Changing the value /mode of pots

[Oldcodger]

My post https://www.vintage-radio.net/forum/...d.php?t=132814


reminded me of an article in the back pages of an old ETI Circuits Book No1, published in 1976 .Those pages are long gone, thanks to an old rescue Collie who decided that making a bed in my electronics mags for her ( unknown to me when I homed her) litter of pups. Dog of good taste ?
One idea was to change the value/mode of a pot by fitting a parallel resister .
I've always been tempted to have a look at this idea from a mathematical view.

Anyone remember this article / looked at it's ideas in detail?

[Boater Sam]

I looked at doing this and asked on the forum. Was told it was not a good idea and that it would not achieve what I wanted, i.e. a changed value pot either log or lin.

[ms660]

Not seen that particular article but modifying a pots behavior has been covered a lot on the forum, here's a popular link to the subject:

http://www.geofex.com/article_folder...s/potscret.htm

Lawrence.

[Oldcodger]

I'm more interested in the content of the article. At the time, I glanced over it. But these days it might have more interest. Thanks, ms660.

[m0cemdave]

"Slugging" a linear-track audio level pot to obtain a log law is a legitimate technique. The curve is determined by the parallel combination of the added resistor from slider to earth and the following circuit load impedance.
I remember some old Neve mixing consoles used a preset trimmer that was adjusted to give the required fader law.

[Terry_VK5TM]

I've got the book somewhere in the shed.

If i can find it, I'll scan the page for you.

Terry

[Terry_VK5TM]

Found the book, no log/lin article in this one.

I do remember seeing something along those lines in another book/mag somewhere.

Just to confirm I have the correct book, front cover below.

[Oldcodger]

TERRY- that is the book.I raised the point as I thought other members might like a look at this article.
Your shed must be a lot better than mine. BUT then, I had an old Collie looking to make a bed for herself away from a brood of hungry pups ---and my stack of mags must have looked a decent den ---possibly a dog with a lot of taste

[Hartley118]

Originally Posted by m0cemdave

"Slugging" a linear-track audio level pot to obtain a log law is a legitimate technique. The curve is determined by the parallel combination of the added resistor from slider to earth and the following circuit load impedance.
I remember some old Neve mixing consoles used a preset trimmer that was adjusted to give the required fader law.

Quite right. Back in the day at Neve we found log pots pretty inconsistent in their law, so it was common instead to 'slug' a 10k linear pot with say 5k1 from wiper to ground so that the centre position was more like 10dB down rather than the 6dB down of a linear pot. The use of a preset would be unusual though - maybe it was on something like a twin gang balance control that needed precise centring.

Martin

[Terry_VK5TM]

Originally Posted by Oldcodger

Your shed must be a lot better than mine.

Don't bet on it, I know I've got all sorts in there, but finding it is the problem. At least the dog here knows better than to get into my stuff, although lengths of wire intrigue him.

The article may have been in one of the others from the series, it rings a bell (we need a smiley with a ringing head ). They are in there (the shed) somewhere, will keep an eye out for them.

Terry

[lesmw0sec]

Originally Posted by Hartley118

Back in the day at Neve we found log pots pretty inconsistent in their law, so it was common instead to 'slug' a 10k linear pot with say 5k1 from wiper to ground so that the centre position was more like 10dB down rather than the 6dB down of a linear pot. The use of a preset would be unusual though - maybe it was on something like a twin gang balance control that needed precise centring.

Yes, we used this method in MTE mixers (Queensland). We used it on the pan pots where a log/anti-log combination was difficult to obtain and pricey.

Les.

[Stockden]

Well after a bit of digging in my archive (and even more messing about with the scanner to get it to PDF) I've found the attached, which came from Elektor nearly 40 years ago. It seems to cover the ground fairly well so I hope that it proves useful.

Hugh