Piezo Blend?

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Burjor, Feb 7, 2008.


  1. Burjor

    Burjor Member

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    I have a custom guitar with two humbuckers and a piezo setup. The piezo has a on off switch and its own volume. When I run the guitar through a PA or a bass amp the piezo sounds very smooth and really brings forward a nice acoustic flavor. However when I play through any and all guitar amps the piezo is overwhelmingly bright, even if the piezo volume is under 1 on the dial.
    I put a VOM meter on the piezo volume pot to check out the values. It starts at 0 very quickly jumps to 300 (1 on the volume knob) goes to 1580 (6-7 on the volume knob) and then tapers back down to 0 (10 on the volume knob). Even though it shows 0 when the volume is at 10 on the VOM meter the piezo is still clearly coming through loud and strong, it just has a slightly different tone. The volume pot also has a 333 capacitor soldered to it.

    What would be the best way to create a gradual increase in piezo volume?
    Should I change the capacitor value?
    Is the volume pot a 1.5M pot? Should I change it to a lower value?
    I am new to this stuff so any info is much appreciated.

    I do not care how my guitar sounds through a PA or bass amp as I play through my guitar amps 100 percent of the time.

    Thanks
    Burjor
     
  2. Bob V

    Bob V Member

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    I might be confused by your title, but if you "blend" your piezo into a mono signal with your magnetic pickups you run into huge problems with impedance mismatch unless there's a buffer (something with a battery). Does your "on/off" switch toggle back and forth between magnetic and piezo, or is there a middle position that combines them? If you do have an onboard preamp, see if there's a trim pot on the circuit board that you can turn down a little. If there's just a volume pot, it's usually a much higher value than a magnetic circuit, something like 1 meg ohm instead of 250K.

    I'd recommend the Fishman PowerChip which is an onboard preamp/buffer with a volume control for the piezo. It will also blend the two signals if you use a mono cord. It will also keep them separate if you use a stereo cord (it detects which kind of cord is plugged in). It has plenty of juice, and I find I needed to turn down the trim pot quite a bit to get the piezo down to a level comparable to the magnetic signal when they're played together (mine is on a tele style partscaster with a Fishman powerbridge and Lindy Fralin blues special in the bridge position and an Unbucker in the neck).

    Problem is, I can never be satisfied with the sound of a piezo run through a guitar amp, particularly not at the slightly dirty settings I'd use for the magnetic pickupss. Best solution is a second amp for your acoustic signal or a direct box to the PA. Now you can start panning back and forth and using volume pedals on each signal, and you get a clean acoustic sound with a warm electric sound. And of course there's a separate effects chain for each one. Acoustic amps with onboard digital signal processing are getting cheaper now, there's one on my GAS list.
     
  3. Burjor

    Burjor Member

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    Really excellent point. Let me clarify. When the piezo toggle is on the humbuckers are still on with their own separate volume. Both humbuckers and the piezo are passive, their is no extra power, preamp etc.
    Since the piezo is so much hotter than the humbuckers would switching to a lower value pot help?
    I am also wondering if I can somehow voice the piezo to sound better through the guitar amp with a different value capacitor. I am trying to keep my gear down to a minimum when I gig and I would prefer not having to take a separate amp for the piezo.
    Thanks for the help.
     
  4. pipedwho

    pipedwho Member

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    Using a lower value pot for the piezo will make it sound brighter. A piezo acts like a capacitor, so by putting a resistive load on it's output, you'll effectively roll of the bass side of it.

    If you want to tailor the output to be less bright, you can add a tone pot to the output of the piezo. The tone pot will need to be a high value like 1.5Meg and the capacitor will be on the order of 5nF. Unfortunately, piezos don't respond to tone controls in the same way magnetic pickups do, and I haven't tried this before.

    As Bob V has said, an active buffer will help dramatically. I used the Graphtech preamp to mix the two signals. This let me put a decent value tone pot on the piezo side of things for use with an electric amp.

    But, I have to say, dual amping (mags to the guitar amp, and piezos to the acoustic amp) is the bees knees and worth the price of entry! :AOK
     
  5. Big0range

    Big0range Member

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    Spot on. I've got a couple of guitars with a piezo / magnetic pickup mix, and I'm never happy with the piezo unless I'm running it into an acoustic or clean amp (preferably an acoustic amp). I gotta lug an extra amp and the minimal hassle of two guitar cables instead of one, but it's worth it. I figure why bother having the piezo if you can't use it exactly the way you want?
     
  6. walterw

    walterw Gold Supporting Member

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    +1 piezo without proper impedance-matching preamp = brittle, useless yuckiness.
     
  7. Burjor

    Burjor Member

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    If I install the preamp would my humbuckers also be active or would they stay passive?

    Thanks
     
  8. Bob V

    Bob V Member

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    Well, lessee. If you run a stereo cord, then there are two independent signals - passive magnetic and active piezo. If you use a mono cord, the PowerChip mixes the two but I have not heard any difference in the output of the magnetic signal (and I have a digital amp with a trim control that is sensitive to different outputs for different guitars so I'd probably have noticed). Theoretically it has a unity gain buffer on it. If you turn the PowerChip volume down all the way the piezo goes away and it behaves like a regular electric guitar except that you do need a battery to keep it working.

    If you have a LP/ES335 type four-knob layout and you don't want another knob on there for the Powerchip, you can make room for it. One suggestion is to make one of your magnetic tone controls a master tone for the magnetic side (instead of one tone for the rhythm pickup and one for the lead). That frees up a knob location for the powerchip while staying stealthy.

    It's interesting that your guitar would have a passive setup blending the two signals into a mono output. Fishman warns vehemently against it in their instructions for the various Power-Bridge models. L.R. Baggs I believe is the other major manufacturer of electric guitar replacement saddles with piezos in them, but they also have a matching onboard preamp. All the acoustic guitar undersaddle pickups now come with onboard preamps, so it's pretty much state of the art to have your signal buffered before it tries to travel along any cables outside the guitar, and again it's necessary before you can mix it with a magnetic pickup signal.

    I got my PowerChip from Ted Blue at Blue Star Music; they're $79.95. Cheaper than many of the outboard preamp gizmos and certainly cheaper than a stomp box.
     
  9. pipedwho

    pipedwho Member

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    It depends on how you wire it up. I tried both, but ended up buffering both the piezo and the magnetics. The only difference in sound is that the cable has less 'tone suck', and fuzz pedals don't load down the pickups. The key is to make sure you have tone controls on the guitar for both pickups - I used a couple of dual concentric pots to achieve that without adding extra pots to the guitar.
     
  10. Burjor

    Burjor Member

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    It looks like the best thing is to install a buffer/preamp. Thanks to everyone for the information.
    Burjor
     
  11. walterw

    walterw Gold Supporting Member

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    i like the fishman powerchip, which replaces a tone knob with a piezo volume control and automatically blends or splits the piezo and the magnetics depending on if you use a mono or stereo cable.
     

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