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Input impedance, too much of a good thing?

Discussion in 'Amps and Cabs [BG]' started by cassius987, Apr 21, 2010.


  1. cassius987

    cassius987 Banned

    Apr 20, 2007
    Denver, CO
    I have always read that you should avoid input impedance on an amp below a certain level of ohms, for instance 250 K is supposed to be a minimum according to some. However lots of amps--for instance the one I use--supply an impedance load several times higher than this. My amp's impedance is actual a whopping sixteen times higher than this at nearly 4 mega-ohms. Now, at this point, what am I gaining from that--other than that much more "cable color" from enhanced cable capacitance?

    My thinking is that after a certain point high input impedance becomes a burden, rather than a benefit...
     
  2. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    It depends very much on what you are plugging into it. Impedance effectively has no meaning except in relation to the device being connected.

    With a typical Fender bass and common cables, I sometimes find anything over 1.5 Meg to be too "brittle" or noisy or other qualities associated with having too much high frequency content. But again I wouldn't generalize that to all basses and all cables; and of course any pedals in the chain would affect that relationship significantly.

    Some people prefer the natural rolloff of high frequencies associated with a passive bass into an amp with low input impedance--the REDDI exhibits this effect.
     
  3. will33

    will33

    May 22, 2006
    austin,tx
    Subscribed...like I didn't have enough crap to worry about.:D

    I've done nothing but plug passive basses into amps, actually I did have 2 active ones over my life thus far but they weren't that great.

    Let me do a little searching for input specs of different amps I've had and could maybe add something to this thread.....or totally confuse it.;)
     
  4. cassius987

    cassius987 Banned

    Apr 20, 2007
    Denver, CO
    Well I know this much, the Markbass I'm looking at is 500K and my Ashdown ABM EVO II 500 is 3.9M, almost eight times higher than the Markbass.

    Just seems to me like if anything above a certain impedance is equally good at getting rid of the problems of high impedance instrument input, why go higher? Doesn't that make cable capacitance worse?
     
  5. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    No, it reduces the effects of cable capacitance. But I do have to agree that 3.9M is insanely high for most electric basses, and there is no benefit to it.

    It may have been designed that way for use with piezo-equipped basses, especially uprights. Very often they need super-high input impedance like that in order to transmit the signal strongly. I've even seen some buffers for EUB's with 10M z in.

    Anyway, even though 3.9M may not be benefitting you, it won't harm your signal either--just turn down the treble EQ a bit.
     
  6. In audio, as a rule of thumb, the input impedance of a device should be a decade above the impedance driving it. Thus if the output impedance of an instrument is say 1MΩ then the input impedance of the amplifier it's plugged into should be 10MΩ minimum. Your amp with sixteen times is a good match.

    Paul
     
  7. cassius987

    cassius987 Banned

    Apr 20, 2007
    Denver, CO
    Weird. A recent issue of BP must have it wrong because after explaining that you don't want your signal to be loaded down they write that "Impedance doesn't just affect level--the higher the input impedance, the greater the tone control's effect. This is why a tone knob can seem very effective with some amps and not with others. Cable capacitance is also more of an issue with higher input impedances." This led me to believe that as impedance went up, cable capacitance would too.

    If you are referring to the impedance of the pickups mine are usually around 10K. I haven't actually measured them in Henries yet...
     
  8. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I'm not aware of any bass instrument with an output impedance anywhere near 1M. Typical z out from a passive bass is closer to 10K, so the decade rule would suggest the amp should have a z in of 100K (or higher of course). You can use the resistance reading of the pickup as a general approximation of the z out, since the inductance and capacitance in the circuit is very small. Active basses have much lower z out, for example the Aguilar OBP has 100 ohms z out. So the decade rule suggests an amp with at least 1K input. We are nowhere near the Meg range in most cases.
     
  9. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    my UA 6176 channel strip has two settings for the input impedance-47K and 2.2M. i prefer the 2.2M as the 47K, even though it sounds great, rolls off the highs a bit too much for me. it also drops the gain a bit too. it may be an 'illusion', but i also seems to have a tiny bit more lows with the 2.2M setting too. neither setting seems to have any more noise from the cable.
     
  10. Jefenator

    Jefenator Supporting Member

    Aug 22, 2008
    Oregon
    I haven't had any bad experiences with too high an input impedance yet. That's with my upright piezo & various passive electrics in to a Countryman DI @ 10MΩ. (Also fdeck's 10MΩ buffer pre.) If the passive electrics seem too "crisp", that's what the tone knob is there for. :smug:

    I have had issues with too low an input impedance. IME the upright piezo doesn't work nearly as well, straight in to my 500KΩ Markbass head, and the passive electrics don't respond right, run straight in to a Roland keyboard amp (which is probably way down there).

    Put more simply, IME it's better to err on the high side.

    Having said that, I don't require extra-high impedance in my amps. IME, 500K is fine for passive electrics (and active, actually) and my upright piezo much prefers to get buffered before the 15' cable.

    Summit Audio has that little preamp with a variable input impedance control. That would be fun to try!
     
  11. The 1MΩ figure was an example only Bongo. I could have used 100KΩ or even 10KΩ just as easily. Active basses have impedances much lower than a passive instruments.

    Paul
     
  12. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    ??? You might need some extra coffee or green tea this morning. :p
     
  13. cnltb

    cnltb

    May 28, 2005
    So, a high input impedance on the amp means that the cable has less of an impact-like using a buffer into a low impedance amp?
     
  14. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Right, I like to describe impedance as being like "efficiency". If you're carrying a load uphill, you can get to the top faster if you either carry less of a load, or exert more energy. Greater capacitance means greater loading, and greater loading means your signal will have to work a lot harder to get across to the next device without losses. The greater the gap between output impedance (on the low side) and input impedance (on the high side), the easier it is for your signal to pass without losses. So a very high input impedance can significantly reduce the amount that extra loading (e.g. from a long cable) has any audible effect on your signal. Also, as in the hill-climbing illustration, you can mitigate that loading by turning your signal level up higher before the cable--but since gain almost always requires an active device, and an active device will almost always have a lower output impedance, there's really no practical difference.
     
  15. cassius987

    cassius987 Banned

    Apr 20, 2007
    Denver, CO
    So that's just a misprint then?
     
  16. AlexanderB

    AlexanderB

    Feb 25, 2007
    Sweden
    As the magnetic/current fields from strings and pickups interact, a current flows through the pickup windings and input stage of the amplifier, thus generating voltage across the input stage. (And across the pickups, too.)
    The resulting voltage is following Ohms law and is split over pickups and input stage in proportion to the impedance of the devices, respectively. A high ratio between input impedance and pickup impedance will yield a higher ("stronger") input voltage to the amp.

    An unnecessarily high input impedance will not add any sonical benefits but instead add significantly more noise to the amplified signal.

    A slightly high impedance will sound crisp and clear but be prone to noise.

    A somewhat low impedance will sound dull and unresponsive but will not have much noise.

    Too low an input impedance will be virtually noise free but also no generate a clear or strong enough signal for the following stages in the amp to work with.

    /Alexander
     
  17. AlexanderB

    AlexanderB

    Feb 25, 2007
    Sweden
    Any conventional, well designed, dedicated instrument cable in standard lenghts will have virtually zero effect on the sound of the bass.
    Don't let evil marketing people make you believe something else... :meh:

    /Alexander
     
  18. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    The fine point there is that not all cables are conventional, and not all conventional cables are well-designed. I agree about the evil marketing people, but just because they are liars it doesn't actually follow that instrument cables won't have higher or lower capacitance, or that there can be no audible effect on the sound of the bass. A passive bass through a standard cheap cable can very well sound different than the same bass through a very-low-capacitance cable. The audible difference between those two cables will be greater at long cable lengths, lesser at short lengths, and it may be barely audible or nonexistent with an active bass (or buffered line).
     

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