Do guitar pedals draw guitar signal when there is no amp connected at their output?

Discussion in 'The Projects Forum' started by icydash, Jun 30, 2011.

  1. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    Ok so this is kind of an odd question, but here it goes:

    I'm trying to do something like this:
    [​IMG]

    So basically, my guitar signal is being routed both into the input of a guitar pedal (in series with a few other guitar pedals), and it's also being routed to the amp. It's hard to explain the reason I'm doing it this way, but trust me, this is what I have to do for a project.

    Anyway, notice that the guitar pedals aren't connected to an amp at the output; they are left floating. Will the guitar signal that IS being routed to the amp be diminished in any noticeable way? Since the guitar pedals are "open" in terms of the guitar signal, do they act just like any other open circuit and have no effect on the signal? Or, will the op-amps and other things in the pedals cause some of my guitar signal to be drawn away / distort it somehow?

    If the pedals will cause distortion or signal to be drawn away, how much distortion or signal will be drawn away (and why)?

    Thanks!
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If I understand you; you have the output from a pedal, that is split into two signal paths.

    One path goes to something, the other is just left unplugged?

    If that is the case, the unplugged wire should have no effect.

    But if the output of one pedal is connected to two inputs, it *might* cause some signal loss which will typically make it a little more quiet and slightly less treble. Generally pedals are designed to have an output that is ok to drive a couple of inputs, as people connect them in lots of weird ways.
     
  3. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    Kind of. I have an output straight from the guitar (or a pedal, depending on where in the chain you decide to split the signal), that is split into two paths. One path goes into the rest of the pedals, which are actually left unconnected to an amp/anything. The other signal path goes to an amp. I'm wondering if the unconnected pedals will draw any of the signal away or distort it.
     
  4. #12

    Expert

    Nov 30, 2010
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    Short answer, Yes.

    The input stage of a stomp box has almost nothing to do with the output. The input is is isolated by the amplifier, or whatever effect is in there, from the output stage. The input stage will typically have a resistor to its internal ground, and that sucks some of the signal all the time. With a normal guitar, the resistance of the volume pot causes so much impedance (when it's not on 10) that even the cord bleeds off high frequencies of the audio range!

    My cure is to use a pre-amp plugged directly into the guitar, and not a $10 preamp. You need a low impedance driver to keep the signal pristine as it feeds the leakage of cord capacitance and effect inputs. I have never found a low impedance line driver that anybody else built, so I built my own.

    I now turn the floor over to anybody that is familiar with products available in 2011, as my knowledge is from the 1970's.
     
  5. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    Ok thanks for the input. Let me switch around the question now:

    What if instead, I had something like this:

    [​IMG]

    As you can see, the input from the guitar goes directly to the the amp, and is not connected in any way to the input of the pedals, but is connected to the output of the pedals.

    There is NO connection between the guitar and the input of the pedals.

    Would the pedals have any affect on the audio with the signal being connected to their outputs?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The pedals have a relatively high INPUT impedance, but a low OUTPUT impedance. The low output impedance would act more or less like a short to your guitar signal; ie: you would get very little signal, if any, to your amp.
     
  7. #12

    Expert

    Nov 30, 2010
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    Yes. The outputs of the pedals aren't even trying to preserve the signal. You have just made things worse.
     
  8. SgtWookie

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    Assuming that the signal return/grounds are common throughout (they usually are) you could do something like this:

    [​IMG]

    S1 is a single-pole, double-throw toggle switch. The common terminal carries the signal to your amplifier. Connected this way, your signal can come either from your guitar through your effects pedals, OR directly from the guitar.

    Here is an example of a SPDT switch available at Radio Shack stores:
    http://www.radioshack.com/product/index.jsp?productId=2062498&filterName=Type&filterValue=SPDT

    Image: [​IMG]

    With this type of switch, the center terminal on the right is the common. With the switch handle in the position shown, the lower terminal is connected to the common.

    [eta]
    If the DC signal levels are significantly different between the guitar out and the stomp box out, you might wind up with a pretty good *POP* from the amp when you flip the switch. If so, it might help to add a 1k to 10k resistor in series with the common terminal.
     
    Last edited: Jul 1, 2011
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I think it will help a lot if you say exactly what you are going to do! Because it looks like we are all guessing... :)
     
  10. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    Thanks a lot for all the input! I'd try to explain it, but it's way too weird and complex. I think I might go with the switch setup SgtWookie recommended, with a resistor to prevent any popping. Instead of using a SPDT switch there, though, i'd like to use an IC that is controlled by a microcontroller. Do you guys know of any relays or ICs that would work well with the circuit shown in SgtWookie's diagram?
     
  11. SgtWookie

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    You could use a 4000 series CMOS 4066, which internally has four independent SPST switches. The signal being switched must be entirely between the Vdd and GND rails.
     
  12. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    Oh thanks. Like this guy? http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1366-5-ND

    I wish it wasn't a quad because I'm kind of low on space on my PCB and only need one switch, but since I can't surface mount solder, I doubt I'll find a smaller DIP package.

    What do you mean it must be entirely between the Vdd and GND rails? I'll be switching my audio signal between the output of the guitar pedal chain and the straight guitar signal, per your diagram, while keeping the ground signal constant all the way through (like your diagram).
     
  13. SgtWookie

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    Your audio input signal is most likely centered around 0v/GND, with excursions above and below 0v. If the 4066's GND pin is also connected to GND, the portion of the waveform that goes below 0v will be truncated. You will probably have to isolate the DC level of the input waveform using capacitors.

    Also, microcontrollers are inherently noisy. Placing microcontrollers on an analog audio board is sort of like inviting a punk band to play in a public library. In order to keep the digital portion from contaminating the analog with noise, you may need to use optoisolators/optocouplers to control the analog side from the digital side.

    But, we don't know what you're doing with your circuit, nor are aware of your size restrictions.
     
  14. Ruptor

    Active Member

    Apr 26, 2009
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    Normally guitar pedals have a bypass switch built in so the effect can be switched out of circuit to preserve the audio signal integrity. So what is the point in another switch. Adding an external switch will be nothing but bother in terms of noise pickup IMHO.
     
  15. #12

    Expert

    Nov 30, 2010
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    I worked with a band in L.A. for years. I've seen stupid, so don't worry about that. If you want good answers from really smart people, you have to let down your guard and tell what you want and what you have to work with. The worst that can happen is that you get some education. More likely is that it will all get rearranged so it actually works.
     
  16. SgtWookie

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    There was another poster on here some months back that wanted to switch their effects unit in and out quite rapidly; so they really needed to use some kind of electronic switch. Don't recall where that thread is now, or how it was resolved. Relays would have been far too noisy, and would not have switched quickly enough.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    I would never use a cheap 3A AC switch for line level audio. The contacts are made of silver and when they corrode (silver turns black and becomes an insulator), 125VAC will spark through the corrosion but a few hundred millivolts at almost no current will not work.

    Use a name-brand switch that has gold-plated contacts (the gold does not cost more than silver contacts) that will switch audio perfectly for a very long time.
    I wish my Yamaha stereo had gold-plated contacts because the silver contacts on the input selector switch are intermittent.
     
  18. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    Thanks everyone for the advice. I mean, from my posts, I thought it was kind of obvious what I was trying to do: I'm trying to use a microcontroller to switch an AC signal between just the guitar output (a bypass) and the output from a bunch of pedals. I'm using an ATMEGA48 for the microcontroller, I just need to find an IC or whatever that works as a switch.
     
  19. #12

    Expert

    Nov 30, 2010
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    Most musicians don't program microcontrollers. In fact, most musicians don't remember to give me their noisy guitar cords so I can repair them before the gig. We could never have guessed you level of competence from, "will the pedals cause some of my guitar signal to be drawn away".
     
  20. icydash

    Thread Starter Active Member

    Jan 14, 2009
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    I mean sure, I totally understand from my beginning posts that what I was trying to do, as well as my competence level, was unclear. But at this point we've been talking about ICs, switches and relays, noise, DIP packaging and surface mount soldering, PCB space, etc...

    I have a bachelors (from a while ago, I'm now in law school) in EE, so I have a pretty good grasp on things, though a lot of it is rusty. If you guys explain, though, I'll probably understand it :)

    The switching is only going to be, at most, once or twice a second, so whatever component I use doesn't have to be able to switch all that fast. As far as size is concerned, the smaller the better; I'm going for something guitar pedal sized. :)

    The optoisolators comment is interesting; I have never heard of one before. After reading about it on wikipedia, though, it may be a good idea to throw it in, though I'm not fully sure yet *where* in the circuit it goes. Is it something I'd like put on the control signal, as this signal (coming from the microcontroller) would be used as the control on the switch (which controls the path of the audio, and the only real point of "connection" between the digital and audio circuitry), and I want to keep those two circuits as separate as possible (since the microcontroller circuit will be noisy)?
     
    Last edited: Jul 4, 2011
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