Connect with us

Just When I Thought it was Going to Be Easy

Discussion in 'General Electronics Discussion' started by SparkyCal, Jul 25, 2021.

Scroll to continue with content
  1. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    As many of you know, I successfully built an overdrive pedal using the 4558D double op amp IC, with the help of the kind folks here.

    My journey can be found on this thread: https://www.electronicspoint.com/forums/threads/back-to-the-breadboard.295296/

    The schematics I used can be found here:
    https://www.electronicspoint.com/forums/threads/back-to-the-breadboard.295296/page-4

    and here:
    https://www.wamplerpedals.com/blog/...ow-to-design-a-basic-overdrive-pedal-circuit/

    So, I thought, since I built this successfully with a dual op amp, why not try it with a single op amp.

    I have a ton of the TL081CP op amps, which are single op amps. Datasheet here:

    https://www.ti.com/lit/ds/symlink/tl082h.pdf?ts=1627155765274&ref_url=https%3A%2F%2Fwww.google.com%2F

    So, I build the entire circuit, and the last part was to insert the op amp into the socket. I did so, but because my eyesight sucks, I accidentally put it in upside down. I only mention this because I am not sure if it caused the problem I am about to describe.

    I took the op amp out of the socket and threw it out. I inserted a new one, this time, correctly.

    However, I am getting the following Pin readings- and yes, I did remap the pins according to the pin mappings for a TL081CP op amps:

    These readings are in Volts:

    Pin1. 7.6
    Pin2. 4.0 and counting down
    Pin3. 4.5
    Pin4. 0
    Pin5. 1.1
    Pin6. 4.4
    Pin7. 9.02
    Pin8. 8.17

    I don't think these readings make sense. Could I have fried some components when I inserted the op amp into the socket the wrong way?

    Thank-you

    UPDATE: Forget it for now. I think I may have screwed up my wiring. I will update o once I sort this out.
     
    Last edited: Jul 25, 2021
  2. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    the TL 082 was my go to op amp for this sort of thing back in the day.
    Speced out similarly to the 072, trading off very little for some cost savings ... Mild bonus when you anticipate silicon to carbon conversion at every turn.
    every migration seemed to be graced by what you describe ... user error and pinout confusion.
    you have to embrace the sweet release of the magic smoke
     
  3. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    I have 50 of them, so it’s not a big deal.
     
  4. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    Well, that's a fair fist full of action really.
    I'm sure you have a rough understanding of how things function, particularly how the gain pot alters the ratio between it and the bias resistor to control the amount of distortion.
    With a parts mine like what you have there, you might try amending the NFB loop with a jfet connected to an envelope follower so the distortion can be controlled by pick attack.
    It might mimic tube amp dynamic behavior.
     
  5. Audioguru

    Audioguru

    3,382
    726
    Sep 24, 2016
    Input pin 2 is supposed to have the same voltage as the output pin 6 which feeds the voltage to it through the gain pot. Maybe the gain pot is missing a connection?
     
  6. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    Thanks guys. It turns out that my mistake is the result of two things:

    Soldering in the early hours of the morning when the brain is tired

    Cramming on too small a PCB

    I am doing a rebuild and will; report back.
     
  7. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    Thank-you but when you say:you might try amending the NFB loop with a jfet connected to an envelope follower so the distortion can be controlled by pick attack., it is like talking Mandarin to me. I need it is simpler terms ;-)
     
  8. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    drive knob replaced with sensitivity.threshold and limit knobs.
    these define a sort of virtual drive knob that changes with your pick attack.
     
  9. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    When you say replace the drive knob with sensitivity threshold and limit knobs, which is is the drive knob? the 500K one? and are these sensitivity threshold and limits knobs, special kind of knobs?
     
  10. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    You said you needed simple terms, so I over simplified it. so we could focus on operational result .... dynamic overdrive.
    your 500K can be replaced by a JFET type transistor which will do the same thing, but instead of changing value with a knob twist, it'll change with a voltage level.
    Now ... if that voltage is determined by the strength of the input signal, your playing style influences the amount of distortion.
    To get that control voltage, we build an envelope follower.
    Basically ... I'm doing you the favor of getting you to transition into circuit design
     
  11. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    Don't get me wrong. I appreciate the advice and prompting. When I ask for clarity, it is truly because I don't understand what is being suggested.
     
  12. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    This is strange

    I cannot get this circuit to work with the single op amp I am using.

    On the op amp, the following is written:

    19ALE3M
    TL081CP

    From this, I assume PIN 2 is the - IN, Pin 6 is the Out, Pin 3 is the + IN , Pin 7 is the VCC in and Pin 4 is ground.

    Yet, I have built this many times and it does not work; Further, I get strange Pin readings. Here they are in Volts:

    Pin1. .445
    Pin2. .096
    Pin3. 8.15
    Pin4. .454
    Pin5. .123
    Pin6. .096
    Pin7. .517
    Pin8. 0

    Obviously, something is very wrong. I am wondering if I have an op amp at all, or if the PIN mappings are something other than what I think they are. But I looked up the Pin mappings, and what I stated above, is what I found.

    Any advice?

    I even attached a pic of the op amp
     

    Attached Files:

  13. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    Pin 7 is your + power Pin 4 is - power pin 8 is correct, because it's basically there for decoration
    Given your readings, Your just crossed up on your pin connections
     
  14. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    I honestly don't know how I keep screwing this up. I think you;'re right though. I'll report back once I have had a chance to give my head a shake
     
  15. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    So, my eyesight needs a check. I again plugged the op amp in to the socket upside down. I threw it out and put a new one in correctly. Still does not work. Here are my new readings:

    Pin1. 6.68
    Pin2. 1.2 and falling
    Pin3. 3.68
    Pin4. 0
    Pin5. 0
    Pin6. .1.52 and falling
    Pin7. 8.12
    Pin8. 7.28

    Could it be that I damaged a component when the op amp was in the socket upside down?
     
  16. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    You sure you didn't just get decaf by mistake?
    Pin 8 cannot be anything but 0 ... per data sheet, it's has no internal connection
     
  17. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    When I first did this, it was for 2 reasons:

    1. I have about 100 of these op amps.
    2. I Figured, if I can now build a dual op amp pedal (thanks to you folks- especially AG, Bertus and Harald- trust me, they have been coaching me for longer than I care to admit;-)) , then it should be simple building it with a single op amp, as I won't have to worry about buffering the second op amp (less connections)

    So....it is not turning g out as such. I think I may have to give myself a break from this particular build and try it again once I have regained my composure toward it.

    BTW VB, I also very much appreciate your help and guidance.
     
  18. VenomBallistics

    VenomBallistics

    283
    56
    Aug 30, 2018
    ran into the same issue with different parts back in the day.
    It was a quad op amp version of the 741. It could be that my data sheets sucked. Ultimately I just migrated to building tube amps because I beat the op amp, hex inverter, and transistor fuzz distortions to death.
    Due to the quantity of stock you have, it's probably worth figuring out how to use your 081's though.
    Probably a unity gain test circuit to remove any and all variables will get you using them right
     
  19. SparkyCal

    SparkyCal

    831
    30
    Mar 11, 2020
    @VB. You're right. As I own about a hundred of these op Amos, I would like to find out how they work. You referred to a unity gain test circuit. I have no idea what that is, but I would be very interested in building one, so I can get to the bottom of this. Is there a schematic you could direct me to?

    Thank-you
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-