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input impeadance matching

Discussion in 'Amps and Cabs [BG]' started by Pbassred, Oct 30, 2013.


  1. A little knowlege is a dangerous thing certainly but....

    I have been looking at input impedances. The littlemark 800 tube spec states that its return input is 33KOhm, but the Markbass effects units have many different output impeadances. Should I be concerned? Does unmatched input output impeadance matter?

    ....At least its not another thread about speaker ohms :bag:
     
  2. No, impedance matching is most important in maximizing power transfer. You aren't trying to power the pedals from your signal. Also, the circuits in the pedals are all active or buffered so you aren't worried about your guitar being the only source in your signal chain.
     
  3. gumtown

    gumtown

    May 7, 2007
    New Zealand
    As long as your effect pedal output impedance is not higher than the Return input impedance, you will be alright.
    If the pedal output impedance is too high (much higher than the return impedance), then loading will occur and change the tone character and reduce the level.
    The amp (preamp) input impedance matching is more importaint, especially with a HiZ passive bass.
     
  4. 1958Bassman

    1958Bassman

    Oct 20, 2007
    The output impedance of one piece isn't supposed to be the same as the input impedance of the next piece in the chain.
     
  5. chienmort

    chienmort

    Aug 15, 2012
    As a rule of thumb use a 10:1 ratio. So a 330k input impedance could be fed from say 33K although most devices will have an output impedance much lower than that.
     
  6. Ok. A fairly wide tolerability then. I only thought about it because I made an attenuator to insert in the effect loop. Its resistance between signal and earth is 19Mohm. Not so worried about tone. More worried about overloading the circuits and blowing the amp.
     
  7. funnyfingers

    funnyfingers

    Nov 27, 2005
    One of the biggest reasons to have an input impedance be at least a couple multiples of the output impedance is to allow splitting without much of a voltage drop among the devices.
     
  8. Agreed. A lot of SS power amps have input impedances in the 10-25KΩ range. This is fine for pre-amps or mixers having 600-2KΩ output impedances.

    What you have to watch is tube pre-amps. Those with outputs taken from a cathode follower will normally work well enough. Those where the output is taken from a plate will be loaded down detrimentally by the following input impedance.
     
  9. And with modern opamp circuits, this is easy to do. Look for circuits with a JFET front end. The input impedence of these is usually in the megaohm range. For my passive bass, I built a one-JFET buffer based on the Tilman circuit that had this feature.
     
  10. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    You don't want a maximum power transfer; you want a good voltage transfer. That means the input impedance (the load impedance) should be many times more than the output impedance (source impedance) driving it, ideally by a ratio of 10:1 or higher.
     
  11. Exactly, that's why I said 'no' - he doesn't want to match impedances. What's more important than tranferring voltage is not coloring the ouput as others have pointed out.
     

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