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Piezo preamp impedance, my theory

Discussion in 'Amps, Mics & Pickups [DB]' started by fdeck, Dec 1, 2005.


  1. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    The recent discussion of Acoustic Image Series III reminded me that I researched the impedance issue when I designed my DIY preamp. Here is what I learned...

    It appears to me that a piezo pickup can be modeled as a voltage source in series with a capacitor. Equivalently, you could use a current source in parallel with a capacitor. The capacitance of the pickup, coupled with the input resistance of the amplifier, forms an RC high pass filter. For this filter to be benign to the overall frequency response of the system, the cutoff frequency has to be below the audio band. The cutoff frequency is equal to 1/(2 pi R C) [equation 1], so you can see that increasing R, or increasing C, will lower the cutoff frequency. Thus, if you know nothing about C, then you have your best chance with the highest possible value of R.

    Another way of writing that equation is that if the cutoff frequency is fc, then the minimum value of R should be 1/(2 pi fc C) [equation 2]. With a capacitance meter, I measured 0.025 uF on my K&K Bass Max pickup. Assuming I want fc to be somewhere around 20 Hz, the minimum value of R would be roughly 0.32 MegOhms. Anything above 0.32 MegOhm should not significantly alter the sound of the pickup. Thus, the new Clarus head should be OK. Another calculation is that 1 MegOhm would be the bare minimum if the capacitance of the pickup is 0.008 uF.

    Of course I am assuming that you don't want a high pass filter to affect your sound. It is entirely possible that some pickups sound better when they are "matched" to a particular load resistance. By trying different preamps, you may be inadvertently using that resistance to EQ your pickup. I am certainly making no value judgement against doing it this way.

    Now we get into a bit more murky waters... during my research, I discovered that there is a way to "pad" the output of a pickup, in case its signal is actually distorting the preamp. A typical resistive attenuator is the wrong way, as it requires high resistance values that add noise. Instead, take advantage of the pickup capacitance. Pad the pickup by putting a capacitor in parallel with the pickup output. For instance, putting 0.025 uF in parallel with my Bass Max would pad it by 3 dB.

    But when I modeled this circuit, I noticed that the padding capacitor does another thing... it renders the pickup less sensitive to the impedance of the preamp. If you think about it, putting 0.008 uF in parallel with a 1 MegOhm input would guarantee that no piezo pickup would be affected by the preamp impedance. Of course this would be at the expense of gain, but most piezo pickups have enough signal that you can give up some gain. You could hide the capacitor (preferably a polymer film type) in the output jack of the pickup. Let's make it 0.01 uF, which is a commonly available value. Indeed, an evil thought is to use much larger values of capacitance to match the lower input impedances of cheap electric bass amps.

    Here is a thought about signal cable length... the fact that the pickup is a capacitor means that some additional capacitance introduced by the cable probably has negligible effect on the frequency response of the system.

    A final note... there are some subtleties about raising the input resistance of the preamp. Under ideal conditions, increasing this resistance reduces the input noise of the preamp. However, this effect is buried under the noise floor of the input transistor or op amp on probably any commercial preamp. A much larger source of noise in an amplified bass is probably acoustical noise in the room coupled through the bass and into your amplifier.

    And one more thing... it is my experience that really high input resistance amplifiers tend to be rather squirrely. Keeping the resistance to a moderate level may make good sense for the stability of the amplifier.
     
  2. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    Hmm. Would this not cut the voltage in half? In that case it would be 6 dB of attenuation, not 3 dB (because voltage is a "20-log" quantity).
     
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Oops, you're right. My brain is on ice.
     
  4. Thanks fdeck - a very helpful article.
    I always thought low amp impedance attenuated the highs so I didn't worry about it too much for bass, but you've set me straight - now I know why the piezo my friend plugs into his old Peavey sounds so thin.
     
  5. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    No, I think your brain works very well, indeed! I was remiss in not telling you what I fine post that was.

    I'll be building your simple pre-amp tomorrow. Can't wait!
     
  6. mje

    mje

    Aug 1, 2002
    Southeast Michigan
    I ordered some of those thin-film piezo transducers, and hopefully I'll have time over Christmas break to put a few together. I'm going to see if I can't put a one-FET preamp as close to the transducer as possible, and run power from a box on the tailpiece. FIngers crossed.

    Somewhere in my box of musical junk I think I still have a FRAP pickup that encapsulates an op-amp into a tiny accelerometer type pickup, with a remote battery box. A quick search indicates that FRAP was succeded by http://www.tranceaudio.com/.

    Hmm. Maybe I'll order some accelerometer transducers, too.
     
  7. Francois Blais

    Francois Blais Supporting Member

    Dec 11, 1999
    Québec, Canada
    Yesterday, I found an old "It by FRAP" pickup in a box.
    I bought it about 20 years ago.
    I glued it on my EUB's bridge with silicone.
    Will try it tonight.

    The small "preamp" holds two nine volts batteries, and there's a small printed circuit, but no active electronics as far as I've seen.
    An electrolytic capacitor and a couple of resistors I think.
    More later.

    François
     
  8. Francois Blais

    Francois Blais Supporting Member

    Dec 11, 1999
    Québec, Canada
    Well, it works, but not to my liking.
    The main problem is to find THE sweet spot to put it, in order to get an even tone.
    The device is very sensitive, and in all three axis, so you get a lot of low frequencies signal too.
    A high pass filter is mandatory, but you still have to find the sweet spot.

    I consider it more as a laboratory accelerometer than a useful musical device...
     
  9. mje

    mje

    Aug 1, 2002
    Southeast Michigan
    That was my experience as well. I think the IT has the electronics in the transducer head- I can't recalll offhand if the batteries are in series, whcih would imply a simple FET follower, or if they're wired seperately, whcih would suggest an opamp.

    BTW, I built fdeck's preamp into my Fender P-bass A/E the other night to buffer the piezo pickup- in stock form it mixes the magnetic pickup with the piezo through a 500K pot, which is really dumb. Adding the preamp made a big improvment. I also added one to my Azola some time ago.