Linear pot is default?

Discussion in 'General Electronics Chat' started by upand_at_them, Jun 19, 2015.

  1. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    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?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    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.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    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).
     
    Last edited: Jun 19, 2015
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  4. DickCappels

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    Aug 21, 2008
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    Either will work but you would find an arrangement per crutschow's recommendations to feel more natural.
     
  5. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    Thanks, guys.
     
  6. upand_at_them

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    The Silicon Chip folks replied:

    Isn't the load resistance (headphones) always between the wiper and one end of the pot?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    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.
     
  8. ian field

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    Oct 27, 2012
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    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.
     
  9. crutschow

    Expert

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    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).
     
  10. ian field

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    I just assumed it would be a normal application.

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

    Thread Starter Active Member

    May 15, 2010
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    It's from a magazine article; I wasn't sure if I'd violate the copyright.
     
  12. ian field

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    Oct 27, 2012
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    You could freehand sketch the circuit around the pots and scan it.

    There's something about reproducing a whole work - but the whole schematic might count as that!
     
  13. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    Output:

    6-22-2015 5-15-18 PM.png
     
  14. crutschow

    Expert

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    Is the OUTPUT connected to the headphones?
     
  15. upand_at_them

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    Yes. In the schematic it's just a headphone jack.
     
  16. crutschow

    Expert

    Mar 14, 2008
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    Then that would give a non-linear voltage level output with respect to pot rotation, depending upon the headphone impedance.
     
  17. ian field

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    Oct 27, 2012
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    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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