+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Advanced Member level 1
    Points: 1,621, Level: 9

    Join Date
    Aug 2016
    Posts
    444
    Helped
    3 / 3
    Points
    1,621
    Level
    9

    high voltage to audio

    I have a signal which can be from 40-90v and the power audio amp takes up to 5v signals.

    It seems logarythmically scaling the high voltage signal is the ideal way to get it to the amp.

    There are very expensive high voltage and high power oamps out there but they are huge and very expensive, whats a minimalistic method to get the signal logarythmically scaled?

    Perhaps there is a transistor logarythmic scaling circuit but Im not sure which is ideal.

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 74,732, Level: 66
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,164
    Helped
    3450 / 3450
    Points
    74,732
    Level
    66

    Re: high voltage to audio

    Hi,

    For audio one uses linear voltage dividers (resistive voltage divider). If you use something non-linear like logarithmic you get lots of distortion.

    Maybe you mix the phrase with "logarithmic audio volume control potentiometers".
    With these pots
    * not the audio_input to audio_output is logarithmic but
    * the moving_angle to gain is logarithmic.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 1
    Points: 1,621, Level: 9

    Join Date
    Aug 2016
    Posts
    444
    Helped
    3 / 3
    Points
    1,621
    Level
    9

    Re: high voltage to audio

    Quote Originally Posted by KlausST View Post
    Hi,

    For audio one uses linear voltage dividers (resistive voltage divider). If you use something non-linear like logarithmic you get lots of distortion.

    Maybe you mix the phrase with "logarithmic audio volume control potentiometers".
    With these pots
    * not the audio_input to audio_output is logarithmic but
    * the moving_angle to gain is logarithmic.

    Klaus
    Wouldnt linearly scaling the signal ommit much of the fine details were as log scaling would preserve them?

    This might be intended for other signal types aside from audio.



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 74,732, Level: 66
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,164
    Helped
    3450 / 3450
    Points
    74,732
    Level
    66

    Re: high voltage to audio

    Hi,

    You could simply use Excel to simulate this.

    Let's imagine a sine waveform input.
    Now if you use V_out = A × log(V_in)
    You will see that there is a problem with input voltages <1

    You may choose another mathematical nonlinear formula.

    Maybe square_root.
    It will work with positive input voltages only, thus you need to solve this problem, maybe this way:
    V_out = sqrt(|V_in|) × V_in / |V_in|

    If you simulate this you see that the sine shape becomes more like a square wave. This means overtones = distortions, THD.

    What you need is some kind of "audio compressor".

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

  5. #5
    Advanced Member level 5
    Points: 16,348, Level: 30

    Join Date
    Nov 2012
    Posts
    2,993
    Helped
    727 / 727
    Points
    16,348
    Level
    30

    Re: high voltage to audio

    Now if you use V_out = A × log(V_in)
    This is a common problem; we use the conventional db (decibel for these messy problems) in such messy cases.

    If the input voltages is less than 1 (V, I presume), you express the voltage in uV - but then other problems (like negative voltages) will certainly give headaches.

    The problem is that log function argument must be a pure number and hence V or A are not proper arguments. We often use a ratio for this: log (test_V/ref_V)

    The problem of negative voltages still do not go away and we use power (depends on the voltage squared) but it cannot be negative - helps certainly.

    But in the log scale 0 to 1 is given the same space as 1 to infinity. So it is inherently more useful in the 1 to infinity range.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  6. #6
    Super Moderator
    Points: 256,166, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,684
    Helped
    13593 / 13593
    Points
    256,166
    Level
    100

    Re: high voltage to audio

    You should really tell more about the signal you want to process. How is the problem related to audio? I doubt that "logarithmic scaling" can be usefully applied to audio signals. The nonlinear characteristic will severely distort it. A pure log characteristic doesn't work at all, because it's undefined for negative half waves. You need at least a symmetrical completion of log function like y = sign(x)*log(abs(x)). If the nature of audio signal should be maintained, an envelope manipulation like a compressor function may be more appropriate.


    1 members found this post helpful.

--[[ ]]--