Op Amp choice?

Discussion in 'General Electronics Chat' started by Tonyr1084, Mar 9, 2016.

  1. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    I have on hand some various operational amplifiers. LM324N, LM339N, LM358N & LM386N. I don't need a quad op amp, so the 339 is probably out. Same of the 324. The 358 is a dual op amp, and I may use both of them, but all I really need right now is the single low voltage audio power Op Amp, the 386.

    Here's my goal: To strobe an LED from a pre-amplified audio source. The LED will actually be a chase light circuit I've built before, but instead of simply chasing lights across from end to end I want them to strobe to music. Frequency ranges from 30 Hz to 20,000 Hz. I've done this before using the output of one of the speaker leads, but that means I have to control voltage by turning up the volume (or down) to achieve the level of light display I want. I'd rather do it from the pre-amp (before volume control). That way I can run the speakers without regard for how bright the LED's get. Setting the voltage via a divider network will probably solve that part (Lighting the LED's to the brightness I want) but I have to use an op amp to achieve this.

    What would you recommend? I haven't decided on A or B yet, but this is where I'm going with this. Am I on the right track?

    single supply Op Amp.png
     
    Last edited: Mar 9, 2016
  2. mcasale

    Member

    Jul 18, 2011
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    What does the signal at the input look like (amplitude, freq., etc.)?

    What are you driving with the op amp outputs?
     
  3. #12

    Expert

    Nov 30, 2010
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    For low noise audio, I have a woodie for the TL-071, but you don't have a noise problem and humans can't see at 20,000 Hz. That pretty much puts you in the range of, "anything that will work with the voltage I want". After the pre-amp is called, "line level". About 1 volt. This is a non-problem.
     
  4. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    The input is an audio input. Just the pre-amp signal from a stereo (or another similar audio source). Frequency ranges from 30 Hz to 20 kHz. The amplitude (not measured yet) may be in the milli-volts range. The output is driving a chase light (LED's)(one at a time).
     
    Last edited: Mar 9, 2016
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    B.

    Note that a 386 is not a normal opamp. You do not bias the inputs to Vcc/2 and you do not set the gain with a traditional negative feedback loop.

    Separate from that, you will get better sound-to-light performance if you full wave rectify the audio before using it to control an LED current driver. That is assuming you don't want the LEDs on at half-brightness when the audio is silent. Do you?

    ak
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    Also note the the LM339 is a comparator, not an opamp.
    It will oscillate if you try to use it as an amplifier with negative feedback.
     
  7. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    @AnalogKid: Thank you. I have noticed (from web drawings) that the 386 is not wired as I've shown, and am considering that more fully. However, regarding the LEDs, it's precisely the effect I want (not rectifying the signal. I know rectifying the signal will modulate the LED based solely on amplitude. What I want is a dual effect; amplitude AND signal (frequency). By strobing across a series of LED's with both amplitude AND frequency at certain points (based on the strobe rate) you get blank spots. Change the frequency and you change the size of the blank spots. What also goes on is that the blank spots can move left or right - dependent on the frequency and strobe rate. With audio you can even get spots moving in both directions at the same time (when multiple signals are present).

    About 35 years ago a friend and I were fooling around with LED's. Mike (that's his name) hooked an LED to an audio source (not pre-amp'd) and noticed it flashed to the amplitude. However, when he moved it across his vision he noticed it also strobed. So he and I got the idea to mount an LED on an old album (record) and mount it to a motor. We created some brushes and then gave it a spin. We LOVED the effect. So we built several machines. The one thing we did not like was the noise from the motors and brushes. So for a number of years I have toyed with the idea of doing a chase light instead of a motor and an LED. Now that I have time to pursue this silly idea I want to give it a go.

    One other project that may also depend on an LED is using a laser pointer modulated from an audio source. Shine that against a mirror mounted on a motor with a slight angle to it and you can cast a circular beam on the wall. Modulate it and you'll get the same effect.

    If I rectify the incoming signal then all I get is a circle that flashes to the music. Not much different from the old "Color Organ's" we used to build in high school electronics class, using christmas lights.

    @crutschow: Thanks for the 339 comment. I'll keep the oscillation point in mind for when I need an oscillator.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    I don't know the specs of all the amps you listed but I do know about the 358. One useful feature it has is sensing (and output) near the lower power rail. So it may be useful for low-level audio signals. The comparator can also sense very near ground.

    But you may want to use +/- power anyway, with the audio signal biased between the two power rails.
     
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