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"db" question

Discussion in 'Amps and Cabs [BG]' started by DLM, Jan 26, 2005.


  1. DLM

    DLM

    May 25, 2004
    California
    Does "db" work this way?

    If an external rackmount preamp has 9 db of gain and my bass' Bartolini 18v 3-band EQ has, I'm' guessing, 15 db of gain, would I get 24 db of total gain when maxing both out?
     
  2. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Not exactly. First of all I don't like it when manufacturers used dB to measure signals that happen before the amplification stage. dB is supposed to be a measure of Sound Pressure Level, and we all know there's no sound until the signal is amplified and then projected from a speaker.

    Somewhere along the line they started using dB to measure signals that are pre the amp. In doing so, they basically adopted the stance that if you double something, you've just increased it by 10dB. In the case of your rack mount preamp, the 9dB boost is boosting your signal from maybe 0.5Volts to just under 1 volt. Wether or not that translates to a 9dB increase at the speaker depends on a lot of variables.

    As for your EQ - a 15dB gain merely means you can increase that frequency band by 1.5. Most rack mount graphics have 12dB of gain/cut. 15dB for a bass preamp EQ is not out of the question since they tend to grab a group of frequencies and boost/cut them simultanously.

    As for maxing both out, I wouldn't. Chances are you'll end up clipping the input stage of the preamp, and amp will amplify the clipped signal, sending some pretty nasty stuff to your speakers.
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    dB is a measurment of SPL true, but it is also a measurement of relative signal level and gain and is quite a bit more useful than talking about volts. The total gain from pickup to speaker can be a thousandfold; instead of multiplying the voltage by a thousand it's more convenient to call it 30dB.

    10dB does refer to a doubling of perceived volume, but it is not a doubling of signal strength. 10dB is equal to ten times the voltage, or a gain factor of 10; 20 dB is one hundred times the voltage. In the case of the 9dB boost that translates to a gain factor of 8, not somewhat less than two.

    One nice thing about dBs is that they are additive throughout the signal chain. Assuming no power compression a boost of 1dB at the source or anywhere else along the way will result in a boost of 1dB at the output. That 1dB at the source may be a thousandth of a volt, while at the power amp output it may be ten volts, but it remains 1dB in both cases. Doing it this way one can accurately predict what will happen to the dB sound pressure level of the system by altering the signal at any point in the signal chain.
     
  4. DLM

    DLM

    May 25, 2004
    California
    Thanks for both responses.

    I guess my question is more of a volume question (as opposed to technical question). The reason I'm asking is that I'm considering buying a preamp that has 9 db of bass boost as opposed to my current amp having 15 db. I normally have the EQ on my bass set flat. But if I increase my bass EQ, would I then be able to compensate for the 6db differential between the two preamps and notice no loss of bass from a volume/listening perspective?
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yes but in a typical gig situation how often are all things equal? You mentioned amp power compression and that's just one factor to take into account. What about speaker efficiency, speaker position relative to X-max, amplifier gain, boundary loading, room acoustics, all of which would affect the ability of a 1dB pre-amp gain to translate to a 1dB SPL gain.

    Yep you're correct. I cant see you having any problems. Who knows you may find the new preamp has a different voicing that doesn't need any bass boost. And to be honest if you need 15dB of boost at any frequency, I'd be very surprised.
     
  6. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    I think Bill was just trying to correct some factual errors, and not attemping to explain all possible ramifications of the issue.

    dB can measure just about anything, and are commonly used to measure voltages as well as power (eg SPL). The trick is that a doubling of power is represented by a 3dB increase (regardless of the perceptual rule of thumb 10dB). However, a doubling of voltage is represented by a 6dB increase. Has to do with math and whatnot [;)].
     
  7. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Thanks, Ben. Working with volts versus power is confusing as it brings square roots into the equation and that's taxing on my 6th grade math abilities, another good reason for using dBs and thus making only simple addition and subtraction required.

    Pete, while what you say is true it's well beyond the scope of the question. If you add or cut by a finite number of dBs anywhere in the signal chain then the final acoustic output will be altered by the number of dBs if the system is perfectly linear. The fact that systems aren't linear in the real world opens up a different can of worms entirely.

    DLM, the answer to your question is that 15dB is 15dB and it doesn't matter the source. So if you run your current rig with the amp EQ at full and your onboard flat you have 15dB of bass boost. Run your new rig with the EQ full for 9dB of boost, along with 6dB boost from the onboard, and the total boost will be 15dB.

    What Pete was alluding to is that adding 15dB of boost at any point in the signal chain doesn't always equate to 15dB of additional acoustic output; it will do so only if the amp and speaker both have at least 15dB of headroom available throughout the signal chain. If not then both compression and distortion are possible consequences. With bass that's usually not desireable. With guitar it's what makes the world go round.
     
  8. DLM

    DLM

    May 25, 2004
    California
    Thanks, guys. All the info. was really informative and helpful. I can be rest assured that I won't be lacking for any bass adjustability from a volume perspective between the two preamps when using my on-board preamp.