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DI Impedance

Discussion in 'Effects [BG]' started by seanm, Jun 1, 2005.


  1. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    I have been looking at DIs, and notice that a lot of very popular ones, such as the Radial JDI, have a very low imput impendance. A lot of passive ones have input impedances in the 10k range and actives have impedances in the 100k range.

    Shouldn't these low impedances be affecting tone? Especially with a passive bass or guitar? Do people put a buffer (or a buffered effect) in front of the DI?

    For an active DI, it would be trivial to put a fet or opamp based buffer in front that would have 1M to 10M input impedance. Why don't they?

    :confused: :confused: :confused:
     
  2. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    A basic transformer based DI's input impedance is dependant on the load it's driving, in addition to voltage and current, transformers transform impedance. The radial's are rated 10k input-Z (sorry, I'm tired of writing it out :p) feeding a 600R input-Z mic pre, if your mixer/mic pre has 10k input-Z the bass will see somewhere around 165K.

    All the active DI's on radial site have fairly high input-Z.

    And finally, yes, a 10k load on a passive bass will have a significant effect, 100k and up is a fairly standard input-Z for amps, and as such has little effect on tone, as I mentioned in a thread in amps, beyond a certain (fuzzy) level cable capacitance and the control pot on the bass dominate any loading effect.
     
  3. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Ok, I didn't know that.

    I use SS amps, so I am use to input Zs around 1M. Even 220k seems low to me considering that pickups can have output impedances of 500k to 900k. I realize this is mainly at higher frequencies.

    Do you remember which thread you posted about the loading?
     
  4. Audere

    Audere Supporting Member Commercial User

    Apr 7, 2005
    South Beach, OR
    Owner: Audere Audio
    Yes it would be trivial to buffer.

    Yes it changes the sound.

    FYI - many of the DIs do not meet their printed specs.

    I have seen one (SS big bucks studio unit) which claims > 3M which is rather hard to achieve when the input has two 1M resistors in parallel to ground.

    Welcome to world of audio...
     
  5. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    I guess 1M || 1M > 3M for VERY BIG values of 1M ;)
     
  6. bassman314

    bassman314 I seem to be a verb, an evolutionary process... Gold Supporting Member

    Mar 13, 2005
    Bay Area, CA
    New Signature!!!
     
  7. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    There's not much in the thread I mentioned of real use, (really don't know why I mentioned it :eyebrow:) but if you still wan't it here it is http://www.talkbass.com/forum/showthread.php?t=182780


    Yes pickups do have a high output impedance at high frequency, but they're loaded down by the volume knob on your bass, the tone knob on your bass and the capacitance of your cable (no matter how expensive).

    The impedance of 1 nf (fairly low) of cable capacitance at 10khz is 15915.5 ohms
    (Capacitive reactive impedance is equal to 1/(2*PI*F*C) where F is frequency, C is capacitance in farads and the result is in ohms)

    15915.5 ohms in parallel with 10megs is equal to 15890.2 ohms
    15915.5 ohms in parallel with 1meg is equal to 15666.2 ohms
    15915.5 ohms in parallel with 100k is equal to 13730.3 ohms
    As you can see while amp input impedance is important, beyond a certain point increasing input impeadance makes little difference as other effects become the dominate factors.

    P.S. I realize that this is somewhat of an oversimplification treating the pickup as a resistive source and ignoring resonance, but it gets the point across well enough.
     
  8. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    Could be bootstrapped, got a link to the schematic? Of course they could be blatently lying! :D
     
  9. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Ok, that makes sense. I forgot about the volume control.... And the cable.
     
  10. Audere

    Audere Supporting Member Commercial User

    Apr 7, 2005
    South Beach, OR
    Owner: Audere Audio
    Definitely not bootstrapped. Schematic is not on the web.

    There were other problems with the design which I considered to be major - the company however informed me that the design had worked that way for 12 years ... :scowl:

    Just so I do not smear Radial (the only company listed in the thread), they are not the manufacture.