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Need help interpreting something on a schematic

Discussion in 'General Electronics Discussion' started by SparkyCal, Jul 30, 2020.

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  1. SparkyCal

    SparkyCal

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    Mar 11, 2020
    I wonder if you could help me with a few things on the attached schematic:

    1. There is the word "Drive" right below the 150k text. Is he simply saying that a 150K resistor needs to be placed there? I am confused because having that arrow pointing down makes it look like a potentiometer. Can someone help me interpret what is going on in that area?

    2. I see where the Output leaves and attached to the jack, but where does the input pin attach to? In other words, where does the pin coming from the guitar cable connect to?

    3. This circuit calls for a LM308 op amp. Provided I make allowances for any variation in pin mappings, can I use an alternative op amp? I don't have LM308s
     

    Attached Files:

  2. Bluejets

    Bluejets

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    Oct 5, 2014
    1. 150k pot
    2. Input..... (input) positive pin LM308
    3. No idea...depends on the application(no details of input other than "guitar" provided)
     
  3. AnalogKid

    AnalogKid

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    Jun 10, 2015
    This is an overdrive distortion circuit. It drives a linear amplifier into clipping intentionally, a form of a "fuzz box".

    As above, the 150K part is a pot. It adjusts the amount of overdrive distortion.

    The + input must have a resistor to GND to establish the operating point of the opamp circuit. If the circuit is powered by two batteries or a +/- power supply, then something simple like a 100K resistor to GND is fine. If the circuit is powered by a single voltage such as one 9 V battery, then you want two 100K resistors, one to +9 V and one to GND. In either case, add a 10 uF coupling capacitor between the + input pin and the input connector.

    Note that the Tone control will affect the output volume level also.

    Guitar effects people *love* the "sound" or "color" of the older opamp designs, and the 308 is a very old part. Something with a similar sound would be the LM741. The 741 has internal compensation, so you won't need the 30 pF capacitor. Newer parts with greater bandwidth still will overdrive, but will have a more harsh sound.

    ak
     
  4. SparkyCal

    SparkyCal

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    Mar 11, 2020
    Thanks to you both for the grest insights. I initially thought this would be a very simple circuit to create, as it dors not look like there is much to it. I'm not sure I will tackle this one. It sounds as thoiught there are things missing frommit that you have graciously pointed out. being a beginner, I would not be comfortable doing this one. Thanks for the info. It helped me better understand this.
     
  5. Audioguru

    Audioguru

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    Sep 24, 2016
    Another old circuit using an obsolete opamp. It produces the same distortion as all the other overdrive circuits you posted.
    The opamp is not powered so it will not work. It also will not work if it is powered by one 9V battery and the guitar pickup grounds its input. The circuit assumes that the opamp has a positive and negative supply. It can have its input biased like all your other circuits so it can be powered by one 9V battery.
    The designer has TWO series RC strings in parallel that does not make sense.

    The antique LM308 has a high input resistance that an electric guitar needs, an antique 741 opamp has a low input resistance that will muffle the guitar sound. A TL081 or TL071 opamp can be used.
     
  6. SparkyCal

    SparkyCal

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    Mar 11, 2020
    Thanks Audioguru. I have a ton of 4558Ds and was lookign for a use for them. i also read this on this web page.https://www.neatcircuits.com/doctor_pedal.htmhttps://www.neatcircuits.com/op_amp.htm

    The 4558 opamp is probably the most popular audio opamp in most pedals. Particularly the JRC4558 device. All 4558 devices are the same from manufacturer to manufacturer but some claim the JRC4558 has a more "desired" sound. I have not witnessed that but will go along with this tale for the time being. The 4558 consists of two independent operational amplifiers with common power pins. Pin 8 is (+) power and pin 4 is (-) power. Other dual opamps can and will work in this type of pedal. A MC1458, TL072, TL082, LM318, LF356, CA3140, etc.

    [​IMG][​IMG]

    The nice thing about these amplifier chips is that they can be programmed to provide the desired gain and don't require any external frequency compensation. They have a very high input impedance and a very low output impedance. Meaning they will not load a preceding stage and can provide a lot of drive for the next stage. If you are confused or want to learn more about opamps, study my Basic Opamp Design page.


    Having said that, i don't doubt your advice. I am just in a learning mode and just trying to make circuits work and learn from my failures.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Reworking the Proco circuit for a 4558 is not at all difficult. What is your intended power source(s)?

    ak
     
  8. SparkyCal

    SparkyCal

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    Mar 11, 2020
    Well, I thought I was using one 9v battery, but I assume I can't
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    The 4558 is popular in pedal circuits because it old but is better than the very old 741 or dual 1458.
    Any opamp uses two resistors (or a pot) to set its gain.
    Opamps designed 60 years ago (LM308 in your other thread) needed an external frequency compensation capacitor because in those days they could not make one in the IC.
    Most opamps have a very high input impedance and some have extremely high.
    All opamps have a very low output impedance.

    ANY opamp can be powered from a single positive supply if its + input is biased at half the supply voltage and input, output and feedback to ground coupling capacitors are used like in all your other pedal circuits.

    This circuit will make the same distortion as all the other pedal circuits you posted. The diodes make the distortion.
     
  10. SparkyCal

    SparkyCal

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    Mar 11, 2020
    Thanks Audioguru. For the moment, all the projects i tackle, are just excersizes for me to build my knowledge level and skill. I've come a long way. although it may not be apparent ;-)

    as an aside, i am a performing musician. When I play on stage, I hav a small pedal board which consists of:

    Boss tuner pedal, Ernie Ball Volume pedal, Fulsome OCD , Boss Chorus and sometimes. Boss Digital Delay.

    I'm not a big fan of huge pedal boards, nor do I like the integrated units (BOSS ME 80, Helix and the like). I find those units too programmable, sucking the life out of spontaneous performance.

    I think there is value in keeping it simple and let the playing do the talking.

    I don't see myself ever using the pedals i am trying to build, on stage. (however, who knows?). For the moment, I need true and tried solutions and the Boss stuff and Fultlone OCD sound very good. When using the 18 volt option, the Fulltone OCD is unbeatable. in my opinion
     
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