Linear pot is default?

Thread Starter

upand_at_them

Joined May 15, 2010
940
The June 2015 issue of Silicon Chip has an "infrasound snooper" project that allows you to hear sounds below 20Hz by frequency shifting.

Neither the article, the schematic, nor the parts list says whether the two pots used should be linear or logarithmic.

One pot controls the gain of the input signal. The other pot controls the output volume (to headphones).

Should we assume linear pots unless otherwise specified? Or should we assume logarithmic because audio is involved?
 

dl324

Joined Mar 30, 2015
16,846
I would assume linear taper unless specified otherwise. Audio circuits use linear tape for faders, so it isn't safe to assume audio taper because audio is involved.
 

crutschow

Joined Mar 14, 2008
34,285
The pot that controls the input signal can be linear or logarithmic but the pot that controls the audio output volume should be logarithmic (audio taper).
An audio taper pot matches the logarithmic response of the ear so that the change in loudness is perceived to be linear with respect to pot rotation.
A linear pot will cause the apparent loudness to change very rapidly at the start of the rotation, with only a small apparent change after that (only 6dB change from the 50% point to the 100% point, for example, whereas an audio pot will have about a 14dB change).
 
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Thread Starter

upand_at_them

Joined May 15, 2010
940
The Silicon Chip folks replied:

That's true [that the volume pot is usually logarithmic], however in this case because the pot is unbuffered and the load resistance is between the wiper and one end of the pot, it has a strange control law anyway and a logarithmic taper wouldn't have much effect.

Basically, it's a pretty crude method of volume control but works well enough for this project. You simply need a way to limit the maximum volume so it isn't painful.
Isn't the load resistance (headphones) always between the wiper and one end of the pot?
 

crutschow

Joined Mar 14, 2008
34,285
Normally a volume control pot is at the input to the headphone amplifier, not the output to the headphones.
Putting it at the output is a crude way to control the level.
 

ian field

Joined Oct 27, 2012
6,536
The June 2015 issue of Silicon Chip has an "infrasound snooper" project that allows you to hear sounds below 20Hz by frequency shifting.

Neither the article, the schematic, nor the parts list says whether the two pots used should be linear or logarithmic.

One pot controls the gain of the input signal. The other pot controls the output volume (to headphones).

Should we assume linear pots unless otherwise specified? Or should we assume logarithmic because audio is involved?
Whichever pot you decide is the volume control, should be a log pot.

I'd try a lin for the other one and change it if there's a problem with it.
 

crutschow

Joined Mar 14, 2008
34,285
Whichever pot you decide is the volume control, should be a log pot.

I'd try a lin for the other one and change it if there's a problem with it.
It there is a significant load on the pot (significantly lower impedance then the pot value such as with a headphone load) then a linear pot will have a quasi-log output versus pot rotation. So you don't need a log or audio-taper pot under those conditions.
(See this for a writeup I did on a pot with a load).
 

ian field

Joined Oct 27, 2012
6,536
It there is a significant load on the pot (significantly lower impedance then the pot value such as with a headphone load) then a linear pot will have a quasi-log output versus pot rotation. So you don't need a log or audio-taper pot under those conditions.
(See this for a writeup I did on a pot with a load).
I just assumed it would be a normal application.

If the TS had posted the schematic, there'd have been more to go on.
 

ian field

Joined Oct 27, 2012
6,536
Then that would give a non-linear voltage level output with respect to pot rotation, depending upon the headphone impedance.
Well I would imagine that depends on the headphones impedance - the 2k - 4k variety are pretty much valuable collectors items now, probably won't be higher than 600R by any stretch of the imagination, telephone earpieces are often somewhere around 140R and the Walkman style ear buds are sometimes as high as 16 - 32R.

The old pocket radio earphones could be anywhere between 3 - 8R, apart from a couple I managed to stash in the spares box - I haven't seen any of those since my school days.

Last time I checked, Maplin still had crystal earpieces (expensive!) - that wouldn't load the pot at all, you can actually hear faint mains hum if you put it in your ear and touch just one of the two wires - and the Maplin one is less sensitive than the one that came with a Philips Electronic Engineer kit.

The TS could always stick an emitter follower after a log-pot.
 
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