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Input impedance thread

Discussion in 'Amps, Mics & Pickups [DB]' started by Chris Fitzgerald, Jan 6, 2019.

  1. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    After having reviewed a lot of gear over the years, two things jump out at me as being really important: Input impedance, and sweepable hi pass filtering. I currently use amps with a roughly 5MegOhm input impedance because that's what tends to sound best with my basses, and carry a preamp with me that has the same when I travel. Part of the reason that the impedance is so important is that it represents a kind of baked in HP filtering before you even hit the preamp.

    Here's my question/topic starter - since a lot of people seem to feel that these things are important, what is involved in creating a particular input impedance? Could this be incorporated into, for instance, a patch cable?

    I would ask similar questions about HP filtering: How big does that circuit really need to be, and does it need to be active (i.e. - involve a battery?). I would be interested in finding out what the smallest form factor for carrying my own input impedance and HP filter with me would be.
    Povl Carstensen likes this.
  2. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    I definitely agree with the @5 Meg Ohms for the input impedance. Since the Grace Felix and Headway EDB-2's have a setting that's right around 5 I can get that.

    Most of the amplifiers around now are set at 1 Meg Ohm, but that's not enought, for my particular situations. According to Double Midi, it's quite difficult to built

    a variable inpedance preamp section that would cover from 1-10. So, it's more likely that some one like Bruce Broughton, could construct a box with an HPF and switchable

    1/5/10 Meg Ohm buffer preamp. Might be something to consider. My edumacated guess is that it would have to be an active device, with a battery.
    Chris Fitzgerald and Randy Ward like this.
  3. jlmorgan84

    jlmorgan84 Supporting Member

    Feb 16, 2014
    Clemson, SC
    Here's a link to just such a buffer in a cable: FET Preamp Cable. This one's 3megs, phantom powered and DIY so you'd have to make it yourself. The way I understand it a fixed hpf is just a resister and a capacitor so maybe could be squeezed in there. Interesting project anyway.
  4. And if the resistor or the capacitor is variable, it is a variable hpf? Or..?
  5. fdeck's (Francis Deck) hpf preamp is documented on his website. His is design is set to 10 Meg ohm by a resistor of that value on the input of the first amplifier stage. You could switch out that resistor to whatever value you wanted, i.e. 5 Meg ohm. The reason that it needs to be active is because the high input impedance of a passive circuit would be adversely loaded down by any circuit that followed it. It also allows the hpf circuit after it to have practical and obtainable component values. To my knowledge, fdeck's product has the smallest footprint commercially available for it's given features. Radial Engineering's AC Driver has a 10 Meg ohm input impedance with similar features with the addition of a direct out, but has a larger footprint.

    I own fdeck's series 3 and use it when I can't use my preamp with it's built in hpf or just want to simplify my setup.
  6. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    All great and important stuff. So, you could either get Francis to switch out the resistor, or do it yourself. Interesting idea.
  7. jlmorgan84

    jlmorgan84 Supporting Member

    Feb 16, 2014
    Clemson, SC
    beyond my pay grade I'm afraid :)
  8. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    There's only one truly variable inpedance buffer and it's on the Euphonic Audio Doubler. As I recall it goes from 1 Meg Ohm to 4 Meg Ohms. The Grace Designs Felix has a three way switch and so does the Headway EDB-2
    As of this post. The Radial OD has a switch.
    Zbysek likes this.
  9. or switch IN another 10 Mohm in parallel with the existing one to result in 5Mohm.

    Generally the problem with a passive HPF is that the simplest design has insufficient slope (6dB / octave IIRC), and designs with steeper slopes diminish the incoming signal significantly, requiring more gain and reducing signal to noise ratio.
  10. For a variable input impedance you could replace the 10 Meg ohm with a rotary 10 Meg ohm linear taper conductive plastic potentiometer in series with a 1 Meg ohm resistor. So the minimum would be 1 Meg ohm, the max would be 11 Meg ohm (1 Meg ohm + 10 Meg ohm).

    Chris... make friends with a member of your EE faculty with soldering skills. :) Actually, any competent technician should be able to do this.
    Ric Vice likes this.
  11. The Schalltechnik Vong Filterung has a really handy combination of features including 10M input impedance (which could easily be changed with a resistor swap), sweepable high and lowpass filters, polarity switch and a DI out with ground lift. Though it only comes in kit form, which will be a deal breaker for some. I haven't gigged with mine much yet, but it's quiet and clean and all the functions work as expected.
  12. leonard


    Jul 31, 2001
    Fdeck HPF and Fishman Platinum both have a high enough input impedance and a high pass filter. I think they are both 10M. I doubt the difference between 5 and 10 megohms makes a big difference. A typical bass guitar amp might be 500k. A high quality DI box like Countryman is 10M. If you chain your DI to your amp the pickup sees the lower impedance.

    Gollihur's site is golden:

    PREAMPS: Do You NEED a Preamp with Your URB Pickup? - FAQ courtesy of GollihurMusic.com

    OHMS & IMPEDANCE from the INPUT perspective - FAQ courtesy of GollihurMusic.com
    Last edited: Jan 6, 2019
    Zbysek and DoubleMIDI like this.
  13. The problem is getting a 10 MegOhms pot. I got trim pots to experiment with (if I got time for this) of 5 MegOhms, but that was the highest I could find. To get a range of 1 to 10 Megohms, an additional switch and a 1 and 4.7 MegOhms resistor are needed and you can get a 1 to 6 MegOhms range and a 4.7 to 9.7 MegOhms range (use a 5.6 MegOhms resistor if you think you need to get to 10 MegOhms. Serial resistances add up.
    A logarithmic pot would be nice, but even a lot harder to find if at all than a linear one. (But a logarithmic one needs to be connected in the right way otherwise it's much worse than a linear one.)
  14. I think that a linear pot is better, in that this is NOT a volume pot. The input resistance values derived based on pot position, would not indicated reliably or rendered smoothly.

    53C110MEG Honeywell Sensing and Productivity Solutions | Potentiometers, Variable Resistors | DigiKey

    The link above is for a 10M rotary linear taper pot from digikey (albeit in the US). Values of 3, 3.5, 5 M are also available.

    Caveat: I'm of the school of contributor agedhorse, whereby I don't worry about the input impedance if it's 1 M or higher. That's not to say that there isn't a difference of opinion or experience of the loading of one's instrument piezo pickup system with anything greater than 1M. If it's an issue for a user, the options being discussed are user's choice.
    JeffKissell and Ric Vice like this.
  15. I don't know if this has anything to do with input impedance or is even relevant to this thread, but I am intrigued...

    I use this with my piezo...
    Chris Fitzgerald likes this.
  16. shwashwa


    Aug 30, 2003
  17. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    Reading the fine print, the “Drag” control will vary the input impedance between 22 K Ohms and 1 Meg Ohm. So it’s not what we’re looking for.
    Chris Fitzgerald likes this.
  18. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    It definitely does, but the specifications don't reveal precisely what different loading values the Neutrix NP2RX "Timber Plug" achieves. They show a line graph that shows the relationship between frequency response and impedance but offer no numbers.

    Screen Shot 2019-01-07 at 9.41.54 AM. b
    bassmanbrent likes this.
  19. peterpalmieri

    peterpalmieri Supporting Member

    Apr 19, 2005
    Babylon, NY
    I’m not directly answering your question but my Fdeck is velco’d To the inside of my tailpiece and travels with bass in its bag. It makes input impedance, HPF and a volume control non issues all the time.
    jmlee and Chris Fitzgerald like this.
  20. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Right, and I have thought of this. If the input impedance could be made to be 5MegOhm one way or another, it would be a reasonable solution.

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