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Active / Passive What do they mean?

Discussion in 'Basses [BG]' started by brianerwin, Dec 6, 2012.

  1. brianerwin


    Oct 21, 2012
    My bass is active SR 505.

    What does the active part do to a bass sound and does it pertain to the way it uses its preamp?

    What does the preamp do? I would think it gives the sound a boost.

    I tried something that I thought was a pretty good idea but it turns out that it didn't work out like I planned.

    I have headphones and I figured that my bass is "active" I would be able plug in the phones and hear it just fine..... not the case. Can someone please explain this to me?
  2. gdavis


    Oct 18, 2012
    Active may or may not boost. Many active preamps have a trim pot or knob to adjust the level which can be set to the same level as passive or give a large boost or anything in between.

    Aside from the boost, it extends the high and low frequency range giving it a more "hi-fi" sound compared to passive. Also, the tone controls of an active preamp can boost as well as cut, whereas a passive preamp can only cut.

    It's not going to be strong enough to drive headphones (even if boosted) since it's still in the instrument level range and a low power signal intended to drive a high impedance input, not speaker impedance.
  3. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    gdavid hit the nail on the head.

    Active - Runs on batteries. The switch that turns the preamp on is actually in the jack, so don't leave your bass sitting around plugged on or it will drain the battery. Plus, most active circuits will boost AND cut, whereas passive only cuts. You should feel a little "tick" in the middle of the range of the knobs. That will be at the point that the knob you are adjusting is neither cutting nor boosting. Many of us call that "flat". You may want to try starting out with all of your tone knobs set to flat and adjust from there, only boosting or cutting what you need to. Signal is not strong enough to power headphones. But there are plenty of cheap practice headphone amps out there.

    Hope that helps.
  4. "Active" is generally too vague to mean anything other than that there is a buffer/preamp stage that requires a DC power source. Often, it means there is an onboard preamp with equalization. To characterize the specifics, however, there are a number of uses for buffers and preamps on basses.

    1. Lowered output impedance. The use of long runs of instrument cable creates a parasitic capacitance that interacts with the resistive component of the signal impedance to create a low-pass filter. Lowering output impedance thus shifts the frequency cutoff point up, reducing the "tone suck." This is one reason why people often incorrectly assume that active components have an inherent tonality.

    2. Constant input impedance. The pickups will see a constant input impedance regardless of what you plug the bass into. This means that various amps, effects, cables, etc. will not influence the part of the circuit between the pickups and preamp input.

    3. Multiple buffered inputs. This is relatively rare, as most preamps have a single input, but in some cases, such as when mixing pickups of significantly dissimilar output impedance (Piezo and magnetic pickups, for instance.), it may be necessary that each source be given its own input that is not influenced by other parts of the circuit. This may also be useful for things like buffered blends, and volume adjustment of individual input sources.

    4. Equalization. This is what most people are instantly familiar with. Two, three, or four band bands of equalization with or without adjustable frequency centers is common.

    5. Fixed voicing. Many players prefer a tonally transparent preamp, but some preamps may be designed to apply a particular voicing, even with the EQ pots set flat. This may be the signature of a particular manufacturer, or it may be a design choice based on the use of pickups that are naturally lacking in some regard, or in need of tonal compensation from a preamp. EBMM's Stingray pickup and preamp pairing comes to mind.

    6. Gain boost. Some preamps provide a boost of gain. When a preamp offers this feature, it is often adjustable with a trim pot on the PCB module. Many people incorrectly assume that "active" is synonymous with "louder," however, and this is not necessarily true. Preamps may be unity-gain.
  5. There is a difference between a mere buffer/preamp gain stage and a proper power amp. Headphones present a much lower impedance load than what a preamp is designed to drive. Furthermore, there is a relatively significant current demand that increases the total power use. What you want is a headphone amp.
  6. I understand everything you are saying here but I'm afraid you may have caused the OP's head to explode. :)

    Rick B.
  7. The preamp in my 1992 Carvin bass was advertised as being able to drive headphones. I remember trying it and it worked. The bass doesn't have an unusually high output when plugged in to an amp so I don't know how they did it. I have not tried plugging headphones into EMG or Lane Poor preamps, my only other sources of reference.

    Rick B.
  8. Stilettoprefer


    Nov 26, 2010
    My stock Schecter stiletto will power headphones, very quietly, but it will make noise through headphones. These Pre amps are designed after EMGs.

    And I tried my passive guitar into a fuzz pedal then to headphones. With the fuzz activated, there was signal through the headphone, without there was no signal.