Do You Need a Preamp?
Your amp or other device you plug your pickup into will determine that -- the input impedance
of the amp affects how a pickup sounds.
Piezo (most common) and similar acoustic instrument pickups are very different from the magnetic pickups you find in electric guitars and basses, and most amps are aimed at that market. 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. There are three general impedance values, as follows:
- 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 with XLR connectors (three pins inside a circular shell) match best with an input around 600 ohms.
- Piezo transducers want an input load in the 1-10 million+ ohms (1-10 megohms) 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.
A preamp is placed between the bass pickup and the input of your amplifier:
Here are some reasons why you should have or should consider using a preamp between the pickup and amplifier:
- the input impedance specification of your amplifier is under 1 megohm
...also expressed as 1,000,000 ohms or 1Mohm; being close to that value, like 800K, is usually ok. This is probably the most common reason for using a preamp.
- there are features available from the preamp that are not on your amp, that can enhance or help with problems
...like more precision or flexible tone controls, phase reversal, a DI output, high pass filter, etc.
- you want/need a convenience volume control (and tone controls?) close to you, for when your amplifier is not at your feet
since at higher volumes you will likely need to place the amplifier further away from you, so avoid feedback and other undesirable sounds
- your pickup's output is very low, so you need a preamp to increase its signal level to better match your amplifier
Even with an amp that seems to handle the ultra-high impedance of a piezo pickup, I often 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 (turned off) 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.