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Is there an amp FAQ somewhere?

Discussion in 'Amps and Cabs [BG]' started by rabid_granny, May 29, 2002.


  1. I'm looking for an amp FAQ that can teach me about what to look for in an amp. Example, I've never heard anyone talk about ohms before, even my guitarists friends don't talk about that.

    I'm hoping that if I sound smart then I can fool salespeople into giving me a good deal.
     
  2. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    Courtesy of Joris

    PT 1

    Frequency.

    The first issue that concerns us, bassists, is frequency. Of all tonal instruments, we cover the lowest frequency range. The frequency range of a bass guitar's notes is 41.2 - 392 Hz. With today's 6 string basses, it ranges from 30.9 - 523 Hz (based on 24 frets). However, a great deal of the sound comes from harmonics or overtones. Especially when playing close to the bridge or with a pickup that's close to the bridge. These harmonics and their relationships are what determines the tone of an instrument, its character. They range to about 2 kHz. Fret sounds like slapping go well above 5 kHz.

    Most bassists find that if the lowest part of this range is heard as loud as the rest of the range, the sound becomes too heavy or muddy. That's why bass guitar cabinets are often equipped with 10" loudspeakers. This 10" size speaker, with a few exceptions, would hardly qualify as a bass speaker, from a technical point of view. In fact, in sound reinforcement terms, they are considered midrange speakers. They hardly go below 60 Hz. In case of a low B on a 5 string, there's no actual low B audible, just harmonics. But most bassists prefer this - if you will - bass light sound. Most of the not so wide-spread 18" speakers cover the entire low bass range. But they need a heavy, large cabinet, which is of course unpractical. A possible alternative is to use hifi speakers, which have great depth and use rather compact cabinets. But they require a lot more power (more on that later).

    But if a 10 is technically so bass light, why not simply boost the bass a lot on the equalizer? This is why not: apart from maybe frying the speaker because of the increased power, it sounds floppy. The speaker has to move 1-2 cm to produce this low bass. No speaker can really handle that. A few millimeters at max, after that it will distort (which may be desirable, by the way).

    Power.

    A watt is a watt. It doesn't matter from which amplifier it comes. Valve amplifiers do not sound louder than solid state, when their power ratings are the same (valves APPEAR to sound louder, because they react differently). You put a watt in a speaker, you get a certain amount of sound. That's how it is.

    If I want to explain something about power, I have to talk decibels first. Sound pressure is measured in decibels. If you double power (twice the watts) the sound becomes 3 dB louder. Unfortunately, an increase of as much as 10 dB is heard as a doubling of volume. This equals ten times the power. So if you put to work 100 watts and you want it twice as loud, you need 1,000 watts. If you're considering upgrading from 300 to 400 watts: DON'T. You gain 1.25 dB. You can't tell the difference. You should at least double the power.

    So how much power do I need? This is no simple question. Obviously, it depends on the playing style, the band's music style and thereby its overall volume level, the type of sound you want, solid state or valves, etc. etc. Some bass players will do fine with a 50 W combo, for me personally it takes 300 watts. A good speaker cab and a 1,000 watts amp head will do 130 dB when standing just in front of it. You won't survive.

    As mentioned earlier, a watt is a watt. No matter what amplifier supplies it. Valve amps appear to play louder because of the way they distort: gradually. As opposed to solid state amps that just stop pumping more current into the load, which sounds like a speaker being ripped apart. It won't damage the bass speakers, but if you have a tweeter that's not well protected, it WILL fry. A moderate level of valve distortion isn't that apparent. In fact, it may even sound nice, putting an edgy, growly quality to the sound. But most valve amplifiers distort way before that point. Almost every valve amp uses an output transformer, through which all output power has to go. There's no such thing as a linear transformer, so they all add colouring to the sound. In my opinion, this is what makes the vintage characteristics of a valve amp, especially old amps, which were mostly equipped with tin plate core transformers. Both transistors and valves have their distinct advantages and disadvantages. It's what suits the individual. A fact is: they sound and react completely different.

    If you're looking for a watt or two more than the average bass head, a PA amp, combined with a separate preamplifier, is a valid choice. PA amps are often of very high quality standard. Much higher than bass heads in the same price range. Or you could put it this way: they are much cheaper per watt, and less likely to fail.
     
  3. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    Pt 2

    Speakers.

    A speaker (or better: electrodynamical sonic transducer) is in essence: a magnet (the heavier the better) and a moving coil (the larger the better) with an attached cone (the stiffer the better). Alternating current is sent through the copper or aluminium windings of the coil, which moves back and forth inside the magnet. The attached cone puts air into motion and we hear sound. But once the air is in motion, it travels to the back of the cone and shorts out the sound, especially the low frequencies. That's why we put the speaker in an air-tight box. The box makes low frequency reproduction possible. Different box sizes lead to different bass responses. Most of the boxes also have a reflex port, of which the working principle is extremely complicated, which increases the low frequency performance of the box even more. A bass reflex cabinet is not just a box with a tube stuck in it. It's a well-balanced resonator in which speaker, internal volume and port interact to obtain an even bass response. Not every speaker is suited for bass reflex. It has to have a certain way of resonating. Specific values like resonance bandwidth (Q) and the tightness of the suspension are of essence. Every speaker should come with these values.

    Efficiency.

    These 10" speakers mentioned above may not do a good job on very low bass, they however require much less power than 10" speakers that ARE capable of delivering very low bass, like hifi speakers. The lower the bass, the more power you need. Unfortunately, "much less power" still means less than 3% efficient. Which, in turn, is why most of us need several hundreds of watts to obtain the sound pressure we need on a gig. If speakers were to be 100% efficient (which is physically impossible due to impedance and friction), regular bass combos would have 5 watts of amplifier power instead of 150. Some brands build bass cabinets with very impressive low bass response, using 10" drivers. But, as said above, they need at least 4 times as much power, compared to the average speaker cab.

    A normal, reasonable value for efficiency is 95 dB. This means that the (housed) speaker produces 95 dB at a distance of 1m, when 2.83 volt AC is applied to it. For an 8 ohms speaker, this corresponds to 1 watt. This 95 dB speaker is 2% efficient. In other words, 98% of the power put into it, turns into heat. If you were to replace it with a speaker that's 98 dB (that's twice as much, see explanation of dBs) efficient, you could cut your amplifier power in half. For speakers with impedances other than 8 ohms, the efficiency is not measured at 1 watt. Therefor, these numbers can't be compared directly. You have to compensate for the different impedance. If you connect two speakers to one amplifier, the efficiency gain is 3 dB (twice as much) because of the increased cone area. And in case of a parallel connection, the amplifier can deliver 3dB more power with half the impedance. Your total gain is 6 dB (=4 times the power). That is, if your amp HAS a double power rating for 4 ohms. It's only theory that you get twice as much power when you have half the impedance. Most amps just can't put up with the extra heat and current. So in most cases you get only 50 percent more power.

    An extreme example (based on efficiency ratings supplied by the respective brands). If you would compare Acme's low B2 cabinet to Trace Elliot's Green Giant 8x10, you'll find that the Acme needs 30 times as much power as the Trace Elliot to obtain the same sound pressure. However, in respect, the Acme delivers astonashingly more low bass than the Trace Elliot. It goes one whole octave lower. And here's the catch: if you'd boost the bass on the Trace Elliot cab to compensate, it would lose all of the advantage, because that much power would be needed to get that amount of depth out of it.

    Impedance.

    All speakers have an impedance. In fact, all electronic parts have an impedance. Simply put, it is the rate at which they convert the input voltage into power. A lower impedance means more power at the same voltage level. But this also means more current (it has to come from somewhere). As if it weren't complicated enough, the impedance is not a constant. It changes along with the frequency (a bit like a graphic EQ with a smiley setting). But to keep it simple, we'll say it's a constant. Most amplifiers can work with 4 ohm loads. Some are limited to 8 ohms and some go as low as 2 ohms. Some car amplifiers do 1 ohm. The impedance of a speaker can NOT be changed (except, maybe, when it's burned out). However, some speakers have two voice coils, which can be switched in parallel (1/2 imp.), in series (2x imp.) and sometimes in single (1x imp.). In the last case, you lose half the efficiency. Multi-speaker cabs sometimes have an impedance switch, which rewires the cab internally. Usually you have a choice of 4 or 16 ohms. Other cabinets have the possibility to have their impedance altered by the manufacturer from 4 to 8 ohms or from 8 to 4. This means replacing all speakers. The most important advantage of a lower impedance is: the amp delivers more power, provided it can handle the load. If an amp has a certain load impedance capacity, you can ALWAYS go higher. NEVER lower.

    You can always combine cabs on one amplifier. Running two cabs on a 2-channel amplifier is the nicest way. Just make sure the cabs have an impedance equal to or higher than the load impedance capacity of the amp. If the cabs have different impedances: no problem. But keep this in mind: the higher the impedance the lower the amp power. Most amps only have 1 channel so it gets more complicated. Never combine two cabinets with different impedances, unless you really know what you're doing. Two 8 ohms cabs can be parallelled if the amp can handle 4 ohms. Two parallel 4 ohms cabs would get you 2 ohms, which would burn out a 4 ohms amplifier. Two 4 ohms cabs in series (you need special cables for this or cabs with a series output) give you 8 ohms, which is a very safe value. It is inadvisable to connect two different cabs in series: they will influence each other. Combining cabs with a very different efficiency (more than 4 dB apart) isn't very useful. The "louder" one will completely outperform the other. Unless you have a 4x10 stacked on a 1x18. The two cabs have different frequency ranges. The 18" will put in some serious low end, and the 4x10 will provide the mids and highs.

    So, if you want to buy a cabinet, you probably want a 4 ohms one. That way, you can make use of the maximum output power of the amp. But, if you're planning on ever getting a second cab, you probably want an 8 ohms as your first, and later a second 8 ohms.
     
  4. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    Again, Props for this work goes to Joris
     
  5. Whoa. Cool! Was this in the forum?
     
  6. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    I think so...ages ago, and in a Zip file, I think.
     
  7. I just finished reading it.

    My head hurts...
     
  8. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    :D
     
  9. I'm still working on a really, really, long version of that article. Covers virtually everything. Shouldn't take much longer now.

    PS. It should make your head blow up :D