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Thread: Bass amps and pedals

  1. #1
    Member Andy123's Avatar
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    Bass amps and pedals

    Rookie question:
    Can anyone tells me what the main difference is between a guitar amp and a bass amp?
    Likewise with bass specific pedals?

  2. #2
    Member Cliff Rogers's Avatar
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    The tone controls are voiced differently.
    Typically bass amps are closed cab designs.
    A bass amp is generally cleaner than a guitar amp.
    Cliff

  3. #3
    Member PJSprog's Avatar
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    Frequency response, mostly. The preamps are designed to manipulate a different frequency range. Guitar amps also usually have a different gain stage.

    Combo amps and speaker cabinet designs differ as well, also to cater to the frequency range differences.
    What Did You Play Today? ~PJS~

  4. #4
    Mentor blinddrew's Avatar
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    If you're just talking about the amp, then it's as stated above. Cabinets and speakers have a very different set of build requirements though.

  5. #5
    Member Andy123's Avatar
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    Thanks for that, I suspected as much. There was a bit of an urban legend going around among rookie musos when I first started out that playing bass through a guitar amp would damage the guitar amp. I never understood why that might be - it is just an audio signal passing through an amp.

    Hypothetical situation:
    You're roped into filling in on bass in your mate's band. Available for you to plug into onstage is:
    • a 50watt solid state bass practice amp
    • a Marshall JTM45


    ...what do you do?

  6. #6
    Member JimC's Avatar
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    A bass guitar signal can most certainly damage the speakers in a guitar amp. Especially if its an open backed cabinet. The cone movement is greater at low frequencies, and there's not much restraining the cone, so the voice coil can actually pop out from between the magnets or the cone tear. A bass speaker will typically be ported or sealed, which provides some protection for the driver.

    I have seen an explanation of how a bass signal with its extra energy could (IIRC) somehow overwhelm the output transformer of a valve amplifier, but I didn't understand it and can't remember it so your guess is as good as mine how much truth there is in it. My utterly uninformed guess is you'd need to be thrashing the amp though.
    Last edited by JimC; 11-10-2019 at 07:10 AM.

  7. #7
    Mentor fender3x's Avatar
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    I think I agree with most of what's been said here. This is more to clarify. It's easy to get confused about just what is meant by "amp." A combo amp combines a pre-amp, a power amp and a speaker cabinet. A typical bass "head" combines a pre amp and a power amp. It's also increasingly common to separate the pre- and power sections for bass as well.

    It's unlikely that you could damage a pre or power amp just by playing a bass through it, especially a modern one. Speakers are another story, but it is primarily about power.

    The pre amps are typically different because the EQ emphasizes lower frequencies, particularly for a vintage sound. For more modern stuff they also reproduce overtones, which is why modern cabs tend to have tweeters. Bass overtones sound harsh when distorted, which is why they tend to be a bit more like PA or hi-fi amps than guitar amps that are designed to distort.

    The main difference in a bass power amp is the power. Bass notes require a LOT more power than guitar. Ampeg, Sunn were popular mostly because they could keep up with relatively small guitar amps. Most guitar players I know gig with amps that are around 50 Watts or less. By contrast small class D amps for bass produce 250 Watts or more, and 1000 Watt amps are common.

    The cabs and speakers are the biggest difference. Again this is mostly about power. The lower the note the more air you have to move to produce it. No matter how much power you put into the speakers, volume limited by the amount of air the cab can move. One way they do this is by moving the cone. Bass woofers tend to have much more travel (the spec is called "xmax") than guitar speakers. They generally have more robust voice coils as well to keep the amps power from frying them.

    Low power voice coils an low xmax speakers are super easy to fry with a bass, even at low power levels.

    The cab is also different because it tends to contribute more to the sound and power. Bigger closed cabs reproduce low frequencies better. Ports are a way of tuning the cab to produce low frequencies while maintaining optimal pressure in the cab. The cab works sort of like a spring or resonator to move more air. I don't know that that protects the speaker in a direct way. I think it is mainly protective in that the bass player can use a lower power setting to get the volume he/she wants and is less likely to fry the speaker by Turing it up too high.






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  8. #8
    Member PJSprog's Avatar
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    To throw a bit of a wrench into this conversation ... Sometimes, there's literally no difference at all. When I worked for St. Louis Music, we built an updated version of the Ampeg V4B head (late '90s, early 2000s, can't remember exactly when). It was literally a modified version of the Ampeg SVT-CL tube preamp with a modified version of the Crate Blue Voodoo 100w tube power amp section. So, in a sense, it was kind of both.
    What Did You Play Today? ~PJS~

  9. #9
    Overlord of Music Simon Barden's Avatar
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    But I expect the EQ sections were altered, with certainly the bass and mids being voiced an octave down to suit a bass guitar?

    Modern bass guitar amps will often incorporate a high-pass filter, to both stop the amplifier wasting power on amplifying sub-sonic rumble, and also to help protect the bass drivers from trying to oscillate at low frequencies and high excursions which can damage them.

    Historically some (not all) valve guitar amp output transformers were rather undersized for the task, and as Jim mentioned, putting bass through them (normally frequencies below 60Hz) can cause them to present a very high impedance to the power valves, similar to running the amp without speakers and damage the valves and then the transformer itself.

    And bass cabs are almost always either sealed enclosures or ported enclosures, which prevents over-excursion of the cone at loud volumes. You can use open-backed cabs for bass, but you lose a lot of bass energy by doing so, and the speaker coil can easily be pushed to overextend its rated travel limits unless the speaker power rating is significantly higher than that which the amp can supply.

    Human hearing is less sensitive at bass frequencies, so a bass sound needs to be significantly louder (in physics terms) in order to sound the same as a guitar sound.

    Relative loudness levels are normally shown on a Fletcher-Munson curve (it's an average value as people's hearing responses will always be slightly different, and also what they perceive as the same volume). So down in the lower frequencies, a 50Hz note (around a bottom G on a bass) needs to be around 8dB louder than a 100Hz note (around a bottom G on a guitar) to be heard to be at the same volume.

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    8dB may not seem like much, but in amplifier power (given equally efficient speakers), you need 6.3x the amplifier power for a 50Hz note to sound the same volume as a 100Hz note. So a 50W solid state guitar amp is best matched by a 350W bass amp. And a 50W valve amp, which when pushed will output nearer 100W, is best matched by a 700W+ bass amp. Add in a very efficient guitar speaker and a less efficient bass speaker, and the required power differences become even larger. So you can see why high powered bass amps are required if they want to compete with a loud guitar on stage.

    Luckily we tend not to use such loud stage volumes these days, and even PAs are generally used at much lower levels than they used to be (though the sound is of much higher fidelity), so we shouldn't need to use all the capabilities of our amps, but it's nice to have the headroom.

  10. #10
    Overlord of Music DrNomis_44's Avatar
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    Here's an interesting side-note, the original 5F6-A Fender Tweed Bassman amp was originally designed for use as a bass amp with the Fender Precision Bass, but, some blues guitarists found that it turned out to be a pretty good guitar amp, so much so that Marshall based their first amp, the JTM45, on the Fender Bassman design.
    Last edited by DrNomis_44; 17-10-2019 at 11:08 PM.

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