Do You Need a Preamp?
Your amp will determine that -- some very few newer and even some older ones seem to get along reasonably well with the ultra-high impedance of piezo pickups, but they are rare. Examples are:
Acoustic Image Contra, Coda, and Clarus; SWR California and Strawberry Blonde; some acoustic amps by Epiphone and Trace Elliot; etc. (check the specifications of your device). In most cases, the need for a "preamp" isn't because the pickup needs a boost -- it's a matter of matching the impedance of the pickup to the amplifier's input.
Keep in mind that when you plug something into an amplifier, it becomes part of the circuit, and that circuit is designed for a specific impedance range. If there is a mismatch, problems result.
- Most bass amplifier inputs usually aim for around 50,000 ohms, which is considered "high impedance" -- electric guitar/bass pickups and general use dynamic microphones' impedances are designed to be compatible with that input impedance.
- Low impedance (like condenser mics and higher quality mics) is around 600 ohms.
- Piezo transducers can be in the 1-2 million+ ohms range. ULTRA-High impedance.
So it's no wonder odd things happen. They can also often have a frequency response bump that further contributes to the odd sound -- that brittle, raspy, all highs and no lows sound that makes you think you have a bad pickup. Probably not -- you just need a buffering/impedance matching preamp. You can improve the sound by EQ'ing it to death, but it still isn't the same. See my FAQ on
OHMS & IMPEDANCE from the INPUT perspective for further details.
Even with an amp that seems to handle the ultra-high impedance of a piezo pickup, I like the buffering/matching effect of a preamp, and I think in most cases they are necessary for the best and most realistic-sounding results. They can also give you convenient control of volume and tone without having to move towards your amp, which can cause feedback problems depending on volume. I have experimented and found the Bass Max can be more "acceptable" without a preamp, but the Double Big Twin really benefits/needs a preamp for best results. Acceptable? that's a call you'll have to make - if you buy without a preamp, try to borrow one to try.
I've also experimented with the K&K Twin Spot on my old '27 "the gibson" tenor guitar, and while it is more acceptable than the bass pickup experience (it's tuned very high), the warmth and improved character that a preamp adds confirms my personal decision to never plug into an amp without one, unless it is designed to handle the ultra-high impedances of piezo-based pickups.
So, unless you have one of those special amps that are designed to accommodate piezo pickups, I suggest that you use some sort of buffering preamp with
any piezo pickup, or you won't be getting everything these and other pickups have to offer. Some "stomp boxes," when not in bypass mode and other preamps may be ok; check the input impedance specification of your device. If it is 1 megohm or higher, it should do the buffering/matching job.
There are other reasons for considering a preamp-
- When you are playing louder and need to locate your amp away from you to avoid feedback, a preamp can give you easily accessible volume and tone controls without having to drag you and your bass over to the amp.
- Some preamps have more sophisticated tone and other controls that can be useful in tone-shaping and feeback avoidance.
