Tech questions? Ask away.......

Discussion in 'Amps and Cabs [BG]' started by Joris, Nov 19, 2000.

  1. This is an article I wrote quite some time ago. With the last Talkbass board upgrade it was corrupted. I decided to repost a slightly edited version of the article. I haven't had many reactions from the old thread lately (which was called "Tech questions? Ask me!!!", and after it was damaged, it didn't have my initial post on top anymore), so I figured: let's start a new one, with the initial post on top this time!


    Across this board I stumbled upon some technical questions and misconceptions. The following is for the people who are interested in the electronic part of bass playing and amplifying. I am a micro-electronics engineer both by profession and hobby; I built my own rig. Pictures of it were on the old board and on my personal homepage (see below). I think I can make a valuable contribution to this board by putting some of my technical knowledge on it. I hope it isn't too technical. Anyway, I hope it will help you in understanding this difficult stuff.


    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).


    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.


    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.


    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.


    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.

    Well, that certainly raised more questions than it answered, but hey, this is a discussion, so let's have it.

  2. Sampoerna

    Sampoerna Guest

    Oct 9, 2000
    W. KY, USA
    I'm going to have to read this about 30 more times and take some courses in electronics before I begin to comprehend.

    Heck, my primary way of practicing is by plugging my bass into my computer's sound card. [Computers...there's something I understand!] Sometimes I use hubby's guitar amp (at low volume levels, of course...don't want to make him amp-less!).

    This is another example of why these forums are so valuable. All my experienced musician friends have moved away and I'm at the mercy of my local retailers and whomever they decide to hire.

    Thanks, Joris! Even though I don't understand it all, I still appreciate it and will make efforts to put this information into use.


  3. Flatwound

    Flatwound Supporting Member

    Sep 9, 2000
    San Diego
    Well, Joris, I understood it. Uh, mostly ;) . I'm currently using a 100 watt head on a Peavey Black Widow cab and it seems to work fine, but we don't play at super high volumes. I also run a line out to the PA. Peavey doesn't seem to publish efficiency ratings on their cabinets so I don't know how good this one is (115 BW) but it seems to do pretty well. You'll probably get some flak from people who feel that tube amps are always louder and better. I don't have much experience with tube amps, but solid state has always worked well for me. I figure if I ever need high power, I'll probably go with a pre-amp/power-amp configuration.
  4. phil_chew


    Mar 22, 2000
    Thanks for all the technical info. It is much appreciated.
  5. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Thanks for re-posting that Joris. It should be required reading for all newbies. It's a good refresher for me, too.
  6. VicDamone


    Jun 25, 2000
    Is there a simple (with a voltmeter) way to tell if a component, such as a preamp or amplifier, which has XLR connections is an actual balanced circut or its simply accommodating the XLR connector?
  7. Good post Joris. Tell me, why does my 500watt @ 4ohm, 400watt @ 8ohm, SWR SM400S not seem to be as loud as some other amps?. It's not a problem, I take a feed to the PA anyway, but I am curious as to why one maker rates an amp so differently to another.
  8. CS


    Dec 11, 1999
    Glad this has been re-vamped, I dont want to get in the way of Marty's question but I have one of my own.

    I posted here and at Webers and E mailed the manufacturer last week.

    I have just bought a Trace Eliiot V4 it has 2 speaker connections, 1 1/4 jack and 1 XLR with a painted line from one t'other. Can I use one or two cabs? It makes a difference to what cab(s) I buy. Thanks in advance.
  9. CS


    Dec 11, 1999
    Trace Elliot have just replied-it has parallel outputs
    and I can use one or both (impedance switch as well)

    I would pull the post but I want everyone here to know that Paul at Trace Elliot is a great bloke. 3 days to support a discontinued product that they did not make a profit on is excellent customer care, respect.

    Back to Marty :)

  10. pierce

    pierce freethinker

    May 25, 2000
    San Francisco, Ca
    ok, lets see if i understand watt:) your saying about power.

    lets say that x = power, and y = the sound pressure that an x-rated:D amp produces on a particular speaker. is this how the power to pressure progression would look?

    x = y
    2x = y + 3db
    4x = y + 6db
    8x = y + 9db
    >>>>>9.33x = y + 10db (double audible volume)
    16x = y + 12db

    or did i totally miss it?
  11. MikeyD


    Sep 9, 2000
    Yes - this isn't a linear algebraic problem, but one of a logarithmic (powers of 10) variety. Power in decibels is P(dB) = 10*log(P/Pref), where Pref is some reference power level. If P is twice Pref, you have 10log2 = 3.01 db.

    Sound Power Level is acoustic power in watts, as expressed in dB. At a particular point in space, it is proportional to the Sound Pressure Level (which is actually derived from pressure squared). If the reference pressure and the reference power are chosen to produce the identical sound level, then Sound Power Level and Sound Pressure Level are equal quantities. I don't have my acoustics book handy, so I will refrain from giving the reference levels right now.

    What we hear is related to a psychoacoustic phenomenon, not math, although the logarithmic scale approximates it best. Our useable (non-threshold) hearing extends over about 120 dB SPL, from quietest to loudest (pain threshold). That corresponds to a factor 1.0 trillion difference in power level! That one trillion does not sound to us like a trillion times difference from softest to loudest. It sounds more like a 12-fold (4096 times) difference, which is why the logarithmic dB scale works so well.

    Be careful equating amplifier power to SPL. For a given amplifier/speaker combo, in a particular orientation, in a particular room, with the ear (or microphone) at a fixed point, you can then make a reasonable 1-to-1 correlation between amplifier power and SPL - and then only at a particular, fixed frequency. If anything above changes, all bets are off. Usually a very small percentage of the amplifier power reaching the speakers gets "transduced" to acoustic power. Once that bit of power is fully converted to acoustic, then it has to be propagated out to the listener - and is subject to all kinds of other phenomena (absorption, interference, reflection, etc.) which affect how much actual sound power (or sound pressure) is achieved at the listening position.

    - Mike
  12. mikemulcahy


    Jun 13, 2000
    The Abyss
    Thanks for re-appering oh great one of tech knowledge. Here's my story: i use 2 Eden 410 and 2 Peavey 118 with 400w and 800w amps respectively. The problem is I have to cut the volume to the horn due to the fact it "honks" and any respectable volume. The cabs are rated @ 540w RMSwith a frequency response of +/- 2db 80hz-14khz.Mind you i dont play at rediculous volume, but i have to keep up with 2 Marshall stacks @ 100w each and the noise only comes out of the horns and not the speakers.

    Any thoughts recomendations?

  13. How is the sensitivity measured with 4 ohm cabs? I think you said that it was different from the 8 ohms? That 100db 8 ohm is not the same as 100db 4 ohm sensitivity? Or have I got this all wrong Joris?

    Good to have this "Tech questions" back again" :)
  14. rickbass

    rickbass Supporting Member

    Joris - Thanks so much for your kind invitation to take advantage your considerable knowledge. Perhaps you, (or someone), could comment on few things that would help me out tremendously, and perhaps, others -

    1. In Acme's "Short Discussion of `Maximum Output' Specifications" it says, "...consider the JBL E120, a highly efficient 12" guitar speaker. The JBL120 is rated by the manufacturer to have a sensitivity of 103db 1w/1M. This would be considered a very high sensitivity for a moving coil loud speaker. Additionally, with a maximum input power of 300 watts, it is a very loud 12" speaker indeed. In practice, the JBl E120 is loud enough to kill a small child." To a rube, JBL is saying that, in theory, the speaker will produce 309dB's if one pumps 300W into it and stands 3 meters away. However, I hope they don't think anyone is that naieve. Can you explain how JBL is B.S.'ing in laymens' language ?

    2. I hated algebra, geometry, trig, never even grasped the rest. Are there manufacturer spec's I could replace in the equation "Om = (10 x log Pm) +S" to get a grip on loudspeaker sensitivity ?

    3.When you look at manufacturer spec.'s for bass speaker cabinets and amp head, which spec.'s do you pull out and what are the calculations you plug them into, (and where ?), so that you're not impulsively impressed when, for instance, you see a catalog which says "our awesome Hiroshima BassEuphoria head puts out 9800W RMS" and I'm not sitting there trying to follow the intricate plot of a "The Dukes of Hazard" episode, thinking, "I gotta have me one of them!" ?

    Printed out your thread. Very appreciative of your generosity. You're :cool:


    [Edited by rickbass1 on 11-22-2000 at 01:52 PM]
  15. Matthias


    May 30, 2000
    Vienna, Austria
    rickbass, here is an interesting thread for you (I guess):

    Joris, Mike:
    Why is it, that 15" and 18" loaded cabs are better heard in the distance?
    Is speaker size ITSELF the reason, or is it just because larger speakers produce more lows, wich are better heard farther away in general, no matter from wich speaker they come?
    In the latter case (wich I believe is true) it should be possible to achieve about the same result with say a 410 with boosted lows (on the EQ) and a 115 or 215 run flat.
    Hope this is not too weird and you know what I mean...


    BTW: I know that this is very 'academic', that you have to use your ears to know what's best for you, that it all depends, and so on. In this case I'm just interested in the theory behind.

    [Edited by Matthias on 11-23-2000 at 03:11 AM]
  16. Hi Joris,

    Could you explain what a limiter is meant for and what it does, and why I should (or shouldn't!) use it. If it protects my cabs I'll use it but if it also playes the role of a compressor I won't. Are there a way inbetween, so I could turn up the limiter just to the point where it would protect my cabs, but not interfer with my sound? My rig is a SWR Bass 750(has a built-in adjustable limiter)and two Goliath III.


  17. MikeyD


    Sep 9, 2000
    I doubt JBL claimed 309 dB. That is probably close to the sound level of a supernova at a distance of 1 meter (i.e., loud enough to destroy the solar system!). A Saturn 5 rocket is somewhere around 180 dB (as I recall), and that sound level would probably kill any human being standing nearby (assuming one could survive the heat and blast force [NOT!]). Anyway, the formula for decibel power level is Lp(dB) = 10*log(P2/P1). In your example above, P1, the reference level, is 1 watt, and P2 is 300 watts. So the increase in dB over the reference level is 24.8 dB. If the reference power level yielded 103 dB, then the 300-watt level would yield 103 + 24.8 = 127.8 dB (assuming the speaker is linear in acoustic output vs. power input over that range).

    Unfortunately there is no real shortcut to having technical/mathematics knowledge when it comes to understanding technical specs. If one wants to understand what specs really mean, one has to study some math, physics, acoustics, and electronics. Some of us have done so and are glad to help others as much as we can reasonably can. It is very easy to oversimplify, because acoustics and audio can be very daunting topics for which one can easily spend 6-8 years in college and decades studying. There are probably some books out there than can help you get started - or just read a lot of posts here and/or in Usenet newsgroups on the Internet. Some specs, as you are guessing, can be misleading if one doesn't look at the whole picture. Sometimes we have to rely on others' experiences and/or the reputation of the manufacturer.
    - Mike
  18. MikeyD


    Sep 9, 2000
    An interesting phenomenon on which I've read other bass players' comments, but have yet to experience definitively myself. I do believe that speaker size (the aggregate size of all the radiating surfaces, that is) does have a material effect on the development of low wavelengths of bass signals. It is a bit puzzling why one 18" driver might differ noticeably from, say, four 10" drivers in one cabinet - other than the basic cabinet frequency response. I'm now wondering if it might have something to do with acoustical impedance. There is a reactive effect going on close to a cone wherein air particle velocity and pressure are not in phase; yet eventually, they align in phase at a distance from the cones so that the acoustical impedance becomes resistive, not reactive. I'm really rusty in this area, but I'm wondering if that might be part of the difference. There may be some funny things going on between drivers of a 4x10" speaker - some mass transport phenomena in the space between the drivers that does not occur with a single 18" because the entire cone is moving (with no dead baffle space between, as in the case of a 4x10). I'm very curious about the effect you describe. I've read posts to the effect that Ampeg's 8x10" won out over big folded horn designs in the 1970s because the bass became more well developed in the confines of a music store room than with the folded horn; yet the folded horns tended to have better "throw" in bigger spaces. Really interesting, and I will continue to think about it and do some reading. Any acoustics experts out there? Let's jam on this one!
    - Mike
  19. I had prepared this reply, but was unable to post it yesterday, some answers are already given by MikeyD, thank for that, just read over it.......
    VicDamone -- Yes there is. The following method gives you absolute proof. Put a music signal on the XLR. Put an AC voltmeter (millivolts) first on pins 1 and 2 of the XLR, if you measure something fluctuating, it's OK, there's music. Second, measure on pins 1 and 3. If you measure the same kind of thing, no doubt, it's balanced.

    Marty Forrer -- There are a lot of parameters that influence the "loudness" of an amplifier: the sensitivity of the connected speaker(s), the honesty of the power measurements of the amp (max, peak or rms) and, most important, EQ setting. Low frequencies EAT power. Slide the EQ fader up 3 dB and lose about half your power.

    MikeyD -- nice to see you back! Thanks for your post.

    mikemulcahy -- looks like a blown horn, or a blown crossover. If the horn is still making any sound, it may just be torn or something, not fused. But maybe, just maybe you're clipping the amps. Distortion, even from solid state, at high volume (doesn't even have to be ludicrously high) isn't very audible, 'specially with a switched off horn.

    Mesa Man -- When you buy a 4 ohms cab (I was referring to speakers only), the manufactorer should have corrected the sensitivity rating already (subtracting 3 dB). Only when they want to brag about their specs, they haven't... Unfortunately there's very little way of telling, but I suspect only low-budget brands will ly about it. If a 4 ohms cab is specified as XX dB (1W/1m [on axis]), you got the right number. If you see XX dB (2.83V/1m [on axis]), pro'lly not.....

    rickbass1 -- you're quite welcome :D.

    1. About the Acme/JBL story: gotta be a misprint, it's complete bull. 309 dB is physically impossible. Air can't vibrate that strongly. They should have printed (you might have come to the same conclusion) 128 dB at 1 meter. 3 meters isn't even a valid measurement distance. Come to think of it, it's not a misprint, it's just freakin' BS. They just multiplied dBs. YOU CAN NEVER DO THAT. 309 dB is literally several zillion times the pain barrier.

    2. The proper way to calculate: SPLmax=u0+10*log(Pmax), substitution gives 128dB=103db+10*log(300W)

    3. That's kinda hard to answer. As long as there are dozens of measurement methods, it's not just picking out, for instance, the rms value. With amps, though, it's fairly simple. Full bandwidth (pink noise) power is the most valuable spec. An 1 kHz rating is useless, at least for bass it is. Speakers are very different. Good speakers can handle 4 times (sometimes 10 times) their rms value. That's what they call "crest factor". For convenience, I take the long-time rms value, knowing that it's OK to go far over it. I design cabinets at rms power, because most speakers will distort anyway above it. A port in a 2x10 cab doesn't have to sound clean at 3000 watts. Who'll notice anyway....
  20. the_iceman


    Nov 19, 2000

    I read the posts here and I believe I have developped a basic understanding of the theories. So what you are saying is that speakers with a higher efficiency (a higher decibel rating) will require less power to run effectively, but they will not produce the absoulute BEST sound right (sorry I'm pretty new to most of the technical stuff)?

    With this in mind, I am thinking about looking for a 15" cab and a good(efficient) 2X10 to be powered by my 150w @4 ohm head. I remember reading that most 15" cabs are pretty efficient, so I thought that this setup would be the most favorable because of the low power amp I am working with. I want to be sure to avoid clipping in the speakers especially, which I read comes with speakers being under-powered. What do you think about this, and can anyone recommend any highly efficient 2X10's? Thanks