone control it all happens between 0 and 4, with the range between 5-10 being inconsequential. That is where adherence to the true power function would provide a more gradual tone response.

author: dmills
author_rank: Super Contributor
timestamp: January 10, 2020, 05:34:58 pm
content: For volume controls the Baxandall volume control circuit is reasonable if you can tolerate an active device to do the buffering and the tendency of the law to fall to bits at the very low end. Uses a linear pot too, which is helpful.Regards, Dan.