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n00b question about input and output of a guitar effects box

Discussion in 'General Electronics Discussion' started by dietermoreno, Dec 30, 2012.

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  1. dietermoreno

    dietermoreno

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    Dec 30, 2012
    N00b question about input and output of a guitar effects box:

    I have encountered a band which at a show attempted to connect the output jack of an effects box to the guitar and then connect the input jack of the effects box to the guitar amp.

    It sounded terrible.

    I think there is a good reason why input and output are not interchangeable, but I am not quite sure why, and I found the Guitar World forums to be full of people who don't care about electrical engineering and want the retard explanation, so I came to these forums for help. (Here is an example of a thread on Guitar World forums where the forums members were spamming asking for retard explanation asking what an ohm is:http://www.ultimate-guitar.com/forum/showthread.php?t=1571081&page=1&pp=20).

    What is the electrical engineering reason why input and output jack of a guitar effects box can not be mixed and matched incorrectly, only one way works?

    Does it have to do with impedence matching to block feedback from the applied effect in the effects box from oscillating back to the guitar pickups in a continuous oscillation?
     
    Last edited: Dec 30, 2012
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi ya
    welcome to the forums :)

    no it has to do with the way the circuit is designed
    the signal will only flow correctly in one direction, as with all electronic circuits

    Dave
     
  3. dietermoreno

    dietermoreno

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    Dec 30, 2012
    What is the reason that all electronic circuits are designed to only flow correctly in one direction? (I do remember my physics teacher telling me that in a circuit analysis for DC the sign + or - does NOT matter as long as they are opposite of each other, but the current direction DOES MATTER, but that was for DC circuits, and the circuit in question is a guitar amp that uses AC.)

    Alternating current flows in two directions oscilating back and forth at a frequency.

    Does it have to do with transformers, like alternating current in an electric power transmission line does oscilate back and forth, but the order matters what side of the transformer you are on because one side has a higher voltage and one side has a lower voltage?
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    no, it has mainly to do with specific components that are designed so that the signal will only flow in one direction
    eg ... transistors, IC's, diodes etc

    think about it....
    You cannot connect your guitar to the speaker connections of an amplifier and expect to get a sound out of the input socket to the amplifier

    Dave
     
  5. dietermoreno

    dietermoreno

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    Dec 30, 2012

    For me to play devil's advocate, what if the circuit has no such devices that make the signal flow in only one direction, then wouldn't connecting to either input or output work?

    Do I have it correct that amplifiers whether they be vacuum tube, transistor, or IC, they all use DC for all 3 of those components, so that is the reason why no amplification occurs if I connect my guitar to the output jack of my guitar amp?

    What if the effects box has no components that only let current travel in one way and only uses oscillator circuits to amplitude modulate the guitar signal? Is the reason in that case why the input and output which one chose matters is because you do not want to apply the effect to the original guitar signal before the effect is supposed to be applied?
     
  6. davenn

    davenn Moderator

    13,833
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    Sep 5, 2009
    the few times where a circuit may be bi-directional ... without active devices
    ... filters ... mix of capacitors and inductors
    ... attenuators ... just resistor networks

    these circuits are quite lossy and need active devices before and after them to make up for the losses ... and that once again results in a directional circuit

    There will be a DC power supply, BUT the signal going through is AC, right from the input to the output and on to the speaker
    No, the reason it doesnt work connecting you guitar to the output is as already stated,
    the signal is guided from input to output through directional devices that amplify the signal in stages

    you are still not getting it :) ....
    if there are no active devices its not going to work as an effects box.
    it can only pass the signal with attenuation

    Dave
     
  7. BobK

    BobK

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    Jan 5, 2010
    Revesing the input and output of an electronic circuit makes as much sense as trying to water your lawn by forcing water into the tiny holes on the sprinkler head.

    Bob
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    Here is "Steve's Simplest Guitar effects pedal":

    [​IMG]

    It is a simple low pass filter. The signal source is normally connected to the input, and has an output impedance much lower than the resistor shown. This allows the resistor here to dominate, and the 3db point is determined by the resistor and the capacitor.

    If I connect it the wrong way around, there is no resistance other than the output impedance of by guitar in series with the capacitor. Ignoring the effect that may have on the output signal, this will change the 3db point to a much lower frequency.

    The effect of connecting this the wrong way may be to change the 3db point from 2000Hz to 100Hz. Other unintended effects will be the loading of the signal connected to it, possibly introducing distortion.

    If the device has amplifiers to buffer the input and output, the effect would be similar to singing into your speaker stack and pointing the microphone to the audience for them to listen to. (i.e. anything they hear is due to dumb luck)
     

    Attached Files:

  9. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Thanks guys! I think I get it now!

    So a passive device that is all AC using only inductors, capacitors, and resistors, is bi-directional.

    So a passive device that has DC in any point in time even if the input and output are AC is directional.

    So an active device is directional because it uses external power and the external power supply must be DC so as to not add harmonic distortion into the AC signal from the AC wave forms of the AC power supply.

    So a passive device that is all AC using only inductors, capacitors, and resistors is directional if a low pass filter is used because "low pass" implies that some current goes one way and some current goes another way.

    So tubes, transistors, and ICs must use DC when amplifying a circuit because (1) they only work with DC because "valve" implies current is flowing one way, and (2) AC power supply of amplifier even if possible would cause harmonic distortion from the AC wave forms of the AC power supply. So any circuit with tubes, transitors, and ICs is a directional circuit.
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    yes ... BUT the layout of the components may mean that the effect on the signal is NOT the same in either direction ... note that basic cct that Steve posted above
    one direction the resistor is encountered first, the other way the capacitor

    that doesnt make sense to me
    you can have passive devices in a DC only circuit... say a resistor and a light bulb
    the resistor will "restrict" the current through the light bulb. but it doesnt matter which way around the DC is applied as neither the resistor nor the light bulb (globe) are polarity conscious devices

    its directional because of its design ... eg a diode, current will ONLY flow 1 way

    No, Low Pass doesnt imply that, low pass means it passes lower frequencies as per a given design

    The DC is only use for the supply, the signal in is AC or at least a varying DC

    Dave
     
  11. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Okay, everything that you said makes sense, except I am still confused on one point that you said.

    You said that amplifiers use a DC power supply but are otherwise powered by AC or DC pulses?

    Can you be more specific? When you say "powered by AC", are you refering to the fact that the amplifer in question is a guitar amp and guitar pickups are generating AC?

    Do you mean that the tube or transitor or IC uses a DC power supply to pass the current in one direction through the amplifing component, and the DC current is connected to the AC signal generated by the guitar pickups, so that what passes through the amplifing component when no guitar signal is generated is continous DC, and what passes through the amplifing component when a guitar signal is generated is DC pulses, and each amplifing component has a duplicate wired perfectly out of phase by 90 degrees so that one component will amplify DC pulses for the positive waveform and the other component will amplify DC pulses for the negative waveform and then the DC pulses will be combined back together into AC after the DC pulses have been amplified?

    I'll talk about a tube for simplicity. With a tube amp, does the thermionic effect only work with a DC signal? With a tube amp, is the DC power supply what powers the tube heating element just like in a light bulb (Edison got the idea after observing what part of a light bulb burnt out first and his light bulbs were of course using DC), and the DC power supply IS NOT THE SIGNAL, rather the pulses of DC that the AC signal has been converted to 90 degrees out of phase with a pair of tubes using diodes that passes from anode to cathode through the grid heated by the filament IS THE SIGNAL?



    I have a new related question: say I want to combine guitar effects pedals to be all in the circuit at the same time, a distortion pedal and a hi-pass filter, what is the correct way to do it?

    Could the pedals be wired in series, or would they need to be wired in parallel using a mixer and a splitter?
     
    Last edited: Jan 1, 2013
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    I didnt say anything about powered by AC or DC pulses...
    were you referring to this ?...

    Quote you:
    So tubes, transistors, and ICs must use DC when amplifying a circuit because (1) they only work with DC because "valve" implies current is flowing one way, and (2) AC power supply of amplifier even if possible would cause harmonic distortion from the AC wave forms of the AC power supply. So any circuit with tubes, transitors, and ICs is a directional circuit.

    my answer...
    The DC is only use for the supply, the signal in is AC or at least a varying DC


    .... the signal may be AC, varying DC or even DC pulses as in a logic type cct

    again emphasis on the word SIGNAL


    It really sounds like you need to do some reading on basic transistor theory

    work your way through these tutorials also available via the Tutorials drop down menu near the top of this page



    lets just stick with transistor theory for a start
    have a look through the tutorial I linked to :)


    havent tried that on my system .... ask a guitarist who uses lots of effects pedals ;)


    Dave
     
  13. Rusty

    Rusty

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    Nov 30, 2012
    Why does a drop of water fall downwards and not upwards?
    Its science. Things are made with special reasons and purposes.

    Guitar pedals are usually always wired in series, like most electronic amplifiers. If you put a filter before a distortion pedal, you will distort/amplify the formed/filtered signal.
    If you put a filter after the distortion, you will distort/amplify the original guitar signal and then filter it.

    Whats the best way to do, is really up to you. There is no 100% correct answer for this. Most distortion pedals already have filters inside the circuit. Boosters/overdrives that are used to boost the signal and add a little midbump are often placed before the amplifier/main distortion. While filters and graphic EQ's are placed in a loop between the preamp and poweramp.

    My advice to you is to read about transistors, op amps, buffers and filters. Learn the basics, study simple guitar circuits and learn from that. Learn the difference between signal path, biasing and compansating for noise, DC-blockage, LEDS ++

    Placement of a filter in the circuit really depends on what you want to amplify. If you want to amplify the original guitar sound, or the modified signal with delays, reverbs and other effects.

    Usually a guitar circuit contains lots of gain stages and lots of filters with different parameters for each use/purpose.

    Simple Example:
    1. Guitar with filter => Boost with filter => Overdrived preamp with EQ ( EQ = filters) => delay/reverb/wah with filters and amplifiers => Power Amp => Speaker Cabinet.

    2. Guitar with filter => Distortion with filter => clean preamp with EQ ( EQ = filters) => delay/reverb/wah with filters and amplifiers => Power Amp => Speaker Cabinet.

    Here are some simple guitar circuits you can study:

    http://diy.musikding.de/images/stories/plusv2/plusschalt.pdf
    http://diy.musikding.de/images/stories/birdie/birdieschalt.pdf

    Those are really simple. Most pedals contains more than one gain stage.
     
    Last edited: Jan 2, 2013
  14. dietermoreno

    dietermoreno

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    Dec 30, 2012
    So I have read some tutorials on your forum for AC, the definition of electronics, biasing, transistor theory, and tube theory.

    Oh okay, so the reason that not even a passive AC circuit will work the wrong way is because THE WIRE HAS ITS OWN IMPEDENCE IN ADDITION TO THE IMPEDENCE OF THE RESISTOR.

    So the reason that an active AC circuit (only analog circuits considered for simplicity) is directional is because tubes and transistors use DC biasing to use a DC control voltage to be directly proportional to the voltage outputed of the tube or transistor provided by the power supply of the tube or transistor since energy can neither be created no destroyed.

    So the direction of the circuit analysis for a tube or transistor is:

    quiet signal AC (lower voltage)
    V
    tube or transistor <--bias DC
    <--power supply DC
    V
    louder signal AC (higher voltage)





    The simple guitar circuits will be penetrable by me if I can understand how to "fix" Ohm's Law so it works for AC since every guitar circuit uses AC at some point.

    So after I have read some tutorials on your forum for AC, is it correct that Ohm's Law works for AC if the voltages are first converted into the rectangular form of a complex number?

    So if my guitar amp is 15 watts and the output to its speakers is 16 ohms, how much current in amps does it uses?

    If we assumed that my guitar amp used DC, then P=IV and I=P/V and V=IR and P=(I squared)(R) and (I squared)=P/R and I= square root of (P/R) so I=square root of (15 watts/16 ohms) so I= square root of (15/16) amps.

    but my guitar amp doesn't use DC...

    Does Ohm's Law work if RMS values are used?

    Is the 15 watts printed on the guitar amp by the manufacture an RMS value or a peak value?

    Is the 16 ohms printed on the back of my guitar amp where the speakers are connected to is that an RMS value or a peak value?

    So are complex numbers not needed if RMS or peak values are selected?

    If complex numbers are not needed then why did you devote an entire chapter of tutorials to complex numbers?
     
    Last edited: Feb 11, 2013
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Ohm's law works regardless of the waveform of the signal as long as your impedances are resistive. You will need complex numbers only if the impedance has a reactive part (capacitor or inductor). In that case, however, you may represent the complex number in any mathematically valid form you like, not necessarily rectangular. E.g. polar representation is also useful in some applications.

    A useful simplification is the concept of RMS value (Root Mean Square). Regardless of the waveform of a signal, the RMS value is the value that an equivalent DC source would need to develop the same power.

    Therefore: P=Irms^2*Rload=Vrms^2/Rload

    That should be stated in the amp's datasheet/specification. If it were a Hifi component, you could assume it's the peak power at best (Hifi manufacturers like to boast with BIG numbers). I don't know whether that's a valid argument for a guitar amp.

    This question doesn't make sense. A resistance is (in first approximation) a fixed value and has no RMS or peak value. Typically a speaker's resistance is a nominal value that you can use for simple calculations just like an ohmic resistor.
    The truth is more complex (pun intended): A speaker has a complex impedance that varies with frequency. The complex impedance is a result of the construction combiningh electrical parameters (e.g. inductive coil to drive the speaker's membrane) and mechanical parameters (e.g. stiffness of the membrane, resonances of the speaker).
    For many purposes you can just insert the 16 Ohm into the above equation for power and treat the speaker as a real impedance (resistive, not complex).
    There are other instances (e.g. a cross over network for a multi-way speaker box) where you need to take into account the frequency response of the speaker's impedance.
     
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