Determining Input/Output Impedances of Pedals?

Discussion in 'Effects, Pedals, Strings & Things' started by ZeroChan, Jan 25, 2012.

  1. ZeroChan

    ZeroChan Member

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    3 questions guys.

    1 - Other than looking at the manual for the specs, is there any other way to determine input and output impedances of buffered or true bypass pedals that are turned on?

    2 - I read somewhere (maybe here?) as a 'general' rule, the input impedance should be at least 10x more as the output impedance of the pedal before it to play 'well' together. As in from 10k (or less) output into 1m input. Is this generally true?

    3 - Is a buffered pedal with 500k input impedance considered a low input impedance? As in it loads the guitar like fuzz faces do? If it is, this is bad for my signal as the buffer's always on, no?

    And please save the "If it sounds good to you, why does it matter?" talk. This is for my own knowledge. Thank you!
     
  2. chervokas

    chervokas Member

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    Not really

    To minimize voltage loss at the input of the second device it's input impedance should be at least 10X the output impedance of the device in front of it. That will still give you a 1/10th voltage loss. But that's the conventional ratio that's considered acceptible among audio engineers for a signal chain (note, this has nothing to do with impedance matching speaker loads to tube amp outputs). There are other issues with changes to frequency balance, not just voltage loss, that can occur when one part of a circuit loads down another with a really low input to output impedance ratio.

    But your math is incorrect. 100K ohms (= 100,000) ohms into a 1M ohm (= 1 million ohms) would be a 1:10 ratio; 10K ohms into 1M ohms would be a 1:100 ratio.



    Well, without getting deep into it, the relative impedance are more important than the absolute impedance. A guitar has a variable output impedance -- it changes with the frequencies you play, with the pickups you use, it rises a lot as you roll back the guitar's volume knob. So it can be 10K ohms, say, or it can be over 200K ohms when you get the volume knob rolled way down. But say a guitar, knobs on "ten," has a nominal output impedance of 5 or 10K ohms... a 500K ohm input impedance on a pedal is well over 10X that nominal output impedance.

    The input impedance you most commonly see on guitar amps is 1M ohm. Loading the guitar with 1M ohm impedance might result in sound that's very slightly brighter and very slightly hotter than loading it with a 500K ohm inpedance.

    You need to get down into things like the original Big Muff circuit, with it's hardwire bypass 100K ohm input impedance IIRC; or an original Univibe, which as also hardwire bypass w/ a 69K ohm input impedance to get into something that's really problematic.

    Here's a way to think about it. In the old BF amps there are two inputs. Input one is a "normal sensitivity" input with a 1M ohm input impedance. Input two is a "low sensitivity" input with a 68K ohm input impedance IIRC. The first his hotter and much brighter; the second is considerably quieter and much warmer.
     
  3. jkokura

    jkokura Member

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    1. Yes there is. The hard but most effective way is do some complicate math after measuring the parts used in your pedals.

    http://en.wikipedia.org/wiki/Input_impedance
    http://en.wikipedia.org/wiki/Output_impedance

    2. Perhaps, but I can think of specific pedals that countermand this idea and are pretty great pedals, like the Fuzz Face for instance. Many pedals do have an input impedance and output impedance that are carefully balanced, but other circuits are happy accidents or too simple to really implement this sort of rule.

    3. I don't think so. I think in most cases 500K is common and enough. Many Boss pedals are at 470K I think, and often they're considered more than adequate. I think going lower than that would be too low for some guitars. 1M is a very safe number, and 10M is often considered way to high. Not that you really need to worry about it. Use your ears.

    Long and the short, trust the manuals. If they don't have the info for you, ask the manufacturer. If they can't be reached, try google.

    If you'd like, I'm sure you could spend hours learning over at one of the DIY sites. If you want some direction to one, feel free to ask!

    Jacob
     
  4. chervokas

    chervokas Member

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    I think that's a good point. I presume the OP is asking about the effects of pedal in bypass. The Univibe for example has an input impedance of 69K ohms, it really loads down the guitar and delivers that characteristic dark, chewy tone. I don't like that pedal with a 1M ohm input impedance, it sounds too bright, like a generic modern phaser. But I wouldn't want that in impedance in bypass though, that's why pretty much all the modern clones are made with true bypass switching.
     
  5. ZeroChan

    ZeroChan Member

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    Thanks for the replies guys.

    What I'm getting from the responses is that a device with a lower input impedance warms up the sound (slightly) compared to a higher impedance. Can be a good thing or a bad thing, but doesn't really matter much as long as it's not too low?
     
  6. chervokas

    chervokas Member

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    It's a little more complex than that. A big part of the tonal balance of a guitar with passive pickups comes from the fact that a resonanting circuit is formed by the inductance of the pickups, the resistance of the pots and the capacitance of the cable. The frequency of the resonance -- will determine a lot about how bright the tonal balance of the system will be. The input impedance of the device loading the circuit won't change the frequency of the resonance. It will damp the amplitude of the resonance though, so it will have an impact on the sound but a lot will depend on what the frequency of the resonant peak is. So within the range of typical guitar amp/buffer input impedances 470K ohms to 1M ohms will all sound pretty darn similar with things like cable capacitance having a bigger impact on tonal balance (esp. with single coils). But if you get to extreme values (10M ohm input impedances or 60K ohm inputm impedances) you'll get much more dramatic impacts on frequency balance.
     
  7. ZeroChan

    ZeroChan Member

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    To be honest, did not really get the first part of the paragraph. No offense, haha. So the cable capacitance has more impact on the tone than the impedance values of pedals then?

    For that one, low capacitance cables yields more highs, right? The cable legth is also a factor?
     
  8. chervokas

    chervokas Member

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    No. Not necessarily. If you didn't get what I said before don't worry about it. If you're concerned with choosing a buffer impedance value, anything from 470K ohm to 1M ohm will be fine and you might not be able to tell much difference between buffers of those values in terms of frequency balance. If you get something with a very high or very low input impedance though, the frequency balance and signal level changes will be much, much more apparent and potentially problematic. That's exactly what "tone suck" originalyl referred to -- hardwire bypass pedals that loaded the guitar with low impedances in bypass causing a drop in level and a change in frequency balance. (Nowadys "tone suck" has become a meaningless term used indiscrimently by folks to refer to just about anything.)

    The amount of total capacitance between a guitar with passive pickups and the first active device -- be it a buffer, a switched-on pedal or the amp -- will have an impact on the upper harmonics of an electric guitar. (The can happen for two reasons, but I'll skip explaining them.)

    The lower the total capacitance, the more open and bright the system will tend to sound. The higher the total capacitance, the warmer and more mid-focused the system will tend to sound.

    Length matters to the extent that it affects total capacitance. So, say a cable has a capacitance of 25 pF/ft. 20 feet of that will have 500 pF of capacitance in total and will deliver a particular sound. Now, if you have a cable with a capacitance of 50 pF/ft, 10 feet of that will also have a total capacitance of 500 pF and will pretty much sound identical to the 20 feet of 25 pF/ft cable. But if you have 20 feet of the latter cable you'll have twice as much capacitance in line and the sound will be warmer.
     
  9. ZeroChan

    ZeroChan Member

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    Thank you so much man, you've been very informative.

    Makes sense. Thanks. Your saying though is that the cable and the first device the cable 'sees' matters most for it's tonality?

    I'm using my Catalinbread DLS as an always on 'buffer' and it works well right now. From what I'm getting from your quote above is that it doesn't really matter much if I put another buffered pedal after the DLS as the DLS is the one doing the buffering? Or am I taking it out of context?
     
  10. chervokas

    chervokas Member

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    Correct. The always-on pedal is loading the guitar with its input impedance and driving the next device that's active with it's output impedance. Nothing after that first always-on pedal -- not the input impedance of subsequent devices nor the capacitance of subsequent cable -- is able to load the guitar. You'll get no benefit from adding another buffer in the signal chain unless you have a problem with impedance mismatched between devices in the signal chain that needs to be managed.

    It's worth noting that some ODs have highly variable ouput impedances (the ouput impedance can change significantly as you change the output volume pot position in a pedal that doesn't have an output buffer in its effect circuitry -- note, these pedals don't necessarily need to have buffered bypass).

    So it can happen with some of these (the OCD is the famous case) that you get one tone out of it when nothing after it is switched on, but when you put a buffer after it or switch on a pedal after it with a 1M ohm input impedance suddenly its tone gets very bright. That's why ODs aren't necessarily great substitutes for stand alone buffers or a buffered bypass pedals as devices for managing loading in a signal chain;. I don't know about the DLS circuit but if it's working for you it's fine, right?
     
  11. Angelfire

    Angelfire Member

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    I thought about creating a new topic for this, but "Determining Input/Output Impedances of Pedals" seemed general enough, actually, this should be official for this kinda thing, lol.

    Anyway, I was considering getting a booster/overdrive pedal to drive my amp a little more, I play neoclassical stuff a'la Malmsteen, Joe Stump, Tony McAlpine etc. Their tone is very bright, and one customary pedal to use is the DOD 250 (I actually have the DOD YJM), these pedals have a 4 MOhm input impedance, this is very very high compared to all other booster/od out there.

    I want to change my tone a little though, not only keep the ever DOD thing, but 4 MOhm has set by standard a little high with regards to input impedance. I considered the Fulltone OCD and MI Audio Crunch Box, both I thought sounded good, but nowhere was I able to get information about their input impedance.

    So, anyone here knows the input (output also if possible) impedance of these pedals? If not, any advice on high input impedance boost/od pedals?

    Thanks
     

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