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At what point is a high impedance input too high?

Discussion in 'Amps and Cabs [BG]' started by Flux Jetson, Aug 27, 2012.

  1. Checking specs, I see that most bass amps spec out at around 1meg-ohm input impedance.

    One or two of the Sans Amps gizmos spec at 4.7mohm input impedance (almost 5x that of a typical bass amp).

    Yet, some mic preamps spec at around 640k to 750k input impedance.

    And some DIs spec at roughly 500k.

    What is the point of diminishing returns? I mean how high is too high? What differences in sound can be expected with some of these really high impedances?

    Obviously speaking of how this relates to passive electronics in a bass, not so much active electronics. Is it safe to assume that active electronics aren't as sensitive to input impedances?

    Thank you. :)
  2. megafiddle


    May 25, 2011
    Active electronics have a very low output impedance and can drive virtually anything, so they are pretty insensitive to amp input impedances. Even passive pickups are a relatively low impedance and if the bass guitar volume is near "10", the bass can drive any typical amp.

    There are settings of the volume control that can raise the output impedance of the bass to around 125 kohm with a 500 K volume pot (62 kohm with 250 K pot). In that case though, I would expect a bigger problem from cable capacitance than from amp input impedance.

    The very high input impedances may just be a characteristic of the input stages uses. Off hand I can't think of any benefit, but I might be unaware of some device that needs it. For one thing, you would use a very high input impedance with a high output impedance device, and that would tend to be very sensitive to noise.
  3. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    At some point thermal background noise, aka white noise, aka hiss, would become an issue. All resistive impedances generate the same noise power but the noise Voltage increases as the resistance increases. In a radio system where the incoming signal level is often a few micro-Volts an impedance as high as 1 meg-Ohm would kill the signal to noise ratio. In a bass amp the incoming signal is hundreds of milli-Volts to Volts so impedances in the meg-Ohm range are not an issue or not a big issue. You might be able to hear a noise difference between a few hundred kOhms and a few MOhms if you listen carefully. I haven't tried so I don't know for sure.

  4. megafiddle


    May 25, 2011
    The bigger problem would be 60 cycle hum and buzz pickup. A long cable with the instrument end disconnected and hanging off the amp input would be similar to a very high impedance source. Equivalent to a 2 Megohm source driving a 2 Megohm input (1 Megohm total), for example). That can be pretty noisy, as anyone who has pulled the cord out of a bass with the amp on has probably noticed.

    My Acoustic B100 is dead quiet at full gain with the input shorted (zero ohms in). However if I lift the ground at the input with a blank 1/4" plug, I get a noticable hiss. That is of course with no signal. At that gain, an actual signal would completely drown out the noise. And that's also worst case. It is demonstatable but as you said, not an issue.
  5. Ok .. thanks fellas. I'm just trying to add more stones to my city of knowledge. After my collective experiences in audio electronics (since 1978) I've built a tiny village with the amount of info amassed. Certainly a minimal construct compared to the mighty metropoli that other folks have built over their lives. But it all adds up in the end as yet more information that helps me better understand this music electronics stuff.

    I'm neck deep in designing a new amplification system and preamp system for my own bass guitar rig, and this question is but one of many on my list. The answers enable me to do this as right as possible, with fewer backsteps and do-overs.

    The mic preamps I've been using on the prototype 2-channel bass have an input impedance of 640k. They seem to work very well with the way I have the bass itself wired up. Each pickup is wired to it's own volume control, and each volume control is sent to it's own output jack. The use of a N/C switching jack in the works provides standard 2-into-1 output -- or -- if both jacks are used each pickup is sent to it's own output. There is no tone control in this prototype. In the actual completed bass I may utilize stacked volume/tone pots on each pickup. But for now, as everything is still prototypical, this system is fine.

    I've been considering the use of one of the SansAmp products in the input module. So far that module only consists of the two mic preamps. Consideration of the SansAmp units is what prompted this thread. They report to have 4.7meg ohm input impedance. I was wondering how that much of a difference between the SansAmp and the mic preamp would affect things. You both have well-answered my question.

    Thanks .. I certainnly appreciate your help in this matter. :)
  6. Roscoe East

    Roscoe East

    Aug 22, 2011
    One of the best sounding DIs ever made -- the Demeter VTDB-2b -- has an input impedance of 20Megohms (!). So I would conclude that the diminishing returns threshold is pretty freakin' far up there.

    (Not that I'm suggesting that the VTDB sounds so good because its input impedance is so high...just that being an order of magnitude higher than other DIs that are already considered to have a high input impedance is clearly not an obstacle to good sound.)
  7. tech21nyc

    tech21nyc Commercial User

    Aug 17, 2010
    Manufacturer: Tech 21
    The 4.7MOhm input is used on our Para Driver DI and RPM units. These were designed to get the maximum dynamics and signal response from piezo pickups.

    The idea behind the 1MOhm input impedance in most guitar amps is so that the high Z passive pickups won't be loaded down at the input. If you plug a high Z instrument into a low Z mixer it will work but there will be some tonal loss. The instrument will sound somewhat dull.

    As stated, high Z signals can be plugged directly into most sources and work fine. One exception is certain vintage efx like older fuzz tones and boosters that require the guitar's signal to be high Z in order to work with those particular circuits.

    I can't say that there will be a benefit to using the higher impedance circuit with your bass. It will work just fine but you would have to hear for yourself to determine any tonal difference.
  8. Yea, pretty much have a handle on that, but thanks for the refresher course.

    My main question was where do things get beyond usefulness. The 4.7m was higher than most of what I'd seen and since I've just purchased my first passive bass (of the twenty six basses I've owned in the past) I wasn't certain if the nearly 5x higher impedance would produce an audible difference with passive pickups. You've squared me away on that, thank you.

    I've been looking at the Para Driver, mainly due to it's inclusion of the semi-parametric EQ in the midrange function. It is essentially the same circuit as the RPM from what I can tell, "Sans" a few features, and $120 less buckolas.

    On the other hand, your "VT" series also seems well done. Having owned an SVT rig myself for many years the VT has it's own set of attractions. If you're still watching this thread, can you tell me where the midrange control's freq center is? And if I may dig a little deeper, what order of filter is it (4 pole/24db or 2 pole/12db or other, and so on). I do not live in an area where I can personally audition anything.

    Thanks once more for the help.
  9. tech21nyc

    tech21nyc Commercial User

    Aug 17, 2010
    Manufacturer: Tech 21
    The super high input impedance is really for instruments with piezo pickups. The Para Driver DI is a multi-purpose preamp that originally started out as our Acoustic DI. The RPM is the rackmount version.

    In terms of tone the Para Driver DI is more open ended while the VT Bass is more centered around the Ampeg sounds. There is a certain amount of overlap in sounds but only you can decide which is better.

    The VT Bass' mids are at 500Hz and there is a 4 pole custum tuned filter used in the speaker sim.

    To get back to your original post, mic preamps are designed for low Z mics. Hence the need for DI's when using high Z instruments. Piezo pickups are yet another category and many standard DI's are not up to the task to deal with those super high Z pickups. Also, you need a fair amount of gain with a piezo and you can't use a very long cable length with a passive piezo element. FYI.
  10. Thank you. :)

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