question for you engineer types

Discussion in 'Amps and Cabs [BG]' started by LakLnd5, Mar 12, 2002.

  1. Hello bass players! I have a question for those of you who are engineer types. I'm attempting to calculate the average difference in SPL between a 100 watt amplifier into an 8 ohm cab and the same amp into a 2 ohm load.
    I know that the dB in relation to power is a logarithmic relationship and I have a book that goes through the formula's, but it's confusing.
    Using the book, it seems that I have to convert my units to volts in order to get to dB. But I get confused between dBu, dBm, dBV. .
    I've come up with the voltage for each scenario (8 ohms gets 28 volts and some change, 2 ohm gets half of that). I used OHMS LAW to get the voltage but now I'm stuck.
    I understand that I would be coming up with an approximation due to all the other "real world" factors. I just want to get a grip on the conversions. If anyone could help me out, I'd appreciate it.

  2. This one belongs in amps. Thanks Oysterman for bringing it to my attention. I would have caught it when i came online next, but it's nice to have it pointed out so i can take care of it immediately.
  3. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania
    Well, in your hypothetical question, it would make a difference between tube and solid state, since tubes amps can hold the same wattage at each ohm rating.

    I'd say if it's solid state you would be getting 300 watts then, and it would be a 5.5db increase?

    I could be wrong, but that's how I've always seen things work.

    Each double in wattage usually equates to a 3db increase. I'm not so sure about the ohm thing, though.
  4. chucko58


    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    You haven't given nearly enough information. For instance, is the amp 100 watts at 8 ohms and at 2 ohms, or is it a typical solid-state amp that produces more power at 2 ohms? Are they the same speakers in the same cabinet? How efficient are the speakers? Are you using decent speaker cable, or a guitar cable? Etc.

    If it's 100 W into both loads, and you are using the same speakers in the same cabinet, and they are connected with a short cable of adequate gauge wire, the SPLs will be the same. Change any one of those assumptions and you will have to run the numbers.
  5. OK, maybe I wasn't clear. I have a 100 watt head (a marshall w/ tubes) that is plugged into a marshall 8 ohm 4x10 cab. If I plug the exact same head into a DIFFERENT cab that is 2 ohms with the exact same speaker cable, what is the difference in dB (approximately).
    Please, let me know how you came up with the numbers.


  6. I'm not as expert as many here, but I'll give it a whack, subject to correction by the more knowledgeable ones (in whose general direction I genuflect). First, it's my understanding that the resistance of a cab, *in and of itself*, has nothing to do with how loud a cab can get. That, AFAIK (someone correct me if I'm off, please), is determined by (a) the amount of power delivered into the speaker and (b) the efficiency of the speaker.

    Most tube amps, I believe, deliver about the same power into all the loads they can drive. Which means that however much you fiddle with the cabs, you're not going to get the amp to give you appreciably more or less power (unless you intentionally mismatch cabs to resistance settings, but that's another story).

    Where cab resistance does come into play is with SS amplifiers, for which, all else being equal, output power decreases as cab resistance goes up and increases as cab resistance goes down. So cab resistance is only relevant to overall volume insofar as it affects (a) above, which it doesn't ijn your case AFAIK.

    Thus, since you have a 100 W *tube* amp, you're still only going to get about the same amount of power. That is, (a) above won't change. This means that your only way of getting more volume is to increase (b)--speaker efficiency. Just because a speaker has a lower resistance, that doesn't mean it's more efficient. Example: two 4 ohm Acme 2-10s (total 2 ohms in parallel) is still going to be less efficient--and therefore not as loud with equal input--as one 8 ohm Eden D410XLT.

    So there's no way of answering your question without knowing the efficiency of the 2nd cab. If we assume that it has the same efficiency as the 1st cab, then your volume gain would be zero in the scenario you describe, because as stated before, neither (a) above nor (b) would be changing.
  7. Interesting. Can you explain why a tube amp and a solid state amp react differently to varying impedences? I've never heard that before.

    My situation is that I have 2 guitar players in my band who both use the same head (the 100W marshall). One plays through a marshall 8-ohm 4x12. The other plays through two 4-ohm 2x12's that are custom made. The latter is SIGNIFICANTLY louder than the other. I'm trying to figure out why.

    But according to you, it's simply due to driver efficiency. Is that right? Because they're both tube amps, they are unaffected by impedence in regards to power output?

    Their must be more to it. The difference in volume is drastic. What else could it be?

    I appreciate the input.


  8. Lots more to it. Too many variables to generalize. Tell that guy not to run a Marshall tube head into 2 ohms. He'll fry it. Hook them in true series to make an 8 ohm load. The amp will live longer.

  9. The one is louder because there are more speakers. (Also, the individual 4-12 cabs may be each more efficient than the other guy's single 4-12.) Adding speakers, all else being equal, ups your efficiency in practical terms. Put two identical cabs together, and the resulting efficiency is maybe 3 to as much as 6 dB greater than either alone.

    Remember, watts aren't units of volume, they're units of power. 50 watts into a very efficient cab or cabs is louder than 100 into a very inefficient cab or cabs. With both of the guys in your band, the amp puts out 100 W, more or less. But the guy with the two 4-12s is converting his 100 W to acoustic output with greater efficiency, that's all. (I'm assuming it's not just a matter of his turning up higher.)

    I am embarrassed to say I don't know why impedance doesn't interact with power output the same way for tube amps as for SS amps. But there are certainly other folks here who do.
  10. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania
    Has anyone noticed that he's using a guitar head, and not a bass head?

    He's also comparing the same exact amp which is intended for guitar, in a playing guitar on it vs playing bass on it.

    As a rule of the thumb, you need 3-4 times more power in your bass amp, to match the guitarist. For an 100 watt guitarist, I'd use at least 350 watts.
  11. Throbbinnut's got it covered!

    Basically, tubes are high voltage/low current devices (400-600 V, is common in tube amp power sections). Speakers tend to work better at low voltages/higher currents (10-40 V). The transformer just steps the voltage down (and the current up) so that the amp's output is useable by speakers. One of the effects of this is that the power amp always sees only the impedence of the primary winding of the transformer and not the impedences of the speakers. Running tube amps at impedences other that what they are rated for is not a really good idea, as Throbbinnut said.

    Different speakers can have radically different SPLs irrespective of impedence. Say you have two speakers one with a sensitivity of 95dB 1 watt/1 meter and another with a senstivity of 105dB. That's 10dB more, which we hear as double the volume. So yes, speaker efficiency can cause drastic differences in sound pressure levels.
  12. Well, thanks for clearing that up. I never knew that about tube amps.
    I think there are other factors causing the difference in in volume between the two guitar players. But I'm sure the difference between the speaker cabs is the main culprit. Thanks again.

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