Potentiometers, Log and Lin. - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 13th Dec 2006, 8:51 pm

If a pot is described as having a Logrithmic track (i.e. a volume control), is the track symmetrical?
Are both ends the same if you wire the thing the other way up?

paulsherwin · 13th Dec 2006, 9:00 pm

Quote:

Originally Posted by Neil Purling

If a pot is described as having a Logrithmic track (i.e. a volume control), is the track symmetrical?
Are both ends the same if you wire the thing the other way up?

No and no. The resistance is skewed towards one end of the track - it increases logarithmically

Paul

Neil Purling · 13th Dec 2006, 9:53 pm

Stupid mistake made upon re-assembly. Reaction was strangely 'knife edge', and right at the bottom ende of the pot. For reaction I suspect one would be better with a Linear pot. The reaction pot is a wire-wound example, from a forum member as it happens. I was unaware of what sort of track it had, I just assumed that it was linear.

paulsherwin · 13th Dec 2006, 11:44 pm

Try wiring it the other way round, though this will involve turning the control the opposite way to operate it.

Paul

Neil Purling · 14th Dec 2006, 8:57 am

The fact I had to turn the pot the other way made me realise the thing was probably wired the wrong way round.

Neil Purling · 14th Dec 2006, 8:12 pm

I know the present 1 Leg log volume pot is suspect, and I am looking for a replacement.
I have a 1 Meg Linear pot in my spares box. What'd be the audible difference to the ear? I have tried ebay for Old Stock 1 Meg Log pots, but they ended at very high sums. Maybe they are in demand for restorers of vintage amplifiers?

paulsherwin · 14th Dec 2006, 8:48 pm

Try it. Log pots are normally used for volume controls because the log track gives a subjectively evenly spaced volume scale. THere are no hard and fast rules though.

You can tweak the characteristics of the control by adding fixed resistors between the slider and the bottom or top of the track. Have a play around

Paul

Neil Purling · 18th Dec 2006, 10:12 am

I just stuck my AVO on a spare logrithmic pot and the resistance changes at a extreme rate in the first quarter of the pot's rotation.
Yet the effect seems linear to the ear.
I know I have re-stated the contribution of Mr Sherwin.
Would it be right to say that substitution of a Linear pot for a logrithmic pot in a volume would have the effect that volume would seem to increase greatest over a small part of the pot's travel?

You know, idle curiosity means I might just try it. The 1 Meg Linear pot I have seems to be of a better rating.

ppppenguin · 18th Dec 2006, 10:36 am

Quote:

Originally Posted by Neil Purling

I just stuck my AVO on a spare logrithmic pot and the resistance changes at a extreme rate in the first quarter of the pot's rotation.

This sounds the wrong way round. The change should be gentler at the low volume end. If you use a linear pot as a volume control, the low volume end will seem too abrupt with a small rotation causing too much change in volume.

Many (most?) so-called log pots are actually nothing of the kind. Their track is in 2 sections, one with high resistance, the other with low and a fudged join between them.

Neil Purling · 18th Dec 2006, 11:14 am

I stated what I saw on my AVO. I would expect a dramatic difference in the way the pot behaved if connected with the top and bottom wires switched of course.
Anyone here tried a linear pot as a volume control, perhaps because it was all you had at the time?

Why on earth are the logrithmic pots rated half that of the same type with a linear track?

ppppenguin · 18th Dec 2006, 11:19 am

Quote:

Originally Posted by Neil Purling

Why on earth are the logrithmic pots rated half that of the same type with a linear track?

Because half the track is disspating almost all the power. This is evident if you think of the common arrangement where the track is in 2 sections, one of which has high resistance and disspates most of the power.

Tom_I · 18th Dec 2006, 12:04 pm

This has been posted before on this forum, but here's a handy page on tapering linear pots if you can't find a suitable log one:

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

Tom

13th Dec 2006, 8:51 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Potentiometers, Log and Lin. If a pot is described as having a Logrithmic track (i.e. a volume control), is the track symmetrical? Are both ends the same if you wire the thing the other way up?

13th Dec 2006, 9:53 pm	#3
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Potentiometers Stupid mistake made upon re-assembly. Reaction was strangely 'knife edge', and right at the bottom ende of the pot. For reaction I suspect one would be better with a Linear pot. The reaction pot is a wire-wound example, from a forum member as it happens. I was unaware of what sort of track it had, I just assumed that it was linear.

13th Dec 2006, 11:44 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Potentiometers Try wiring it the other way round, though this will involve turning the control the opposite way to operate it. Paul

14th Dec 2006, 8:57 am	#5
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Potentiometers The fact I had to turn the pot the other way made me realise the thing was probably wired the wrong way round.

14th Dec 2006, 8:12 pm	#6
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Potentiometers I know the present 1 Leg log volume pot is suspect, and I am looking for a replacement. I have a 1 Meg Linear pot in my spares box. What'd be the audible difference to the ear? I have tried ebay for Old Stock 1 Meg Log pots, but they ended at very high sums. Maybe they are in demand for restorers of vintage amplifiers?

14th Dec 2006, 8:48 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Potentiometers Try it. Log pots are normally used for volume controls because the log track gives a subjectively evenly spaced volume scale. THere are no hard and fast rules though. You can tweak the characteristics of the control by adding fixed resistors between the slider and the bottom or top of the track. Have a play around Paul

18th Dec 2006, 10:12 am	#8
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Potentiometers I just stuck my AVO on a spare logrithmic pot and the resistance changes at a extreme rate in the first quarter of the pot's rotation. Yet the effect seems linear to the ear. I know I have re-stated the contribution of Mr Sherwin. Would it be right to say that substitution of a Linear pot for a logrithmic pot in a volume would have the effect that volume would seem to increase greatest over a small part of the pot's travel? You know, idle curiosity means I might just try it. The 1 Meg Linear pot I have seems to be of a better rating.

18th Dec 2006, 11:14 am	#10
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Potentiometers I stated what I saw on my AVO. I would expect a dramatic difference in the way the pot behaved if connected with the top and bottom wires switched of course. Anyone here tried a linear pot as a volume control, perhaps because it was all you had at the time? Why on earth are the logrithmic pots rated half that of the same type with a linear track?

18th Dec 2006, 12:04 pm	#12
Tom_I Heptode Join Date: Jul 2006 Location: North Herts. UK. Posts: 549	Re: Potentiometers This has been posted before on this forum, but here's a handy page on tapering linear pots if you can't find a suitable log one: http://www.geofex.com/Article_Folder...s/potscret.htm Tom