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Someone please explain 18 V headroom

Discussion in 'Pickups & Electronics [BG]' started by RED J, Nov 4, 2013.


  1. RED J

    RED J Lol

    Jan 23, 2000
    Hey guys,
    I'm puzzled when I see 18V onboards spoken of as having headroom. I've had active basses, all 9 volt, and though I understand what headroom is related to amplification, there is nothing in my experience that can relate it to an active circuit on a bass. Please 'splain. Thanks.
     
  2. Higher headroom means you can handle a hotter signal before distorting.
     
  3. All gain stages have headroom. It's a measure, not a feature.
     
  4. RED J

    RED J Lol

    Jan 23, 2000
    Thanks guys I understand the concept but give me a scenario where that becomes an issue while playing an active bass, I guess that is what I don't get.
     
  5. With cheaper preamps, you'll notice distortion when you play hard. This is especially true of basses with hotter pickups.
     
  6. Gain is a multiplier... Gain is usually measured in dB which is a log scale but certainly can be expressed in a linear scale for simplicity's sake.

    So if a stage (a preamp) has X gain, and peak voltage of the the signal coming in is A volts then the output of the preamp is AX volts. If AX is greater than 9 volts, and you have a 9 volt system, it will distort.

    Headroom is the amount of room before distortion... so if you have a 9 volt system and your output is 8 volts and then you have 1 volt of head room.

    Mind you, preamplifier systems cannot utilise the entire rail-to-rail voltage (9 volts in my example), so head-room and distortion thresholds are actually lower. This is because there is trade-off in circuit design. Lower current designs (longer life) will generally have lower thresholds. Faster response (better fidelity) will also have lower thresholds.

    18volt systems allows for better use of these design criteria. But certainly, there are MANY great 9volt systems out there.
     
  7. megafiddle

    megafiddle

    May 25, 2011
    Headroom is like a safety margin. If gives you some room to get a bit louder beyond your
    normal playing level without clipping.

    If your amp is barely big enough, and you are running your level right at full output, then
    you have no headroom. If you had to get any louder than that occasionally, you wouldn't
    have the room to do it; you would wind up clipping the output.

    Now if you switched to a bigger amp, but kept your actual sound level the same as it was
    with the smaller amp, you would now have some headroom. The headroom is now there
    because the bigger amp has additional power that is not normally being used. But if you
    raised your normal level up to the maximum that the bigger amp could handle, then that
    headroom would be gone.

    It's the same thing with a preamp. For a given average playing level, the larger voltage
    swings available with 18V gives you the headroom to produce higher levels occasionally.

    So headroom is not a diifferent type of power or voltage swing that has been added or
    something. It is simply that you have more than you need and you are reserving it for
    those occassional louder peaks.

    -
     
  8. RED J

    RED J Lol

    Jan 23, 2000
    This is the answer. It just never occurred to me that it would happen. I've always compensated for a signal hitting a preamp too hard because I hate distortion, ( e.g. really hot humbuckers hitting an amp's preamp section ) I guess I just instinctively did it on onboards too. I tend to dig in a lot too, guess it's something I just naturally compensate for with volume/gain controls. Many thanks. FWIW, just as you can do the same thing with passive pickups depending upon how you balance things out.
     
  9. mrbell321

    mrbell321

    Mar 26, 2012
    N. Colorado
    To add a bit, an amplifier essentially takes the signal and uses it as a valve to control the output of a battery. So, normally, you can't exceed the voltage or amperage of the battery.

    The raw signal from a pickup is fairly likely to swing between -1V and +1V RMS. That doesn't seem like much, but there's more...

    To amplify both positive and negative sides of the signal(which you want to do), the 9v from the battery is "divided" so it represents -4.5 and +4.5 That +/- 1V seems alot closer now, eh?

    But there's more still. There are inefficiencies in the system that mean that it's actually closer +/- 4V.

    And then, if you consider most circuits can't swing all the way to the rails(that +/- 4V I mentioned), so you're probably looking at something like +/-2.5V maximum input from the coils to the preamp circuit.

    And if you dig in, you might even be pushing +/-1.5V RMS, that's pretty tight but it's not out of the question. Now RMS means that's not an actual peak value. I can't remember the math right now, but I think +/-1.5V RMS would actually mean over +/-2V peak-to-peak.
    There's still some room there and that why many many preamps get away w/ 9V systems.

    But as batteries die, they produce lower voltage, so you might have to go back to the beginning of the math and start w/ 8V and that extra +/-1V headroom can evaporate.

    Also, I've seen some pickups hit RMS of 2V. And remember, this is an amplifier, right? So it's not unreasonable that there is more output from the circuit than came on the input signal(usually, for an onboard preamp, it's not much in terms of voltage; current is often the goal, but you're unlikely to be limited there.)

    So some people like 18V to get a little more voltage, a longer life, and, if you're aggressive or run hot pickups with a preamp, less distortion.
     
  10. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    This is one of the keys to understanding the value of 18V. If you want to get the full rated capacity out of your 9V battery, the capacity that is used to tell you that your battery will last XXX hours of use, then the terminal voltage of your "9V" battery when it is exhausted will be about 4.8V. Rerun all the calculations above at 4.8V!

    But, before you do that consider the other key to understanding the value of 18V. If your preamp lets you run 10dB of boost and you boost the bass range that much you will have a gain of about 3 and you should multiply your estimated signal by 3 before running any headroom calculations. If you can and do run 20dB of boost then you must multiply your signal levels by 10.

    Now you can calculate your headroom requirements ....

    18V is starting to sound like a good idea! I have a 9V Fender and an 18V Fender. The 9V Fender has a lot lower output than the 18V. The 18V is about like a passive bass, maybe a little hotter, until I boost it and then it is way hotter. The 9V is a quiet bass. The 9V was built to have a low output so that you can run 12dB boost on a 4.8V battery without distortion. If you have a 9V bass with normal passive level output with the preamp set to flat then you can expect that it will distort well before the battery has been exhausted if you run a lot of boost. Not a huge issue but I prefer 18V.
     
  11. Sav'nBass

    Sav'nBass Supporting Member

    Jan 18, 2009
    Northern Va.
    So that begs the question If I had a 9v preamp a d I hooked up another 9v battery to it in series boosting it to 18v could that damage the pre amp eventually or would it have no effect? I ask this because I have seen 18v mods for MTD basses that seems to work with no ill effects but I am curious about longterm effects.
     
  12. mrbell321

    mrbell321

    Mar 26, 2012
    N. Colorado
    It would depend on the particular circuit topology and components used.
    If the components are rated properly and the circuit is built correctly, there should be no ill effects.
    If the components are not rated properly or if the circuit isn't set up right, it could be fine or it could fry immediately or it may just not behave properly or any combination of those.
     

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