Log pot using a linear one

[llama]

I want to build a little audio mixer for my Grand-son. Ideally passive. I bought some cheap pots off ebay - all the cheap ones seem to be linear. I opted for 10k but I think 100k would have been a better choice.

Be that as it may, I'd like to approximate a log progression by simply using a fixed resistor from wiper to one end.

The attachments show my calculations using various fixed resistors. Is there an easy way to tell if any of them come anywhere near a log curve? Could be more widely useful than just this query.
Graham

[ms660]

Originally Posted by llama

The attachments show my calculations using various fixed resistors. Is there an easy way to tell if any of them come anywhere near a log curve? Could be more widely useful than just this query.

Yes, Google> pot log curve

Then click on Google images.

EDIT: Taper resistor usually 20% of the track value.

Lawrence.

[pmmunro]

Is this any help?

PMM

[Ambientnoise]

Pse don't forget that the impedance of the pot + resistor with vary with pot position, reaching a minimum of about the 80% of the resistor value at max volume. You will end up with 1.6k for the example above.

Ken

[Tim]

It will be a reasonable approximation of a log pot. It'll be good enough I expect.

[Wellington]

Some links here that might be of use, particularly the last one…

[llama]

Useful comments and links all. Thanks so much. The Google search suggested also provided this link:

https://www.maximintegrated.com/en/a...dex.mvp/id/838

which suggests another configuration, namely series resistor plus linear pot wired as a variable resistor. I guess that's a current/voltage "dual" cf the normal arrangement and might help with the variable input z which I hadn't considered.
As a simple try-out, the 2k2 option looks like it'll behave well.
Thanks again. Hope others find this refresher useful!
Graham

[mhennessy]

If you want to use the ones you've got, you could also use an active Baxandall gain control circuit, which gives a log law with a linear pot. See here (scroll down about a third of the way): http://sound.whsites.net/project01.htm

This is also good bed-time reading: http://www.ti.com/lit/ug/tidu034/tidu034.pdf

It's easy to be put off by these circuits as they require op-amps, but you'd require those anyway to make a mixer. Trying to keep it entirely passive is fraught with complexity. More about mixing circuits here: http://sound.whsites.net/articles/audio-mixing.htm

Alternatively, buy some 100k pots, and feed each one of those into the 10k input resistors of a virtual earth mixer (figure 4 at above link). Only 1 op-amp needed (2 for stereo), and Zin remains at 9k or more (put 1k in series with the 100k pot to ensure it never goes below 10k, or use 12k instead of 10k for the mix resistors). As a rule of thumb, line inputs should be 10k or more, so this will be fine.

If you want to use the 10k pots in a circuit like this, you'll need to buffer them first, and then use ~1k mix resistors. An emitter follower would be OK, and the biasing components could be shared between channels...

Whichever you use, watch out for DC currents in the pots - these wreck 'em!

[David G4EBT]

Un-switched log pots (and lin) in a range of values are only 85 pence each from Bowood Electronics, (no VAT, Min order value £5.00). £2.85 P&P:

https://www.bowood-electronics.co.uk...cfbe639965e0fd

Hope that's of interest.

[Stockden]

Does this help?