Modify amps input impedance?

Discussion in 'Pickups & Electronics [BG]' started by SteveMoodie, Jun 21, 2011.


  1. SteveMoodie

    SteveMoodie

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    Hey guys,

    Anyone know if it's possible to modify the input impedance of SS amp? I'm using an LMII and the 500 Kohm impedance doesn't match with my double bass very well. (using a buffered pedal at 1 Mohm seems to fix the problem). Would be nice to be able to plug straight in though!

    Is this possible? Anyone think it's a bad idea?
     
  2. Downunderwonder

    Downunderwonder

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    I'm guessing you have a piezo pickup up around 2 Mohm? I never heard of anyone modifying an amp to fit that situation.

    While it's up for discussion, I have inherited a 1974, or earlier, "D'Armond Bass Microphone" pickup limpet thingie with an upright bass I just bought. Don't know anything about it. It has two bare wires for a lead.
     
  3. staindbass

    staindbass Gold Supporting Member

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  4. SGD Lutherie

    SGD Lutherie Banned

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    No amps have that high of an input impedance. But it is often 1M. You should use a preamp with piezo pickups.
     
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  6. line6man

    line6man

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    I really don't understand. Why would you want to increase the input impedance, when your output impedance is clearly too high? Your focus should be on fixing the problem the right way. I'm not familiar with double bass piezo buffers, but perhaps you need one with a lower output impedance.

    500k Ohms does sound like a fairly low input impedance, however.
     
  7. SGD Lutherie

    SGD Lutherie Banned

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    He's trying not to use a buffer for the piezo and was hoping to be able to plug right into the amp. That's not going to happen though.

    A small JFET buffer would work fine and is small enough to put right in the bass if he wanted to go that route.

    I was thinking that can't be right, but that's what the specs say:

    Markbass - Little Mark II

    Maybe building a buffer into the amp would be a good way to go.
     
  8. line6man

    line6man

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    I can't see it being desirable to run piezos without a buffer. Their output impedance is just too damn high! If the bass doesn't already have an onboard buffer, it needs one.
     
  9. SGD Lutherie

    SGD Lutherie Banned

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    I agree.
     
  10. SteveMoodie

    SteveMoodie

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    Thanks for the comments. I must be misunderstanding something.
    I thought that I wouldn't need a buffer if the input of my amp had a high enough impedance to match with my piezo pickup...

    Isn't a buffer just a high impedance input/low impedance output device that you plug a piezo into so that you don't have a problem with an amp's relatively low input impedance?
    Amps that are designed for upright bass always seem to have a higher input impedance (1 - 10 Mohms) so that you don't need a buffer before it.

    Or have I got that completely wrong?
     
  11. line6man

    line6man

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    You want the impedance low before the signal has to run a length of cable. One disadvantage of high impedance signals is that if the impedance is too high, the parasitic capacitance of the cable will start to suck out the treble. Also, low impedance signals tend to be less noisy. (Unrelated, but if your input impedance is too high, you get more thermal noise, because thermal noise is proportional to resistance.)
     
  12. teleharmonium

    teleharmonium

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    No, you've got that right. But you hit the nail on the head - amps designed for upright (specifically, for directly connecting piezo pickups) have much higher input impedance than general purpose bass amps that have an input impedance that makes sense for a magnetic pickup.

    Input impedance just is not a characteristic that is within the normal spectrum of how you can modify an existing circuit, it's not like there's a single part in the amp that controls it, and it's important to understand that doing the impedance conversion at the amp is not the ideal place to do it because it's (usually) going to be after a relatively long cable run.

    For that reason, preamps or buffers to do this impedance conversion on the bass or on a belt clip box or a nearby stand work better and that's why there are so many of those on the market. It's not that players like having an extra, awkwardly placed gadget to deal with on stage.

    I'm not necessarily saying that the amp couldn't be modified for higher input impedance, if you understand the cable length issue and were going to use a reasonably short cable. I don't know about that.

    Personally I often use a 10 foot cable, no preamp, and a 1965 Vox AC50 for the piezo pickup on my upright. So I guess you could say I'm sold on the idea of just not worrying about the less than optimal impedance situation, if you like the sound. What does it sound like when you plug straight in ?
     
  13. walterw

    walterw Supportive Fender Gold Supporting Member

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    IIRC, straight-up tube preamps have a naturally high input impedance; i do know they tend to do a better job with passive piezos than regular solid state amps.
     
  14. SteveMoodie

    SteveMoodie

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    Cool, thanks for the help guys.

    Ok, seems like more work to modify than I thought it was - I thought it was just a component at the input of the amp.

    There are other amps I know that use a 1 mOhm input that aren't double bass specific amps. Eg. TC RH450.

    I used to think that my LMII impedance matching was fine, until I started using a Boss TU-2 tuner in front of the amp. The difference was HUGE! I couldn't understand why at first. With a bit research I realised that it was the TU-2's impedance/buffer (1 Mohm).

    If changing the amps impedance is going to involve changing the whole circuitry/characteristics of it, then I'm definitely not going to head down that route. I'll just stick to using the TU-2.

    (a while ago I used to have a Fishman Pro Plat which I sold because I thought I didn't need it anymore.... hmmmm.... damn!)
     
  15. walterw

    walterw Supportive Fender Gold Supporting Member

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    Yeah, for piezo pickups on a bass I would want more like 6-10M input, so no low end gets lost.
     
  16. craig.p

    craig.p Supporting Member

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    It won't hurt to experiment a little, as long as you understand that adding any series resistance before the input is asking for noise. You can use a 4.7 Mohm or higher resistor in a metal bud box, or wire it into the input jack of the amp if the topology will permit, or even make up a short cable with the resistor in-line, and put a phone plug at one end of it and a phone jack at the other. If you really want to get fancy, you could use a 10 Mohm pot (two legs only, placed in the signal path the way a series resistor would be) so you could vary the impedance. But again, noise considerations. The more "stuff" you add, the larger the antenna you're creating. Regardless of how you go about it, shielding will be crucial.
     
  17. Jazzdogg

    Jazzdogg Less barking, more wagging! Supporting Member

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    I bought one of FDeck's HPF/Pre-amps (version 2) to use with my DB, and it's great: it functions as a high-impedance buffer, volume control, and variable high-pass filter. I strap it to my tailstock with velcro.
     
  18. walterw

    walterw Supportive Fender Gold Supporting Member

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    I'm not sure it works that way.

    The impedance issue is the resistance between hot and ground, in that the lower that number, the more signal gets lost.

    Adding series resistance in front isn't gonna change that, is it? I'd think it would just attenuate the signal even more.
     
  19. SGD Lutherie

    SGD Lutherie Banned

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    Adding series resistance to a piezo transducer acts like a high-pass filter and removes all the low end. That's how you wire up piezo tweeters in PA bins.

    There is no way around using a buffer.
     
  20. line6man

    line6man

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    If you add a series resistance, it will raise the input impedance, as far as what's seen by what you plug into the amp, but not the impedance at the input gain stage.

    For example, if you have a 1M Ohm input impedance, and you add a 1M series resistance between the bass and the amp, that means the input gain stage of the amp will have a 1M impedance, but the bass will see a 2M load from the amp. The impedance after the series resistance, at the input gain stage is still going to be 1M, regardless of the series resistance, and the series resistance is only going to attenuate the signal. At the end of the day, the amp's input impedance is too low, and as David just pointed out, there is no way around using a buffer.

    I don't understand this aversion to using a buffer. I would not want to have a high impedance signal from piezos coming straight out of my bass without a buffer. Buffers are your friend! They do wonderful things!
     
  21. PhiDeck

    PhiDeck

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    Not mentioned in your post is that you've just created a voltage divider which significantly attenuates the bass' signal, requiring the amp to operate at higher gain, thereby further amplifying noise.

    The correct solution is to buffer the piezo at the bass' outputp, which:
    - lowers the source impedance
    - allows use with any amplifier
     

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